Heterocyclization and Spirocyclization Processes Based on Domino Reactions of N‐Tosylhydrazones and Boronic Acids Involving Intramolecular Allylborylations of Nitriles

Polycyclic molecules featuring all‐carbon quaternary bridgehead centers were synthesized through domino cyclizations between N‐tosylhydrazones and boronic acids. Variations of the general cascade have been applied for the preparation of 3‐quinuclidinones and related alkaloid‐like scaffolds through transannular heterocyclizations. Moreover, the employment of 3‐cyanopropyl and 4‐cyanobutylboronic acids and α,β‐unsaturated N‐tosylhydrazones led to spirocycles through unprecedented formal [n+1] cyclizations, including the stereoselective spirocyclization of the Hajos–Parrish ketone. The common feature of all the new reactions described is the creation of an all‐carbon quaternary center by formation of two Csp3−C bonds on the hydrazonic carbon atom. DFT‐based calculations suggested the occurrence of cascade processes, which involve a diazo compound carboborylation followed by a 1,3‐borotropic rearrangement on an intermediate allylboronic acid and a novel bora‐aza‐ene cyclization. Two bonds on the same carbon are formed in transannular cyclizations and [n+1] spirocyclizations, involving properly designed N‐tosylhydrazones and boronic acids, that lead to polycyclic structures featuring an all‐carbon quaternary bridgehead center. The proposed mechanism for the domino processes involves carboborylation of a diazo compound, a 1,3‐borotropic rearrangement, and an intramolecular bora‐aza‐ene reaction.