synthesis of (2r,5r)-2-amino-5-hydroxyhexanoic acid by intramolecular cycloaddition

The title compound was synthesized from sorbic acid by an eight step sequence. The key step was the stereospecific intramolecular Diels-Alder reaction of the acylnitroso derivative of N-sorbyl-L-proline (5). L-Proline served as a temporary tether which directed both the stereochemistry and the regiochemistry of the cycloaddition.