Carotenoids with 7-Oxabicycloheptyl End Groups. Attempted Synthesis of Cycloviolaxanthin (=(3S,5R,6R,3'S,5'R,6'R)-3,6:3',6'-Diepoxy-5,6,5'6'-tetrahydro-β,β-carotin-5,5'-diol)

Article abstract of DOI:10.1002/hlca.19900730719

Starting from our recently described synthon (+)-24, the enantiomerically pure 3,6:4,5:3',6':4',5'-tetraepoxy-4,5,4',5'-tetrahydro-ε,ε-carotene (34) and its 15,15'-didehydro analogue 32 were synthesized in eleven and nine steps, respectively (Scheme 4).Chiroptical data show, in contrast to the parent ε,ε-carotene, a very weak interaction between the chiral centers at C(5), C(5'), C(6), C(6'), and the polyene system.Diisobutylaluminium hydride reduction of 32 lead rather than to the expected 15,15'-didehydro analogue 35 of cycloviolaxanthin (8), to the polyenyne 36 (Scheme 5).We explain this reaction by an oxirane rearrangement leading to a cyclopropyl ether followed by a fragmentation to an aldehyd on the one side and an enol ether on the other (Scheme 6).This complex rearrangement includes a shift of the whole polyenyne chain from C(6), C(6') to C(5), C(5') of the original molecule.