Chiral Lewis acid-catalyzed asymmetric Baylis-Hillman reactions

Organocatalytic Synthesis of Highly Functionalized Heterocycles by Enantioselective aza-Morita-Baylis-Hillman-Type Domino Reactions

Organocatalytic enantioselective domino reactions are an extremely attractive methodology, as their use enables the construction of complex chiral skeletons from readily available starting materials in two or more steps by a single operation under mild reaction conditions. Thus, these reactions can save both the quantity of chemicals and length of time typically required for the isolation and/or purification of synthetic intermediates. Additionally, no metal contamination of the products occurs, given that organocatalysts include no expensive or toxic metals. The aza-Morita-Baylis-Hillman (aza-MBH) reaction is an atom-economical carbon-carbon bond-forming reaction between α,β-unsaturated carbonyl compounds and imines mediated by Lewis base (LB) catalysts, such as nucleophilic phosphines and amines. aza-MBH products are functionalized chiral β-amino acid derivatives that are highly valuable as pharmaceutical raw materials. Although various enantioselective aza-MBH processes have been investigated, very few studies of aza-MBH-type domino reactions have been reported due to the complexity of the aza-MBH process, which involves a Michael/Mannich/H-transfer/β-elimination sequence. Accordingly, in this review article, our recent efforts in the development of enantioselective domino reactions initiated by MBH processes are described. In the domino reactions, chiral organocatalysts bearing Brønsted acid (BA) and/or LB units impart synergistic activation to substrates, leading to the easy synthesis of highly functionalized heterocycles (some of which have tetrasubstituted and/or quaternary carbon stereocenters) in high yield and enantioselectivity.

Synthesis of phenyl 2-acryloyloxybornane-10-sulfonate diastereomers

Several routes to phenyl 2- exo- and 2- endo-acryloyloxybornane-10-sulfonate have been investigated. Preparation of the former product is complicated by concomitant formation of 10-isobornylsultone, while addition of HCl is observed when the alcohol precursors are treated with acryloyl chloride in the presence of Al2O3. An X-ray crystal structure has been determined for one of the chlorinated derivatives.

From Conventional Lewis Acids to Heterogeneous Montmorillonite K10: Eco-Friendly Plant-Based Catalysts Used as Green Lewis Acids

The concept of green chemistry began in the USA in the 1990s. Since the publication of the 12 principles of this concept, many reactions in organic chemistry have been developed, and chemical products have been synthesized under environmentally friendly conditions. Lewis acid mediated synthetic transformations are by far the most numerous and best studied. However, the use of certain Lewis acids may cause risks to environmental and human health. This Review discusses the evolution of Lewis acid catalyzed reactions from a homogeneous liquid phase to the solid phase to yield the expected organic molecules under green, safe conditions. In particular, recent developments and applications of biosourced catalysts from plants are highlighted.

α-Isocupreine, an enantiocomplementary catalyst of β-isocupreidine

Complementary chemistry! α-Isocupreine (-ICPN) was synthesized for the first time in one step from quinine by treatment with CF3SO3H (see scheme). This compound serves as an enantiocomplementary catalyst to β-isocupreidine (β-ICD) in the Morita–Baylis–Hillman reaction.

Organocatalytic asymmetric synthesis of substituted 3-hydroxy-2-oxindoles via Morita-Baylis-Hillman reaction

We report a highly enantioselective Morita-Baylis-Hillman (MBH) reaction of isatins and acrolein to provide enantiomerically enriched 3-substituted 3-hydroxyoxindoles, which could serve as valuable synthetic building blocks. This is also the first time that a ketone has been used as the electrophile and acrolein as the nucleophile in a highly enantioselective catalytic asymmetric MBH reaction. Hatakeyama's catalyst, β-isocupreidine (1), turned out to be a powerful catalyst for this transformation.

Gases as guests in benzocyclotrimer cage hosts

An imine-linked (+)-syn-benzotricamphor derivative gives access to chiral unimolecular cages exhibiting internal cavities of new shapes and volumes. One of these hosts hydrocarbon gases at low temperatures in solution through CH-pi attractive interactions. No encapsulation is observed when the cage structure is too narrow or too large for the guest.

Acceleration of the Morita-Baylis-Hillman reaction by a simple mixed catalyst system

By using a catalytic amount of 4-dimethylaminopyridine (DMAP) as a nucleophile in the presence of an equal amount of tetramethylethylenediamine (TMEDA) and MgI(2), Morita-Baylis-Hillman adducts can be obtained in good to excellent yields from various aromatic and aliphatic aldehydes and cyclic enones/enoates at room temperature after convenient reaction times.

Theory-Guided Design of Brønsted Acid-Assisted Phosphine Catalysis: Synthesis of Dihydropyrones from Aldehydes and Allenoates

The phosphine-catalyzed addition of 2,3-butadienoates to aldehydes has been extended to the formation of disubstituted dihydro-2-pyrones. The requisite shift in equilibrium of the intermediate zwitterionic beta-phosphonium dienolates toward the s-cis intermediate was accomplished through the use of a Brønsted acid additive, which disrupts the favorable Coulombic interaction present in the s-trans intermediate. The detailed nature of the synergistic interactions involving the Brønsted acid additives and phosphine involved in the formation of s-cis beta-phosphonium dienolates was analyzed through a series of DFT calculations. Unlike previously reported annulations of aldehydes with allenoates, where trialkylphosphines are optimal catalysts, in this study triphenylphosphine was also found for the first time to be a suitable catalyst for the synthesis of dihydropyrones. This method provides a one-step route toward functionalized dihydropyrones from simple, stable starting materials. In addition, new reaction pathways of phosphine-catalyzed allene annulations are unveiled, with the formation of dihydropyrones being the first example of dual activation in this sphere.

The enantioselective Morita-Baylis-Hillman reaction and its aza counterpart

The development of asymmetric Morita-Baylis-Hillman (MBH) reactions has evolved dramatically over the past few years, parallel to the emerging concept of bifunctional organocatalysis. Whereas organocatalysis is starting to compete with metal-based catalysis in several important organic transformations, the MBH reaction belongs to a group of prototypical reactions in which organocatalysts already display superiority over their metal-based counterparts. This Minireview summarizes recent mechanistic insights and advances in the design and synthesis of small organic molecules for enantioselective MBH and aza-MBH reactions.

Nonenzymatic Acylative Kinetic Resolution of Baylis−Hillman Adducts

The first efficient nonenzymatic acylative kinetic resolution of Baylis-Hillman adducts is reported. Chiral pyridine catalyst 1a and an optimized analogue 1e are capable of promoting the synthetically useful enantioselective acylation (the efficiency of which is outstanding for sp(2)-sp(2) carbinol substrates, s = 3.5-13.1, ee up to 97%) of Baylis-Hillman adducts derived from recalcitrant precursors which are currently difficult to synthesize utilizing benchmark asymmetric Baylis-Hillman reaction catalyst technology. A novel one-pot synthesis-kinetic resolution process involving a DBU-catalyzed Baylis-Hillman reaction and subsequent 1e/DBU-mediated enantioselective acylation has also been developed.