Stereochemical Diversity in Asymmetric Cyclization via Memory of Chirality

Abstract

An enantiodivergent asymmetric cyclization of N-Boc-N-omega-bromoalkyl-alpha-amino acid derivatives has been developed. With potassium amide bases in DMF, cyclization proceeds with retention of configuration, while inversion of configuration was observed with lithium amide bases in THF. Chirality of the parent amino acids was preserved during enolate formation and cyclization to give aza-cyclic amino acids in up to 98% ee with retention of configuration or inversion of configuration, depending on the reaction conditions. Thus, both enantiomers of cyclic amino acids with a tetrasubstituted stereocenter were prepared in high enantiomeric purity from readily available l-alpha-amino acids. This protocol is also applicable to a spirocyclization and an intramolecular conjugate addition of alpha-amino acid derivatives, giving either of the enantiomers of a diazaspiro compound and a tetrahydroisoquinoline derivative, respectively, in up to 99% ee.