HETEROCYCLES

Paper | Special issue | Vol. 79, No. 1, 2009, pp. 1043-1060
Received, 17th November, 2008, Accepted, 18th December, 2008, Published online, 22nd December, 2008.
DOI: 10.3987/COM-08-S(D)81

■ Catalytic Asymmetric Synthesis of Both Enantiomers of Pyrrolizidines 223H’, 239K’, 265H’, and 267H’ Found in Madagascan Frogs (Mantella) and Their Affinities for Nicotinic Acetylcholine Receptor

Yukako Saito, Seiki Takahashi, Nehad Azer, Amira T. Eldefrawi, Mohyee E. Eldefrawi, and Hiroki Takahata*

Tohoku Pharmaceutical University, 4-4-1 Komatsushima, Aoba-ku, Sendai 981-8558, Japan

Abstract

The asymmetric synthesis of pyrrolizidines 223H’, 239K’, 265H’, and 267H’ has been achieved starting from 1,5-hexadiene via a common synthetic intermediate 5. The affinity of both enanthiomers of 1–4 for nicotinic acetylcholine receptor was evaluated.

INTRODUCTION
A diverse array of biologically active, lipid-soluble alkaloids have been discovered in amphibian skin. During the past 30 years, over 800 alkaloids belonging to more than 20 structural classes have been detected.1 Among them, a variety of ‘izidines’ (pyrrolizidines, indolizidines, and quinolizidines) have been reported. In 1993, four 3,5-disubstituted pyrrolizidines 223H’ (1), 239K’ (2), 265H’ (3), and 267H’ (4) were isolated from Madagascan frogs (Mantella).2 It is known the relative structure of pyrrolizidine 223H’ is identical to that of xenovenine found in an ant (Solenopsis xenoveneum).3 It was recently reported that the absolute configuration of both alkaloids from frog and ant has the same absolute configuration, as shown in Chart 1.1 However, the absolute configurations of the other three pyrrolizidnes remain unknown. The syntheses of chiral forms of xenovenine have been up to now reported several times,4 whereas the syntheses of the other three have never been reported in either racemic or chiral forms. Among the poison-dart frog alkaloids, 3,5-disubstituted, 5,8-disubstituted, and 5-substituted indolizidines5 appear to represent an atypical and potent class of noncompetitive blockers for muscle-type and ganglionic nicotinic receptor-channels. However, the biological activity of homologues such as 3,5-disubstituted pyrrolizidines has not been investigated. Our current interest is in binding tests for the nicotinic acetylcholine receptor (nAChR) of these alkaloids, along with any detectable differences between the enantiomers. Accordingly, we developed a comprehensive synthetic program for preparing these alkaloids. In this paper we wish to report on the asymmetric synthesis of 3,5-disubstituted pyrrolizidines 223H’ (1), 239K’ (2), 265H’ (3), and 267H’ (4) from trans-2,5-disubstituted pyrrolidine as a common synthetic intermediate together with the synthesis of their enantiomers. In addition, their binding to the nicotinic acetylcholine receptor (nAChR) was examined.6

RESULTS AND DISCUSSION
Our retrosynthetic analysis is shown in Scheme 1. The formation of 3,5-disubstituted pyrrolizidines could be stereoselectively accomplished by the reductive annulation of a ketopyrrolidines III via transient iminium ions II.4d The pyrrolidines could be constructed from the common intermediate, trans-2-hydroxymethyl-5-(6-heptenyl)pyrrolidine (5), prepared by the kinetically controlled intramolecular amidomercuration4d of a chiral 4-pentenylcarbamate 6, which can be obtained from achiral 1,5-hexadiene (8) using the Sharpless asymmetric dihydroxylation (AD)7 as a chiral source. The synthesis of the pyrrolidine was carried out as shown in Scheme 2.
The (S)-diol 7 (70% ee)9 prepared by the (DHQ)2-PYR8 ligand-induced AD reaction of 1,5-hexadiene (8) was successively subjected to epoxidation,10 and the regioselective cleavage of the resulting epoxide ring without further purification with 6-hexenylmagnesium bromide in conjunction with a cuprous iodide to give the alcohol 9 in 42% overall yield from 7. The secondary alcohol 9 was converted via a four-step sequence (1. mesylation; 2. azidation; 3. reduction; 4. carbamation) into the desired N-benzyloxycarbonyl group (N-Cbz) 6 in 56% yield. The mercuric acetate-mediated cyclization of diene 6 followed by treatment with aqueous NaBr afforded the organomercurial, which was subjected to oxidative demercuration11 to provide the trans diastereomer 5 in 73% yield without the concomitant formation of the cis isomer and the azocine. This annulation proceeded stereoslectively and regioselectively.

Having the common synthetic intermediate 5 in hand, we began with the elaboration of the ring appendage (hydroxymethyl). A Swern oxidation followed by the Horner–Emmons reaction using Masamune-Roush's conditions gave the α,β-unsaturated ketones 10 and 11 in 72 and 71% yields, respectively. A sequence of hydrogenolysis, the formation of immonium intermediate, and the reduction by the treatment of 10 with hydrogen in the presence of Pd(OH)2 as a catalyst in methanol gave the desired (+)-pyrrolizidine 223H’ (1) {[α]26D +10.2° (CHCl3)}12, lit.4d{[α]23D +10.9° (CHCl3)}, in 54% yield along with its C-5 epimer 12 (10%). The spectral data for 1 were in agreement with previously reported data.4d The Wacker oxidation of the terminal olefin on 11 gave the diketone 13 in 73% yield. Exposure of the diketone to hydrogen under the above conditions proceeded regioselectively to give (+)-pyrrolizidines 265H’ (3) {[α]27D +3.50° (CHCl3)}, in 55% yield together with its C-5 epimer 14 (13%)}.

We then focused on the synthesis of pyrrolizidines 239K’ and 267H’. The hydroxyl group of the C-3 appendage was installed via the AD reaction. Unfortunately, the AD reaction of 10 resulted in a mixture of diol and the tetraol. In advance of the AD, the conjugate olefin 10 was reduced with Red-Al, in the presence of CuBr13, thus converting it to the monoolefin 15 in 74% yield. The (DHQ)2-PYR8 ligand-induced AD reaction of 15 afforded diol 17 in 84% yield. The selective monotosylation of primary hydroxyl group was accomplished in a two-step sequence (1. Bu2SnO 2. p-TsCl) to give tosylate 21 in 58% yield. The formation of a pyrrolizidine ring was performed by exposing 21 to hydrogen in the presence of Pd(OH)2 as a catalyst. The tosyloxy group of the resulting pyrrolizidine, without purification, was reduced with Super-Hydride®, to the desired pyrrolizidine 239K’ (+)-(6’R-hydroxy)-214 in 58% yield15 from 21. Similarly, (+)-(6’S)-2 was obtained from 15 via the (DHQD)2-PYR8 ligand-induced AD reaction of 1514 The two pyrrolizidines 267H’ [(+)-(6’S)-and(+)-(6’R)-4]14 were next prepared from 16, as shown in Scheme 4 by a similar procedure.15

The analogous synthesis of six enantiomers of 1, (6’S)-2, (6’R)-2, 3, (6’S)-4, and (6’R)-4 was performed starting from ent-7 (80% ee) (Scheme 5).6

With twelve chiral pyrrolizidines in hand, we turned our attention to examining their biological activity. The interaction of the prepared pyrrolizidines with the binding sites on carbamylcholine-activated nicotinic acetylcholine receptor (nAChR) channel complex from Torpedo californica electric organ was investigated using radiolabeled probe, [3H]-thienyl-cyclohexylpiperidine ([3H]-TCP). The Ki values for inhibition of [3H]-TCP, compared to those for the 3,5-disubstituted indolizidines 25 and 26,5a are shown in Table 1. Interestingly, the affinity of 1 was increased by one order compared with the corresponding indolizidines 25 and 26. Since the introduction of a hydroxyl moiety in a side chain dramatically decreases the affinity, the structure–activity relationships suggest that hydrophobic interactions are important factors. As a result, the stereoconfiguration of the hydroxyl group had little effect on ion channel interactions. Surprisingly, no significant differences in affinities were observed between the two enantiomers.

