An enamine method for the synthesis of 1-azaazulene derivatives. Reactions of troponimines with enamines1

Abstract

A short new synthesis of 1-azaazulene derivatives consists of the enamine alkylation of troponimines 4–7 with pyrrolidino enamines, which are derived from cycloalkanones, aliphatic ketones, and heterocyclic ketones, to lead to formal [8 + 2] cycloadducts and subsequent aromatization under the reaction conditions. The reactions are quite general, and N-hydroxy- and N-methoxytroponimines, 4 and 5, are less reactive than N-mesyloxy- and N-tosyloxytroponimines, 6 and 7, which react smoothly even at room temperature. In the reaction of the pyrrolidino enamine, which is derived from cyclopentanone, forcing conditions are required, probably because of ring strain in the [8 + 2] cycloadduct 11a. Furthermore, in the reactions of 4–7 with isomeric mixtures of two pyrrolidino enamines, 15/16 and 20/21, only enamines 15 and 20 can intervene in the cycloaddition reactions, giving 1-azaazulene derivatives. In the context of the reactivity of 4–7 and pyrrolidino enamines, and the selectivity observed in the reactions with isomeric mixtures of enamines, minimal neglect of differential overlap (MNDO) calculations on troponimines 4–7 and pyrrolidino enamines, as well as isomeric mixtures of two enamines, 15/16 and 20/21, were performed to gain further insight into the reactions via a theoretical interpretation based upon frontier molecular orbital theory (FMO). Troponimines 6 and 7 have lower LUMOs and they are more reactive than troponimines 4 and 5; the energy level of the HOMO of the pyrrolidino enamine derived from cyclopentanone is relatively higher as compared to those of other pyrrolidino enamines. Enamines 15 and 20 are less stable and energy levels of the HOMOs are higher as compared with those of the corresponding isomers 16 and 21, respectively.