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- 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).

oxidative transformations ; purines ; uric acid ; covalent adducts ; ethers ; stereoelectronic control ; allantoin ; uricolysis ; mechanism

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