Regio- and Stereocontrolled Selective Debenzylation and Substitution Reactions of C<i>-</i>2 Formyl Glycals. Application in the Synthesis of Constrained β-Sugar Amino Acids

O-Benzyl-protected C-2 formyl glycals are regioselectively deprotected and acetylated first at C-3 and then at C-6 upon treatment with a ZnCl2−Ac2O−AcOH reagent combination. Treatment of the thus-obtained C-3 acetate with various nucleophiles leads to substitution at C-3 with retention of stereochemistry in the galactal series, whereas mixed results were obtained with glucal-derived compounds. Two of the azido-substituted products were converted into the corresponding β-sugar amino acids.经氯化锌-乙酸酐-乙酸（ZnCl₂−Ac₂O−AcOH）试剂体系处理后，O-苄基保护的C-2甲酰基糖烯可实现区域选择性脱保护，并依次在C-3位、随后在C-6位发生乙酰化反应。将由此得到的C-3位乙酸酯与各类亲核试剂反应，可在半乳糖烯系列中实现C-3位的立体构型保持的取代反应；而对于葡萄糖烯衍生的化合物，则得到了混合的反应结果。其中两种叠氮取代产物可被转化为对应的β-糖基氨基酸。