New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Stereoselective Synthesis of 2,3,5-Trisubstituted Tetrahydrofurans Initiated by a Titanium-BINOLate-Catalyzed Vinylogous Aldol Reaction

The enantioselective synthesis of 2,3,5-trisubstituted tetrahydrofurans 3 has been achieved using a chiral titanium-BINOL complex as catalyst for the vinylogous Mukaiyama aldol reaction of bis(silyl) diendiolate 1 and an aldehyde. The ensuing BF₃·OEt₂-mediated Prins-type cyclization with a second aldehyde gave rise to 2,3,5-substituted tetrahydrofurans 3 with generally good yields and excellent stereocontrol. In this process, three new σ-bonds and three new stereogenic centers were generated in a one-pot process.

Modular Synthesis of Dipyrroloquinolines: A Combined Synthetic and Mechanistic Study

A straightforward synthesis of [1,2-a][3',2'-c]dipyrroloquinolines has been developed generating up to eight new σ-bonds and five new stereogenic centers in a simple and modular one-pot operation. Generally good to excellent yields and moderate to good stereoselectivities in favor of the all-cis stereoisomer were observed. A detailed investigation combining synthetic studies, analytical measurements, and theoretical calculations has been conducted to elucidate the reaction mechanism using ESI- and liquid-beam IR-laser desorption mass spectrometry as well as DFT calculations. Key steps of this sequential transformation include a Lewis acid-catalyzed vinylogous Mukaiyama-Mannich reaction of bis(silyl) dienediolate 1 and a Brønsted acid-promoted Mannich-Pictet-Spengler reaction cascade reaction to complete the synthesis of the dipyrroloquinoline core of the target compounds.