Conformational and stereoelectronic control in ring‐transformations of cis ‐4,5‐dialkoxytetrahydropurine‐2,6,8‐triones

Divergent acid‐catalysed ring‐openings of 4,5‐dimethoxytetrahydropurine‐2,6,8‐triones 2 at position 4, yielding 1‐(5‐methoxyhydantoin‐5‐carbonyl)ureas 4 (R 7 = Me) or 5‐methoxy‐5‐ureido‐2,4,6‐pyrimidinetriones 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 presumably occurs via a bicyclic acid aminal type intermediate 3 , heretofore misassigned as the reaction product. A curious base‐catalysed rearrangement was encountered with the 5 (R 1 = R 3 = Me, R 7 = H) cases, which afforded 5‐methoxy‐1,5‐bis(methylaminocarbonyl)hydantoins 7 . Remarkable stability of the conformationally rigid propellane type 4,5‐ethylenedioxytetrahydropurine‐2,6,8‐triones 9 toward acids, shows that the mode of ring‐opening at position 4 is controlled by powerful Stereoelectronic factors. However, an alternative ring‐opening at the 1,6‐bond has occurred on heating aqueous solutions of 9a (R 7 = H); the ensuing decarboxylative rearrangement leads to 1,3‐dimethylallantoin ( 12 ) and its precursor, 1‐(2‐hydroxyethoxy)‐2,4‐dimethyl‐3,7‐dioxo‐2,4,6,8‐tetraazabicyclo[3.3.0]octane( 11 ).