HETEROCYCLES

Paper | Special issue | Vol. 84, No. 1, 2012, pp. 657-667
Received, 19th June, 2011, Accepted, 12th September, 2011, Published online, 20th September, 2011.
DOI: 10.3987/COM-11-S(P)41

■ Flash Vacuum Pyrolysis of Naphthalen-1-yl and -2-yl Prop-2-ynoate

Vit Lellek and Hans-Jürgen Hansen*

Institute of Organic Chemistry, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland

Abstract

In contrast to phenyl prop-2-ynoates, which give on flash vacuum pyrolysis (FVP, 650 °C, 0.13 Pa, o-xylene) the corresponding 2H-cyclohepta[b]furan-2-ones in acceptable yields, leads the FVP of naphthalen-1-yl prop-2-ynoate (5a) to the formation of benzo[1,2-b]furan (6, 17%) and naphthalen-1-ol (25%). A blue component (1.8%), which is found in the product mixture, turned out to be (E)-2-(4-metyl-2H-benzo[h]chromen-2-ylidene)naphthalen-1(2H)-one ((E)-7). On the other hand, naphthalene-2-yl prop-2-ynoate (5b) behave on FVP “normal” in that a cycloaddition dimer 11 (15%) of the primarily formed benzo[4,5]cyclohepta[1,2-b]furan-2(2H)-one (10) was isolated from the pyrolysate.

INTRODUCTION
2H-Cyclohepta[b]furan-2-ones 1 are excellent starting materials for the synthesis of azulenes by thermal [8 + 2] addition with enol ethers or enamines under loss of carbon dioxide and HX (X = OR and NR2, respectively).1 There are two main accesses to 1, namely, following the Nozoe way, by base-catalyzed condensation reaction of tropolone derivatives 2 with C–H acidic compounds2,3 or by flash vacuum pyrolysis (FVP) of phenyl prop-2-ynoates 3 according to original experiments of Trahanovsky et al.,4,5 whereby the last procedure permits a broader variation and higher number of substituents at the seven-membered ring (Scheme 1).6,7 The onset of the alkyne-ethenylidene equilibrium 3 ⇄ 3’, which is necessary for the formation of 1 by intramolecular carbene addition to the benzene ring of 3’ followed by ring enlargement, lies at temperatures >600 °C. Ideal conditions for the FVP of 1 are therefore 650 °C at a pressure of 0.13 to 0.013 Pa with xylene as carrier of 3. However, substituents at C(3) of the prop-2-ynoates such as Ph or Me hinder markedly the establishment of the discussed equilibrium at 650–670 °C, so that the yields of 3-substituted cyclohepta[1,2-b]furan-2-ones 1 are low or the furanones are not formed at all (see8 and literature cited there). In the following part, we report on the FVP of a highly substituted phenyl prop-2-ynoate, and naphthalen-1-yl and 2-yl prop-2-ynoate where the alkyne-ethenylidene equilibrium should work, but which offer in part other problems as we found.

RESULTS AND DISCUSSION
Naphthalen-1-yl prop-2-ynoate and naphthalene-2-yl prop-2-ynoate (5a and 5b, respectively) as well as 3,5-bis(ethoxymethyl)-2,4,6-trimethylphenyl prop-2-ynoate (3a) were prepared from the corresponding naphthols and 4,6-bis(ethoxymethyl)-mesitol according to our earlier described procedure in excellent yield.6
The result of the FVP of 3a was not surprising since we found the expected cyclohepta[b]furan-2-one 1a in the pyrolysate, collected in the cooling trap (Scheme 2), albeit in a yield distinctly lower than in the comparable case of the FVP of 2,3,4,5,6-pentamethylphenyl prop-2-ynoate, where yields in the range of 15 – 40% of the corresponding penta­methylcyclohepta[b]furan-2-one 1 are realizable.6 Nevertheless, the FVP of 3a indicates that C–O bonds in benzylic position do survive the FVP at 650 ­– 670 °C (Scheme 2).

The result of the FVP of 5a was surprising by the fact that we found no indication for the presence of 2H-benzo[6,7]cyclohepta[1,2-b]furan-2-one (8) or a dimer of it (see later) in the cooling trap after warming (Scheme 3). Instead, we isolated by chromatography naphtho[1,2-b]furan (6) and 1-naphthol in moderate yields. Both compounds were identified by comparison with authentic samples spectroscopically.

