Synthesis of Boc-protected bicycloproline

The synthesis of a highly constrained quaternary carbocyclic α-amino acid, (+)-N-Boc-bicycloproline, has been achieved starting from sodium cyclopentadienylide. Key steps include a rhodium-catalyzed nitrenoid C–H insertion to install the tert-alkylamine and a ring-closing metathesis reaction to form the pyrrolidine ring.