Competition of Endoperoxide and Hydroperoxide Formation in the Reaction of Singlet Oxygen with Cyclic, Conjugated DienesRose‐bengal‐sensitized photooxygenation of (−)‐(R)‐α‐phellandrene (1) in MeOH at room temperature yielded a complex mixture of products, contrary to previous reports describingcis‐(3S, 6R)‐epidioxy‐p‐menthene (2) andtrans‐(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 endoperoxides2(yield 39%) and3(26%), all those hydroper‐oxides, which can be deduced from an ene reaction of1O2with1, were isolated,i.e.4β‐p‐mentha‐2,5‐dien‐1β‐yl hydroperoxide (4) (14%), 4β‐p‐mentha‐2,5‐dien‐1α‐yl hydroperoxide (5) (9%), (2R, 4R)‐p‐mentha‐1(7), 5‐dien‐2‐yl hydroperoxide (6) (2,1%), (2S, 4R)‐p‐mentha‐1(7),5‐dien‐2‐yl hydroperoxide (7) (1,5%) and (1R)‐p‐mentha‐3,6‐dien‐yl hydroperoxide (8; 1,5%;Scheme 1). Furthermore, the constantcis/transratio for all diastereoisomeric pairs (2/2,4/2,6/2) was striking. With the help of the two possible conformers1aand1bof the starting material a model of a common first step for endoperoxide as well as for hydroperoxide formation is developed. A photooxygenation at −50° supports this model. The absolute