Most interesting was the isolation of a third blue component, the color of which developed just on warming of the yellow colored pyrolysate. It was characterized spectroscopically as (E)-2-(4-methyl-2H-benzo[h]chromen-2-ylidene)­naphthalene-1(2H)-one ((E)-7). Most informative was its UV/VIS spectrum in CHCl3 (Figure 1), since it showed in the long-wavelength region around 560 nm an intense almost symmetric band with clear recognizable vibrational splitting of Δν = ca. 1200 cm–1 (IR (KBr): 1227 cm–1), which spoke for the presence of an almost rigid structure. In the presence of 1% (v/v) TFA, the solution

took a red-violet color and the structured band had vanished and a new unstructured broad band appeared hypsochromically shifted to 523 nm (Figure 1), a fact, which spoke for the protonation of the C=O group and formation of 2-(1-hydroxynaphthalen-2-yl)-4-methylbenzo[h]chromenium trifluoro­acetate (9) (Scheme 4). The structure of (E)-7 was finally solved by NMR spectroscopy with full assignment of all

H- and C-atom positions, of which the 13C-NMR shifts were compared with those obtained by DFT calculation for (E)-7 (for the used program, see9). The agreement was excellent (Figure 2). The structure of (E)-7 was also supported by the spectroscopic data of a dimethoxy derivative of it, which has been prepared together with its (Z)-analogue by oxidation of correspondingly substituted naphthopyranes with AgO or chloranil.10

The pyrolysate of the FVP of 5b exhibited in the cooling trap an intense yellow color, which changed to almost colorless on warming to ambient temperature. Chromatography gave beside 30% starting material 5b as colorless solid, which showed the [M + NH4]+ ion with twice the mass of 10 in the CI-MS, however, with the basis peak at m/z = 197 in correspondence with a [10 + 1]+ ion (Scheme 5). Therefore, there was

little doubt that the new product, formed in 15% chemical yield, represented a dimer of the preliminary formed benzo[4,5]cyclohepta[1,2-b]furan-2(2H)-one (10). An in depth analysis of the 1H- and 13C-NMR spectra suggested the structure of a cycloaddition dimer 11 of 10 with the suspension of the 1,2-quinodimethane partial structure of both molecules of 10 (cf. Scheme 5). Dimer 11 crysta­llized from CH2Cl2 in colorless prisms, and one of them was well suited for an X-ray crystal-structure analy­­sis, which fully supported the structure of 11 (Figure 3). The asymmetric unit contained one molecule of 11 plus two molecules of CH2Cl2 without disorder.

Finally, it can be said that the phenyl propynoate 3a and the naphthyl propynoate 5b behave normal on FVP at 650 °C by undergoing the alkyne-ethenylidene equilibrium with the result of the formation of cyclohepta[1,2-c]furans 1a and 10, respectively. On the other hand, the pyrolysis of naphthyl propynoate 5a at 650 °C does not lead to products derived from benzocyclohepta[1,2-b]furan-2-one 8 as primary intermediate, despite the fact that the AM1 calculated ΔHf° values of 8 (28.70 Kcal·mol–1) and 10 (28.84 Kcal·mol–1) are quite similar. Also a possible dimer of 8, built by [10 + 8] cycloaddition with bond formation between C(4)/C(10b’) and C(10b)/C(6’), shows ΔHf° = 20.72 Kcal·mol–1 close to that of 10 (AM1: ΔHf° = 18.33 Kcal·mol–1). Therefore, it can be concluded that the isolated products of the FVP of 5a result from a thermal process, which takes place before the alkyne-ethenylidene equilibrium of 5a (cf. Scheme 1) starts to become product determining. The relatively high amount of naphthalen-1-ol that is formed speaks for a sigmatropic process accompanied by cleavage of the migrating σ-bond. It seems therefore that the original idea of Trahanovsky et al.4 is realized in the case of 5a, namely that the first step of the FVP of phenyl propynoates represents a “Claisen type rearrangement” (Scheme 6), which leads in the present case to the formation of 2-(3-oxapropa-1,2-dienyl)naphthalen-1(2H)-one (12). Propa-1,2-dien-1-ones are known to undergo decarbon­ylation under FVP conditions to corresponding ethenylidene intermediates.11 However, 12 may undergo wall-catalyzed enolization (prototropic shift) to naphthol 13 before decarbonylation to 14 takes place. The latter, on [1,5]-H shift to prop-2-enylidene 15, opens by cyclization the way to naphthofurane 6 and by [1,2]-H shift to 2-vinylidenenaphthalen-1(2H)-one (16), which then is engaged in the trap in a [4+2] dimerization to (E)-2-(4-methylene-3,4-dihydro-2H-benzo[h]chromen-2-ylidene)naphthalen-1(2H)-one (17). A 1,3-prototropic shift in 17 concludes finally the formation (E)-7. On the other hand, it could also be that intermediate 16 results by [1,5]-H shift of 2-ethynylnaphthalen-1-ol (19), which itself is an out­come of the decarbonylation of 12 accompanied by [1,2]-H shift and enolization (Scheme 7).12

