STEREOISOMERISM OF 4A,4B-DIHYDROPHENANTHRENES

We reported previously that sterically hindered photochromic 4a,4b-dih ydrophenanthrenes can exist in two modifications, These labile interme diates are designated as L (long) or S (short) according to the positi on of their visible absorption band. We presently suggest that these a re two conformers ap (L) and ac (S) of the central C(4a)-C(4b) ethane unit ap is the more planar while ac is decidedly distorted, Spectral c haracteristics-the position and intensity of the visible absorption ba nd-provide the basis for the conformation assignment, The relative sta bilities of L and S depend on the type and magnitude of the steric int eraction X(4)-X(5) vs. X(4)-H(4b)/X(5)-H(4a), (C) 1997 Elsevier Scienc e S.A.