In summary, the total synthesis of both enantiomers of the six 3,5-disubstituted pyrrolizidines 1-4 has been asymmetrically achieved starting from a symmetrical 1,5-hexadiene. Their affinities for the nAChR channel of T. californica were evaluated for the first time.

EXPERIMENTAL
General. Melting points are uncorrected. NMR spectra were recorded at 300 MHz (1H), 75 MHz (13C) using CDCl3. As an internal standard, tetramethylsilane was used for CDCl3. Mass spectra were obtained by EI or FAB mode. Silica gel for chromatography was Fuji Silysia PSQ 100B. When the reagents sensitive to moisture were used, the reaction was performed under argon atmosphere.

(R)-5-Hydroxydodeca-1,11-diene [(R)-9].
To a solution of (S)-7 (70% ee)9 (4.374 g, 37.6 mmol) and PPTS (94.5 mg, 0.38 mmol) in CH2Cl2 (47.7 mL) was added trimethyl orthoacetate (6.22 mL, 48.88 mmol) at rt and the mixture was stirred at the same temperature for 15 min. After evaporation of the nixture, triethylamine (0.52 mL, 3.76 mmol) was added in a solution of the residue in CH2Cl2 (47.7 mL). To a solution was slowly added acetyl bromide (3.34 mL, 45.12 mmol) at 0 °C and then the mixture was stirred for 1 h. To the reacion mixture was added 1M HCl (52 mL). The mixture was extracted with CH2Cl2, dried over anhyd. Na2SO4, and evaporated. Pulverized NaOH (3.308 g, 82.72 mmol) was added a solution of MeOH (2.29 mL, 56.4 mmol) in Et2O (92.1 mL) and the reaction was stirred for 2.5 h. The mixture was filtered over K2CO3 and washed with Et2O. The filtrate and washings were distilled at an atmosphere to yield the crude epoxide. To a suspension of Mg (1.113 g, 45.9 mmol) in THF (18.7 mL) was added 1-bromohexene (7.343 g, 45.9 mmol) with heating. The prepared Grignard reagent was added to a suspension copper(I) iodide (5.83 g, 33.66 mmol) in THF (52.0 mL) at –78 °C and then the temperature was raised to –40 °C. To the mixture was added a solution of the crude epoxy in THF (6 mL) at the same temperature and the mixture was stirred for 3 h. The reaction was quenched with sat. aq. NH4Cl and separated. The aqueous layer was extracted with Et2O. The combined solutions were successively washed with sat. aq. NH4Cl, water, and brine, dried over anhyd. Na2SO4, and evaporated. The residue was purified by silica gel column chromatography (n-hexane : EtOAc = 30 : 1) to give (R)-9 (3.553g, 42%) as an oil; [α]27D+0.86° (c 2.75, CHCl3); IRνmaxcm-1 (neat) 3354, 2930, 2856, 1640, 909; 1H-NMR (300 MHz, CDCl3) δ 1.10-1.79 (10H, m), 1.96-2.25 (4H, m), 3.57-3.63 (1H, m), 4.91-5.08 (4H, m), 5.74-5.91 (2H, m); 13C-NMR (75 MHz, CDCl3) δ 25.60, 28.97, 29.26, 30.18, 33.83, 36.55, 37.50, 71.45, 114.35, 114.77, 138.74, 139.11; Anal. Calcd for C12H22O : C, 79.06; H, 12.16. Found: C, 78.95; H, 12.31.
(S)-9; (71%); [α]28D-0.93° (c 1.14, CHCl3).
(S)-5-Benzyloxycarbonylaminodeca-1,11-diene [(S)-6].
To a solution of (R)-9 (3.93 g, 18.61 mmol) and DMAP (340.7 mg, 2.79 mmol) in pyridine (31.4 mL) was dropwise added methanesulfonyl chloride (3.24 mL, 27.92 mmol) at 90 °C and the whole was stirred for 2 h. The mixture was raised to rt and was further stirred for 30 min. Then the reaction was diluted with Et2O and acidified with aq. KHSO3 at 0 °C. The whole was separated and the aqueous layer was extracted with Et2O. The combined solvents were successively washed with water and brine, dried over anhyd. Na2SO4, and evaporated. The residue was purified by silica gel column chromatography (n-hexane : EtOAc = 30 : 1) to give the mesylate (4.521 g, 93%) as an oil;
A suspension of the mesylate (4.46 g, 17.13 mmol) and NaN3 (1.67 g, 25.7 mmol) in DMF (33.8 mL) was heated with stirring at 45 °C for 3 h. After addition of water at rt, the whole was extracted with Et2O. The extract was dried over anhyd. Na2SO4, and evaporated. The residue was purified by silica gel column chromatography (n-hexane) to give the azide (2.486 g, 70%) as an oil; [α]26D+13.21° (c 3.30, CHCl3); IRνmaxcm-1 (neat) 2933, 2857, 2097, 1641, 1458, 1342, 1256, 994, 912; 1H-NMR (300 MHz, CDCl3) δ 1.28-1.65 (10H, m), 2.02-2.25 (4H, m), 3.27 (1H, qui, J = 6.59 Hz), 4.92-5.09 (4H, m), 5.73-5.87 (2H, m); 13C-NMR (75 MHz, CDCl3) δ 26.07, 28.88, 29.02, 30.46, 33.80, 34.53, 62.48, 77.41, 114.50, 115.50, 137.62, 138.98; HRMS calcd for C12H21N3 207.1735, found 207.1743.
