Studies towards the preparation of sparteine-like diamines for asymmetric synthesis

A route for the preparation of sparteine-like diamines starting from naturally occurring amino acids has been explored. Starting from the amino acids (S)-proline and (S)-phenylalanine, two novel sparteine-like diamines 2 and 3 have been prepared. The synthetic route involves Dieckmann condensation followed by a double Mannich reaction to set up the tricyclic structure with control of the relative stereochemistry. During the Dieckmann and Mannich reactions it was found that racemisation occurred either via retro-Michael or retro-Mannich processes. Conditions for preventing racemisation in the Dieckmann reaction were uncovered but it was not possible to prevent racemisation during the double Mannich reaction. Thus, the two novel sparteine-like diamines 2 and 3 have been prepared in racemic form.