Evans–Saksena reduction explained

The Saksena–Evans reduction is a diastereoselective reduction of β-hydroxy ketones to the corresponding anti-dialcohols, employing the reagent tetramethylammonium triacetoxyborohydride (Me₄NHB(OAc)₃). The reaction was first described by Anil K. Saksena in 1983^[1] and further developed by David A. Evans in 1987.^[2]

The reaction is thought to proceed through the 6-membered ring transition state shown below. The intramolecular hydride delivery from the boron reducing agent forces the reduction to proceed from the opposite face of the chelating β-alcohol, thus determining the diastereoselectivity.

This can be contrasted with the Narasaka–Prasad reduction which similarly employs a boron chelating agent but undergoes an intermolecular hydride delivery, favouring the corresponding syn-diol product.

The Saksena-Evans reduction has since been used in the synthesis of several products, particularly the bryostatins.^{[3] [4]}

See also

• Evans–Tishchenko reaction

Notes and References

1. Saksena. Anil. Mangiaracina. Pietro. 1983. Recent studies on veratrum alkaloids: a new reaction of sodium triacetoxyborohydride [NaBH(OAc)3]. Tetrahedron Letters. 24 . 3. 273–276. 10.1016/S0040-4039(00)81383-0 .
2. Evans . David . Chapman . K.. Carreira. E.. 1988. Directed reduction of β-hydroxy ketones employing tetramethylammonium triacetoxyborohydride. Journal of the American Chemical Society. 110. 11. 3560–3578. 10.1021/ja00219a035.
3. Masamune . Satoru . 1988. Asymmetric synthesis and its applications: Towards the synthesis of bryostatin 1. Pure Appl. Chem.. 60. 11. 1587–1596. 10.1351/pac198860111587. 95655097 . 30 December 2012.
4. Nakagawa-Goto . Kyoko. Crimmins. Michael. 2011. Synthetic Approaches to the Bottom Half Fragment for Bryostatin 11. Synlett. 2011. 11. 1555–1558 . 10.1055/s-0030-1260784.