Aza-analogues of pteridine. Part II. The novel use of silver oxide in transetherification of alkoxy-1,2,4,6,8-penta-azanaphthalenes, alkoxynitropyrimidines, and related systems

Abstract

Alkoxy-derivatives of 1,2,4,6,8-penta-azanaphthalene (pyrimido[5,4-e]-as-triazine) undergo transetherification when treated with boiling alcohols in the presence of silver oxide. Appropriate methoxy-compounds give 5-ethoxy-3-methyl-, 5-propoxy-3-methyl, and (more slowly) 5-isopropoxy-3-methyl-penta-azanaphthalene; also 5,7-diethoxy-, 5,7-dipropoxy-, 5,7-di-isopropoxy-, 5,7-diethoxy-3-methyl-, and 5-ethoxy-3,7-dimethyl-penta-azanaphthalene. Synthetic routes to the methoxy-substrates and to one of the products are reported; other structures are confirmed by u.v. and ¹H n.m.r. spectra.

4-Methoxypteridine undergoes transetherification similarly but simple alkoxypyrimidines and alkoxypyridines are resistant. However, the more highly activated 2-methoxy-5-nitro-, 4-methoxy-5-nitro-, and 2,4-dimethoxy-5-nitro-pyrimidine easily give their respective propoxy-homologues, and so does 2-methoxy-3,5-dinitropyridine. In contrast, 2,4-dimethoxy-6-methyl-5-nitropyrimidine gives only its 4-methoxy-2-propoxy-homologue, and both 4,6-dimethoxy-5-nitro- and 4-methoxy-2,6-dimethyl-5-nitro-pyrimidine remain unchanged.

The classical transetherification agent, ethanolic sodium ethoxide, reacts with 5,7-dimethoxypenta-azanaphthalene to give the 7-ethoxy-5-hydroxy-analogue. This was identified by spectral comparison with its 5-hydroxy-7-methoxy-homologue, itself synthesised by 5,6-addition of methanethiol to 7-methoxypenta-azanaphthalene followed by dehydrogenation and selective hydrolysis.