Enantioselective synthesis and absolute configurations of the enantiomers of o-carboranylalanine

We report two new asymmetric syntheses of o-carboranylalanine, [3-(1,2-dicarba-closo-dodecaborane(12)-1-yl)-2-aminopropanoic acid] (1), using the Fitzi–Seebach imidazolidinone and the Oppolzer–Lienard sultam procedure, respectively. Both methods gave high diastereoselectivity but some racemisation of 1 (EP, 91–96%) was observed after the final hydrolysis step in the imidazolidine procedure. The Oppolzer procedure gave 1 with EP>99%. The absolute configuration of the (−)-1 (CH3OH) was established as S. The preparation of (S)-Boc-1 is reported. Attention is drawn to a spontaneous self-degradation of the zwitterionic form of 1 in water and methanol solutions.