Hydroboration of Terpenes. 9. A Simple Improved Procedure for Upgrading the Optical Purity of Commercially Available α- and β-Pinenes. Conversion of (+)-α-Pinene to (+)-β-Pinene via Hydroboration-Isomerisation.

Article abstract of DOI:10.1021/jo00252a032

An improved method for the preparation of optically pure diisopinocampheylborane (Ipc2BH) from commercially available (+)- and (-)-α-pinene (91-92percent ee) is described.The procedure, which is based on selective incorporation of the major enantiomer of α-pinene in crystalline dialkylborane, is both simple and efficient.Treatment with benzaldehyde liberates the parent olefin in very high enantiomeric excess (>99.5percent).The intermediate Ipc2BH can be thermally isomerized (130 deg C, 12h) to dimyrtanylborane, which is readily converted into the otherwise inaccessible (+)-β-pinene (>99.5) percent ee).In the course of this study it was established that the optical purification of commercial (-)-β-pinene too can be easily achieved by the formation and recrystallisation of tri-cis-myrtanylborane.Thus, simple manipulations via hydroboration provide easy access to all four enantiomers of α- and β-pinenes in very high optical purity.