The postulated appearance of 13 in the FVP of 5a leads necessarily to the question whether it also opens a further reaction channel, which will lead by [1,5]-H shift to (Z)-2-(3-oxaallylidene)naphthalen-1(2H)-one (20) in an s-trans (t-20, Θ (H–C(1)–C(2)–H) = 179.8°) and s-cis (c-20, Θ (H–C(1)–C(2)–H) = 0.1°) conformation, which differ distinctly in their heat of formation (Scheme 7). The latter may cyclize to 2H-benzo[h]chro­men-2-one (21). However, this compound was not present in isolable amounts in the mixture of products (Scheme 3), and it seems that the FVP of coumarines and their benzo forms has not been investigated so far. Nevertheless, the rotation of the 3-oxaallylidene group of 20 around the C(1)–C(2) bond shows local minima at an H–C(1)–C(2)–H torsion angle of about 45°, ideal for binding interaction of the n-electron pair of the naphthalen-1-one O-atom with C(2) and thereby entering the hypersurface of thermal gas-phase decarbonylation reactions of aromatic and α,β-unsaturated aldehydes at temperatures >400 °C (see16 and literature cited there). The result in the present case would be the formation of 6 by loss of CO from 23 formulated as zwitterion.

In summary, we can say that the important steps of the pyrolysis of 5a are the cleavage and [3,3]-migration of the C(1)–O bond leading to the formation of naphthalene-1-ol and 2-(3-oxapropa-1,2-dienyl)naphthalen-1(2H)-one (12), respectively. The next steps towards 6 and 16, the precursor of (E)-7, depend on the interplay of decarbonylation reactions and H-shifts, and it would need further FVP experiments to elucidate the sequence of these steps.

EXPERIMENTAL
General: Melting points (mp) were measured on a Büchi FP5 apparatus. They are not corrected. TLC on alumi­num sheets coated with silica gel 60 F254 (Merck). Column chromatography (CC) was performed on silica gel 60 (40-63 µm; Chemie Uetikon AG). IR spectra were recorded on a Perkin Elmer 1600 FT-IR spectrometer. Band positions are given in wave-numbers (cm–1). Transmissions are classified as vs = very strong (< 10%), s = strong (10-30%), m = middle (30-50%), and w = weak (> 60%). 1H-NMR and 13C-NMR spectra (CDCl3) were measured at 300 K on Bruker instruments at 600, 500 or 300 MHz; δ in ppm related to internal TMS (= 0 ppm) and adjusted to the solvent signals 7.26 ppm and 77.00 ppm, respectively, J in Hz. Assignments of the signals are based on additional DEPT 90, DEPT 135, COSY, NOSEY, NOE, HSQC, HMBC, and TOCSY measurements. Mass spectra (MS) were measured on a Finnigan MAT 95 instrument; chemical ionisation (CI) with NH3, 70 eV, at a temp. of 250 °C.

Synthesis of the Prop-2-ynoates. – The prop-2-ynoates were prepared according to our described procedure [6] from the corresponding phenols via their arylcarbonochloridates and reaction of them with sodium prop-2-ynoate in overall yields of >80%.
3,5-Bis(ethoxymethyl)-2,4,6-trimethylphenyl prop-2-ynoate (3a). Crystallized from hexane/Et2O, mp 108.7–109.3 °C. 3,5-Bis(ethoxymethyl)-2,4,6-trimethyl­phenol (mp 85.5–88.1 °C (hexane/Et2O)) was prepared by bisbromome­thylation of mesitol with 1,3,5-trioxane/HBr in AcOH,17 followed by heating of the resultant 3,5-bis(bromomethyl)mesitol with EtONa in EtOH/MeCN (1:1).
Naph­thalen-1-yl prop-2-ynoate (5a). Crystallized from hexane/t-BuOMe, mp 56.0–57.8 °C.
Naph­tha­len-2-yl prop-2-ynoate (5b). Crystallized from hexane/t-BuOMe, mp 73.5–75.0 °C (mp 74°).18

