Preparation of 3,3-Bis(alkylthio)-1,1-dipiperidino-2-propenylium Iodides by S-Alkylation of Alkyl 3,3-Dipiperidinodithioacrylates

Methylation of propyl 3,3-dipiperidinodithioacrylate with iodomethane afforded a 93 : 7 mixture of (E)- and (Z)-3-methylthio-3-propylthio-1,1-dipiperidino-2-propenylium iodides nearly quantitatively, while propylation of methyl 3,3-dipiperidinodithioacrylate with 1-iodopropane produced a 6 : 94 mixture of the above isomers. Study of these iodides led to the conclusions that (a) free rotation about the C2–C3 bond is frozen for these carbenium ions, thus allowing the clearly distinguishable E- and Z-isomers to exist and (b) the alkylation takes place in such a manner that the newly formed S-alkyl group and the carbenium carbon atom become cis to each other. Such stereoselectivity was general for methylation of a series of alkyl 3,3-dipiperidinodithioacrylates and was explained by examining the preferred conformation of the starting dithioacrylates with NMR and X-ray diffraction analyses.