Chiral synthesis via organoboranes: XVI. Boroxazolidones derived from α-amino acids and borinic or boronic esters. A simple procedure for upgrading borinates and boronates to materials of high optical purity

Abstract

The synthesis of boron heterocycles from borinic and boronic esters with α-amino acids was explored as a means of upgrading the optical purities of intermediates from asymmetric hydroboration. B-Methoxy-9-borabicyclo[3.3.1]nonane, methyl dicyclohexylborinate and (+)- and (-)-methyl diisopinocampheylborinate react with various α-amino acids to form the corresponding crystalline chelates. Recrystallization of the chelates derived from methyl trans-2-phenylcyclopentylisopinocampheylborinate of 85% ee with l-phenylalanine, methyl isopinocampheyl- exo-norbornylborinate of 83% ee with l-proline and both optical isomers of methyl diisopinocampheylborinate of 92% ee with l-proline yield the corresponding products with optical purities approaching 100% ee. Dimethyl cyclopentylboronate, dimethyl exo-norbornylboronate and dimethyl isopinocampheylboronate upon treatment with iminodiacetic acid and N-methyliminodiacetic acid form the corresponding bicyclic boronates. Recrystallization of the chelate derived from diethyl 3-tetrahydropyranylboronate and iminodiacetic acid yields a product of essentially 100% ee. Consequently formation of boroxazolidones of asymmetric hydroboration products provides one possible route to upgrade such products to materials of high optical purity.