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[摘要]：A series of oxazolidines derived from tris(hydroxymethyl)aminomethane (TRIS; 1), have been prepared efficiently. Geometries optimized at the B3LYP/6-31G(center dot) level of theory, along with the crystal data of compounds 9 and 12 and NOESY correlations, point to a strong endo anomeric effect that anchors a preferential conformation and subsequently dictates the completely regioselective ring-opening of the oxazolidine moiety under acetylating conditions to afford imines instead of N-acetyloxazolidines. This process was monitored by H-1 NMR spectroscopy, corroborated by synthesis, and rationalized by the intermediacy of an iminium ion. Oxazolidine imine equilibria are also described for TRIS and other aminopolyols. The equilibria are shifted to the heterocyclic partner as the number of reactive hydroxy groups increases. The structures of an unprotected imine (31) and a per-O-acetylated derivative (43) have also been established by crystallographic analyses. |
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