On the Optical Activity of Steroidal 5 , 7 - Dienes

Circular dichroism (CD) data are reported of a series of 9,10stereoisomeric steroidal 5,7-dienes. In general the effects in the longest wavelength transition (270-280 nm) are large (Mm" 10---30) and consignate with respect to the diene helicity rule. The magnitude of the CD appears to vary markedly with the substituent at C-3 and at C-17, and with solvent. In the case of the 9a,10p-H dienes, variation of solvent and temperature can affect even the sign of the Cotton effect. This is explained from a change of geometry of the diene ring: solvation, substitution and temperature can affect the average geometry of the ring including the values of the angle of twist of the diene (ef> (6-7». The relevance of the observed chiroptical data for the theoretical description of the optical activity in the So-+ Sl transition of homoannular cisoid dienes is discussed.