Stereospecific Synthesis of Cyclohexenone Acids by [3,3]-Sigmatropic Rearrangement Route

Herein we report a modular synthetic method for the preparation of diaryl-substituted cyclohexenone acids starting from phenyl pyruvate and suitable enones. When the reaction is carried out in alkaline tert-butanol or toluene solutions in microwave-assisted conditions mainly anti configuration products are obtained with up to 86% isolated yield. However, when the reaction is carried out in alkaline water, a mixture of products with anti and syn conformations is obtained with up to 98% overall isolated yield. Mechanistically the product with anti conformation forms by a hemiketal-oxy-Cope type [3,3]-sigmatropic rearrangement-intramolecular aldol condensation route and syn product by an intermolecular aldol condensation-electrocyclization (disrotatory type) route.

Origins of Stereoselectivity in Mannich Reactions Catalyzed by Chiral Vicinal Diamines

The origins of the enantio- and diastereoselectivities in the Mannich reactions between aldehydes and ketimines catalyzed by chiral vicinal diamines have been determined by density functional theory calculations and distortion-interaction analysis. Computational results indicate a strong energetic preference for hydrogen-bonded nine-membered cyclic transition states. The favored transition states involve eight heavy atoms in the crown (chair-chair) conformation using the nomenclature of the analogous cyclic hydrocarbons. Energetic discrimination in the chirality-imparting step arises from pseudogauche-butane-type interactions in the disfavored transition states, as well as steric clashes between the N-Boc protecting group and the ammonium N-substituents.

Enantioselective Access to Robinson Annulation Products and Michael Adducts as Precursors

The Robinson annulation is a reaction that has been useful for numerous syntheses since its discovery in 1935, especially in the field of steroid synthesis. The products are usually obtained after three consecutive steps: the formation of an enolate (or derivative), a conjugate addition, and an aldol reaction. Over the years, several methodological improvements have been made for each individual step or alternative routes have been devised to access the Robinson annulation products. The first part of this Review outlines the most relevant developments towards the formation of monocarbonyl-derived Robinson annulation products (MRA products, MRAPs) and activated monocarbonyl-derived Robinson annulation products (AMRA products, AMRAPs). The following sections are then devoted to the diastereoselective and enantioselective synthesis of these products, while the last section describes the enantiomeric resolution of racemic mixtures.

Enantioselective C-H Olefination of α-Hydroxy and α-Amino Phenylacetic Acids by Kinetic Resolution

Significant progress has been made in the past decade regarding the development of enantioselective C-H activation reactions by desymmetrization. However, the requirement for the presence of two chemically identical prochiral C-H bonds represents an inherent limitation in scope. Reported is the first example of kinetic resolution by a palladium(II)-catalyzed enantioselective C-H activation and C-C bond formation, thus significantly expanding the scope of enantioselective C-H activation reactions.

Condition-determined multicomponent reactions of 1,3-dicarbonyl compounds and formaldehyde

By means of changing the reaction parameters, different products could be generated selectively starting from the same combination of substrates involving 1,3-dicarbonyl compounds and formaldehyde. This strategy enabled us to access diverse molecules without changing both starting material and reactor, maximizing thus the multifunctionality of the synthetic system. For example, starting from a 1,3-dicarbonyl compound, formaldehyde and 1,1-diphenylethylene, two kinds of products could be selectively formed including (i) a densely substituted dihydropyran and (ii) a C2-cinnamyl substituted 1,3-dicarbonyl compound. A one-pot three-component reaction of phenacylpyridinium salt, 1,3-dicarbonyl compound, and formaldehyde was also investigated, which produced either 2,4-diacyl-2,3-dihydrofuran or 2,4-diacyl-2-hydroxylmethyl-2,3-dihydrofuran in good to excellent yield.