Flash Vacuum Pyrolysis (FVP) of the Prop-2-ynoates. – The pyrolysis experiments were performed as described in detail in ref.6 However, the heating device (oven for combustion analyses in6) was exchanged by a Thermolyne® 21100 tube furnace of Sigma-Aldrich, which formed with the quartz tube inside an angle of inclination of 45° (vertical in6). The solution of the prop-2-ynoates (in each case 2.0 g) in o-xylene (2.0 mL) was passed through the quartz tube at 650 – 670 °C with a residual stream of nitrogen. After warming of the cooling trap to rt, the o-xylene in the resulting pyrolysate was evaporated, and the residue was subjected to chromatography on silica gel with hexane/Et2O mixtures.

ACKNOWLEDGEMENTS
We thank Dr. D. N. Laikov for the DFT calculation of the 13C shifts of (E)-7. Support of this work by the Swiss National Science Foundation is gratefully acknowledged.

FVP of 3,5-Bis(ethoxymethyl)-2,4,6-trimethylphenyl prop-2-ynoate (1a). – 5,7-Bis(ethoxymethyl)-4,6,8-trimethyl-2H-cyclohepta[b]furan-2-one (3a). CC of the pyrolysate gave 0.184 g (9.2%) 3a as yellow solid.
Data of 3a: Lemon yellow crystals, mp 136.8–140.4 °C (toluene). UV/VIS (MeCN): λmax 403 (4.10), 265 (4.28), 249 (4.28); λmin 318 (3.32), 256 (4.27). IR (CHCl3): 3027w, 3022w, 3012m, 2980w, 2930w, 2877w, 1721vs, 1609w, 1575w, 1480m, 1445w, 1412w, 1375w, 1351w, 1286w, 1253w, 1235w, 1198w, 1171w, 1125w, 1095m. 1H-NMR: δ 5.76 (s, H–C(3)); 4.44 (s, CH2–C(7)); 4.27 (s, CH2–C(5)); 3.63 (m, MeCH2OCH2–C(5)); 3.60 (m, MeCH2OCH2–C(7)); 2.55 (s, Me–C(8)); 2.52 (s, Me–C(6)); 2.39 (s, Me–C(4)); 1.31 (t, MeCH2OCH2–C(7)); 1.28 (t, MeCH2OCH2–C(5)). 13C-NMR: δ 168.88 (C(2)); 151.79 (C(3a,8a)); 142.37 (C(5)); 141.80 (C(7)); 137.90 (C(6)); 137.19 (C(4)); 127.50 (C(8)); 98.20 (c(3)); 70.70 (EtOCH2–C(5)); 69.30 (EtOCH2–C(7)); 66.18 (MeCH2OCH2–C(5,7)); 21.15 (Me–C(8)); 20.59 (Me–C(6)); 16.31 (Me–C(8)); 14.94 (MeCH2OCH2–C(5,7)). EI-MS: 304 (9, [M]+), 258 (17, [M – EtOH]+), 212 (100, [M – 2 EtOH]+).

FVP of Naphthalen-1-yl prop-2-ynoate (5a). – Naphtho[1,2-b]furan (6), Naphthalen-1-ol, and (E)-2-(4-methyl-2H-benzo[h]chromen-2-ylidene)­naphthalen-1-(2H)-one ((E)-7). CC of the pyrolysate gave 0.291 g 6 (17%), 0.367 g (25%) naphthalen-1-ol, and 0.0031 g (1.8%) (E)-7. 1% of 5a was recovered.

