Heating a mixture of 2,5-dimethyl-3-hexyne-2,5-diol(3a) with elemental sulfur in benzene at 190-200 "C in an autoclave affords 3.6-dimethylthieno[3,2-blthiophene (5a) in 26% yield.The reaction provides a practical one-pot synthesis of several gram quantities of 5a, even if the yield is moderate, since 3a is commercially available and inexpensive.The reaction proceeds via 2,5-dimethyl-1.5-hexadiene-3-yne(4a) as one of probable intermediates, which is produced by dehydration of 3a.Addition of p-toluenesulfonic acid promotes the dehydration of 3a and thus can lower the reaction tempera-.In addition, the reaction of 4a, prepared from 3a independently, with sulfur affords 5a in a somewhat improved yield (33%).The reaction of 3a with elemental selenium also provides a one-pot synthesis of 3,6-dimethylselenolo[3,2-blselenophene (9a) in 16% yield.Reactions of some other acetylenic diols with elemental sulfur and selenium are also described.
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Nakayama et al. (1994) studied this question.