Alkynes thiocarbonyl derivatives

The benzo derivative (128) reacts as a thiocarbonyl ylide. Addition of N-(p-tolyl)maleimide gives a mixture of the exo (71%) and endo (16%) adducts (129 Ar=p-tolyl), which in hot acetic acid eliminate hydrogen sulfide giving the pyrido[l,2-a]benzimidazole (130 Ar=p-tolyl). Analogous 1 1 cycloadducts (131) are formed with dimethyl maleate, dimethyl fumarate, methyl crotonate and methyl acrylate. In contrast to the transformation (129) —> (130), treatment of the adducts (131 R = H, Me) with hot acetic acid gives the tetracyclic compounds (133) via the benzimidazole derivatives (132 R = H, Me). Reaction with alkynic 1,3-dipolarophiles gives pyrido[l,2-a]benzimidazole (134) by desulfurization of the primary adducts (80CL1369). 

Few reactions of alkynes with C—X bonds are valuable for the preparation of four-membered hetero-cycles. Ynatnines react with aldehydes and ketones in the presence of Lewis acids to give unstable oxetene derivatives which undergo electrocyclic opening (Scheme 43). Open-chain products are also obtained with thiocarbonyl compounds,Schiff bases and iminium salts. Reactions of ynamines with carbon dioxide, ketenes, - ketenimines and isocyanates " often give mixtures of products and are of little preparative value. 

Reduction of the ketone moiety in 24 with NaBH4 provides the corresponding alcohol as a 1 1 mixture of diastereoisomers, which is then transferred into the pentalene derivative by a Barton deoxygenation using thiocarbonyl diimidazole and tributyl tinhydride. Finally, desily-lation with AgNOs and KCN in EtOH provides the desired deprotected alkyne 25. 

No systematic studies of the reaction mechanism have been undertaken, but it is suggested that the 1,3-dithiole derivative is formed directly by a 1,3-dipolar cycloaddition where the thiocarbonyl serves as the negative end of the dipole. The formation of l,6,6a/l4-trithiapentalenes may start with a 1,2-addition of the alkyne to the thiocarbonyl group, followed by a rearrangement to give 136. 

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