Data of 6 (see19,20): Colorless oil. IR (CHCl3): 3305m, 3064vs, 3013vs, 1593m, 1513vs, 1456m, 1439m, 1411m, 1393vs, 1323vs, 1296vs, 1223m, 1208m, 1170vs, 1129vs, 1069vs, 1040s, 1022vs. 1H-NMR: δ 8.35 (br. d, 3J(9,8) = 8.3, H–(9)); 7.96 (dt-like, 3J(6.7) = 8.2, H–C(6)); 7.79 (d, 3J(2,3) = 2.05, H–(2)); 7.69 (s, H–C(4,5)); 7.62 (td, 3J(8,7) = 3J(8,9) = 8.2, 4J(8,6) = 1.20, H–C(8)); 7.52 (td, 3J(7,8) = 3J(7,6) = 8.2 4J(7,9) = 1.25, H–C(7)); 6.93 (d, 3J(3,2) = 2.05, H–C(3)).

Data of (E)-7: Dark blue solid, mp >165 °C (decomp.). UV/VIS (CHCl3; Figure 1): λmax 256 (4.33), 267 (4.34), 277sh (4.25), 297 (4.21), 307sh (4.13), 320 (4.08), 347 (3.89), 394 (3.75), 474sh (3.74), 508sh (4.00), 540 (4.15), 579 (4.15), 623sh (3.89); λmin 262 (4.33), 317 (4.08), 368 (3.71), 441 (3.46), 560 (4.11). UV/VIS (CHCl3 + 1% (v/v) TFA): λmax 253sh (4.43), 282sh (4.22), 291 (4.25), 331 (4.02), 377 (4.01), 400sh (3.84), 419sh (3.78), 523 (4.26); λmin 271 (4.18), 321 (3.98), 351 (3.91), 443 (3.63). IR (KBr): 3053w, 2956w, 2922w, 2852w, 1715m, 1639s, 1604s, 1592s, 1575s, 1543vs, 1514vs, 1498s, 1479s, 1459vs, 1447vs, 1406m, 1385m, 1366vs, 1314s, 1258m, 1227s, 1150m, 1138m, 1093s, 1023m, 933s, 886m, 862m, 812s, 787s, 745s, 714m. 1H-NMR: δ 9.07 (br. s, 4J with Me–C(4’), H–C(3’)); 8.38 (2 superimp. d with 3J ca. 7.9); 8.383: H–C(10’) and 8.381: H–C(8)); 7.80 (dd, 3J(7’,8’) = 7.4, 4J(7’,9’) = 1.4, H–C(7’)); 7.63 (d, 3J(3,4) = 9.4, H–C(3)); 7.62 (td, 3J(9’,10’) = 3J(9’,8’) = 7.8, 4J(9’,7’) = 1.4, H–C(9’)); 7.60 (d, 3J(6’,5’) = 7.3, H–C(6’)); 7.59 (td, 3J(8’,7’) = 3J(8’,9’) = 7.3, 4J(8’,10’) = 1.4, H–C(8’)); 7.49 (td, 3J(6,7) = 3J(6,5) = 7.8, 4J(6,8) = 1.4, H–C(6)); 7.44 (d, 3J(5’,6’) = 7.3, H–C(5’)); 7.39 (dd, 3J(5,6) = 7.7, 4J(5,7) ~ 2 × 5J(5,8) = 1.2, H–C(5)); 7.34 ((td, 3J(7,8) = 3J(7,6) = 1.3, H–C(7)); 6.69 (d, 3J(4,5) = 9.5, H–C(4)); 2.49 (d, 4J(Me–C(4’),3’) ∼ 1, Me–C(4’)). 13C-NMR (Figure 2): δ 182.49 (C(1)); 162.98 (C(2’)); 149.44 (C(10b’)); 148.90 (C(4’)); 137.79 (C(4a)); 134.92 (C(6a’)); 132.76 (C(8a)); 131.36 (C(6)); 128.72 (C(8’)); 128.00 (C(7’)); 127.32 (C(9’)); 127.14 (C(5)); 126.82 (C(8)); 126.01 (C(7)); 124.97 (C(6’)); 123.15 (C(10a’)); 123.02 ((C(3)); 122.07 (C(10’)); 120.02 (C(3’)); 119.56 (C(5’)); 118.50 (C(4a’)); 118.47 (C(4)); 109.91 (C(2)); 19.70 (Me–C(4’)). EI-MS: 336 (76, [M]+), 321 (44, [M – Me]+), 265 (39, [(M – Me) – 2 CO]+), 171 (60), 168 (39), 115 (100), 91 (99, [C7H7]+). CI-MS: 337 (100, [M + 1]+).