To a suspension of LiAlH4 (1.591 g, 41.88 mmol) in THF (130.8 mL) was slowly dropwised a solution of the azide (2.894 g, 13.96 mL) in THF (20.0 mL) at 0 °C and the whole was stirred for 1 h. To the mixture were successively added water (1.6 mL), 2N NaOH (1.6 mL), and water (4.8 mL). The whole was filtered through Celite and washed with ether. To the combined solvents was added 2N NaOH (6.2 mL) and CbzCl (2.18 mL, 15.36 mmol) at 0 °C and the whole was stirred at rt for 2.5 h. The mixture was extracted with Et2O. The extract was successively washed with 20% KHSO3, sat. aq. NaHCO3, and brine, dried over anhyd. Na2SO4, and evaporated. The residue was purified by silica gel column chromatography (n-hexane : EtOAc = 60 : 1) to give (3.787 g, 86%) (S)-6 as an oil; [α]27D-0.88° (c 1.99, CHCl3); IRνmaxcm-1 (neat) 3311, 3067, 3035, 2978, 2922, 2852, 1682, 1542, 1290, 1259, 916, 726, 696; 1H-NMR (300 MHz, CDCl3) δ 1.20-1.66 (10H, m), 2.00-2.14 (4H, m), 3.62-3.66 (1H, m), 4.48-4.52 (1H, m), 4.91-5.04 (1H, m), 5.09 (2H, s), 5.73-5.85 (2H, m), 7.30-7.37 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 25.21, 25.71, 28.84, 29.06, 30.21, 33.75, 34.71, 35.42, 50.99, 66.50, 114.35, 114.88, 127.90, 128.07, 128.52, 136.80, 138.16, 139.01, 156.18; HRMS calcd for C20H29NO2 315.2199, found 315.2171.
(R)-6; (61%); [α]26D+1.24° (c 2.30, CHCl3).
(2R,5S)-1-Benzyloxycarbonyl-2-hydroxymethyl-5-(6’-heptenyl)pyrrolidine (5).
To a solution of (S)-6 (3.226 g, 10.23 mmol) in THF (186 mL) was added mercuric acetate (3.419 g, 10.74 mmol) and then the reaction mixture was stirred for 18 h at rt. Sat. aq. NaHCO3 (878.5 mg, 10.23 mmol) was added to the mixture with ice cooling. After 30 min of stirring, sat. aq. KBr (5.127 g, 40.92 mmol) was added to the mixture. After 2 h of stirring, the THF layer was separated. The aquous layer was extracted with EtOAc. The combined organic layers were washed with brine and dried. After evaporation, the resulting residue was purified by silica gel column chromatography (n-hexane : EtOAc = 15 : 1) to give the organomercurial (5.307 g, 87%) as an oil;
Oxygen (O2) was bubbled into a suspension of NaBH4 (466 mg, 12.28 mmol) in DMF (125 m) for 1 h, and to this was dropwise added a solution of organomercurial (5.219 g, 8.77 mmol) in DMF (363 mL) over 3 h with continuous introduction of O2. The bubbling of O2 into the mixture was continued for 1 h, and Et2O was added. The reaction mixture was filtered through Celite, and the filtrate was evaporated. The residue was purified by silica gel column chromatography (n-hexane : EtOAc = 5 : 1) to give 5 (2.445 g, 84%) as an oil; [α]26D+31.49° (c 2.75, CHCl3); IRνmaxcm-1 (neat) 3440, 2926, 2855, 1682, 1498, 1415, 1355, 1197, 1112, 1050, 909, 772, 732, 697; 1H-NMR (300 MHz, CDCl3) δ 1.21-2.13 (14H, m), 3.80-4.00 (1H, m), 4.01-4.11 (1H, m), 4.91-5.09 (2H, m), 5.11-5.37 (2H, m), 5.70-5.84 (1H, m), 7.28-7.40 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 26.42, 26.70, 26.85, 28.33, 28.91, 29.02, 32.78, 32.89, 33.69, 33.84, 59.17, 60.51, 67.35, 114.49, 128.16, 128.24, 128.34, 128.68, 139.10, 156.81; HRMS calcd for C20H29NO3 331.2148, found 331.2121.
ent-5; (63%); [α]26D-38.61° (c 1.03, CHCl3).
(2R,5S)-1-Benzyloxycarbonyl-2-(3’-oxo-1’-butenyl)-5-(6’-heptenyl)pyrrolidine (10).
To a solution of oxalyl chloride (482.5 μL, 5.56 mmol) in CH2Cl2 (7.0 mL) was dropwise added DMSO (783.8 μL, 11.12 mmol) at –78 °C, and the mixture was stirred for 10 min. To the whole was added a solution of 5 (922 mg, 2.78 mmol) in CH2Cl2 (7.0 mL) at the same temperature, and the mixture was stirred for 30 min. Then triethylamine (2.31 mL, 16.68 mmol) was dropwise added to the mixture and the whole was raised to –30 °C. After raising to rt, the mixture was stirred for 10 min and quenched with water. Then the whole was diluted with Et2O and separated. The aqueous layer was extracted with Et2O. The combined organic layers were dried and evaporated to yield the aldehyde. Without further purification, dimethyl (2-oxopropyl)phosphonate (245.9 mL, 1.78 mmol) and N,N-diisopropylethylamine (310.7 mL, 1.78 mmol) was successively added to a suspension of LiCl (75.5 mg, 1.78 mmol) in MeCN (12 mL) at 0 °C. To the whole was added a solution of the aldehyde in MeCN (9 mL) and the mixture was stirred at rt for 20 h. The whole was quenched with sat. aq. NH4Cl at 0 °C, diluted with Et2O, and separated. The aqueous layer was extracted with ether and the combined layers were dried over anhyd. Na2SO4, and evaporated. The residue was purified by silica gel column chromatography (n-hexane : EtOAc = 10 : 1) to give 10 (394 mg, 72%) as an oil; [α]26D+59.60° (c 2.15, CHCl3); IRνmaxcm-1 (neat) 2927, 2856, 1698, 1630, 1456, 1405, 1351, 1255, 1187, 1104, 698; 1H-NMR (300 MHz, CDCl3) δ 1.23-2.24 (14H, m), 2.10, 2.24 (6 : 5 ratio 3H, s), 3.85-3.92 (1H, m), 4.45-4.58 (1H, m), 4.91-5.16 (4H, m), 5.75-5.81 (1H, m), 5.91, 6.03 (6 : 5 ratio 1H, d, J = 15.9 Hz), 6.58, 6.67 (6 : 5 ratio 1H, dd, J = 15.9, 6.0 Hz), 7.36-7.28 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 26.53, 26.59, 26.67, 27.51, 27.62, 27.67, 28.47, 28.85, 28.96, 29.00, 29.14, 29.49, 32.75, 32.87, 33.80, 33.83, 34.00, 58.11, 58.17, 58.26, 58.76, 66.84, 67.06, 114.40, 114.46, 128.16, 128.19, 128.28, 128.36, 128.56, 128.60, 129.44, 129.57, 136.69, 136.72, 139.04, 139.15, 146.45, 146.88, 154.20, 154.41, 198.21, 198.56; HRMS calcd for C23H31NO3 369.2304, found 369.2304.
