REARRANGEMENTS OF BRIDGED DIALLENES - A FACILE SYNTHESIS OF NOVEL CONDENSED HETEROCYCLES BY TANDEM [3,3]-SIGMATROPIC REARRANGEMENT AND DOUBLE INTRAMOLECULAR MICHAEL ADDITION OF DIALLENYL DISULFIDES AND DISELENIDES - ISOLATION OF STABLE DIALLENYL DISELENIDES

Thienothiophene 14 and selenoloselenophene 16 have been synthesized by the action of lithium methoxide on τ,τ-dimethylallenyl thiocyanate and selenocyanate, respectively. A multistep mechanism involving bis-τ,τ-dimethylallenyl disulfide (20) or diselenide as key intermediates, is suggested. The latter are believed to undergo consecutive 3,3]-sigmatropic rearrangement and double Michael addition, to the observed products. This mechanism is supported by the isolation of bis-τ,τ- diisopropylallenyl and bis-τ-methyl-τ-isopropylallenyl diselenides, under the same reaction conditions, and the rearrangement of the latter to the expected selenoloseleophene 33. The synthesis of the novel mixed selenolothiophene 35 by treatment of a mixture of allenyl thiocyanate 13 and selenocyanate 15 with lithium methoxide, is also described. © 1990.