Recent advances in the boron route to asymmetric synthesis

Chiral organoboranes, readily available via asymmetric hydroboration or the Matteson asymmetric homologation reaction, have been successfully utilized for a general asymmetric synthesis of pure enantiomers. The recent continued successes achieved in our three-pronged approach to asymmetric synthesis (1) asymmetric hydroboration, (2) asymmetric reduction, and (3) asymmetric allyland crotylboration, coupled with developments in asymmetric homologation, reveal the broad scope of these methodologies for the synthesis of stereoisomers in purities approaching 100 I ee. INTRODUCTION It was serendipity that we discovered the hydroboration reaction (l), the remarkably facile addition of diborane in ether solvents to alkenes and alkynes (eq 1). This reaction was initially received without enthusiasm. Little was known about the chemistry of organoboranes. But, we studied the characteristics of hydroboration systematically (2). These studies gave birth to several new reagents. Systematic study of the organoborane products made available by hydroboration revealed their remarkable versatility (3) (Fig. 1). An unexpected feature revealed by these studies was the fact that the great majority of the substitution reactions of organoboranes proceed with complete retention of configuration in the organic group that is transferred from boron to some other element or group (Fig 2). t cis3 cis: cis, t m : t m 9 t m , etc. RCH=CHCH=CHR R 4 ROH