Non-Equilibration of Excited-state Rotamers

The photochemically induced ring closure of (1) to (2) has also found relatively little synthetic use due to tile seemingly complex nature of the photopnxlucts obtained upon irradiation of a hexatriene. " Some of the following principles or factors which can affect tile course of a photochemical process have been reviewed by Laartioven and these include the following, (a) The NEER (non-equilibration of excited state rotamers) principle indicates tiiat each conformer of a polyene yields its own characteristic... 

The photochemistry of vitamin D (see also Special Topic 6.4 above and Scheme 6.8) has also played a central role in the development of modern organic photochem-istry.564,598,617 618 The concept of non-equilibration of excited rotamers (NEER Section 6.1.1) has been used to explain the excitation-wavelength dependence of E Z isomerization (Section 6.1.1) of previtamin D3 (41).619 Whereas the quantum yield for E Z isomerization decreases with increasing wavelength, the formation efficiencies of the 6jt-electron conrotatory ring-closure products, diastereomeric 7-dehydrocholester-ol (provitamin D) (64) and lumisterol (65) (Scheme 6.22), increase dramatically. This was found to occur on the basis of a participation of both the Si and S2 excited states in the photoreaction.620 For example, the quantum yields of 64 and 65 formation were [Pg.244]

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