6.1.2 Reduction of Allylic alcohol
Allylic alcohols can be reduced with high selectivity using chiral Ru-( S )-BINAP as a catalyst. For example, the reduction of geraniol can be accomplished with 94% ee (Scheme 6). The reduced product is used for the large scale synthesis of L-(+)-menthol. Under these conditions, nerol undergoes reduction to give ( S )-citronellol in 99% ee. Chiral iridium-based catalytic systems have also been subsequently explored for the asymmetric reduction of allylic alcohols. For example, the complex bearing chiral phosphanodihydrooxazole L1 catalyzes asymmetric reduction of an allyl alcohol, which is used as a key step in the synthesis of lillial (Scheme 7). Scheme 8 illustrates the synthesis of chiral phosphanodihydrooxazole L1.
Scheme 6. Synthesis of (S) and (R)- Citronellol by Chiral Reduction of Geraniol and Nerol
Scheme 7. Asymmetric Synthesis of Lillial.
Scheme 8. Synthesis of Phosphanodihydrooxazole L1