Rr. Monaco et al., SEMIEMPIRICAL STUDIES OF RING TWISTING IN CIS-STILBENE AND RELATED BIOMOLECULES, International journal of quantum chemistry, 71(1), 1999, pp. 57-62
The geometric. and energetic predictions of the MNDO, AMI, and SAM1 mo
dels as they describe rotation of the dihedral angle between the plane
of one of the phenyl rings and the plane of the olefin core of cis-st
ilbene (cis-1,2-diphenylethylene) were tested for a variety of constra
ints. An three models predict that distortions away from the stable st
ructure fixed by a compromise between ir-electron and steric repulsion
s lead to small (at most 1-2 kcal/mol) strain energies and geometry re
laxations. Extensive peripheral substitution on the phenyl rings prese
nt in a prototype natural product having the cis-stilbene structure, C
ombretastatin A-4 4,5-trimethoxy-3'-hydroxy-4'-methoxy(Z)-stilbene), d
istorted the shapes of the cis-stilbene barriers to conformation chang
e only minimally. It is concluded that natural products having the cis
-stilbene structure should be expected to interact with other biomolec
ules as if the phenyl twists are barrier free. (C) 1999 John Wiley & S
ons, Inc.