Formal total synthesis of (–)- and (+)-balanol: two complementary enantiodivergent routes from vinyloxiranes and vinylaziridines

Abstract

Formal total syntheses of both enantiomers of balanol have been achieved by the preparation of the protected hexahydroazepine core 2. Two complementary routes have been investigated. The first relied on the regioselective opening of 1,2-epoxycyclohex-3-ene with a chiral-auxiliary version of the Burgess reagent to provide a diastereomeric pair of cis-fused cyclic sulfamidates. The sulfamidates were transformed to trans-amino benzoates with ammonium benzoate and, after separation, converted to (−)- 2 and (+)- 2 by oxidative cleavage and reductive amination. The second approach utilized vinylaziridines derived from 1-bromo-2,3-dihydroxycyclohexa-4,6-diene obtained by the whole-cell fermentation of bromobenzene with Escherichia coli JM109(pDTG601). Stereoselective opening of the aziridines generated the requisite trans-amino alcohol derivatives, which after saturation of the vinyl bromide moieties were converted to (−)- 2 and (+)- 2 by oxidative cleavage of the cis-diol and reductive amination. Experimental and spectral data are provided for all new compounds.