A ONE-POT SYNTHESIS OF SUBSTITUTED THIENO[3,2-B]THIOPHENES AND SELENOLO[3,2-B]SELENOPHENES

Heating a mixture of 2,5-dimethyl-3-hexyne-2,5-diol (3a) with elemental sulfur in benzene at 290-200 degrees C in an autoclave affords 3,6-dimethylthieno[3,2-b]thiophene (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 temperature. 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-b]selenophene (9a) in 16% yield. Reactions of some other acetylenic diols with elemental sulfur and selenium are also described.