HETEROCYCLES

Communication | Special issue | Vol. 79, No. 1, 2009, pp. 403-410
Received, 30th September, 2008, Accepted, 28th November, 2008, Published online, 3rd December, 2008.
DOI: 10.3987/COM-08-S(D)52

■ Total Synthesis and the Confirmation of the Revised Structures of Botcinins A and B

Hiroki Fukui, Keisuke Tsuji, Yuma Umezaki, and Isamu Shiina*

Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-8601, Japan

Abstract

The stereoselective total syntheses of botcinins A (1) and B (2), and homobotcinin E (4), a homologue of botcinin E (3), have been achieved starting from a polyoxygenated tetrahydropyran intermediate 15 via formation of botcinic acid (12) and botcineric acid (13). Through the total syntheses, three actual relationships are revealed, that is, (i) the structure of botcinin A (1) is identical with the revised structure of the natural compound, which was formerly assumed to be 3-O-acetyl-2-epibotcinolide (5), (ii) the structure of botcinin B (2) is identical with the revised structure of the natural compound, which was supposed to be 3-O-acetyl-2-epihomobotcinolide (6), and (iii) the structure of the natural compound formerly assumed to be 2-epihomobotcinolide (8) must be revised to that of homobotcinin E (4).

Botcinins A, B and E (1, 2 and 3) were isolated from Botrytis cinerea (strain AEM 211) as antifungal metabolites against Magnaporthe grisea, a pathogen of the rice blast disease, by Nakajima et al. in 2005-6.1,2 Before the isolation of these compounds by Nakajima, Cutler et al.3-5 and Collado et al.6,7 independently extracted several natural metabolites from B. cinerea, which have significant phytotoxicity to a variety of plants, including three related compounds, called 3-O-acetyl-2-epibotcinolide (5),6 3-O-acetyl-2-epihomobotcinolide (6),7 and 2-epibotcinolide (7).6 By a detailed comparison of the spectroscopic data of the natural products by Nakajima, it was proposed that the structural revision of the reported compounds assumed to be 5 and 7 should be done by replacing them with those of 1 and 3, respectively,2 as shown in Figure 1.

Recently, we synthesized botcinins C-F (10, 9, 3, 11),8,9 botcinic acid (12),9 and botcineric acid (13),9 and confirmed that the structure of the natural compound assumed to be 2-epibotcinolide (7) is incorrect10 and should be revised to that of the corresponding structure of botcinin E (3) as predicted by Nakajima. During the synthetic studies of 3, we noted that the true form of the natural compound supposed to be 3-O-acetyl-2-epihomobotcinolide (6) should also be identified as that of botcinin B (2) (Figure 1). Furthermore, it was additionally anticipated that the structure of the natural metabolite, called 2-epihomobotcinolide (8), should be revised to that of homobotcinin E (4) although 4 was not isolated by other groups.

In this communication, we report the total syntheses of botcinins A (1) and B (2), and homobotcinin E (4), and updated the proposed structures of the botcinolides in order to show the true forms of these natural products.

As shown in Scheme 1, the synthetic route is summarized for the preparation of botcinins D (9), C (10), and F (11) from the corresponding lactones 17a-c.8 These precursors were generated from a highly substituted tetrahydropyran ring 15 via the SmI2-mediated Reformatsky reaction of 2-bromoester 16, which includes formyl part at a suitable position. On the other hand, we accomplished the total synthesis of botcinic acid (12) and botcineric acid (13) through the alternative synthetic route using the six-membered ether 15 as a key intermediate (Scheme 2).9

In the previous paper,9 we also succeeded to transform botcinic acid (12) into botcinin E (3) by the acidic treatment. Because botcinins A (1) and B (2) might have the same stereochemistries at the C-2 and -3 positions to those of 3, it is postulated that 1 and 2 could be synthesized from 12 and 13, respectively.

First, botcineric acid (13) was converted into homobotcinin E (4)11 according to the same procedure for the preparation of botcinin E (3) from botcinic acid (12) as shown in Scheme 3. Next, the hydroxyl groups at the allylic positions in 3 and 4 were selectively protected with TES group, and the remaining hydroxyl groups of 22a and 22b were acetylated using acetic anhydride with pyridine. Botcinin A (1)12 and botcinin B (2)13 were finally produced by the deprotection of the TES group in 23a and 23b, respectively, under acidic conditions using PPTS in CH2Cl2/MeOH at rt.

The spectroscopic data of the synthetic 1 and 2 well corresponded with those of the isolated 1 and 2. As Nakajima reported that the structure of the natural compound, assumed to be 3-O-acetyl-2-epibotcinolide (5), should be revised to that of 1, we could also predict that the true structure of the natural compound formerly assumed to be 3-O-acetyl-2-epihomobotcinolide (6) should be replaced by that of 2. The compared spectroscopic data of 2 with 6 are depicted in Table 1 and these data are well corresponded, therefore, it is apparently concluded that the true structure of the natural compound, assumed to be 6, is identical to that of 2.

