Michael addition sulfa-,1,4-selective

Very recently two comprehensive review articles dealing with organocatalytic carbon-sulfur bond-forming reactions and asymmetric sulfa-Michael additions have been published. Historically, the first catalytic enantio-selective sulfenylations of conjugated enones in the presence of Cinchona alkaloids were published in 1977 by Pracejus where acrylamides and... 

Numerous guanidine-catalysed asymmetric Michael reactions and its related variants such as aza-Michael, oxa-Michael, phospha-Michael, sulfa-Michael have been reported in the literature. A nonexhaustive selection of conjugate addition reactions that is relevant to green chemistry will be presented. Glycine imines are commonly employed in Michael additions. They are protected a-amino acids and must he deprotected if an amino acid derivative is desired (Scheme 23.5). The large molecular mass of the imine group then makes waste generation a problem. 

SCHEME 14.20. Syn- or a tt-selective sulfa-Michael addition to a,p-disubstituted enones. 

Scheme 3 1,4-Selective sulfa-Michael addition to 1,6-keto ester...

Scheme 9 1,6-Selective sulfa-Michael addition via vinylogous iminium activation...

In 2011, Vaccaro et al. and Kobayashi et al. independently reported on chiral Sc(OTf)3/bipyridine-catalyzed sulfa-Michael additions to enones 264 in water. High yields and enantioselectivities were obtained for aliphatic thiols and acyclic enones, but the use of benzenethiol or cyclohex-enone failed to deliver optimal selectivity. 

In a sulfa-Michael reaction of a,p-unsaturated ketones catalysed by the salt derived from 9-amino-9-deoxy-epz -dihydroquinine (9-e/ f-DHQA) and n-N-Boc-phenylglycine, the best results (96% ee) were obtained using fert-bulyl-thiol. ° A similar catalyst, 9-amino-9-deo y-epz-cupreidine (9-epf-CPDA) allowed a diastereodivergent reaction, where selectivity towards both product diastereomers was possible using the same catalyst by changing the acidic additive and reaction conditions (Scheme 15.31). 

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