Stereoselective Syntheses with Sugar Metal Carbenes

Stereoselective Syntheses with Sugar Metal Carbenes 

The manifold chiral information readily available in customary carbohydrates provides an attractive approach to a chiral modification of metal carbenes that may be applied to either metal-mediated stereoselective organic synthesis via sugar auxiliaries or to carbohydrate synthesis via organometallic methodologies [107, 108]. The sugar moiety can be incorporated into metal carbenes by well-established procedures such as nucleophilic addition to the metal-coordinated carbene carbon atom or conjugate addition to the vinylogous position in alkenyl or alkynyl car- 

The straightforward access to O-glycosidic metal carbenes relies on the nucleophilic addition of an unprotected sugar alcohol to the electrophilic carbene carbon. In order to overcome problems arising from the inherent steric bulk of protected sugar nucleophiles the electrophilicity of metal oxycarbenes can be enhanced by 0-acylation. Acyloxycarbene complexes are temperature-sensitive intermediates but readily accessible from tetramethylammonium [acyl(pentacarbonyl)]metalates 

The potential of metal carbenes in stereoselective synthesis is based on both the pronounced acidity of the a-CH in the alkyl side chain - which may be exploited in aldol and Michael-type reactions - and on cycloaddition reactions centered either on the metal or the carbene ligand. The incorporation of a carbohydrate backbone into the carbene ligand generally allows for an asymmetric modification of these carbon-carbon bond-forming reactions. Deprotonation of 2-oxacyclopen-tylidene complexes 76a/b and 270-279 generates the conjugate bases that can be 

The pronounced electrophilic character of the glycosylidene carbon can be ejqjloited in the addition of nucleophiles and applied to a ring-opening aminoly-sis/recyclization sequence to synthesize iminoglycosylidene complexes. 

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Stereoselective Syntheses with Sugar Metal Carbenes 483... 

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