The Catalytic Asymmetric Allylic Alkylation of Acyclic Enolates for the Construction of Quaternary and Tetrasubstituted Stereogenic Centres

To facilitate the discovery and development of new pharmaceuticals, the demand for novel stereofunctionalised building blocks has never been greater. Whilst molecules bearing quaternary and tetrasubstituted stereogenic centres are ideally suited to explore untapped areas of chemical space, the asymmetric construction ofsterically congested carbon centres remains a longstanding challenge in organic synthesis. The enantioselective assembly of acyclic stereogenic centres is even more demanding due to the need to restrict a much wider range of geometries and conformations of the intermediates involved. In this context, the catalytic asymmetric allylicalkylation (AAA) of acyclic prochiral nucleophiles, namely enolates, has become an indispensable tool to access a range of linearα-quaternary andα-tetrasubstituted carbonyl compounds. However, unlike the AAA of cyclic enolates with a fixed enolate geometry, to achieve high levels of stereocontrol in the AAA of acyclic enolates, the stereoselectivity of enolisation must be considered. The aim of this review is to offer acomprehensivediscussion of catalytic AAA reactions of acyclic prochiral enolates and their analogues to generate congested quaternary and tetrasubstituted chiral centres using metal, non-metal and dual catalysis, with particular focus given to the control of enolate geometry and its impact on the stereochemical outcome of the reaction.

Quaternary Stereocenters via Catalytic Enantioconvergent Allylation of Epoxides

The development of catalytic and enantioselective transformations for the synthesis of all-carbon quaternary stereocenters has long been recognized as a significant challenge in organic synthesis. While considerable progress has been made in asymmetric allylations, their potential to functionalize the commonly used synthon, epoxide, remains largely underexplored. Here we demonstrate the first highly regio- and enantioselective allylation of epoxides that delivers a range of quaternary stereocenters in the face of potentially problematic elimination and protonation reactions. The reaction proceeds via a radical approach under mild conditions and benefits from the use of earth-abundant titanium with a highly sophisticated salen ligand, which facilitates remarkable enantiocontrol and suppresses undesired side reactions. The resulting allylation products are multifunctional building blocks that can be elaborated chemo- and stereoselectively to a broad array of stereodefined structural motifs.

Stereospecific Construction of Quaternary Carbon Stereocenters from Quaternary Carbon Stereocenters

Organoaluminum species promote a smooth nucleophilic substitution at the quaternary carbon stereocenter of stereodefined polysubstituted cyclopropyl methyl phosphate with a complete inversion of configuration, even when more reactive functional groups are present. The regio- and diastereoselectivity of the substitution is attributed to the existence of a bicyclobutonium intermediate.

Stereoselective Sc(OTf)3 -Catalyzed Aldol Reactions of Disubstituted Silyl Enol Ethers of Aldehydes with Acetals

Facile and modular access to stereodefined disubstituted aldehyde-derived silyl enol ethers allowed their successful application in a stereoselective aldol reaction affording the products with excellent yields and diastereomeric ratios. The counter-intuitive stereochemical behavior of this Mukaiyama-aldol reaction is accounted for by a non-classical open transition state.

Regio- and Diastereoselective Copper-Catalyzed Carbomagnesiation for the Synthesis of Penta- and Hexa-Substituted Cyclopropanes

Despite the highly strained nature of cyclopropanes possessing three vicinal quaternary carbon stereocenters, the regio- and diastereoselective copper-catalyzed carbomagnesiation reaction of cyclopropenes provides an easy and efficient access to these novel persubstituted cyclopropyl cores with a complete regio- and diastereoselectivity.

Stereodivergent Pd/Cu catalysis: asymmetric alkylation of racemic symmetrical 1,3-diphenyl allyl acetates

A stereodivergent Pd/Cu catalyst system was developed, which was successfully applied to the asymmetric allylic alkylation of symmetrical 1,3-disubstituted allyl acetates with prochiral imino esters in a fully stereodivergent manner.