Asymmetric synthesis of 4-amino-3,4-dihydro-2,2-dimethyl-2H-1-benzopyrans

Highly enantioselective reduction of 3,4-dihydro-2,2-dimethyl-2H- 1 -benzopyran-4-ones 3 a-e by BH3 was achieved in the presence of catalytic amounts of Corey's oxazaborolidine 4 to afford the corresponding 3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-4-ols 2a-e in quantitative yields. These benzopyran-4-ols 2a-e were converted into the chiral 4-amino-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran 1a-e by mesylation, followed by introduction of an azide group by tetra-n-butyl-ammonium azide, and finally by reduction of the azide 6 with triphenylphosphine under very mild conditions without loss of stereo information. (C) 1999 Elsevier Science Ltd. All rights reserved.