Azomethine methylides

Dithiolane isocyanate iminium methylides (55), are a new type of azomethine methylide derived 1,3-dipole, and undergo efficient and regioselective cycloaddition to thiocarbonyls to yield predominandy thiazolidine-2-thiones (56) <96TL711>. 

The desilylation methodology for the generation of 1,3-dipoles, developed by Vedejs and West (29) with regard to azomethine ylides, was successfully applied by Achiwa and co-workers (30) to the field of thiocarbonyl ylides. This approach allowed the generation of the parent thioformaldehyde (5)-methylide (la) and its use for preparative purposes (31,32). Generation of la in the presence of C=C dipolarophiles led to tetrahydrothiophenes (19) in high yield (Scheme 5.4). 

Only a few examples of the [3 + 2] cycloaddition of thiocarbonyl ylides with C=N compounds have been reported so far. By comparison with aldehydes, imines are poor dipolarophiles. For example, Al-benzylidene methylamine and adamanta-nethione (5)-methylide (52) produce 1,3-thiazolidine (129) in only 13% yield (163). An alternative and efficient approach to 1,3-thiazohdines involves the [3 + 2] cycloaddition of azomethine ylides with thiocarbonyl compounds [cf. (169)]. 

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