Total synthesis of cucurbitaxanthin A, cycloviolaxanthin and capsanthin 3,6-epoxide by applying a regioselective ring opening of tetrasubstituted epoxides

摘要

The synthesis of 3,6-epoxy carotenoids cucurbitaxanthin A 1, cycloviolaxanthin 2 and capsanthin 3,6-epoxide 3, was accomplished via the C_(15)-3,6-epoxides 20e and 20f, prepared by the regioselective ring opening of the 3-hydroxy-5,6-epoxides 10e and 10f.rnCucurbitaxanthin A 1, cycloviolaxanthin 2 and capsanthin 3,6-epoxide 3 (Fig. 1), bearing 3,6-epoxy-end groups, were isolated2 from red paprika Capsicum annuum together with the major pigments capsanthin 4 and capsorubin 5 possessing a K-end group. Cucurbitaxanthin A1 was also isolated from the pumpkin Curcurbita maxima as a major pigment. These carotenoids are considered to be formed in nature from 5,6-epoxy carotenoids through ring opening of the epoxy moiety. Previously, we reported the biomimetic type total synthesis of capsanthin 4 and capsorubin 5 via regioselective cleavage of the oxirane ring at the C-5 position (route b) and the subsequent stereoselective ring contraction of the C_(15)-3-silyloxy-5,6-epoxy dienal as shown in Scheme 1.