:Evans–Saksena reduction
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{{cite journal |last1= Saksena|first1= Anil|last2= Mangiaracina|first2= Pietro|year= 1983|title= Recent studies on veratrum alkaloids: a new reaction of sodium triacetoxyborohydride [NaBH(OAc)3]|journal= Tetrahedron Letters|volume=24 |issue= 3|pages= 273–276|doi=10.1016/S0040-4039(00)81383-0 }} and further developed by David A. Evans in 1987.{{cite journal |last1=Evans |first1=David |last2=Chapman |first2= K.|last3=Carreira|first3=E.|year= 1988|title= Directed reduction of β-hydroxy ketones employing tetramethylammonium triacetoxyborohydride|journal= Journal of the American Chemical Society|volume= 110|issue= 11|pages= 3560–3578|doi= 10.1021/ja00219a035}}
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.
File:Evans-Saksena transition state.tif
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.{{cite journal |last1=Masamune |first1=Satoru |year= 1988|title= Asymmetric synthesis and its applications: Towards the synthesis of bryostatin 1|journal= Pure Appl. Chem.|volume= 60|issue= 11|pages= 1587–1596|doi= 10.1351/pac198860111587|s2cid=95655097 |url= http://pac.iupac.org/publications/pac/pdf/1988/pdf/6011x1587.pdf|accessdate=30 December 2012}}{{cite journal |last1=Nakagawa-Goto |first1=Kyoko|last2=Crimmins|first2=Michael|year= 2011|title= Synthetic Approaches to the Bottom Half Fragment for Bryostatin 11|journal= Synlett|volume= 2011|issue=11|pages= 1555–1558 |doi= 10.1055/s-0030-1260784}}