VERSATILE ALPHA-PINENE-BASED BORANE REAGENTS FOR ASYMMETRIC SYNTHESES

The discovery of hydroboration in 1956 made organoboranes readily available. Systematic study of the chemistry of organoboranes made clear their versatility for organic synthesis. An unexpected development was the discovery that the substitution reactions of organoboranes proceed predominantly with retention of configuration, in contrast to substitution in carbon derivatives. Consequently, the ready synthesis of R* B < by hydroboration of alkenes with mono- and diisopinocampheylboranes, IpcBH(2) and Ipc(2)BH, provided a new general route to asymmetric synthesis. B-Ipc-9-BBN (Alpine-Borane(R)) and Ipc(2)BCl (DIP-Chloride(TM)) made asymmetric reduction easy. Ipc(2)BAllyl and Ipc(2)BCrotyl made asymmetric allyl- and crotylboration readily available for asymmetric synthesis. The ring-opening reaction of meso-epoxides with Ipc(2)BI provided a convenient route for optically pure halohydrins. Ipc(2)BOTf has been utilized by Paterson for asymmetric enolboration reactions. Matteson developed asymmetric homologation reactions using pinanediol boronates. All of these developments justify the title: Versatile cr-pinene-based borane reagents for asymmetric synthesis. Examination of the scope of this synthesis of pure enantiomers reveals that the achievement of the synthesis of 34 R*B < systems in greater than or equal to 99% ee makes possible the simple synthesis of over 100,000 essentially optically pure enantiomers.