TOTAL SYNTHESIS OF C-31-METHYL KETONE APOCAROTENOIDS .3. ON THE STRUCTURE OF HOPKINSIAXANTHIN - FIRST TOTAL SYNTHESIS OF (ALL-E)-(3S)-7'-APOHOPKINSIAXANTHIN AND (9Z)-(3S)-7'-APOHOPKINSIAXANTHIN

Optically active (all-E)-(3S)-7'-apohopkinsiaxanthin, previously known as F1, and (9Z)-(3S)-7'-apohopkinsiaxanthin have been prepared by tot al synthesis for the first time in ca. 1% combined overall yield, incl uding two unidentified geometrical isomers, in sixteen linear steps fr om (4R,6R)-actinol, (2E)-3-methyl-2-penten-4-yn-1-ol, (7-formyl-2-meth yl-2,4,6-octatrienyl) triphenyl phosphonium bromide, (3-formyl-2-buten yl)triphenylphosphonium bromide and methyllithium, by use of a C-15 C-10 + C-5 + C-1 approach. By an alternative route from enyl)-3-methyl -2-penten-4-ynl]triphenylphosphonium bromide, (7-formyl-2-methyl-2,4,6 -octatrienyl) triphenylphosphonium bromide and (2E)3-methyl-4-oxo-2-pe ntenal, the same target compounds were obtained in a combined overall yield of > 61%, including four unidentified geometrical isomers, over two steps, by use of a C-15 + C-16 approach. A hypothetical structure for hopkinsiaxanthin is discussed, based on present and previously rep orted spectroscopic and chemical data for (all-E)-(3S)- and (9Z)-(3S)- 7'-apohopkinsiaxanthin and on data previously reported for hopkinsiaxa nthin itself.