Lithium enolates of (R)- or (S)-oxazolidinones 1 (specified in the title) are generated with lithium hexamethyldisilazanide (LHMDS) in THF at −75°C and added to aliphatic or aromatic aldehydes (products 2–18 of hydroxyalkylation, yields mostly over 80%, diastereoselectivities usually over 98%; Scheme 2). The adducts can be cleaved to give threonine analogs (19–32) under salt-free conditions (H₂/Pd-C, then H₂O and evaporation of the solvents, Scheme 3). In some cases, the primary adducts may cyclize with elimination of benzyl alcohol to give bicyclic carbamates 33 which, in turn, can be hydrolyzed to 5-substituted trans-2-oxo-1,3-oxazolidine-4-carboxylic acids 34 (Scheme 4). – The essentially complete threo selectivity of the coupling step is proved by NMR spectroscopy and by chemical correlations. The stereochemical course of the reaction is opposite to that observed with carbocyclic enolates; possible reasons for this behavior are discussed (Scheme 5). – The starting material rac-1, prepared from glycine, pivalaldehyde and benzyl chloroformate, is readily resolved by chromatography on preparative scale by using the stationary phase Chiraspher® and a Prepbar® system (Scheme 1); the undesired enantiomer may be recycled by thermal racemization (heating at reflux in CH₃CN for 8 h).

中文翻译：

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（R）-和（S）-2-叔丁基-5-氧代-恶唑烷-3-羧酸苄酯，用于从醛方便地制备D-和L-苏氨酸类似物

（R）-或（S）-恶唑烷酮1（标题中指定）的烯醇锂是在-6.75°C下于THF中由六甲基二硅氮杂酸锂（LHMDS）生成的，并添加至脂肪族或芳香族醛中（羟烷基化产物2–18，收率大多超过80％，非对映选择性通常超过98％；方案2）。可以在无盐条件（H ₂ / Pd-C，然后H ₂ O和溶剂蒸发，方案3）下裂解加合物得到苏氨酸类似物（19-32）。在某些情况下，初级加合物可环化并消除苄醇，得到双环氨基甲酸酯33，然后可将其水解为5取代的反式-2-氧代-1，3-恶唑烷-4-羧酸34（方案4）。–通过NMR光谱和化学相关性证明了偶联步骤的基本上完全的苏式选择性。该反应的立体化学过程与碳环烯醇盐观察到的相反。讨论了此行为的可能原因（方案5）。–由甘氨酸，新戊醛和氯甲酸苄酯制备的原料rac - 1易于通过色谱法以固定规模使用固定相Chiraspher®和Prepbar®系统进行拆分（方案1）；不需要的对映异构体可以通过热消旋化（在CH ₃ CN中回流加热8小时）再循环。