Chiral N-acyloxazolidines: synthesis, structure, and mechanistic insights

A series of chiral imines derived from 1-amino-1-deoxyalditols such as d-glucamine, a rather unexplored raw material from the chiral pool, have been serendipitiously transformed into a novel family of N-acetyl-1,3-oxazolidines by means of an unexpected acetylation. The structure of these substances is supported by spectroscopic and crystallographic data. The acetylates also trigger a complex dynamic transformation, in which an initially configured trans oxazolidine converts into a more stable cis-configured derivative. Both isomers can also exist as rotational conformers (E,Z) as a consequence of the restricted rotation around the N-acetyl bond. The barriers to rotation have been determined by variable-temperature experiments. Overall, this transformation most likely involves the intermediacy of a chiral iminium ion, which has been documented in the synthesis of nitrogen heterocycles, thus explaining the experimental facts.