Stereocentres multiple

The carbohydrate substituent possesses multiple stereocentres but what is more there are many different carbohydrates available in enantiopure form from which a ligand library can easily be synthesised to explore the potential of this interesting new ligand class in homogenous catalytic apphcations. 

The possibilities of enantioselective sulfa-Michael additions using amino derivatives of Cinchona alkaloids are also demonstrated through tandem (cascade, domino) reactions, thus allowing the formation of multiple stereocentres with up to 99% ee. In comparison to Cfnc/zona-thiourea derivatives, natural Cinchona alkaloids or their ethers gave lower... 

The amino-indanol-derived triazolium salt F5 was also found to be an efficient precatalyst for the intramolecular Stetter reaction of a-substituted cyclo-hexadienones (Scheme 20.17), furnishing tricyclic products 35 bearing multiple stereocentres in up to 96% yield and with >99% enantiomeric excess. ... 

Vibrational Raman optical activity of biomolecules, including carbohydrates, has been reviewed (23 refs). l A general procedure for assigning both relative and absolute configuration to multiple stereocentres in acyclic aminopolyols based on application of the exiton chirality method is presented. The method has been applied to the side chain aminopolyol of a natural bacterial-derived terpenoid derivative. ... 

The catalytic enantioselective ring opening of oxabicyclic alkenes by carbon-based nucleophiles is a useful method for the simultaneous introduction of multiple stereocentres under high stereocontrol ([70] and references cited therein). Recently, several methods have been developed for ring opening of these meso heterobicyclic compounds employing different soft- and organometallic nucleophiles [71, 72]. 

A domino reaction of an aldehyde (R -CHO) with nitromethane followed by a second aldehyde (R -CH2CHO) yields a functionalized two-stereocentre product (74). Using a simple proline derivative, diphenylprolinol TMS ether, des typically over 95% and ees of 99% were obtained. This amine serves as a multiple-hydrogen-bond-donor (MHBD) catalyst, and is proposed to act in iminium mode for the Knoevenagel-type reaction of nitromethane ion, and then as an enamine catalyst of the second step. ... 

Reactions which form multiple bonds, rings, and stereocentres are partieu-larly important tools for the efficient assembly of complex moleeular strue-tures. Of the many families of reactions discovered over the past eentury, cycloaddition reactions hold a prominent place in the arsenal of synthetie methods currently available to organic chemists, and research activity in this field shows no signs of abatement. Among the metals used to catalyse cyc-loadditions, nickel has been found competent to promote enantioselectively the formation of carbo- and heterocycles of various ring sizes. 

The great potential of asymmetric domino processes to generate chemical efficiency through the formation of multiple new bonds and stereocentres in a one-pot system is amply documented. This strategy avoids time-consuming and... 

---