THE DIASTEREOSELECTIVITY OF ZIRCONIUM ALKOXIDE CATALYZED MEERWEIN-PONNDORF-VERLEY REDUCTIONS

The new variation of the Meerwein-Ponndorf-Verley reduction using 1-[4 -(dimethylamino)phenyl]ethanol (DMAPE) (6) or 1-tetralol (12) (3 equiv .) as the reducing alcohols and Zr(O-tBu)(4) as the catalyst (0.2 equi v.) is kinetically controlled and highly stereoselective. Preferential axial attack is achieved with the sterically less bulky alcohol 6 in the case of 4-tertbutyl- and 3-methylcyclohexanones (1a --> 96 % axial and 3a --> 93% axial attack). The combination tetralol (12)/Zr(O-tBu) (4) behaves as a very bulky reducing agent, and the thermodynamically less stable alcohols 2c (80%) and 4c (92-96%) are formed preferentiall y in the reduction of 2a and 4b. The fused bicyclic systems 1-methyl-2 -tetralone (14a) and flavanone (15a) and the steroids 16a-18a are redu ced with high stereoselectivity to the corresponding cis alcohols 14b and 15b and the beta-alcohols 16b-18b. The stereochemical outcome is i n agreement with the Felkin-Ahn model.