ent-10; (70%); [α]26D-72.24° (c 2.59, CHCl3).
(2R,5S)-1-Benzyloxycarbonyl-2-(3’-oxo-1’-hexenyl)-5- (6’-heptenyl)pyrrolidine (11).
By the analogous procedure described for 10, Swern oxidation of 5 (922 mg, 2.78 mmol) with oxalyl chloride (482.5 μL, 5.56 mmol), DMSO (783.8 μL, 11.12 mmol), and triethylamine (2.31 mL, 16.68 mmol) gave the aldehyde, which was converted with LiCl (141.6 mg, 3.34 mmol), dimethyl(2-oxopentyl)phosphonate (539.4 μL, 3.34 mmol), DIEPA (581.8 μL, 3.34 mmol) into 11 (785 mg, 71%) as a solid; mp 69-70 °C; [α]26D+59.90° (c 1.94 CHCl3); IRνmaxcm-1 (KBr) 2927, 2858, 1698, 1630, 1466, 1418, 1376, 1349, 1193, 1110, 771, 755, 702; 1H-NMR (300 MHz, CDCl3) δ 0.86-0.94 (3H, m), 1.23-2.24 (16H, m), 2.32, 2.51 (5 : 4 ratio 2H, t, J = 7.14 Hz), 3.84-3.92 (1H, m), 4.45-4.56 (1H, m), 4.91-5.19 (4H, m), 5.73-5.85 (1H, m), 5.92, 6.04 (5 : 4 ratio 1H, d, J = 15.94 Hz), 6.62, 6.68 (5 : 4 ratio 1H, dd, J = 6.04, 15.94 Hz), 7.28-7.36 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 13.89, 13.93, 17.56, 26.54, 26.62, 27.64, 28.50, 28.87, 28.97, 29.02, 29.09, 29.15, 29.44, 32.87, 33.80, 33.84, 34.03, 42.65, 42.86, 58.15, 58.25, 58.32, 58.76, 66.81, 66.99, 114.40, 114.46, 128.10, 128.15, 128.18, 128.31, 128.52, 128.59, 128.81, 136.70, 136.80, 139.04, 139.16, 145.04, 145.61, 154.26, 154.38, 200.31, 200.59; HRMS calcd for C25H35NO3 397.2617, found 397.2613.
ent-11; (72%); [α]26D-65.25° (c 2.48, CHCl3).
(3R,5S,8S)-5-Heptyl-3-methylpyrrolizidine (1) and (3S,5S,8S)-5-Heptyl-3-methylpyrrolizidine (12).
A suspension of 10 (200 mg, 0.54 mmol), Pd(OH)2 (35 mg) in MeOH (6 mL) was stirred under hydrogen atmosphere for 20 h. The reaction mixture was filtered through Celite and washed with CH2Cl2. The combined organic solvents were evaporated. The residue was purified by basic alumina column chromatography (n-hexane : Et2O = 7 : 1) to give 1 (65 mg, 54%) and 12 (12 mg, 10%) as oils; 1; [α]26D+10.02° (c 0.71, CHCl3); IRνmaxcm-1 (neat) 2955, 2925, 2855, 1461, 1371, 1353, 1130, 1103, 745, 723; 1H-NMR (300 MHz, CDCl3) δ 0.86 (3H, t, J = 7.10 Hz), 1.09 (3H, d, J = 6.04 Hz), 1.26-1.99 (20H, m), 2.57-2.62 (1H, m), 2.71-2.78 (1H, m), 3.59 (1H, qui, J = 6.59 Hz); 13C-NMR (75 MHz, CDCl3) δ 14.28, 22.08, 22.84, 27.45, 29.53, 30.06, 31.89, 32.05, 32.28, 32.61, 34.62, 37.29, 61.83, 65.11, 66.81; HRMS calcd for C15H29N 223.2300, found 223.2315. 12; [α]28D+21.73° (c 0.75, CHCl3); IRνmaxcm-1 (neat) 2926; 1H-NMR (300 MHz, CDCl3) δ 0.86 (3H, t, J = 6.59 Hz), 1.18 (3H, d, J = 7.14 Hz), 1.26-2.10 (20H, m), 2.86-2.91 (1H, m), 3.19-3.27 (1H, m), 3.55-3.61 (1H, m); 13C-NMR (75 MHz, CDCl3) δ 14.31, 17.32, 22.87, 27.44, 29.58, 30.11, 31.08, 32.07, 32.77, 33.45, 38.64, 57.76, 57.93, 66.27; HRMS calcd for C15H29N 223.2300, found 223.2271.
ent-1; (59%); [α]26D-10.93° (c 1.45, CHCl3). ent-12; (16%); [α]29D-28.07° (c 1.02, CHCl3).
(2R,5S)-Benzyloxycarbonyl-2-(3’-oxo-1’-hexenyl)-5-(6’-oxohepthyl)pyrrolidine (13).
A suspension of PdCl2 (11.2 mg, 0.063 mmol) and CuCl (68.7 mg, 0.69 mmol) in a mixture of water and DMF (500 μL, 1 : 7) was stirred under oxygen for 1 h. To the whole was added a solution of 11 in a mixture of water and DMF (182 μL, 1 : 7) and then under oxygen the reaction mixture was heated with stirring at 40 °C overnight. 3N HCl (1 mL) was added to the whole and the aqueous layer was extracted with Et2O. The extract was dried over anhyd. Na2SO4, and evaporated. The residue was purified by silica gel column chromatography (n-hexane : EtOAc = 3 : 1) to give 13 (189 mg, 73%) as a solid; mp 63-64 °C; [α]27D+57.96° (c 1.67, CHCl3); IRνmaxcm-1 (KBr) 2937, 2858, 1698, 1631, 1467, 1420, 1350, 1323, 1192, 1110, 970, 771, 760, 704; 1H-NMR (300 MHz, CDCl3) δ 0.84-0.92 (3H, m), 1.20-1.28 (4H, m), 1.45-1.71 (8H, m), 1.85-1.95 (2H, m), 2.09, 2.11 (unknown ratio 3H, s), 2.27-2.36 (2H, m), 2.40, 2.48 (5 : 4 ratio 2H, t, J = 7.14 Hz), 3.81-3.90 (1H, m), 4.45, 4.51 (5 : 4 ratio 1H, t, J = 7.14 Hz), 4.93, 5.05 (1H, d, 5 : 4 ratio 1H, d, J = 12.64 Hz), 5.13, 5.17 (5 : 4 ratio 1H, d, J = 4.40 Hz), 5.89, 6.02 (5 : 4 ratio 1H, d, J = 15.93 Hz), 6.59, 6.65 (unknow ratio 1H, dd, J = 6.04, 15.93 Hz), 7.25-7.34 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 13.92, 13.97, 17.57, 23.77, 23.90, 26.59, 26.68, 27.68, 28.52, 29.09, 29.21, 29.46, 30.08, 32.81, 33.96, 42.89, 42.96, 43.74, 43.83, 58.06, 58.19, 58.35, 58.69, 66.87, 67.05, 128.15, 128.18, 128.24, 128.34, 128.45, 128.57, 128.62, 128.84, 136.70, 136.83, 145.01, 145.58, 154.29, 154.38, 200.35, 200.64, 209.17, 209.40; Anal. Calcd for C25H35NO4 : C, 72.61; H, 8.53; N, 3.39. Found: C, 72.71; H, 8.58; N, 3.16.

