SYNTHESIS OF (R)-( - )-BUTAN-2-OL

• To a stirred suspension of diisopinocampheylborane (50mmol) (1) in tetrahydrofuran (18 ml) is added 4.5 ml of (Z)-but-2-ene.

• The reaction mixture is stirred at 25 °C for 4.5 hours.

• The solid diisopinocampheylborane disappears and the formation of the trialkylborane is complete.

• The organoborane is treated with 4 ml of methanol, followed by 18.3 ml of 3 m sodium hydroxide and the careful addition of 20 ml of 30 per cent hydrogen peroxide, maintaining the temperature of the reaction below 40 °C.

• The reaction mixture is further stirred at 55 °C for 1 hour, cooled, and extracted with ether (3 x 50 ml).

• The extract is washed successively with water (2 x 25 ml) and brine (3 ml) and dried over magnesium sulphate.

• The organic layer is care-fully fractionated to provide butan-2-ol, b.p. 96-98 °C, 2.9 g (73%), purity > 95 per cent. The last traces of impurities are removed by preparative g.l.c. (2) to yield (R)-butan-2-ol, [a]£ 3 -13.23° (neat), ee 98.1 per cent.

Notes to keep in mind:

1. ( + )-a-plnene [a]£ 3 + 47.1 ° (neat), 92 per cent ee, distilled from a small excess of lithium aluminium hydride and stored under nitrogen, is used for the preparation of Ipc 2 BH.

2. For preparative g.l.c. a 1.8 m x 12.7 mm column packed with 10 per cent Carbowax 20m on Chromosorb W is used.