Aromatic azapentalenes: 1H- and (mesoionic) 2H-pyrrolotetrazoles. Part 1. Synthesis and spectral characteristics

Abstract

Two separate series of the title systems have been prepared by cyclisation of tetrazolium salts having acylmethyl functions attached to both the ring carbon and the adjacent nitrogen atom (3, 4): (i) working in an acetate buffer led to 7-acyl derivatives (5, 6; Scheme 3), and (ii) treatment with anhydride–base gave 5,7-diacyl compounds by a deviating ring closure mechanism (11, 12; Scheme 4). These materials could be defunctionalised to afford pyrrolotetrazoles (7, 8) which were earlier approached in vain from the respective 5-methyltetrazolium salts (Tschitschibabin reaction). Regarding characterisation data, attention is drawn to the conspicuous spectroscopic differences between the 1H- and the 2H-system. 2H-Pyrrolotetrazoles (6, 8, 12) represent a novel class of Ramsden’s ‘type C’ heteropentalene mesoions.