The reduction of α-alkyl-β-hydroxy ketones is highly syn-selective if carried out in THF on their Ti-alcoholate complexes with LiBH4 or L-Selectride® or N-selectride® depending on the bulkiness of the group bound to the carbonyl group.
The Role of theα-Stereogenic Center in the Control of Stereoselection in the Reduction ofα-Alkyl-β-hydroxy Ketones: A Highly Diastereoselective Protocol for the Synthesis of 1,2-syn-2-Alkyl-1,3-diols
作者:Giuseppe Bartoli、Maria C. Bellucci、Marcella Bosco、Renato Dalpozzo、Enrico Marcantoni、Letizia Sambri
Accurate investigations on the role played by an alpha-stereogenic center in controlling the reduction of various classes of beta-hydroxy ketones allowed us to set up a general and highly diastereoselective protocol for the synthesis of 2-alkyl-1,3-diols with 1,2-syn relationship. This methodology is based on the conversion of a beta-hydroxy ketone into the corresponding titanium alcoholate that permits us to organize the substrate in a stable and rigid structure, which stereofacially favors attacking hydride ions. The use of THF as solvent makes available a variety of hydride donors that cover a large spectrum of steric demand: the choice of the more appropriate one depends on the conformational stability of the cyclic intermediate. Excellent results are obtained also in the presence of an additional stereogenic center in the beta-position, even if it exerts a concordant or an opposite steric effect with respect to the alpha-substituent.
Complete Switch of Reaction Specificity of an Aldolase by Directed Evolution In Vitro: Synthesis of Generic Aliphatic Aldol Products
as competing nucleophiles. D6 turned out to be the key residue that enabled activity with non‐hydroxylated nucleophiles. Altogether 25 single‐ and double‐site variants (D6X and D6X/T26X) were discovered that show useful synthetic activity and a varying preference for ketone or aldehyde as the aldol nucleophiles. Remarkably, all of the novel variants had completely lost their native activity for cleavage