Cycloadditions with 2,2-dialkyl-3-thioxochroman-4-one S-sulfides including an unprecedented [3 + 5] cycloaddition

We report the facile generation of the chroman-4-one-derived thiosulfines 10 under mild conditions by our well-established nucleophilic 'unzipping' of the corresponding acetyl α-chloroalkyl disulfides 9 which in turn can be prepared from the corresponding α-chloro sulfenyl chlorides 8. Spontaneous partial loss of sulfur from 10, most probably by the disproportionation of 10 to 11 and 12, generates the expectedly reactive β- oxo thioketones 11 which partly dimerize in a precedented Diels-Alder fashion, to give the 1,3,4-oxadithiins 13, and partly cycloadd to 10 to form the 1,2,4-trithiolanes 14 in a well-precedented fashion. Dimerization of 10 can occur in two competing ways: the precedented nonconcerted [3 + 3] cycloaddition (to give 1,2,4,5-tetrathianes 15) and the unprecedented concerted [3 + 5] addition to give 1,3,4,5,6-oxatetrathiocins 16. In the latter interaction one molecule of 10 behaves as a 1,5-dipole and a second molecule as a 1,3-dipole. In one case (10d) extensive sulfur scrambling takes place with formation of the 1,2,3,4,5,6-hexathiepane 17d. It is remarkable how sensitive the reactions of 10 are to minor variations of the simple alkyl substituents in the 2-position. In addition to the usual spectroscopic characterization all isolated key compounds were subjected to X-ray single- crystal structure determinations.