Reactions of Lithiated Alkynes and Allenes with Isothiocyanates: A Simple and Efficient Synthesis of New Aryl- or Hetaryl-Substituted 3H-Azepines and 4,5-Dihydro-3H-azepines

A novel synthetic approach to the preparation of a wide range of 3H-azepines and 4,5-dihydro-3H-azepines bearing various aryl, hetaryl, alkyl, and heteroalkyl substituents from readily accessible starting materials (aryl- and hetaryl-substituted alkynes or allenes, sec-alkyl isothiocyanates, and alkyl halides) has been developed. The methodology is based on a fast and smooth conversion of conjugated 2-aza-1,3,5-trienes derived from 1-aza-1,3,4-trienes, S-alkylated adducts of isopropyl isothiocyanate and allenic or acetylenic carbanions, into seven-membered azaheterocycles, 3H-azepines and 4,5-dihydro-3H-azepines, using potassium tert-butoxide (THF-DMSO, ca. -30 ˚C, 30 min). The ratio of the heterocycles depends on the nature of the substituent on the allene or alkyne and on the 2-aza-1,3,5-triene derived therefrom.