Surprising reaction of 5-(phenylthio)- and 5-(methylthio)pent-2-en-4-inal with HCl

Contrary to expectations (Scheme 1), 5-(phenylthio)-(1a) as well as 5-(meth ylthio)pent-2-en-4-inal (1b) react with a slight excess of HCl to give 2-[b is( phenylthio)methyl] furan (17a, 77% yield) and 2-[bis(methylthio)methyl] furan (17b, 61% yield), respectively. Structures 17a and 17b are supported by the results of an X-ray crystal-structure analysis, by spectroscopic dat a in comparison to those of model compounds, and by synthesis of 17a. This surprising reaction is tentatively explained by a mechanism (Scheme 4), inc luding a special pyran --> furan ring-contraction sequence, which is in agr eement with a labelling experiment.