Homobotcinin E (4) is a deacetylated compound of botcinin B (2), and the structure of the natural compound assumed to be 3-O-acetyl-2-epihomobotcinolide (6) had been revised to that of 2 as shown above; therefore, we postulated that 4 should be the true structure of the natural metabolite formerly assumed to be 2-epihomobotcinolide (8).

Homobotcinin E (4) is also a homologue of botcinin E (3) with an extra two-carbon in the side chain as well as a relationship between 2-epihomobotcinolide (8) and 2-epibotcinolide (7). In accordance with this correlation, Nakajima reported that the structure of the natural metabolite, which was supposed to be 7, should be identical to that of 3 because the NMR data of 7 and 3 are almost identical; however, there are some exceptional resonances in the NMR spectra that should be clarified by the advanced investigations.

Based on the above examinations, we attempted to compare the spectroscopic data of 1H and 13C NMR on 7, 3, 8, and 4 in detail in order to determine the exact structures of natural products (Table 2). From Table 2, we found that the resonances of the 1H NMR of 7 appeared at a 0.1 ppm lower field compared to those of the isolated and synthetic 3. It was apparently discordant that the chemical shift of H-11 in 7 was observed at δ 7.91 ppm in the former report by Collado’s group,6 but they assigned δ 7.02 ppm to the corresponding proton peak of 8 in the later paper.7 Furthermore, H-11 of our synthetic 3 and 4 appeared at δ 7.02 ppm, and Nakajima also assigned the chemical shift of H-11 in the isolated 3 to δ 7.03 ppm. Therefore, we can expect that the resonance of H-11 in 7 should be observed at around δ 7.02 ppm, and

Collado’s group might have made a typographic error in the preparation of their paper. The resonance at δ 170.1 ppm for C-9 in 7 in CD3OD might also be an error in the preparation of the paper because the corresponding C-9 signals in the isolated and synthetic 3 were observed at δ 165.8 ppm. Consequently, we now suggest that homobotcinin E (4) should be the true form of the natural product, which was first isolated as 2-epihomobotcinolide (8), in the same way as the structure of the natural compound formerly assumed to be 2-epibotcinolide (7) had been revised to that of botcinin E (3).9,10

In conclusion, we achieved the first asymmetric total syntheses of botcinins A (1) and B (2), and homobotcinin E (4) via stereocontrolled reactions. As a result of the synthetic efforts, we determined that the true structure of the natural compound, assumed to be 3-O-acetyl-2-epibotcinolide (5), should be corrected to that of 1, and the structures of the natural extracts, assumed to be 3-O-acetyl-2-epihomobotcinolide (6) and 2-epihomobotcinolide (8), should also be revised to those of 2 and 4, respectively.

ACKNOWLEDGEMENTS
The authors are grateful to Prof. Nakajima (Tottori Univ.) for providing the spectroscopic data of the natural botcinins. This study was partially supported by a Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan. The author thanks the Shin-Etsu Chemical Co., Ltd. (Japan), for kindly providing t-butylchlorodimethylsilane as a bulk sample.