ent-13; (70%); [α]26D-65.15° (c 0.91, CHCl3).
(3R,5S,8S)-5-(6’-Oxoheptyl)-3-propylpyrrolizidine (3) and (3S,5S,8S)-5-(6’-Oxoheptyl)-3- propylpyrrolizidine (14).
A suspension of 13 (187mg, 0.45 mmol), Pd(OH)2 (18 mg) in MeOH (11 mL) was stirred under hydrogen atmosphere for 20 h. The reaction mixture was filtered through Celite and washed with CH2Cl2. The combined organic solvents was evaporated. The residue was purified by basic alumina column chromatography (n-hexane : Et2O = 7 : 1) to give 3 (66 mg, 55%) and 14 (15 mg, 13%) as oils; 3; [α]27D+3.50° (c 0.96, CHCl3); IRνmaxcm-1 (neat) 2929, 2859, 1717; 1H-NMR (300 MHz, CDCl3) δ 0.88 (3H, t, J = 6.59 Hz), 1.15-1.56 (16H, m), 1.84-1.95 (4H, m), 2.10 (3H, s), 2.39 (2H, t, J = 7.69 Hz), 2.56-2.58 (2H, br s), 3.51 (1H, t, J = 6.04 Hz); 13C-NMR (75 MHz, CDCl3) δ 14.61, 20.53, 24.10, 27.16, 29.68, 30.04, 31.87, 31.89, 37.04, 39.51, 43.98, 64.69, 66.70, 66.79, 209.61; HRMS calcd for C17H31NO 265.2406, found 265.2377. 14; [α]25D+7.98° (c 0.81 CHCl3); IRνmaxcm-1 (neat) 2931, 2862, 1717; 1H-NMR (300 MHz, CDCl3) δ 0.93 (3H, t, J = 6.59 Hz), 1.17-1.86 (16H, m), 1.89-2.01 (4H, m), 2.13 (3H, s), 2.42 (2H, t, J = 7.69 Hz), 2.83-2.82 (1H, br s), 2.98-3.10 (1H, br s), 3.51-3.59 (1H, br s); 13C-NMR (75 MHz, CDCl3) δ 14.63, 21.63, 24.07, 27.11, 29.62, 30.03, 30.25, 30.98, 31.79, 32.17, 33.81, 38.64, 43.94, 57.73, 63.88, 65.94, 209.58; HRMS calcd for C17H31NO265.2406, found 265.2390.
ent-3; (55%); [α]26D-4.0° (c 0.60, CHCl3). ent-14; (11%); [α]25D-12.69° (c 0.98, CHCl3).
(2R,5S)-1-Benzyloxycarbonyl-2-(3’-oxobutyl)-5-(6’-heptenyl)pyrrolidine (15).
To a suspension of CuBr (1.05 g, 7.29 mmol) in THF (14.6 mL) was added Red-Al (2.04 mL, 7.29 mmol) at 0 °C and then the whole was stirred 30 min. After cooling to –78 °C, 2-butanol (2.69 mL, 29.28 mmol) was added to the reaction mixture. After being stirred for 5 min, a solution of 10 (541 mg, 1.46 mmol) in THF (4.9 mL) was dropwise added to the mixture. The whole was stirred for 10 min, raised to –20 °C, and stirred for 12 h. The reaction was quenched with water and sat. aq. NH4Cl and extracted with Et2O. The extract was washed with water and sat. NH4Cl, dried over anhyd. Na2SO4, and evaporated. The residue was purified by silica gel column chromatography (n-hexane : EtOAc = 10 : 1) to give 15 (402 mg, 74%) as an oil; [α]29D+42.02° (c 2.29, CHCl3); IRνmaxcm-1 (neat) 2926, 2856, 1697, 1640, 1454, 1406, 1356, 1102, 698; 1H-NMR (300 MHz, CDCl3) δ 1.20-1.35 (6H, m), 1.56-1.66 (4H, m), 1.81-2.14 (6H, m), 1.99, 2.14 (1 : 1.1 ratio 3H, s), 2.27-2.42 (2H, m), 3.74-3.79 (2H, m), 4.94 (2H, t-like, J = 10.44, 18.13 Hz), 5.05, 5.17 (each 1H, ABq, J = 12.09 Hz), 5.72-5.83 (1H, m), 7.27-7.35 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 26.67, 26.88, 27.23, 27.58, 27.86, 28.02, 28.52, 28.97, 29.09, 29.26, 29.76, 29.93, 32.63, 33.89, 34.03, 41.13, 41.23, 41.31, 57.53, 57.93, 58.44, 58.50, 66.67, 114.27, 114.32, 127.92, 127.96, 128.18, 128.46, 128.51, 136.92, 138.98, 139.09, 154.11, 154.46, 208.06, 208.50; HRMS calcd for C23H33NO3 (M+) 371.2460, found 371.2456.
ent-15; (75%); [α]27D-47.31° (c 2.21, CHCl3).
(2R,5S)-1-Benzyloxycarbonyl-2-(3’-oxohexyl)-5-(6’-heptenyl)pyrrolidine (16).
By the analogous procedure described for 15, the reduction of 11 (969 mg, 2.44 mmol) using CuBr (1.74 g, 12.2 mmol), Red-Al (3.40 mL, 12.2 mmol), 2-butanol (4.47 mL, 43.92 mmol) gave 16 (737 mg, 76%) as an oil; [α]26D+39.12° (c 1.85, CHCl3); IRνmaxcm-1 (neat) 2927, 2858, 1695, 1457, 1407, 1355, 1109, 699; 1H-NMR (300 MHz, CDCl3) δ 0.84-0.92 (3H, m), 1.21-1.36 (8H, m), 1.46-1.75 (6H, m), 1.89-2.12 (6H, m), 2.23-2.49 (4H, m), 3.74-3.80 (2H, m), 4.94 (2H, t-like, J = 10.44, 17.58 Hz), 5.06, 5.17 (each 1H, ABq, J = 12.09 Hz), 5.72-5.84 (1H, m), 7.27-7.35 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 14.01, 17.48, 26.67, 26.89, 27.21, 27.50, 27.86, 28.03, 28.55, 28.97, 29.09 29.26, 32.64, 33.81, 33.89, 34.03, 40.22, 40.38, 44.59, 44.66, 57.21, 57.70, 57.90, 58.49, 66.65, 114.27, 114.32, 127.80, 127.90, 127.96, 128.12, 128.45, 136.95, 138.98, 139.09, 154.13, 154.43, 210.26, 210.61; HRMS calcd for C25H37NO3 (M+) 399.2773, found 399.2764.
ent-16; (73%); [α]26D-46.33° (c 1.14, CHCl3).
(2R,5S,6’S)-1-Benzyloxycarbonyl-2-(3’-oxobutyl)-5-(6’,7’-dihydroxyheptyl)pyrrolidine (17).
