Competition of Endoperoxide and Hydroperoxide Formation in the Reaction of Singlet Oxygen with Cyclic, Conjugated Dienes

Article abstract of DOI:10.1002/hlca.19890720107

Rose-bengal-sensitized photooxygenation of (-)-(R)-α-phellandrene (1) in MeOH at room temperature yielded a complex mixture of products, contrary to previous reports describing cis-(3S,6R)-epidioxy-p-menthene (2) and trans-(3R,6S)-epidioxy-p-menthene (3) as the only products.The mixture was separated by prep.HPLC (silica gel, pentane/Et2O 9:1).Besides the known endoperoxides 2 (yield 39 percent) and 3 (26 percent), all those hydroperoxides, which can be deduced from an ene reaction of ¹O2 with 1, were isolated, i. e. 4β-p-mentha-2,5-dien-1β-yl hydroperoxide (4) (14 percent), 4β-p-mentha-2,5-dien-1α-yl hydroperoxide (5) (9 percent), (2R,4R)-p-mentha-1(7),5-dien-2-yl hydroperoxide (6) (2.1 percent), (2S,4R)-p-mentha-1(7),5-dien-2-yl hydroperoxide (7) (1.5 percent) and (1R)-p-mentha-3,6-dien-yl hydroperoxide (8; 1.5 percent; Scheme 1).Furthermore, the constant cis/trans ratio for all diastereoisomeric pairs (2/3, 4/5, 6/7) was striking.With the help of the two possible conformers 1a and 1b of the starting material a model of a common first step for endoperoxide as well as for hydroperoxide formation is developed.A photooxygenation at -50 deg C supports this model.The absolute value of the cis/trans ratio changes in the same way for the endoperoxides and the hydroperoxides.