Tandem Oxidation Processes for the Preparation of Functionalized Cyclopropanes

NHC-Catalyzed Chemoselective Reactions of Enals and Cyclopropylcarbaldehydes for Access to Chiral Dihydropyranone Derivatives

An N-heterocyclic carbene (NHC)-catalyzed chemoselective activation reaction of 1-cyclopropylcarbaldehydes and α-alkynyl enals is reported. NHC selectively catalyzes 1-cyclopropylaldehydes, followed by a [2 + 4] cycloaddition reaction with α-alkynyl enals. The target dihydropyranone derivatives bearing different substituents and substitution patterns can be obtained in good to excellent yields with excellent enantio- and diastereoselectivities under mild conditions.

Nanocrystals perovskites photocatalyzed singlet oxygen generation for light-driven organic reactions

Lead halide perovskite nanocrystals were prepared and used as photocatalysts for the in situ ¹O₂ generation to perform hetero Diels-Alder, ene and oxidation reactions with suitable dienes and alkenes. The methodology has been reasonably standardized and made applicable to a variety of olefinic substrates. The scope of the method is finely illustrated by the results in all the tested reactions, which allowed to obtain desymmetrized hydroxy-ketone derivatives, unsaturated ketones and epoxides. Some limitations were also observed especially in the case of the alkene oxidations as well as poor chemoselectivity was somewhere observed. ¹O₂ generated by lead halide perovskite nanocrystals as photocatalyst in organic reactions.

Water-Mediated Intramolecular Cyclization/Oxidation of α-Carbonyl Sulfur Ylides: Synthesis of Corey-Chaykovsky Reagent Type Heterocycles

A new class of Corey-Chaykovsky reagent type five-membered heterocycles with an oxosulfonium ylide moiety has been synthesized by the water-mediated intramolecular cyclization/oxidation of α-carbonyl sulfur ylides generated in situ from sulfonium salts under mild conditions.

Efficient access to fluorescent benzofuro[3,2-b]carbazoles via TFA-promoted cascade annulations of sulfur ylides, 2-hydroxy-β-nitrostyrenes and indoles

An efficient, TFA-promoted cascade annulation of sulfur ylides, 2-hydroxy-β-nitrostyrenes and indoles was established.

Formal [4+2] annulation of enaminones and cyanomethyl sulfur ylide: one-pot access to polysubstituted pyridin-2(1H)-ones

One-pot synthesis of polysubstituted pyridin-2(1H)-ones by formal [4+2] annulation of enaminones and cyanomethyl sulfur ylide is described involving sequential nucleophilic vinylic substitution, intramolecular nucleophilic cyclization and dealkylation reactions.

One-pot Synthesis of Cyclopropane Derivatives with a Cis:Trans Stereoselectivity by Wittig Olefination-Sulfur Ylide Cyclopropanation Sequence

Cyclopropane derivatives were synthesised in one pot from aromatic aldehydes, phenacyltriphenylphosphonium bromide, and S-phenacylthiolanium bromide by a Wittig olefination-sulfur ylide cyclopropanation sequence. When Cs2CO3 was used as the base and CH3CN/water (8:2, v/v) as the solvent, the major product was the cis-1,2-dibenzoyl cyclopropane. In contrast, when using DBU as the base and MeOH as the solvent, the major product was trans-dibenzoyl cyclopropane. In some cases, the major isomer was obtained in high purity and good yield by simple filtration.

Chemo-, regio-, and diastereoselectivity preferences in the reaction of a sulfur ylide with a dienal and an enone

Mechanistic insights into an interesting class of reaction between sulfur ylides with (i) a dienal, and (ii) an enone, obtained by using density functional theory, is reported. The kinetic and thermodynamic factors responsible for chemo-, regio-, and diastereoselectivities are established by identifying all key transition states and intermediates along the reaction pathway for 1,2-, 1,4-, and 1,6- modes of attack of dimethylsulfonium benzylide to 5-phenylpenta-2,4-dienal. The reaction profiles for 1,2- and 1,4- modes of addition are also evaluated for the reaction between dimethylsulfonium benzylide and pent-3-en-2-one. Our results show that the final outcome of the reaction with both these substrates would be decided by the interplay between kinetic and thermodynamic factors. It is found that the addition of a semi-stabilized ylide to conjugated carbonyl compounds prefers to proceed through a 1,4- (conjugate) pathway under thermodynamic conditions, which is in accordance with the available experimental reports. However, the formation of epoxides via a 1,2- (direct) addition pathway is computed to be equally competitive, which could be the favored pathway under kinetic conditions. Even though the lower barrier for the initial addition step is kinetically advantageous for the direct (or 1,2-) addition pathway, the higher energy of the betaine intermediates--as well as the reversibility of the accompanying elementary step--may disfavor product formation in this route. Thus, high diastereoselectivity in favor of 2,3-trans cyclopropanecarbaldehyde is predicted in the case of the dienal, using the most favored conjugate addition (1,4-addition) pathway. Along similar lines, ylide addition to the enone is identified to exhibit a preference toward conjugate addition over direct (1,2-) addition. The importance of transition state analysis in delineating the controlling factors towards product distribution and diastereoselectivity is established.

Tandem oxidation processes: a combined phosphorus- and sulfur-ylide approach to polysubstituted cyclopropanes

A new manganese dioxide-mediated tandem oxidation process (TOP) has been developed which, by suitable combination of stabilised phosphorus- and sulfur-ylides, allows the direct conversion of allylic alcohols or alpha-hydroxyketones into polysubstituted cyclopropanes.