Diastereoselective aldol condensation of directly generated titanium enolates of activated esters.
摘要:
Simply generated (TiCl4, Et3N, -78-degrees-C) titanium enolates of some thioesters and alpha-thio substituted esters undergo aldol condensations with achiral and chiral aldehydes with low to high stereoselectivity. An H-1-NMR study of the TiCl4/ester complexation and of the enolization process is presented. The relation between the enolate structure and the aldol product stereochemistry is discussed.
and cross-interactionconstant ρXZ is relatively large and positive (0.90). These trends are consistent with the rate-limiting breakdown of a tetrahedral intermediate, T ± . The proposed mechanism is also supported by adherence of the rate data to the reactivity-selectivity principle (RSP). The kinetic isotope effects, kH/kD, are greater than unity (1.3-1.4) suggesting a possibility of hydrogen-bonded
TiCl4-promoted aldol condensations of S-4-chlorophenyl thioesters with enals or dienals led to the production of dienyl or trienyl thioesters in good yields. Due to good crystallinity, products with high E/Z ratios were obtained by simple filtration in many cases. Reduction of the thioester moiety by DIBAL-H afforded the corresponding unsaturated aldehydes while suppressing the E/Z-isomerization and
Ti-direct, powerful, stereoselective aldol-type additions of esters and thioesters to carbonyl compounds: application to the synthesis and evaluation of lactone analogs of jasmone perfumes
thiophenyl esters or thioaryl esters with aldehydes and ketones was performed (total 46 examples). The present method is advantageous from atom-economical and cost-effective viewpoints; good to excellent yields, moderate to good syn-selectivity, substrate variations, reagent availability, and simple procedures. Utilizing the present reaction as the key step, an efficient short synthesis of three lactone
Kinetics and mechanism of the aminolysis of aryl propanedithioates in acetonitrile
作者:Hyuck Keun Oh、Sun Kyung Kim、Hai Whang Lee、Ikchoon Lee
DOI:10.1039/b006974o
日期:——
The kinetics and mechanism of the aminolysis of aryl proanedithioates with benzylamines are investigated in acetonitrile at −35.0 °C. A large magnitude of the Hammett (ρX and ρZ) and Brönsted (βX and βZ) coefficients and exceptionally large cross-interaction constant ρXZ(= 3.5) are consistent with a stepwise mechanism in which leaving group expulsion from an intermediate, T±, is the rate-determining