RING CHAIN TAUTOMERISM OF 4-HYDROXIMINO-HEXAHYDROPYRIMIDINES SUBSTITUTED IN POSITION-2

As shown by NMR spectroscopy, the reaction of 3-aminopropionamide oxime (9) with benzaldehyde or benzaldehyde derivatives 2a - d substituted with electron-attracting substituents affords products which exist as an equilibrium mixture of tautomers involving the open-chain 3-(benzylideneamino)-propionamide oximes 11a - d and the cyclic 4-hydroximino-hexahydropyrimidines 12a - d. Tautomers 11 and 12, isolated also in crystalline form, can be interconverted by using an appropriate solvent. In contrast, in the reaction of benzaldehydes substituted with electron-releasing groups (2e,f) and cinnamaldehyde (2g) with 9 only the open-chain imines are formed, and in aprotic solvents no tautomerism can be observed.