Abstract
We achieved biomimetic type total syntheses of various carotenoids by using oxirane-ring cleavage as a key reaction. The fascinating point of this strategy is as follows; selective preparation of tetrasubstituted olefinic compounds, cyclopentyl ketones, or 3,6-epoxides could be attained by optimization of side chain lengths and the terminal functional groups at the C-6-position of 5,6-epoxides as a result of regioselective oxirane-ring cleavage and subsequent reactions. In particular, Lewis acid-treatment of epoxide carrying an acetoxypropyl group at C-6 preferably provided tetrasubstituted olefinic compound. On the other hand, Lewis acid or aminium salt-treatment of epoxides carrying an acetoxyethyl or a siloxymethyl group at C-6, and epoxy dienal preferentially afforded cyclopentyl ketones. Treatment of epoxy dienoate and epoxy dienonitrile, possessing a free hydroxy group at the C-3-position with aminium salt preferentially afforded 3,6-epoxides. These resulting compounds were successfully utilized for the total syntheses of crassostreaxanthin B, mytiloxanthin, capsanthin, capsorubin, cucurbitaxanthin A, and cycloviolaxanthin, respectively. Total syntheses of amarouciaxanthin A and B, salmoxanthin, and deepoxysalmoxanthin utilizing oxirane-ring cleavage are also described.
