ENANTIOSELECTIVE SYNTHESIS OF S-O-CARBORANYLALANINE VIA METHYLATED BISLACTIM ETHERS OF 2,5-DIKETOPIPERAZINES

A new synthesis of S-o-carboranylalanine by the Schollkopf-Hartwig procedure is described. The metalated bislactim ether of cyclo-(D-Val-Gly) reacted dth propargylbromide to yield with 52 % de the (5S)-propargyl adduct. Its boronation with 6,9-bis(acetonitrile)-decaborane proceeded in satisfactory yields despite the steric hindrance of the bislactim ether, but surprisingly thermal isomerization was found to occur to some extent at the chiral C-5 atom. Although all attempts to separate the diastereomeric 12:1 mixture failed so far, this synthetic approach represents a significant improvement because of the relatively facile access to S-o-carboranylalanine in high enantiomeric purity.