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

Chapter 3 77dry solvents and, is cheap. The general mechanism of the reduction isshown in scheme 2.7 14 :HR OH O H-BH 3O RRO - + HO H + BH 3 OR -Scheme 2.7: General mechanism of ketone reduction by NaBH4The Na+ seems not to participate in the transition state (as does Li+ inLiAlH 4 reductions 15 ) but kinetic evidences showed that an OR group of thesolvent seems to be involved in the reaction and remains attached to theboron 16 .Both classes of prochiral ketones 25a-f and 26a-e were reduced to racemicalcohols 27a-m with NaBH 4 . A 1:1 mixture of ethanol and THF was usedas solvent to achieve a perfect solubilization of the ketones at lowtemperature (scheme 3.8.):'RRNaBH 4'RRO THF/ EtOHOH25 a-f and 26 a-e 27 a-mScheme 3.8: Synthesis of racemic aryl-phenyl methanols (±)27a-f and aryl-2-benzo[b]furanmethanols (±)27g-m.High yields were obtained and no side products were detected neitherduring the reactions nor after the work up. Products, experimentalconditions and chemical yields are reported in table 3.3.(±)-27a-m were used as reference materials for the determination ofthe enantiomeric excess in the enantiomerically enriched products.Furthermore they were used as starting materials in all the new syntheticroutes in order to prove the feasibility of the strategy. Finally also for thesynthesis of separable diasteroisomers.