A solution of 15 (371 mg, 1.0 mmol) in t-BuOH (2 mL) was added to a mixture of commercially available AD-mix-α [used (DHQ)2-PYR as ligand] (1.41 g), t-BuOH (3 mL), and H2O (5 mL) at 0 °C. After the reaction mixture was stirred for 24 h at the same temperature, sodium sulfite (2.058 g) was added to the mixture. The mixture was stirred for 30 min, filtered through a Celite pad, and washed with EtOAc (50 mL) three times. The organic layer was separated, dried over anhyd. Na2SO4, and evaporated. The residue was purified by silica gel column chromatography (n-hexane : EtOAc = 2 : 3) to give 17 (330 mg, 81%) as an oil; [α]26D+36.90° (c 2.20, CHCl3); IRνmaxcm-1 (neat) 3428, 2932, 1694, 1498, 1411, 1356, 1199, 1163, 1106, 772, 751, 699; 1H-NMR (300 MHz, CDCl3) δ 1.22-1.41 (8H, m), 1.53-1.70 (4H, m), 1.85-2.21 (7H, m), 2.00, 2.15 (1 : 1 ratio 3H, s), 2.30 (2H, t, J = 7.69 Hz), 2.37-2.54 (2H, m), 3.40-3.46 (1H, m), 3.61-3.80 (4H, m), 5.06, 5.16 (each 1H, ABq, J = 12.09 Hz), 7.23-7.36 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 25.22, 25.69, 26.36, 26.45, 26.73, 26.92, 27.24, 27.53, 27.88, 28.00, 28.47, 29.02, 29.65, 29.79, 29.88, 29.97, 32.13, 32.33, 33.16, 33.25, 34.03, 41.13, 41.29, 57.11, 57.58, 57.91, 58.26, 66.71, 66.87, 66.96, 72.09, 72.30, 77.40, 127.95, 128.07, 128.18, 128.48, 128.54, 136.77, 136.92, 154.20, 154.51, 208.17, 208.67; HRMS calcd for C23H35NO5 (M+) 405.2515, found 405.2519 .
ent-17; (84%); [α]25D-44.60° (c 2.35, CHCl3).
(2R,5S,6’S)-1-Benzyloxycarbonyl-2-(3’-oxohexyl)-5-(6’,7’-dihydroxyheptyl)pyrrolidine (18).
By the analogous procedure described for 17, the dihydroxylation of 16 (349 mg, 0.88 mmol) with AD-mix-α (DHQ2PYR (1.293 g) in t-BuOH (4.8 mL), and H2O (4.8 mL) gave 18 (261 mg, 70%) as an oil; [α]26D+37.62° (c 1.32, CHCl3). IRνmaxcm-1 (neat) 3420, 2931, 2857, 1694, 1455, 1411, 1357, 1308, 1163, 1105, 772, 752, 699; 1H-NMR (300 MHz, CDCl3) δ 1.19-1.39 (8H, m), 1.47-1.62 (4H, m), 1.82-2.12 (4H, m), 1.94, 2.21 (1 : 1 ratio 3H, s), 2.24-2.66 (4H, m), 3.34-3.39 (1H, m), 3.58-3.76 (4H, m), 5.03, 5.13 (each 1H, ABq, J = 12.09 Hz), 7.23-7.32 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 25.51, 25.60, 25.68, 26.38, 26.47, 26.62, 26.71, 26.89, 27.23, 27.53, 27.88, 27.99, 28.46, 29.47, 29.65, 29.79, 29.97, 32.31, 33.18, 33.25, 34.01, 41.13, 41.29, 57.11, 57.58, 57.90, 58.37, 58.41, 66.71, 66.85, 66.96, 72.09, 72.31, 72.39, 127.96, 128.07, 128.18, 128.48, 128.56, 136.78, 136.90, 154.22, 154.51, 208.18, 208.68; HRMS calcd for C23H35NO5 (M+) 405.2515, found 405.2517.
ent-18; (92%); [α]26D-40.09° (c 1.31, CHCl3).
(2R,5S,6’R)-1-Benzyloxycarbonyl-2-(3’-oxobutyl)-5-(6’,7’-dihydroxyheptyl)pyrrolidine (19).
By the analogous procedure described for 17, the dihydroxylation of 15 (375 mg, 1.01 mmol) with AD-mix-β (DHQD2PYR (1.424 g) in t-BuOH (5.1 mL), and H2O (5.1 mL) gave 19 (357 mg, 87%) as an oil; [α]26D+34.99° (c 2.16, CHCl3); IRνmaxcm-1 (neat) 3365，2932，2856，1691，1466，1412，1355，1331，1310，1213，1106，771, 699, 603; 1H-NMR (300 MHz, CDCl3) δ 0.81-0.90 (3H, m), 1.09-1.43 (10H, m), 1.46-1.63 (6H, m), 1.86-2.07 (4H, m), 2.20-2.44 (4H, m), 2.88-3.14 (2H, m), 3.33-3.42 (1H, brs, m), 3.56-3.77 (4H, m), 5.03, 5.14 (each 1H, ABq, J = 12.64 Hz), 7.23-7.33 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 13.95, 17.44, 25.31, 25.41, 25.51, 25.68, 26.39, 26.68, 26.86, 27.03, 27.17, 27.30, 27.39, 27.50, 27.83, 27.94, 28.44, 29.15, 29.49, 29.65, 32.19, 32.33, 33.13, 33.19, 34.01, 40.14, 40.28, 44.56, 44.65, 57.21, 57.29, 57.67, 57.87, 58.26, 58.34, 58.38, 66.67, 66.77, 66.88, 72.12, 72.27, 72.33, 76.87, 77.40, 127.89, 127.96, 128.04, 128.16, 128.19, 128.24, 128.28, 128.42, 128.46, 136.72, 136.84, 154.20, 154.45. 210.32, 210.78; HRMS calcd for C25H39NO5 (M+) 433.2828, found 433.2808.
ent-19; (81%); [α]26D-42.37° (c 1.49, CHCl3).
(2R,5S,6’R)-1-Benzyloxycarbonyl-2-(3’-oxohexyl)-5-(6’,7’-dihydroxyheptyl)pyrrolidine (20).
By the analogous procedure described for 17, the dihydroxylation of 16 (396 mg, 0.99 mmol) with AD-mix-β (DHQD2PYR (1.467 g) in t-BuOH (5.5 mL), and H2O (5.5 mL) gave 20 (353 mg, 82%) as an oil; [α]27D+34.55° (c 1.22, CHCl3); IRνmaxcm-1 (neat) 3362, 2931, 2856, 1691, 1466, 1412, 1356, 1331, 1310, 1212, 1106, 771, 746, 604: 1H-NMR (300 MHz, CDCl3) δ 0.83-0.92 (3H, m), 1.06-1.48 (10H, m), 1.51-1.81 (6H, m), 1.88-2.20 (4H, m), 2.25-2.51 (4H, m), 3.36-3.45 (1H, m), 3.61-3.79 (4H, m), 5.06, 5.16 (each 1H, ABq, J = 12.09 Hz), 7.27-7.40 (5H, m); 13C-NMR (75 MHz, CDCl3) δ 13.92, 13.95, 17.44, 25.53, 25.68, 26.45, 26.68, 26.82, 27.14, 27.23, 27.36, 27.41, 27.80, 27.92, 28.43, 29.47, 29.64, 29.72, 32.30, 33.18, 33.28, 34.00, 40.14, 40.28, 44.56, 44.65, 57.21, 57.65, 57.85, 58.37, 66.65, 66.76, 66.87, 72.25, 72.33, 127.89, 127.96, 128.02, 128.42, 128.45, 136.70, 136.81, 154.19, 154.43, 210.34, 210.78; HRMS calcd for C25H39NO5 (M+) 433.2828, found 433.2827.
