Stereochemical study of 1,3-N,X-heterocycles derived from α-aminoacids and formaldehyde. Structural evidence for the existence of the anomeric effect

We have shown that condensation of l-cysteine methyl ester () and l-threonine methyl ester () with formaldehyde provides a convenient and efficient access to novel heterocyclic 2:3 adducts. Depending on the amino acid, the condensation leads to either the N,N′-methylenebis(thiazolidine) (, l-cys) or -(oxazolidine) (, l-thr) derivative or to its bicyclo[4.4.1]undecane isomer (, l-ser) as the major product of the reaction. The structure of , and , was unambiguously confirmed by diffraction analysis and/or NMR spectroscopy. The latter proved to be a powerful tool to discriminate between the two possible isomers. X-Ray data emphasized the contribution of stereoelectronic effects to the structure of the above compounds.