Advances in the stereoselective synthesis of antifungal agents and ...

Chapter 2 41An asymmetric reducing agent need not be catalytic if it is inexpensiveand can be easily obtained. The cost factor may be further reduced if thechiral ligand can be recycled. Chiral modifications of metal hydride reagentswith a wide variety of chiral auxiliaries for the asymmetric reduction ofcarbonyl compounds have been studied intensively during the last threedecades. However, asymmetric reducing agents developed in the early stageof this programme commonly provided only low optical induction in theasymmetric reduction of prochiral ketones 8a-c . A wide variety of asymmetricreducing agents has now become available for such application.Unfortunally, no one particular reagent is effective for all the differentclasses of ketones.Presently there are more than twenty promising reagents 9 , howeveronly during the last decade some of these have found a wide application.At the moment, only five of them are extensively used in asymmetricreduction: ( Figure 2.3 ):21a diisopinocamphenyl-chloroborane, IPC2BCl,(DIP-Chloride TM ); 21b B-isopinocampheyl -9- borabicyclo - [3.3.1]nonane, (Alpine-BoraneTM); 21c oxazaborolidines; 21d aluminiumcomplexes derived from optically active 1,1- Binaphthyl-2,2'-diol, such asBINAL-H; and 21e BINAP-Ru complexes.BCl2Ph PhBON B21a DIP-Chloride 21b Alpine-Borane 21c Oxazaborolidines(Brown)(Midland)(Corey)RRO-O AlHORLi +Ar 2OPP RuOAr 2OOR21d BINAL-H(Noyori)21e BINAP-Ru Complex(Noyori)Figure 2.3: Chiral Complex 21a-e for the asymmetric reduction of prochiral ketones.