Asymmetric induction in the inverse diels-alder reaction of chiral 2-methylene-imidazolidines

inverse Diels-Alder reactions of chiral 2-methylene-imidazolidines with 2,4-hexadienoates at ambient temperatures lead to diastereomeric N,N-acetals, which can smoothly be cleaved by dilute acids to optically active cyclohexenones.     

Organic reactions at high pressure. Wittig reaction of hindered ketones with nonstabilized ylides.

The Wittig reaction between n-butylidenetriphenylphosphorane and a series of cyclohexanones of increasing steric hindrance around the carbonyl group has been studied at 7 kbar and 15 kbar pressure (0.75–1.5 GPa) and at 40 °C. In all cases, except 2,2,6,6-tetramethylcyclohexanone and fenchone, 45-60% yields of olefins were obtained, in contrast to poor to no yields which were obtained under ambient conditions.     

Asymmetric induction in thia-Diels-Alder reactions of chiral polyfluoroalkylthionocarboxylates

A series of chiral S- or O-alkyl thionoesters have been synthesized by treatment of trifluorothioacetyl- or 2,2,3,3-tetrafluorothiopropionyl chloride with corresponding thiols or alcohols. The thia-Diels-Alder reaction of the thionoesters with symmetrical 1,3-dienes proceeds with diastereoselectivity up to 60%. Structures of cycloaddition products and corresponding transition states have been studied at the DFT level of approximation. The experimentally observed diastereomeric excess has been referred to differences in activation energies of transition states, preceding formation of the diastereomeric cycloaducts.     

Asymmetric Mannich-type reactions for the synthesis of aspartic acid derivatives from chiral N-tert-butanesulfinylimino esters

Addition of ketene acetals to sulfinimines derived from homochiral N-tert-butanesulfinamide using various Lewis acids furnishes derivatives of aspartic acid in diastereomeric ratios up to 97:3. Following an easy removal of the N-tert-butanesulfinyl chiral auxiliary, optical active beta-amino esters are obtained.     

Asymmetric induction in the cycloaddition reaction of dichloroketene with chiral enol ethers. A versatile approach to optically active cyclopentenone derivatives.

Significant asymmetric induction has been observed in the cycloaddition reaction of dichloroketene with chiral enol ethers. The resultant diastereomeric cyclobutanones have been converted to synthetically useful α-chlorocyclopentenones in optically active form.     

Asymmetric Diels-Alder reactions of cyclopentadiene in the synthesis of chiral norbornene derivatives

The review discusses available data on asymmetric Diels-Alder reactions of cyclopentadiene, which were published in the past decade. Both noncatalytic and catalytic (in the presence of achiral and chiral catalysts) versions of these reactions are considered. Effects of various factors on the chemical and optical yields, stereoselectivity, and optical purity of the Diels-Alder adducts are analyzed. Prospects in the development of this field of organic chemistry are examined.     

Cycloaddition reactions leading to carbohydrate derivatives part I. Hetero diels-alder reaction of monosaccharide O-thioformates

Monosaccharide O-thioformates under thermal or high-pressure conditions react with various dienes to give O-thiopyran-2-yl saccharides     

Highly diastereoselective nitroaldol reactions with chiral derivatives of glyoxylic acid

N-Glyoxyloyl-(2R)-bornane-10,2-sultam (1a) and (1R)-8-phenylmenthyl glyoxylate (1b) react stereoselectively with simple nitroalkanes giving diastereomeric nitroalcohols with high asymmetric induction. The glyoximide 1a is proved to be a highly efficient chiral inducer, superior to glyoxylate 1b. In all cases, the absolute configuration (2S) and relative configuration, syn for the major diastereoisomers, were confirmed.     

Reactions at high pressure,part 10: the volume profile of an intramolecular diels-alder reaction

the intramolecular cyclisation of furans, 3, proceeds with the following activation characteristics: 3(R=Et) has E_A=72.5, H=69.9 kJmol^?1, S^≠=62JK^?1 mol^?1, V^≠=?25 cm₃ mol_?1. The results confirm the existence of a negative volume contribution from the approach of reagents in intermolecular analogues.     

Asymmetric induction in the conjugate addition of thioacetic acid to methacrylamides with chiral auxiliaries

The conjugate addition of thioacetic acid to methacrylamides with chiral C₂-symmetric trans-2,5-disubstituted pyrrolidines afforded the addition products in excellent stereoselectivities (>99% de) and good yields (80–90%). The high selectivity was attributed mainly to the steric effect of the chiral auxiliaries. The cyclic nature of the chiral auxiliaries seemed also important for both the stereoselectivity and the reaction rate. Acidic hydrolysis of the adduct containing (2R,5R)-bis(methoxymethyl)pyrrolidine gave (S)-3-mercapto-2-methylpropanoic acid, a key intermediate for captopril, in 98% ee and 96% yield. The chiral auxiliary was recovered in the demethylated form of N-Boc-(2R,3R)-bis(hydroxymethyl)pyrrolidine in 90% yield.     

Asymmetric synthesis of permethric acid. stereochemistry of chiral copper carbenoid reaction

The most effective optical isomer (1$\text{R}$-$\text{cis}$) of permethric acid ($\text{2}$, R = H), a potent intermediate in the production of synthetic pyrethroid, was enantioselectively prepared.     

A chiral probe for the asynchronous transition state of diels-alder reactions with acetylenedicarboxylate

Diels-Alder reactions took place with modest diastereoselectivity between dimethylcyclo-hexadiene derivative 3a and di-(−)-menthyl acetylenedicarboxylate, whereas the dimethylcyclohexadiene 2 showed no selectivity whatsoever. These results can be rationalized in terms of a complete lack of any endo-exo preference for the carboxylate groups and a more synchronous addition with 3a than with 2.     

The high pressure mediated intramolecular diels-alder reaction of furans: Factors controlling cycloaddition with monoactivated dienophiles.

The Intramolecular Diels-Alder reactions of furans possessing a 2-alkyl substituent with a terminal α,β-unsaturated ketone have been surveyed. The consequences of varying the degree of substitution of the furan, the bridging chain length, and position of the activating group on the dienophile upon the ease and stereochemistry of cycloaddition are reported.     

Asymmetric induction in the reactions of azinones with C-nucleophiles

Effect of acylating agents on the course of addition of C-nucleophiles to 1,2,4- and 1,3,5-triazinones, as well as to quinoxalin-2(1H)-one, was studied. A series of new azinone derivatives was obtained. A method for the preparation of diastereomerically pure addition products of indoles to 1,2,4-triazinones and quinoxalin-2(1H)-one in the presence of N-Ts-L-amino acid acyl chlorides was suggested.     

Asymmetric synthesis of pipecolic acid derivatives using the aza-Diels-Alder reaction

Imines of the type R---N=CHCO₂Et can be coerced into undergoing a (4+2) cycloaddition with substituted dienes if the reaction is carried out in DMF in the presence of both water and acid: these reactions show extremely high regio- and diastereoselectivity. Use of the 1-phenylethyl group as a chiral auxiliary leads to moderate asymmetric induction (typical d.e. ca. 70%); moreover, the diastereoisomers are surprisingly easy to separate, giving a short general route to optically pure substituted pipecolic acid derivatives.