ATROPISOMERISM IN NATURAL-PRODUCTS - ABSOLUTE STEREOCHEMISTRY OF BIFLAVONE, (-)-4',4''',7,7''-TETRA-O-METHYLCUPRESSUFLAVONE, AS DETERMINED BY THE THEORETICAL CALCULATION OF CD SPECTRA

The molecular asymmetry due to the atropisomerism of biflavone, (-)-4',4"',7,7"-tetra-O-methylcupressuflavone (1), has been studied by CD spectroscopy, and its absolute stereochemistry has been theoretically determined by the pi-electron SCF-CI-DV MO calculation of CD spectra. The molecular geometry of compound (aR)-l was calculated by molecular mechanics to give a stable conformation with a counterclockwise screw sense, in which the dihedral angle of the biphenyl part was 91-degrees. The calculated CD and UV curves of (aR)-1 were in very good agreement, including sign, position, and amplitude, with those of observed spectra of the natural product: calcd UV, lambda(max) 322.6 nm (epsilon-66 200), and 226.8 (78 300); calcd CD, lambda(ext) 359.7 nm (DELTA-epsilon +28.6), 317.5 (-45.0), and 263.2 (+21.7); obsd UV, lambda(max) 324.2 nm (epsilon-40 900), 273.0 (41 400), and 225.8 (51 800); obsd CD, lambda(ext) 362.0 nm (DELTA-epsilon +25.6), 326.2 (-54.4), and 267.5 (+21.3). The absolute stereochemistry of biflavone (-)-1 has thus been theoretically determined to be aR (or M helicity).