References

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10. I. Shiina, Y. Takasuna, R. Suzuki, H. Oshiumi, Y. Komiyama, S. Hitomi, and H. Fukui, Org. Lett., 2006, 8, 5279. CrossRef
11. Homobotcinin E (4): [α]D 26 –64.7 (c 0.57, EtOH), [α]D 24 –70.7 (c 1.26, CHCl3) [lit.,7 [α]D 35 –34 (c 2.0, CHCl3)]; IR (neat): 3437, 2930, 2857, 1727 cm-1; 1H NMR (CD3OD): δ 7.01 (dd, J = 15.5, 5.0 Hz, 1H), 6.07 (d, J = 15.5 Hz, 1H), 4.52 (dd, J = 10.5, 9.8 Hz, 1H), 4.37-4.31 (m, 1H), 4.14 (d, J = 9.5 Hz, 1H), 3.73 (dq, J = 9.8, 5.8 Hz, 1H), 3.70 (d, J = 11.0 Hz, 1H), 3.05 (dq, J = 9.5, 7.3 Hz, 1H), 2.18 (ddq, J = 11.0, 10.5, 6.0 Hz, 1H), 1.63-1.54 (m, 2H), 1.48-1.25 (m, 8H), 1.23 (s, 3H), 1.28 (d, J = 7.3 Hz, 1H), 1.11 (d, J = 5.8 Hz, 1H), 1.05 (d, J = 6.0 Hz, 3H), 0.88 (t, J = 6.0 Hz, 3H); 13C NMR (CD3OD): δ 177.4, 167.6, 154.1, 119.8, 79.6, 78.1, 77.9, 75.0, 71.6, 69.5, 39.5, 37.5, 36.8, 32.9, 30.3, 26.4, 23.6, 18.6, 14.4, 14.0, 11.6, 10.5; HR MS: calcd for C22H36O7Na (M + Na+) 435.2353, found 435.2332.
12. Botcinin A (1): [α]D 23 –39.3 (c 0.72, EtOH) [lit.,1 [α]D 25 –33 (c 0.50, EtOH)]; IR (neat): 3440, 2922, 1733 cm-1; 1H NMR (CDCl3): δ 7.01 (dd, J = 15.8, 4.5 Hz, 1H), 6.06 (dd, J = 15.8, 2.0 Hz, 1H), 5.41 (d, J = 9.8 Hz, 1H), 4.53 (dd, J = 9.8, 9.8 Hz, 1H), 4.36-4.32 (m, 1H), 3.80 (d, J = 10.3 Hz, 1H), 3.63 (dq, J = 9.5, 6.0 Hz, 1H), 3.16 (dq, J = 10.3, 7.0 Hz, 1H), 2.20 (ddq, J = 10.0, 9.5, 6.0 Hz, 1H), 2.21 (s, 3H), 1.66-1.54 (m, 2H), 1.44-1.33 (m, 4H), 1.18 (s, 3H), 1.14 (d, J = 7.0 Hz, 3H), 1.07 (d, J = 6.3 Hz, 3H), 1.02 (d, J = 6.0 Hz, 3H), 0.90 (t, J = 6.5 Hz, 3H); 1H NMR (CD3OD): δ 7.01 (dd, J = 15.5, 5.0 Hz, 1H), 6.07 (d, J = 15.5 Hz, 1H), 4.52 (dd, J = 10.5, 9.8 Hz, 1H), 4.37-4.31 (m, 1H), 4.14 (d, J = 9.5 Hz, 1H), 3.73 (dq, J = 9.8, 5.8 Hz, 1H), 3.70 (d, J = 11.0 Hz, 1H), 3.05 (dq, J = 9.5, 7.3 Hz, 1H), 2.18 (ddq, J = 11.0, 10.5, 6.0 Hz, 1H), 1.63-1.54 (m, 2H), 1.48-1.25 (m, 4H), 1.25 (s, 3H), 1.12 (d, J = 7.0 Hz, 1H), 1.09 (d, J = 6.0 Hz, 1H), 1.06 (d, J = 6.0 Hz, 3H), 0.91 (t, J = 6.5 Hz, 3H); 13C NMR (CDCl3): δ 173.1, 170.1, 165.7, 151.8, 119.1, 78.5, 76.1, 75.2, 74.3, 71.1, 68.4, 37.3, 36.3, 35.4, 27.3, 22.5, 20.6, 18.2, 13.9, 13.7, 11.9, 10.2; HR MS: calcd for C22H34O8Na (M + Na+) 449.2146, found 449.2146.
13. Botcinin B (2): [α]D 25 –27.5 (c 0.76, EtOH) [lit.,1 [α]D 25 –35 (c 0.50, EtOH)]; IR (neat): 3480, 2928, 1731, 1652 cm-1; 1H NMR (CDCl3): δ 7.01 (dd, J = 15.5, 4.5 Hz, 1H), 6.06 (d, J = 15.5 Hz, 1H), 5.42 (d, J = 10.0 Hz, 1H), 4.54 (dd, J = 10.5, 9.9 Hz, 1H), 4.36-4.33 (m, 1H), 3.81 (d, J = 10.3 Hz, 1H), 3.68 (dq, J = 9.9, 6.5 Hz, 1H), 3.17 (dq, J = 10.3, 7.3 Hz, 1H), 2.19 (ddq, J = 10.5, 10.0, 6.3 Hz, 1H), 2.13 (s, 3H), 1.65-1.54 (m, 2H), 1.45-1.30 (m, 4H), 1.27 (s, 3H), 1.13 (d, J = 7.3 Hz, 3H), 1.10 (d, J = 6.3 Hz, 3H), 1.07 (d, J = 6.5 Hz, 3H), 0.89 (t, J = 6.8 Hz, 3H); 13C NMR (CDCl3): δ 173.2, 170.1, 165.7, 151.8, 119.0, 78.6, 76.1, 75.2, 74.3, 71.1, 68.4, 37.3, 36.7, 35.4, 31.7, 29.1, 25.2, 22.5, 20.6, 18.2, 14.0, 13.7, 11.9, 10.2; HR MS: calcd for C24H38O8Na (M + Na+) 477.2459, found 477.2448.