Scheme 2. Diastereomeric Enrichment of 2b and 2d.
Reagents and conditions: (a) 2a or 2c, Dess–Martin, DCM, rt, 1 h; (b) NaBH4, 1% methanol in THF, rt 2 h, 2d + 2a (5.9:1) 92%, 2b + 2c (5.7:1) 85%.
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