Asymmetric Organocatalytic Cyclopropanation–Highly Stereocontrolled Synthesis of Chiral Cyclopropanes with Quaternary Stereocenters (original) (raw)

Enantio- and Diastereoselective Synthesis of Highly Substituted Acylcyclopropanes from Homoaldol Products by Stereospecific Homoallylic Cyclization

European Journal of Organic Chemistry, 2005

Catalytic enantioselective synthesis of 1hydroxy-2,3-bisboronate esters through multicomponent borylation/1,2-addition is reported. Catalyst and substrates are readily available, form both a C-B and CC bond, and generate up to three contiguous stereocenters. The reaction is tolerant of aryl, vinyl, and alkyl aldehydes and ketones in up to 95% yield, >20:1 dr, and 97:3 er. Intramolecular additions to aldehydes and ketones result in stereodivergent processes. The hydroxy bis(boronate) ester products are amenable to siteselective chemical elaboration.

Simple Approach to the Highly Stereoselective Synthesis oftrans-1,2-Cyclopropane Derivatives

Chinese Journal of Chemistry, 2007

In the presence of KF•2H 2 O, furoylmethyltriphenylarsonium bromide (1a) or thienoylmethyltriphenylarsonium bromide (1b) reacted with 2-[(un)substituted benzylidene]malononitrile (2) in chloroform at room temperature to give trans-3,3-dicyano-1-furoyl-2- [(un)substituted phenyl]cyclopropane (3a) or trans-3,3-dicyano-1-thienoyl-2-[(un)substituted phenyl]cyclopropane (3b) respectively in good yield with high stereoselectivity. The structures of product 3 were confirmed by IR, MS, 1 H NMR, 1 H-1 H COSY and microanalysis. The relative configuration of product 3 was determined by 1 H-1 H NOESY technique. The mechanism for the formation of product 3 was also proposed.

Stereoselective synthesis of α-hydroxycyclopropanecarboxylic acids

Tetrahedron Letters, 1999

Cyclopropanation of chiral c~-alkoxy acrylamides derived from 1R,2S-ephedrine and a-keto acids provides cyclopropyl morpholinones with good diastereoselectivity. Removal of the ephedrine portion generates a-hydroxycyclopropane carboxamides which are readily converted to enantiomerically enriched a-hydroxycyclopropanecarboxylic acids.

An improved synthesis of cyclopropanes from homoallenic alcohols

Tetrahedron Letters, 1992

of the readily available g-allene tosylates with LDA or nBuLi provides an expeditious, although stereorandom, synthesis of functionalized cyclopropanes which can be easily prepared in high enantiomeric purity. Homoallenic groups are well-known to participate in solvolysis reactions in a manner analogous to homoallylic groups. For example, g-allenic tosylates or halides (1) are readily cyclized under solvolytic conditions to afford cyclopropylketones (2) or methylenecyclobutanols (3) as the major product, mainly depending upon the RI substituent of the starting allenes (Scheme I).* By taking advantage of the relative acidity of the allenyl proton, it occurred to us that treatment of these g-allene tosylates with a base should furnish alkynylcyclopropanes (4) with clean inversion of configuration at the sp3 carbon center.3 This ring formation would lend itself to the synthesis of enantiomerically pure cyclopropanes of defined absolute stereochemistry from the readily available p-allenic alcohols. 4~5 Herein we report the successful implementation of such approach to an expeditious, although stereorandom, synthesis of functionalized cyclopropanes which can be easily prepared in high enantiomeric purity. Scheme I R, =H +Ts *r-i 1 I RI = alkyl base (LDA or nBuLi) Rl COCH, A R2 2 R3 RI HO a R2 t f% 3a As outlined in Scheme II, we have employed two general synthetic methods for the preparation of the requisite starting J3-allenic alcohols (5a-i).6<7 In Method A the starting materials (5a-e) were readily available (75-86%) by the one-carbon homologation of propargylic alcohols (7a-e) by the procedure of Crabbe .s The latter alcohols were prepared either by addition 4703

An asymmetric route to enantiomerically pure 1,2,3-trisubstituted cyclopropanes

The Journal of Organic Chemistry, 1992

Cycloaddition of various sulfur ylides to the chiral unsaturated lactame la, l b led to cyclopropanated products containing a monosubstituted appendage. The stereochemical outcome is such that all the products are mainly (or exclusively) the kinetically controlled endo-syn-8, -9, or endo-anti-10. The latter occurs by virtue of an epimerieation to the thermodynamically favored product. Removal of the chiral auriliary following Wittig reaction on the intermediate carbinol amines (11,16) gave chiral, nonracemic 1,2,3-trisubetituted cyclopropanes containing various functionalities (13, 16).