Veejendra K. Yadav

Cyclopentane synthesis and annulation: intramolecular radical cyclization of acetals

Abstract A general procedure for cyclopentane synthesis based on free-radical cyclization of thioacetals is described. This permits the rapid assembly, by intramolecular annulation, of various ring systems bearing useful functionality for use in total synthesis.

KF–Al 2 O 3 is an efficient solid support reagent for the acetylation of amines, alcohols, and phenols. Impeding effect of solvent on the reaction rate

Abstract KF–Al 2 O 3 brings about rapid acetylation of a range of amines, alcohols, and phenols with Ac 2 O/AcCl. Aliphatic alcohols are acetylated chemoselectively in the presence of phenols. 1° alcohols react several times faster than 2° alcohols. 3° alcohols do not react.

The directional changes in torsion angles alone after complexation of the carbonyl oxygen with a prototypical cation such as H+ predict the facial selectivity of substituted cyclohexanones. An ab initio investigation

Abstract H + was taken as a prototypical cation for complexation with the carbonyl oxygen of 3-oxa-, 3,5-dioxa-, and 3-thiacyclohexanones. The geometries of the complexes were fully optimized using ab initio MO calculations with 6-31G basis set. The complexation desymmetrizes the molecular geometry further to an extent that the torsion angle changes on the axial face can be rationally used for the prediction of the facial selectivity in reactions with nucleophiles. The torsion angle changes are sensitive to the nature, position, and orientation of the substitutents. Based on the theory of stereoelectronic control and corroborated by ab initio MO calculations, a simple approach to the prediction of facial control in reactions of selected substituted cyclohexanones with nucleophiles is described. Some evidence is also presented against the known transition state models.

An enantioselective synthesis of (+)-crotanecine

Synthese a partir de (2S,4R)-hydroxy-4 proline via la (1S,5R,8R)-hydroxy-8 oxa-2aza-6 bicyclo [3.3.0] octanone-3

The Cation Complexation Model Predicts the Experimental π-Facial Selectivity of 2-ax- and 2-eq-Substituted Cyclohexanones. A Detailed Ab Initio MO Investigation

Abstract The geometrical changes on complexation of the carbonyl oxygen with prototypical cations such as H + and Li + are in tune with the polarity features of the C–X bonds in 2-ax-X-cyclohexanones (X=Cl, F, SR, OR; R=H, Me); the stereoelectronic effects apply. While the 2-ax-Cl- and 2-ax-SR-cyclohexanones are predicted to favor axial attacks, the 2-ax-OR- and 2-ax-F-cyclohexanones must capture a nucleophile predominantly from the eq-direction. The eq-selectivity of 2-ax-OR-cyclohexanone is in contradiction with the torsional model of Anh and Felkin. The Houk model will also fail due to the eminent steric interactions arising from the 2-ax-OMe function in the eq-TS. Neither the Anh–Felkin model nor the Cieplak model could be applied to 2-eq-MeO-cyclohexanone. The complexation model, however, predicts eq-selectivity in full agreement with experimental results. Second order perturbation theory analysis of the Fock matrix in NBO basis indicates that the role of the antiperiplanar effects is not as significant as perceived earlier by Cieplak and by Anh and Felkin.

Diastereofacial selectivities of substituted 5-aza-and 5-bora-2-adamantanones. Application of the complexation model and its ab initio MO investigation

N-Substituted 5-aza-2-adamantanones and 5-bora-2-adamantanone have been studied by ab initio MO methods at Becke3LYP level for their diastereofacial selectivities in reactions with nucleophiles by applying the complexation model. The facial predictions are fully consistent with the experiments. This is to be emphasized that in a given substrate there may be centres other than the carbonyl oxygen that may compete for the cation and, thus, contribute to the overall diastereodetermination. Other models such as the Cieplak model. Anh-Felkin model, and Houk model may or may not apply.

A Practical Approach to Homo Trialkyl Phosphonates: A Catalytic Michaelis-Arbuzov Reaction

Homo trialkyl phosphonates are efficiently prepared by iodine catalyzed isomerization of the correspondind homo trialkyl phosphites. The product is isolated by simple distillation.

A smooth rearrangement of N-p-toluenesulfonyl 2-tert-butyldiphenylsilylmethyl-substituted azetidines into N-p-toluenesulfonyl 3-tert-butyldiphenylsilyl-substituted pyrrolidines

The rearrangement of N-p-toluenesulfonyl 2-tert-butyldiphenylsilylmethyl-substituted azetidines into 3-tert-butyldiphenylsilyl-substituted pyrrolidines under Lewis acid conditions in dichloromethane involves 1,2-migration of silicon through a siliranium ion. The formation of siliranium ion was discovered not to be in concert with σ(C-N) cleavage from stereochemical analysis of the pyrrolidine products formed from 3- and 4-substituted-2-tert-butyldiphenylsilylmethyl azetidines and also from the optical rotation data and chiral HPLC analysis of the pyrrolidine product formed from N-p-toluenesulfonyl 2(R)-tert-butyldiphenylsilylmethyl azetidine. The formation of sterically less hindered siliranium ion is followed by its S(N)2 opening by the internal nitrogen nucleophile. Oxidative cleavage of σ(C-Si) bond leads to the formation of 3-hydroxypyrrolidines.

3-Halocyclohexanones. Torsion angle changes after cation-carbonyl complexation dictate the facial selectivity in reactions with nucleophiles: An ab initio investigation

Abstract The geometrical changes that take place after complexation of the carbonyl oxygen of 3-halocyclohexanones with prototypical cations such as H + and Li + were calculated using ab initio MO methods at 6-31G level. The torsion angle changes interpret rationally the experimentally known axial preference of the 3-eq-derivatives and the axial preference (predicted from transition state calculations) of the 3-ax-species. Stereoelectronic effects in 3-ax-halocyclohexanones and 1,3-dipolar interactions in 3-eq-halocyclohexanones after complexation of the carbonyl oxygen with cations such as H + and Li + account for the experimentally observed and the predicted diastereoselectivities, respectively, of these molecules. The complexation induces pyramidalization of the carbonyl carbon and controls the p orbial to adopt an energetically favorable orientation (ax or eq) for capture by a nucleophile.

Lewis acid-catalyzed formation of indene derivatives via tandem reactions of arylacetylenes with the cations generated from 2-silylmethyl cyclopropyl carbinols

Vicinal silylmethyl-substituted cyclopropyl carbinols undergo tandem intermolecular cation-arylacetylene cyclization to generate indene derivatives.