A Torquospecific 1,5‐Electrocyclization

Abstract

Saponification of homodiazepine l a and Ib, in the absence of any proton donors, led to the formation of the 6a electron anionic species A which, by virtue of a 1,5-electrocyclization, is in equilibrium with the allylic anion B. This latter tricyclic species is thermodynamically less favoured than its bicyclic isomer A. Nevertheless, B could be trapped by acylation and led to type-2 compounds which are the major reaction products. This is due to the fact that B is more nucleophilic and, therefore, much more reactive than A. The trunsoid topology of the tricyclic products 2 was demonstrated by 'H-NMR and by an X-ray diagram of 2d. The trunsoid geometry of 2 is a consequence of a torquospecific 1,5-electrocyclization (of A), which is due to a steric, and possibly even to an electronic factor.Introduction. -The 1,5-electrocyclizations have been reviewed and discussed extensively by Huisgen, who showed in particular that their reaction mechanism represents a unifying concept in organic chemistry [I]. In the epilogue of his review article, this author made the statement that 'should the concept of electrocyclization of the pentadienyl anion type prove to be fruitful in heterocyclic chemistry, it would add to the great scientific harvest associated with the names of R. B. Woodward and R. Hoffmann' [l].We describe herein the 1,5-electrocyclization of the diaza analogue A of a pentadienyl anion which, according to the FMO theory [2], proceeds according to a disrotatory mode. We shall demonstrate that this particular electrocyclization proceeds in a torquospecific manner'). This is to say that out of two possible disrotations, only the one which avoids steric crowding takes place (see below)2).