engleski

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

Divergent acid-catalysed ring-openings of 4, 5- dimethoxytetrahydopurine-2, 6, 8-triones 2 at position 4, yielding 1-(5-methoxyhydantoin-5- carbonyl)ureas 4(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 encounterred 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 decarboxilative rearrangement leads to 1, 3-dimethylallantoin (12) and its precursor 1-(2-hydroxyethoxy)-2, 4-dimethyl-3, 7-dioxo-2, 4, 6, 8-tetraaza- bicyclo[3.3.0]octane(11).

oxidative transformations ; uric acid ; covalent adducts ; purines ; allantoin ; conversion ; mechanism ; ethers

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