CONFORMATIONAL AND STEREOELECTRONIC CONTROL IN RING-TRANSFORMATIONS OF CIS-4,5-DIALKOXYTETRAHYDROPURINE-2,6,8-TRIONES

Divergent acid-catalysed ring-openings of 4, 5-dimethoxytetrahydropuri ne-2, 6, 8-triones 2 at position 4, yielding 1-(5-methoxyhydantoin-5-c arbonyl)ureas 4 (R-7 = Me) or 5-methoxy-5-ureido-2, 4,6-pyrimidinetrio nes 5 (R-7 = H), can be rationalized by assuming a preference for one of two conformational isomers of the cis-fused system, associated with the N-substitution effects. Intramolecular transamidation 5-->4 presu mably occurs via a bicyclic acid aminal type intermediate 3, heretofor e misassigned as the reaction product, A curious base-catalysed rearra ngement was encountered with the 5 (R-1 = R-3 = Me, R-7 = H) cases, wh ich afforded 5-methoxy-1,5-bis(methylaminocarbonyl)hydantoins 7. Remar kable stability of the conformationally rigid propellane type 4,5-ethy lenedioxytetrahydropurine-2,6,8-triones 9 toward acids, shows that the mode of ring-opening at position 4 is controlled by powerful stereoel ectronic factors. However, an alternative ring-opening at the 1,6-bond has occurred on heating aqueous solutions of 9a (R-7 = H); the ensuin g decarboxylative rearrangement leads to 1,3-dimethylallantoin (12) an d its precursor, hyl-3,7-dioxo-2,4,6,8-tetraazabicyclo[3.3.0]octane (1 1).