Читать книгу Catalytic Asymmetric Synthesis - Группа авторов - Страница 41

Cordova reported an enantioselective silyl addition to enals using Me₂PhSi‐B(pin) as a silyl source, and the reaction is catalyzed by a combination of diphenylprolinol silyl ether and a copper(I) salt (Eq. 1.47) [79]. The copper(I)–silyl intermediate is a nucleophile that reacts with an iminium ion to afford the Michael product with excellent enantioselectivity. The product is a synthetically useful β‐silylated aldehyde.

(1.47)

The same group extended the reaction using B₂ (pin)₂ and dialkyl zinc, respectively, by a combined use of diphenylprolinol silyl ether and Cu(OTf)₂ to afford β‐borylated aldehydes (Eq. 1.48) [80] and β‐alkylated aldehydes [81], respectively, both with excellent enantioselectivity. The β‐arylation reaction also proceeded using ArB(OH)₂, which is catalyzed by diphenylprolinol silyl ether and Pd(OAc)₂ [82].

(1.48)

It is known that Hantzsch esters are useful hydride donors for the reduction of α,β‐unsaturated aldehydes to saturated aldehydes, and, in 2005, MacMillan reported the asymmetric reduction of β,β‐disubstituted α,β‐unsaturated aldehydes catalyzed by MacMillan’s catalyst (Eq. 1.49) [83].

In 2012, Hori reported an asymmetric hydrogenation of β,β‐disubstituted α,β‐unsaturated aldehyde, which is catalyzed by a combination of 2‐diarylmethylpyrrolidines and heterogeneous Pd/BaSO₄ under an H₂ atmosphere (Eq. 1.50) [84]. This reaction is successfully applied to citral. A mixture of E‐ and Z‐citral in any ratio afforded citronellal with high enantioselectivity. Citronellal is a key synthetic intermediate of l‐menthol.

(1.49)

(1.50)