Condensation of certain 4.5-diaminopyrimidine derivatives with a-bromoisopropyl methyl ketone is shown to give pyrimido [4,5-b] [I ,4]oxazines and not the expected dihydropteridine derivatives. Similar condensations with a number of different a-halogeno-aldehydes and a-halogeno-ketones have been studied, and thefactorswhich control the nature of the product have been determined. Some preliminary studies of the chemical reactivity of pyrimido- [4,5-b] [I ,410xazines have been carried out.of Strathclyde, Glasgow C.l SEVERAL workers 2-6 have reported that condensation (2) involved condensation of 2,4,5-triaminopyrimidinof a 4,5-dianiinopyrimidine with an cc-halogeno-aldehyde B( 1H)-one (3) with wbromoisopropyl methyl ketone (9). or -ketone gives a dihydropteridine derivative. In Unexpectedly, the product was not a dihydropteridine, many cases the dihydropteridine has not been isolated, but a derivative (10) of pyrimido[4,5-b] [1,4]oxazine. since such compounds are readily oxidized to give the Hitchings and his co-workers ' 9 8 have described some fully aromatic pteridine.aspects-of the synthesis of this ring system, but no : ; % & Br-C-' I 'Me M e 1 9 5 : { $ ; P CO-Me R'kN4 3 R2 (3) R1 = K2 = NH, (9) (10) R1 = NH,, R 2 = NH, (4) R1 = NK,, R2 = NMe, (11) R1 = NH,, R2 = NMe, (12) R1 = NH,, Ra = NHMe ( 5 ) R1 = NH,, R 2 = NHMe (6) R1 = NH,, R2 = NH*C,II1, (7) R1 = H, R 2 = NH, (8) R1 = NH,, R2 = H (13) R1 = NH,, R2 = NH*C6Hll (14) R1 = H, K2 = NH, (15) R1 = R2 = NHAc Independent studies in Stuttgart and in Glasgow of the synthesis of the blocked dihydropteridines (1) and t Present address: