Stereoselective Synthesis of (E)-α,β-Unsaturated Esters via Carbene-Catalyzed Redox Esterification

Abstract

[reaction: see text] Stereoselective, carbene-mediated redox esterification of alkynyl aldehydes provides mild and atom economical access to (E)-configurated, alpha,beta-unsaturated carboxylic esters. The organocatalytic method relies on the generation of activated carboxylates via extended/conjugated umpolung in the presence of catalytic amounts of carbene precursor and base.

Organocatalytic transformations of alkynals, alkynones, propriolates, and related electron-deficient alkynes

“…In 2006, Zeitler 22 reported the use of propargylic aldehydes 9 to achieve under carbene catalysis, a versatile synthesis of a,b-unsaturated esters 51 (Scheme 20). The in situ prepared carbene L (see Scheme 21) from the heteroazolium salt 50, using DMAP as base, was the best catalyst in terms of yield and E/Z stereoselectivity.…”

N-Heterocyclic Carbenes of Indazole: Ring Enlargement Reactions by α-Halo Ketones and Dehalogenations of Vicinal Dihalides

“…In carbonyl chemistry, N-heterocyclic carbenes have very recently been utilized as organocatalysts for silyl enol ether formations, 1 enantioselective syntheses of b-lactams by Staudinger reaction, 2 and highly diastereo-and enantioselective additions of homoenolates to nitrones. 3 Carbon-carbon bond cleavages in redox esterifications of chiral formylcyclopropanes were reported, 4 as well as the syntheses of (E)-a,b-unsaturated esters starting from alkynyl aldehydes, 5 saturated esters from a,b-unsaturated aldehydes 6 or ahalo aldehydes, 7 and b-hydroxy esters from epoxyaldehydes, 8 among many other reactions. Some review articles and books have appeared recently.…”

N-Heterocyclic carbene-catalyzed annulation of cyclic β-enamino esters with enals: access to functionalized indolo[2,3-a]quinolizidines

“…Over the past decade, N-Heterocyclic carbenes (NHCs) have been intensively investigated as versatile organocatalysts to promote a variety of unconventional chemical transformations of different aldehydes. [3] In recent years, ,-unsaturated acyl azoliums have emerged as new reactive intermediates to enabled a wealth of reactions with a range of nucleophiles which were firstly discovered by Zeitler in 2006. [4] More recently, Bode [5] , Studer [6] and Xiao [7] independently reported NHC-catalyzed annulations of ynal [8] or enal [8e, 9] -derived ,-unsaturated acyl azoliums with various stable enols to afford functionalized dihydropyranones (Scheme 1).…”

“…[3] In recent years, ,-unsaturated acyl azoliums have emerged as new reactive intermediates to enabled a wealth of reactions with a range of nucleophiles which were firstly discovered by Zeitler in 2006. [4] More recently, Bode [5] , Studer [6] and Xiao [7] independently reported NHC-catalyzed annulations of ynal [8] or enal [8e, 9] -derived ,-unsaturated acyl azoliums with various stable enols to afford functionalized dihydropyranones (Scheme 1). Bode also described the elegant work of the reactions between ,-unsaturated acyl azoliums and enamines to give diverse dihydropyridinones (Scheme 1).…”