An enamine method for the synthesis of 1-azaazulene derivatives. Reactionsof troponimines with enamines

A short new synthesis of 1-azaazulene derivatives consists of the enamine a lkylation of troponimines 4-7 with pyrrolidino enamines, which are derived from cycloalkanones, aliphatic ketones, and heterocyclic ketones, to lead t o formal [[8 + 2] cycloadducts and subsequent aromatization under the react ion conditions. The reactions are quite general, and N-hydroxy- and N-metho xytroponimines, 4 and 5, are less reactive than N-mesyloxy- and N- tosyloxy troponimines, 6 and 7, which react smoothly even at room temperature. In th e 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 deriv atives. In the context of the reactivity of 4-7 and pyrrolidino enamines an d the selectivity observed in the reactions with isomeric mixtures of enami nes, minimal neglect of differential overlap (MNDO) calculations on troponi mines 4-7 and pyrrolidino enamines, as well as isomeric mixtures of two ena mines, 15/16 and 20/21, were performed to gain further insight into the rea ctions via a theoretical interpretation based upon frontier molecular orbit al theory (FMO). Troponimines 6 and 7 have lower LUMOs and they are more re active than troponimines 4 and 5; the energy level of the HOMO of the pyrro lidino 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 co rresponding isomers 16 and 21, respectively.