group containing not more than about 6 carbon atoms, or one of R3 and R4 is a hydrogen atom and the other is a toluenesulfonyl group, or R3 and R4 together with the intervening nitrogen atom form a nitrogenous heterocyclic ring containing no heteroatoms other than said intervening nitrogen atom, which process comprises reacting a corresponding squaric acid derivative of the formula: ##STR22## in w