Photo‐oxygenation of Styrenic Estrogens: Structural Analysis of 8,9‐Didehydro‐, 6,7‐Didehydro‐, and 9,11‐Didehydroestrone Derivatives and Their Reactivity towards Singlet Oxygen

A series of didehydro‐3‐O‐methyl‐estrones having a styrenic framework, with the ring‐A‐conjugated double bond in all three possible positions (8,9‐didehydro‐ (6),9,11‐didehydro‐ (1b), 6,7‐didehydro‐ (9), and the 12,18‐di‐nor‐8,9‐didehydroestrone analog 11), were compared for their reactivity towards singlet oxygen. Under dye‐sensitized photo‐oxygenation conditions, both, products derived from ene‐type reactions with the isolation of a stable hydroperoxide and a fragmentation product, were obtained from 6 (see Scheme 3), while only fragmentation took place for 1b (Scheme 1), Kinetic studies indicated that 6 is more reactive towards ¹O₂ than 1b (β = 9.2·10⁻³ mol·1⁻¹ vs 3.3·10⁻² mol·1⁻¹, resp.). The observed reactivity, apparently, does not match with ene‐type reaction and [2 + 2]cycloaddition being in competition, since the most activated substrate 6 preferentially yields ene‐type products and their derivatives. Conformational analysis on the structure of 6 and 1b, both calculated by molecular‐mechanic techniques (MMPMI) and determined by X‐ray diffraction, show that the allylic H‐atoms satisfy the orthogonality rule for ene‐type reactions. The product distribution is best rationalized by applying Fukui's rule which takes into account a combination of electronic and geometric factors. Substrates 9 and 11 yielded photo‐products arising from ene‐type reaction with no stable primary products isolated (Scheme 4). Geometric considerations based on the calculated structures by molecular mechanics are consistent with the observed results.