Highly Enantioenriched Homoenolate Reagents by Asymmetric γ-Deprotonation of Achiral 1-Silyl-Substituted 1-Alkenyl Carbamates

Recent advances in N-heterocyclic carbene catalyzed achiral synthesis

N-Heterocyclic carbenes (NHCs) have emerged as powerful and elegant organocatalysts in a variety of newly developed and unprecedented enantioselective transformations due to their unique umpolung capacity. As a supplement to conventional enantioselective organocatalysis, NHC-induced non-asymmetric catalysis has gradually attracted much interest in recent years. Herein, this review aims to reveal the recent developments in NHC-promoted non-asymmetric umpolung transformations resulting in the expeditious construction of versatile achiral natural heterocycles, carbocycles and acylated products.

Unprecedented dithiolation of enals via their NHC-catalysed umpolung reaction with organic disulfides

A novel one-pot N-heterocyclic carbene (NHC)-catalysed dithiolation of α,β-unsaturated aldehydes (enals) with organic disulfides is reported. The protocol involves homoenolate reactivity of enals, where the homoenolate attacks on the disulfide as a d(3) nucleophile followed by thioesterification to afford β-aryl/alkylsulfanyl thioesters with complete atom economy.

Nucleophilic heterocyclic carbene catalyzed annulation of enals to chalcones in methanol: a stereoselective synthesis of highly functionalized cyclopentanes

Homoenolates generated from enals by nucleophilic heterocyclic carbene (NHC) catalysis undergo annulation with chalcones in methanol to afford methyl beta-hydroxycyclopentanecarboxylates, stereoselectively. Construction of four contiguous stereocenters in a stereoselective manner is noteworthy.

Recent advances in carbon-carbon bond-forming reactions involving homoenolates generated by NHC catalysis

Homoenolate, a species containing anionic carbon beta to a carbonyl group or a moiety that can be transformed into a carbonyl group, is a potential three carbon synthon. Recent introduction of a protocol for the generation of homoenolate directly from enals by NHC (nucleophilic heterocyclic carbene) catalysis has made it possible to explore the synthetic utility of this unique reactive intermediate. The versatility of NHC-bound homoenolate is illustrated by its annulation with various carbonyl compounds leading to gamma-butyrolactones, spiro-gamma-butyrolactones, and delta-lactones. Interception of homoenolate with imines afforded gamma-lactams and bicyclic beta-lactams. Formation of cyclopentenes and spirocyclopentanones respectively by reaction with enones and dienones is also noteworthy. This tutorial review focuses on these and other types of reactions which attest to the synthetic potential of NHC-bound homoenolates in organic synthesis.

Stereoselective synthesis of spirocyclopentanones via N-heterocyclic carbene-catalyzed reactions of enals and dienones

Homoenolates generated from enals by nucleophilic heterocyclic carbene (NHC) catalysis undergo a conjugate addition/cyclization sequence with cyclic dienones, culminating in the efficient synthesis of spirocyclopentanones.

Asymmetric γ-Deprotonation and Homoaldol Reaction of 1,3-Dien-2-yl Carbamates: Stereo- and Regiochemistry

[reaction: see text] Lithium compounds 7 are configurationally stable intermediates obtained by deprotonation of 1,3-dien-2-yl carbamates 6 with n-butyllithium/(-)-sparteine with a high degree of enantiotopic differentiation at the gamma-position. They react with electrophiles regioselectively giving highly enantioenriched products. Starting with different isomers or changing the double-bond geometries in 6 leads to either of the enantiomers.