Scandium catalysis addition with

The enantioselective addition of ally organometallics to carbonyls has become one of the workhorses of organic synthesis. Dennis Hall of the University of Alberta reports (J. Am. Chem. Soc. 125 10160, 2003) the scandium triflate catalysis chiral allylboronic acids become more effective tools. The best of these, the Hoffmann camphor derivative 2, adds to aldehydes under Sc(OTf), catalysis with excellent enantiomeric excess. The reaction works equally well for methallyl, and for the E and Z crotyl boronic acids. The crotyl derivatives react with the expected high diastereocontrol. A limitation to the boronate additions is that branched chain aldehydes give low yields. 

The enantioselective total synthesis of (-)-hemiasterlin, a marine tripeptide with cytotoxic and antimitotic activity, was achieved by E. Vedejs and co-workers. The asymmetric Strecker reaction was used to construct the key tetramethyltryptophan subunit. The aldehyde substrate was first converted to the corresponding chiral imine with (R)-2-phenylglycinol under scandium triflate catalysis. The addition of tributyltin cyanide resulted in the formation of a-amino nitriles as an 8 1 mixture of diastereomers. Subsequently the cyano group was converted to a primary amide, and the chiral auxiliary was removed under catalytic hydrogenation conditions. 

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