Polyaza heterocycles. Part 1. Halogenation of quinoxalino[2,3-c]cinnolines by hydrogen halides

Abstract

Quinoxalino[2,3-c]cinnoline (2a) reacts in chloroform solution with hydrogen chloride (or phosphorus trichloride) to give a highly insoluble, deep blue adduct, which on treatment with aqueous sodium hydroxide gives 10-chloroquinoxalino[2,3-c]cinnoline (2b) in high overall yield. Other quinoxalinocinnolines lacking a substituent at C-10 are also chlorinated at this position by the above procedure, and the corresponding reactions with hydrogen bromide yield 10-bromo analogues.

If the 10-position in the starting compound is already substituted, a blue adduct is still formed, but this is found to give different types of final product according to the nature of the original substituent. In some cases, 9-halogenoquinoxalinocinnolines are obtained; in others, the starting compound is regenerated; and in others, mixtures are produced.

Possible mechanisms for these novel halogenations are discussed: these are supported by MNDO calculations.