ent-20; (76%); [α]26D-43.54° (c 0.52, CHCl3).
(2R,5S,6’S)-1-Benzyloxycarbonyl-2-(3’-oxobutyl)-5- (6’-hydroxy-7’-p-toluenesulfonyloxy)heptyl-pyrrolidine (21).
A solution of 17 (319 mg, 0.79 mmol) and di-n-butyltin oxide (237 mg, 0.95 mmol) in toluene-THF (10 : 1) (28.4 mL) was refluxed with Dean-Stark apparatus for 6 h. The solvent was reduced to one of third amount under vacuo. To the rest were added triethylamine (2.71 mL, 0.04 mmol) and p-toluenesulfonyl chloride (177 mg, 0.95 mmol) and the whole was stirred at rt for 12 h. The reaction was quenched with brine, filtered through Celite, and washed with EtOAc. The combined organic solvents were dried over anhyd. Na2SO4, and evaporated. The residue was purified by silica gel column chromatography (n-hexane : EtOAc = 5 : 1) to give 21 (390 mg, 88%) as an oil; [α]27D+24.97° (c 1.87, CHCl3); IRνmaxcm-1 (neat) 3446, 2937, 2858, 1694, 1497, 1454, 1410, 1357, 1189, 1176, 1098, 970, 816, 772, 699, 666; 1H-NMR (300 MHz, CDCl3) δ 1.13-1.40 (8H, m), 1.52-1.60 (4H, m), 1.82-2.14 (7H, m), 1.96, 2.10 (1 : 1.1 ratio 3H, s), 2.26 (2H, t, J = 7.69 Hz), 2.39 (3H, s), 3.72-3.87 (3H, m), 3.93-3.96 (1H, m), 5.01, 5.12 (each 1H, ABq, J = 12.09 Hz), 7.24-7.31 (5H, m), 7.75 (2H, d, J = 7.14 Hz); 13C-NMR (75 MHz, CDCl3) δ 21.73, 24.97, 25.19, 26.32, 26.50, 26.77, 27.07, 27.36, 27.74, 27.85, 28.32, 28.99, 29.29, 29.64, 29.79, 32.17, 32.71, 33.84, 40.95, 41.10, 56.96, 57.43, 57.71, 58.14, 66.55, 66.62, 69.01, 69.17, 73.94, 127.63, 127.83, 128.02, 128.12, 128.16, 128.33, 128.37, 129.83, 132.62, 136.69, 136.75, 144.87, 153.99, 154.29, 208.05, 208.46.
ent-21; (58%); [α]27D-29.50° (c 1.53, CHCl3).
(2R,5S,6’S)-1-Benzyloxycarbonyl-2-(3’-oxohexyl)-5- (6’-hydroxy-7’-p-toluenesulfonyloxyheptyl)-pyrrolidine (22).
By the analogous procedure described for 21, the monotsylation of 18 (239 mg, 0.56 mmol) with di-n-butyltin oxide (178 mg, 0.67 mmol), triethylamine (2.30 mL, 0.028 mmol), and p-toluenesulfonyl chloride (133 mg, 0.67 mmol) in toluene-THF (21.3 mL) gave 22 (291 mg, 90%) as an oil; [α]27D+20.49° (c 1.45, CHCl3); IRνmaxcm-1 (neat) 3445, 2935, 1694, 1598, 1497, 1455, 1410, 1357, 1189, 1176, 1098, 971, 815, 754, 699, 667; 1H-NMR (300 MHz, CDCl3) δ 0.80-0.88 (3H, m), 1.13-1.44 (10H, m), 1.47-1.60 (6H, m), 1.82-2.09 (4H, m), 2.19-2.42 (7H, m), 2.39 (3H, s), 3.72-3.88 (4H, m), 3.92-3.96 (1H, m), 5.01, 5.12 (each 1H, ABq, J = 12.64 Hz), 7.22-7.31 (7H, m), 7.75 (2H, d, J = 8.24 Hz); 13C-NMR (75 MHz, CDCl3) δ 13.86, 17.33, 21.72, 24.95, 25.18, 26.30, 26.36, 26.48, 26.77, 27.04, 27.29, 27.73, 27.85, 28.35, 28.97, 29.29, 32.17, 32.71, 33.84, 40.04, 40.17, 44.44, 44.50, 57.11, 57.56, 57.70, 58.11, 66.52, 66.59, 69.01, 69.17, 73.92, 77.40, 127.61, 127.71, 127.83, 127.95, 128.12, 128.34, 129.70, 129.72, 129.81, 132.62, 136.72, 136.77, 144.82, 144.85, 154.01, 154.26, 210.20, 210.55.
ent-22; (93%); [α]27D-28.75° (c 1.70, CHCl3).
(2R,5S,6’R)-1-Benzyloxycarbonyl-2-(3’-oxobutyl)-5- (6’-hydroxy-7’-p-toluenesulfonyloxyheptyl)-pyrrolidine (23).
By the analogous procedure described for 21, the monotosylation of 19 (351 mg, 0.87 mmol) with di-n-butyltin oxide (259 mg, 1.04 mmol), triethylamine (3.20 mL, 0.044 mmol), and p-toluenesulfonyl chloride (209 mg, 1.04 mmol) in toluene-THF (33.5 mL) gave 23 (438 mg, 90%) as an oil; [α]27D+31.25° (c 1.33, CHCl3); 3447, 2937, 2858, 1696, 1498, 1458, 1410, 1357, 1188, 1176, 1098, 970, 816, 699, 668; 1H-NMR (300 MHz, CDCl3) δ 1.11-1.40 (8H, m), 1.52-1.63 (4H, m), 1.82-2.14 (7H, m), 1.99, 2.14 (1.2 : 1 ratio 3H, s), 2.29 (2H, t, J = 7.69 Hz), 2.44 (3H, s), 3.74-3.90 (3H, m), 3.97-4.01 (1H, m), 5.04, 5.15 (each 1H, ABq, J = 12.64 Hz), 7.32-7.35 (5H, m), 7.77 (2H, d, J = 8.24 Hz); 13C-NMR (75 MHz, CDCl3) δ 21.85, 25.28, 26.47, 26.51, 26.59, 26.83, 27.15, 27.45, 27.82, 27.92, 28.40, 28.99, 29.37, 29.75, 29.91, 32.37, 32.74, 33.92, 41.07, 41.22, 57.03, 57.50, 57.79, 58.20, 58.25, 58.32, 66.64, 66.71, 69.37, 69.40, 74.04, 127.75, 127.81, 127.93, 128.12, 128.22, 128.27, 128.42, 128.48, 129.77, 129.81, 129.94, 132.64, 136.78, 136.84, 144.98, 145.02, 154.10, 154.38, 208.15, 208.58.
ent-23; (95%); [α]27D-34.92° (c 2.39, CHCl3).