FVP of Naphthalen-2-yl prop-2-ynoate (5b). – (4R*,4'S*,9R*,10a'R*)-4,9-dihydro-4,9-(4',10a'-dihydro-2’-oxo-2H-benzo[4,5]cyclo­hepta[1,2-c]furan-4',10a'-diyl)-2H-benzo[4,5]cyclohepta[1,2-c]­furan-2-one (11; systematic name: (1R*,2R*,11S*,12R*)-16,28-dioxaheptacyclo­[10.6.6.42,11.02,25.05,10.
013,17.019,24]octacosa-3,5,7,9,13,17,19,21,23,25-decaene-15,27-dione). CC of the pyro­ly­sate gave 0.210 g (10.5%) 11 as colorless solid and 0.601 g (30%) non-reacted 5b.

Data of 11: Colorless prisms from CH2Cl2. IR (CHCl3): 3029w, 1766s, 1664w, 1600w, 1494w, 1228w, 1200w, 1090w, 1004w. 1H-NMR: δ 7.44 (d, 3J(5’,6’) = 7.5, H–C(5’)); 7.37 (td, 3J(6’,5’) = 3J(6’,7’) = 7.5, H–C(6’)); 7.20 (td, 3J(7’,6’) = 3J(7’,8’) = 7.5, 4J(7’,5’) = 1.4, H–C(7’)); 6.93 – 6.88 (m, H–C(6,8’)); 7.15 – 7.11 (m, H–C(7,8)); 6.57 (d, 3J(5,6) = 7.4, H–C(5)); 6.28 – 6.22 (m, H–C(10,3)); 6.02 (d, 3J(9’,10’) = 12.2, H–C(9’)); 5.98 (s, H–C(3’)); 5.78 (d, 3J(10’,9’)) = 12.2, H–C(10’)); 4.67 (d, 3J(4’4) = 5.0, H–C(4’)); 4.47 (d, 3J(4,4’) = 5.0, H–C(4)); 4.35 (d, 3J(9,10) = 9.5, H–C(9)). 13C-NMR: δ 169.62 (C(2’)); 167.75 (C(2)); 164.32 (C(3a’)); 155.32 (C(3a)); 152.88 (C(10a)); 137.68 (C(8a)); 134.72 (C(8’)); 134.19 (C(4a’)); 133.86 (C(4a)); 132.62 (C(8a’)); 131.77 (C(9’)); 130.97 (C(8)); 130.91 (C(5)); 129.80 (C(5’)); 129.41 (C(6’)); 128.23 (C(7)); 128.10 (C(7’)); 127.77 (C(6)); 126.14 (C(10’)); 121.37 (C(3’)); 116.58 (C(3)); 109.77 (C(10)); 89.83 (C(10’)); 53.95 (C(4)); 52.82 (C(4’)); 50.64 (C(9)). CI-MS: 410 (47, [M + NH4]+), 393 (3.6, [M + 1]+), 214 (4.7, [10 + NH4]+), 197 (100, [10 + 1]+).

2. X-Ray crystal structure determination of 11·2CH2Cl2. Crystal data: Crystals obtained from CH2Cl2, C26H16O4·2CH2Cl2, M = 562.27, monoclinic, space group: P21/n, a = 8.073(2) Å, b = 14.822 (3) Å, c = 20.402 (2) Å, β = 97.35(1)°, V = 2420.9(6) Å3, Z = 4, Dx = 1.543 g cm3, θ(max) = 55°, T = ­–83 °C, crystal dimensions: 0.18 × 0.35 × 0.45 mm, Rigaku AFC5R diffractometer, Mo Kα radiation, λ = 0.71069 Å, µ(MoKα) = 0.524 mm-1, 6183 measured reflections, 5558 independent reflections, 3705 reflections with I > 2σ(I), refinement on F with teXsan,21 405 parameters, R(F) [I > 2σ(I)] = 0.0422, wR(F2) [I > 2σ(I) reflections] = 0.0369, goodness of fit = 1.449, Δρmax = 0.33 e Å-3. The asymmetric unit contains one molecule of 11 plus two molecules of dichloromethane. There is no disorder. The non-hydrogen atoms were refined anisotropically. All of the H-atoms were placed in geometrically calculated positions.
CCDC-829303 contains the supplementary crystallographic data for this compound. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

ACKNOWLEDGEMENTS
We thank Dr. D. N. Laikov for the DFT calculation of the 13C shifts of (E)-7. Support of this work by the Swiss National Science Foundation is gratefully acknowledged.

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