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
摘要:
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.
Visible-Light-Mediated Rose Bengal-Catalyzed α-Hydroxymethylation of Ketones with Methanol
作者:Jingya Yang、Dongtai Xie、Hongyan Zhou、Shuwen Chen、Jiaokui Duan、Congde Huo、Zheng Li
DOI:10.1002/adsc.201800467
日期:2018.9.17
A visible‐light‐mediated α‐hydroxymethylation of ketones using methanol as the hydroxymethylating reagent has been developed. Using 1 mol% rose bengal as the photosensitizer and air as the green oxidant, the reactions proceeded smoothly at room temperature. Experimental studies indicate the reaction proceeded via a radical pathway.
Catalytic asymmetric hydroxymethylation of silicon enolates has been achieved. In this reaction, an aqueous solution of formaldehyde can be used to realize an easy and safe procedure, and high enantioselectivities have been obtained. This is the first example of catalytic asymmetric reactions in aqueous media with a chiral scandium complex.
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.
[reaction: see text] Catalytic asymmetric hydroxymethylation of silicon enolates with an aqueousformaldehyde solution has been developed using a chiral bismuth complex. This is the first example of highly enantioselective reactions using a chiral bismuth catalyst in aqueous media. In this paper, we have added Bi(OTf)(3)-1 complex as a "water-compatible Lewis acid". Bi(OTf)3 is unstable in the presence
Scandium(III) Fluoride as a Novel Catalyst for Hydroxymethylation of Dimethylsilyl Enolates in Aqueous Media
作者:Shu Kobayashi、Masaya Kokubo
DOI:10.1055/s-2008-1078409
日期:2008.6
Hydroxymethylation of dimethylsilyl (DMS) enolates using aqueous formaldehyde solution is catalyzed by scandium(III) fluoride (ScF3) in aqueous media to give the corresponding β-hydroxy ketones in good to excellent yields. Whilst the DMS enolate reactions are facile the TMS enolates react sluggishly under the same conditions. ScF3 has been shown to be a unique catalyst for this reaction.