Alkynones, asymmetric reduction

LBADH also catalyzed the asymmetric reduction of a broad variety of differently substituted acetylenic ketones, including aromatic alkynones and a number of aliphatic derivatives [71]. For example, methyl alkynones bearing an aromatic unit attached to the triple bond were reduced to the corresponding (7 )-propargylic alcohols with >99% ee. Similarly, alkylsilyl-substituted... 

Optimum asymmetric reductions of alkynones previously had been achieved employing the chiral legand in a 2 1 ratio with LAH. Dimeric ligands were synthesized from ( 1)-ephedrine and (S)-(-)-prolinol and employed 1 1 with LAH. The results (using a 3-carbon bridge) have not been encouraging (Figure 13). 

Taking a lead later from the work of Brinkmeyer and Kapoor, several amino alcohols have been exploited for the asymmetric reduction of alkynones. One such publication appeared from the group of Cohen, who have also used Darvon alcohol-LAH complex at 70°C to furnish various propargyl alcohols in 34 to 90% e.e. In addition to Darvon alcohol, other chiral ligands were explored (Table 21.2). 

Most alkynones are reduced to the corresponding / -alkynols with high (62— 82%) enantiomeric excess using a complex formed from lithium aluminium hydride and the amino-alcohol (10). The asymmetric reduction of ketones by borohydride in the presence of quaternary ammonium derivatives of (25,35)-l,4-diaminobutane-2,3-diol has been described. When lithium aluminium... 

These can be prepared by a) asymmetric dihydroxylation of allylic chlorides followed by acetonide formation and elimination with BuLi b) catalytic asymmetric transfer hydrogenation and c) reduction of alkynones with chiral metal hydrides, (a) Marshall, J. A. Jiang, H. Tetrahedron Lett. 1998, 39, 1493 Yadav, J. S. Chander, M. C. Rao, C. S. Tetrahedron Lett. 1989, 30, 5455 (b) Matsumura, K. Hashiguchi,... 

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