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

Chapter 4 111which was eliminated easily by chromatography. Unfortunately, however,also in this case the reaction proceeded without any stereocontrol. Allproducts, with the exception of (+)-55d, showed no specific rotation atthe Na and Hg wavelenght. As in the case of the Mitsunobu reaction withDPPA measurement of the e.e. of the products was not possible. The lastattempt to synthetize the chiral azide from the chiral alcohols wasperfomed with zinc azide/bis pyridine complexes. Identical conditions asreported in the literature were used, but no conversion of the startingmaterial was observed.4.5.3. Reduction of the azides 55a-d to the amines.From the literature several methods for the reduction of azides toprimary amines are known. Two methods (Scheme 4.15) were tried on ourcompounds:A) Reduction by Ph3P/H2O in THFB) Reduction by SnCl2/MeOHR R 1R R 1N 355a-dmethod A or BNH 256a-dScheme 4.15: Reduction of azides (±)-55a-d to amines(±)-56a-d.In table 4.8 the results of both methods used for the reduction of theazides 55a-d are reported.entry substrate R1 R2 product yield % A yield %B1 (±)-55a Ph 4PhC6H4 (±)-56a 50 832 (±)-55b 2BrPh 4PhC6H4 (±)-56b 55 803 (±)-55c 2benzo[b]furan Ph (±)-56c 68 874 (±)-55d 2benzo[b]furan Ph (±)-56d 38 855 (+)-55d 2benzo[b]furan 2,4 Cl2Ph (+)-56d 42 84Tab 4.8: reduction of azides (±)-55a-d to amines (±)-56a-d.The best results in terms of chemical yields were obtained withSnCl 2 /MeOH. The reactions were carried out with both the racemic and