2- benzaldehydes, ring chain tautomerism

An important feature of saturated oxazolo[3,4- ]pyridines is their easy epimerization at the aminal C-l stereocenter. A quite explicit example has been reported by Moloney et al. and is depicted in Scheme 46. The reaction between lactam 157 and benzaldehyde produces a mixture of hexahydro-oxazolo[3,4- ]pyridines, the kinetic product 158 being the major one. Equilibration of the mixture with boric acid allows the ratio of diasteroisomers to be reversed since /rarcr-oxazolidine 159 is now the major product <1998TL1025> the equilibration of epimeric oxazolidines via ring-chain tautomerism has been investigated in detail and explains the epimerization observed for some hexahydro-oxazolo[3,4-4]pyridines <1993JOC1967>. 

Oxazolidines are subject to ring-chain tautomerism. A variety of substituted oxazolidines in the solid state exist in the chain form, based on C NMR experiments <85X5919,92X4979). In solution, the two forms are in equilibrium, the position of which depends on the solvent and the substituents. Oxazolidines prepared from meta- and para-substituted benzaldehydes and 2-amino-2-methyl-propanol, norephedrine, norpseudoephedrine, and serine esters all give good linear plots for the equation log -l-log Ax=h where a are the Hammett-Brown values <93X6701,93JOC1967). 

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