The condensation of 2, 6-diaminopyrimidin-4-one (3) with various α-halo carbonyl compounds is examined. The reaction produces two types of products, both regiospecifically. For example, treatment of 3 with α-chloroacetone affords 2-amino-6-methylpyrrolo[2, 3-d]pyrimidin-4-one (5a) and 2, 4-diamino-5-methylfuro[2, 3-d]pyrimidine (6a). Depending upon the nature of the α-halo carbonyl compound, pyrrolo[2, 3-d]pyrimidin-4-one and/or furo[2, 3-d]pyrimidine products were observed. Structural assignments were based on UV, 1H NMR, 13C NMR. 2-Chloropropionaldehyde was found to react with 3 to produce 2-amino-5-methylpyrrolo[2, 3-d]pyrimidin-4-one (7) exclusively, thus providing an entry into the substitution pattern of nucleoside Q (2). © 1978, American Chemical Society. All rights reserved.