An experimental study on the mechanism and stereochemistry of a photochemical (1,3)-OH shift. A non-Woodward-Hoffmann reaction path for photochemical sigmatropic reactions

An experimental study on the photochemistry of the 4-methyl, 4-ethyl disubstituted 3-alkylidene-2-naphthalenol derivatives 1a,b and 5a,b is presented. It is shown that occurrence of a [1,3]-OH shift is dependent only on the ground-state conformation of the substrate. This conformation in its turn is fixed by the chirality at C2 and C4. In case of compounds 1a,b the hydroxyl group Is located in the plane of the exocyclic double bond. Excitation of this favourable conformation results In a 90°-twist of the exocyclic double bond. Due to the interaction between the substituents at C4 and C9 preferential formation of just one twisted geometry takes place. The stereochemical outcome of the resulting [1,3]-OH shift agrees well with the one expected in case of a planar shift. Further evidence In favour of the occurrence of a non-Woodward and Hoffmann reaction path is obtained from the Irradiation of 5a,b; despite a favourable ground-state conformation for a suprafacial shift to occur, this shift does not take place. Instead a 90°-twisted intermediate is formed, from which solely a radiationless transition to the ground state is observable. The stereostructure of the photoproducts formed was established by means of low temperature NOE measurements