Practical Asymmetric Synthesis of a Bicyclic Pyrrolidinol

Abstract

The “butterfly-shaped” bicyclic pyrrolidinol ((2R,7aS)-2-fluorotetrahydro-1H-pyrrolizin-7a(5H)-yl)-methanol (1) is a key building block for drug candidates, and its practical chemical synthesis remains elusive. As such, an asymmetric synthesis for ((2R,7aS)-2-fluorotetrahydro-1H-pyrrolizin-7a(5H)-yl)-methanol (1) that is amenable for scale-up has been developed. The newly optimized process utilizes readily available N-Boc-trans-4-hydroxy-l-proline methyl ester (8) to establish the challenging stereogenic center bearing the fluoride. Subsequent diastereoselective α-alkylation was achieved by leveraging Seebach’s self-regeneration of stereochemistry (SRS) methodology, which has been exploited for the synthesis of proline derivatives. Finally, intramolecular cyclization/deprotection cascade and carbonyl reduction afford the bicyclic pyrrolidinol 1 in nine linear steps from compound 8. This process significantly reduces the overall production sequence and allows the preparation of product 1 on a multikilo scale with a 40% overall yield and perfect control of chirality (>99% ee and de).