Formation of vinyl halides from vinyl cations generated by acetylenic participation in biomimetic polyene cyclizations

Article abstract of DOI:10.1021/ja00391a017

The object of this study was to examine the fate of vinyl cations of type 4 (resulting from biomimetic polyene cyclizations) in the presence of various halides as sources of nucleophiles. Treatment of the dienynol 1 with trifluoroacetic acid in methylene chloride for 45 min at -78°C resulted in the isolation of the rearranged bicyclic vinyl chloride 6 in 56% yield. Confirmation of the structure and isomeric purity of 6 was obtained by exhaustive ozonization to give diketo ester 9 followed by Wolff-Kischner reduction and then esterification to yield ester 10, the structure of which was confirmed by hydrogen-deuterium exchange of the protons α to the carbomethoxy group. The configuration of the ring fusion in 6 was shown to be trans in the following manner. Selective ozonization gave chloro ketone 11, which was converted into chloro olefin 12 by Wolff-Kischner reduction. Further ozonization afforded keto ester 13, which was converted into diacid 15 by sodium hypobromite oxidation. Esterification afforded diester 16, which was identical with the material obtained via Wolff-Kischner reduction of the known keto diester 17. More careful examination of the products from the aforementioned cyclization also showed the presence of 8% ketone 8 (after hydrolytic workup). Identical cyclization conditions of 1, except at 0°C, gave 55% vinyl chloride 6 and 45% ketone 8 while use of a 7.5:1 pentane-1,2-dichloroethane mixture at 0°C led to 11% vinyl chloride 6 and 89% ketone 8. Cyclization of dienynol 1 with stannic chloride in 1,1-dichloroethylene at -35°C for 45 min gave a 53% yield of three isomeric vinyl chlorides, 7b, 7a, and 6, in a ratio of 8:75:12. The successful cyclization of dienynol 1 led to more extensive cyclization studies of substrates such as 2, 3, and 35, which provided tetracyclic products. The most thoroughly studied case was that of trienynol 3, which, on treatment with stannic chloride in 1,1-dichloroethylene at ca. -30°C for 50 min, gave a 64% yield of three tetracyclic products A, B, and C in a ratio of 15:73:12 by VPC. Product A was shown to be chloro diene 22 by hydrogenation of the Δ¹-olefinic bond followed by ozonization to give 5β,13α-androstan-17-one (29), which was compared with an authentic sample prepared by irradiation of 5β-androstan-17-one (25). Products B and C were shown to be chloro dienes 18 and 20 by conversion of the mixture into 5β-androstan-17-one (25) and keto ester 27. A systematic variation of reaction conditions as well as the use of mixed-halide sources led to evidence for a possible mechanism for the formation of all three products from an intermediary tricyclic cation 43.