Reductive cyclization of 6-cyanomethyl-5-nitropyrimidines - An efficient route to 7-alkyl-5H-pyrrolo[3,2-d]pyrimidines and 6-amino-7,7-dialkyl-7H-pyrrolo[3,2-d]pyrimidines. Utilization in the synthesis of 9-deaza analogues of DHPA