Synthesis of the N-(tert-butyloxycarbonyl)-O-triisopropylsilyl-d-pyrrolosamine glycal of lomaiviticins A and B via epimerization of l-Threonine

An efficient synthesis of the N-(tert-butyloxycarbonyl)-O-triisopropylsilyl-d-pyrrolosamine glycal of lomaiviticin A (1) and lomaiviticin B (2) is described. The synthesis is highlighted by the epimerization of the l-threonine-derived oxazolidine 10 to oxazolidine 11. This key epimerization reaction, which serves to establish the correct relative configuration of the carbohydrate unit, was made possible only after conformational analysis indicated that substituted oxazolidines may adopt conformations that preclude enolization.