PHOTO-OXYGENATION OF 3,3-DIALKYLSUBSTITUTED ALLYL ALCOHOLS - OCCURRENCE OF SYN PREFERENCE IN THE ENE ADDITION OF O-1(2) AT E-Z-ISOMERIC ALLYL ALCOHOLS

Dye‐sensitized photooxygenation (1O2‐reaction) of the symmetrically 3,3‐dialkyl‐substituted allyl alcohols 1–6 (A, Table 1) and the E‐ and Z‐isomers of the 3‐methyl‐3‐alkylsubstituted allyl alcohols 7–11 (A1, Table 2) has been studied. The α, β‐epoxyaldehydes B and B1 and the β‐hydroperoxy‐homoallyl alcohols C, C1 and D1 were practically the sole oxygenation products formed. The rate and selectivity of the 1O2 additions were found to be markedly dependent on the degree of substitution (i.e. H‐availability) at the allyl position which is Z‐orientated to the carbinol group. The allyl alcohols with a Z‐3‐methyl group, 1 and the E‐isomers of 7 to 10, showed practically the same reactivity towards 1O2 and formed only the two oxygenation products of type B1 and C1 (ratio close to 60:40). In contrast, the allyl alcohols with a Z‐3‐methylene group, 2 and the Z‐isomers of 7 to 9, reacted more slowly with 1O2 and yielded all the three possible products B1, C1 and D1 in a very different ratio of ∼ 80:10:10. However, from both the E‐ and Z‐isomers of the alcohols 7 to 10 the formations of the two oxygenation products were always strongly favoured (85–100%) which result from 1O2 additions at the disubstituted side of the double bond. This behaviour is thus completely analogous to that of the corresponding trisubstituted olefins previously studied [1] and follows a new selection rule, termed syn‐preference [1], in the ene addition of 1O2. Copyright © 1979 Verlag GmbH & Co. KGaA, Weinheim