(2R,5S,6’R)-1-Benzyloxycarbonyl-2-(3’-oxohexyl)-5-(6’-hydroxy-7’-p-toluenesulfonyloxy)heptyl-pyrrolidine (24).
By the analogous procedure described for 21, the monotosylation of 20 (351 mg, 0.87 mmol) with di-n-butyltin oxide (259 mg, 1.04 mmol), triethylamine (3.20 mL, 0.044 mmol), and p-toluenesulfonyl chloride (209 mg, 1.04 mmol) in toluene-THF (33.5 mL) gave 23 (438 mg, 90%) as an oil; [α]27D+27.57° (c 1.03, CHCl3); IRνmaxcm-1 (neat) 3446, 2935, 1694, 1598, 1497, 1456, 1410, 1357, 1176, 1098, 970, 816, 754, 699, 666; 1H-NMR (300 MHz, CDCl3) δ 0.81-0.90 (3H, m), 1.15-1.44 (10H, m), 1.46-1.62 (6H, m), 1.79-2.09 (4H, m), 2.21-2.55 (7H, m), 2.42 (3H, s), 3.74-3.89 (4H, m), 3.94-3.99 (1H, m), 5.03, 5.14 (each 1H, ABq, J = 12.64 Hz), 7.25-7.33 (7H, m), 7.77 (2H, d, J = 8.24 Hz); 13C-NMR (75 MHz, CDCl3) δ 13.95, 17.41, 21.82, 25.25, 26.45, 26.56, 26.83, 27.12, 27.36, 27.80, 27.92, 28.41, 29.35, 32.37, 32.74, 33.90, 33.96, 40.13, 40.20, 40.28, 44.53, 44.60, 57.17, 57.64, 57.75, 58.29, 66.61, 66.65, 69.11, 69.32, 74.01, 77.38, 77.41, 77.55, 127.78, 127.90, 128.02, 128.40, 128.43, 129.78, 129.91, 130.04, 132.64, 136.84, 144.93, 144.98, 154.10, 154.34, 210.29, 210.64.
ent-24; (92%); [α]26D-32.82° (c 3.21, CHCl3).
(3R,5S,6’R,8S)-5-(6’-Hydroxyheptyl)-3-methylpyrrolizidine [(6’R)-2].
A suspension of 21 (162 mg, 0.29 mmol), Pd(OH)2 (16.2 mg) in MeOH (9.1 mL) was stirred under hydrogen atmosphere for 12 h. The reaction mixture was filtered through Celite and washed with CH2Cl2. The combined organic solvents were evaporated. To a solution of the residue in THF (1.4 mL)was dropwise added 1.0 M Super-Hydride® in THF (1.16 mL) and the whole was stirred at rt for 5 h. The reaction mixture was quenched with ice and basified with 2N NaOH. The mixture was separated and the aqueous layer was extracted with CH2Cl2. The combined organic layers were washed with water, dried over anhyd. Na2SO4, and evaporated. The residue was purified by basic alumina column chromatography (ether) to give (6R)-2 (34 mg, 50%) as an oil; [α]26D+11.37° (c 1.00, CHCl3); IRνmaxcm-1 (neat) 3385, 2927, 2857, 1458, 1374, 1128; 1H-NMR (300 MHz, CDCl3) δ 1.09 (3H, d, J = 6.04 Hz), 1.17 (3H, d, J = 6.04 Hz), 1.21-1.62 (14H, m), 1.85-2.03 (4H, m), 2.61 (1H, qui-like, J = 7.14 Hz), 2.75 (1H, six-like, J = 6.04 Hz), 3.58 (1H, qui, J = 6.59 Hz), 3.78 (1H, m); 13C-NMR (75 MHz, CDCl3) δ 21.88, 23.75, 26.01, 27.45, 30.11, 31.89, 32.27, 32.63, 34.65, 36.91, 39.55, 62.06, 65.26, 66.91, 68.28; HRMS calcd for C15H29NO (M+) 239.2249, found 239.2233.
ent-(6’R)-2; (57%); [α]26D-11.45° (c 0.26, CHCl3).
(3R,5S,6’R,8S)-5-(6’-Hydroxyheptyl)-3-propylpyrrolizidine [(6’R)-4].
By the analogous procedure described for (6’R)-2, the annulation of 22 (291 mg, 0.50 mmol) with hydrogen in the presence of cat. Pd(OH)2 (29.1 mg) in MeOH (16.4 mL) gave the pyrrolizidine, which was converted with 1.0 M Super-Hydride® (1.5 mL, 1.5 mmol) in THF to (6R)-4 (79 mg, 59%) as an oil; [α]28D+7.29° (c 0.63, CHCl3); IRνmaxcm-1 (neat) 3384, 2955, 2927, 2857, 1459, 1374, 1107, 945; 1H-NMR (300 MHz, CDCl3) δ 0.91 (3H, t-like, J = 6.59, 7.14 Hz), 1.18 (3H, d, J = 6.04 Hz), 1.21-1.65 (16H, m), 1.67-1.70 (2H, brs, m), 1.87-2.00 (4H, m), 2.59-2.66 (2H, brs, s), 3.55 (1H, qui-like, J = 6.04 Hz), 3.75-3.82 (1H, brs, m); 13C-NMR (75 MHz, CDCl3) δ 14.72, 20.63, 23.77, 26.06, 27.39, 30.19, 31.98, 37.23, 39.61, 64.74, 66.73, 66.90, 68.35; HRMS calcd for C17H33NO (M+) 267.2562, found 267.2538.
ent-(6’R)-4; (44%); [α]26D-7.14° (c 0.96, CHCl3).
(3R,5S,6’S,8S)-5-(6’-Hydroxyheptyl)-3-methylpyrrolizidine [(6’S)-2].
By the analogous procedure described for (6R)-2, the annulation of 23 (380 mg, 0.68 mmol) with hydrogen in the presence of cat. Pd(OH)2 (38 mg) in MeOH (21.4 mL) gave the pyrrolizidine, which was converted with 1.0 M Super-Hydride® (2.04 mL, 2.04 mmol) in THF to (6S)-2 (95 mg, 58%) as an oil; [α]26D+4.61° (c 0.95, CHCl3); IRνmaxcm-1 (neat) 3354, 2927, 2857, 1458, 1374, 1120; 1H-NMR (300 MHz, CDCl3) δ 1.08 (3H, d, J = 6.59 Hz), 1.16 (3H, d, J = 6.04 Hz), 1.24-1.58 (14H, m), 1.87-2.00 (4H, m), 2.58 (1H, qui-like, J = 6.04 Hz), 2.75 (1H, six-like, J = 7.14 Hz), 3.58 (1H, qui, J = 6.59 Hz), 3.77 (1H, six-like, J = 6.04 Hz); 13C-NMR (75 MHz, CDCl3) δ 22.20, 23.77, 26.04, 27.48, 30.16, 31.98, 32.36, 32.67, 34.69, 37.27, 39.58, 61.89, 65.14, 66.77, 68.35; Anal. Calcd for C15H29NO : C, 75.26; H, 12.21; N, 5.85. Found: C, 75.23; H, 12.37; N, 5.57.
ent-(6’S)-2; (58%); [α]26D-4.47° (c 0.77, CHCl3).
(3R,5S,6’S,8S)-5-(6’-Hydroxyheptyl)-3-propylpyrrolizidine [(6’S)-4].
By the analogous procedure described for (6R)-2, the annulation of 24 (352 mg, 0.60 mmol) with hydrogen in the presence of cat. Pd(OH)2 (35.2 mg) in MeOH (19.8 mL) gave the pyrrolizidine, which was converted with 1.0 M Super-Hydride® (1.80 mL, 1.80 mmol) in THF to (6S)-4 (87 mg, 54%) as an oil; [α]26D+0.90° (c 1.72, CHCl3); IRνmaxcm-1 (neat) 3355, 2955, 2927, 2857, 1459, 1371, 1106, 944; 1H-NMR (300 MHz, CDCl3) δ 0.92 (3H, t-like, J = 6.59, 7.14 Hz), 1.18 (3H, d, J = 6.04 Hz), 1.21-1.73 (16H, m), 1.75-1.84 (2H, brs), 1.87-2.05 (4H, m), 2.60-2.67 (2H, brs), 3.55 (1H, qui-like, J = 6.04 Hz), 3.76-3.83 (1H, m); 13C-NMR (75 MHz, CDCl3) δ 14.60, 20.53, 23.65, 25.95, 27.30, 30.07, 31.86, 31.90, 37.09, 39.48, 64.69, 66.70, 66.87, 68.33; HRMS calcd for C17H33NO (M+) 267.2562, found 267.2535.
ent-(6’S)-4; (61%); [α]27D-1.28° (c 0.54, CHCl3).
Assay protocol17
Membranes enriched in nAChR are harvested from homogenates of Torpedo califurnica electric organs by differential centrifugation as described in the literature. Binding of [3H]-TCP (sp. act 57.6 Ci/mmole) to carbamylcoline-activated nAChR is used as a functional binding assay as described by Kats et al. [3H]-TCP is available from the National Institute of Drug Abuse.
Binding assay
Test compounds 1-4 and ent-1-4 were dissolved in DMSO to make 100 mM stocks. A second stock of 1 mM in buffer (50 mM Tris HCl, pH=7.4) is prepared by diluting 10 μL DMSO stock into 1 mL buffer. Stocks of 100 times the desired test concentrations were prepared by further dilutions with buffer. 25 μg of Torpedo membranes were added to the final volume of 250 μL of buffer containing [3H]-TCP, 100 μM carbamylcoline, and the test drug 1-4 and ent-1-4 at varied concentrations. Nonspecific binding is determined in presence of 5 mM amantadiene HCl. Bound [3H]-TCP is separated by vacuum filtration over GF/B filters presoaked in 0.05% polyethyleneimine. Incubation time was 30 s at 23 °C. Radioactivity retained on the filters, after washing with 5 mL ice cold buffer, was quantitated by liquid scintillation. All tests were performed in triplicate and data reported as % of control. Six concentrations were selected for each test compound covering a range 1000 times the lowest effective concentration. The percent inhibition of carbamylcholine stimulated [3H]-TCP binding was plotted vs concentration. Each compound was tested twice and means +SEM from 6 values at each concentration were used to generate the concentration-response function. The IC50 values determined graphically from the log concentration-response curves were then used to obtain the inhibitory equilibrium constant (Ki) using the Cheng-Prusoff equation (Ki=IC50/l+[D]/KD). The results are shown in Table l.

ACKNOWLEDGEMENTS
This work was supported in part by High Technology Research Program from Ministry of Education, Culture, Sports, Science and Technology of Japan.

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