Total synthesis of C31-methyl ketone apocarotenoids. 3. On the structure of hopkinsiaxanthin: first total synthesis of (all-E)-(3S)- and (9Z)-(3S)-7'-apohopkinsiaxanthin.

Abstract

Optically active (all-E)-(3S)-7'-apohopkinsiaxanthin, previously known as F1, and (9Z)-(3S)-7'-apohopkinsiaxanthin have been prepared by total synthesis for the first time in ca. 1% combined overall yield, including two unidentified geometrical isomers, in sixteen linear steps from (4R,6R)-actinol, (2E)-3-methyl-2-penten-4-yn-1-ol, (7-formyl-2-methyl-2,4,6-octatrienyl)triphenylphosphonium bromide, (3-formyl-2-butenyl)triphenylphosphonium bromide and methyllithium, by use of a C15 + C10 + C5 + C1 approach. By an alternative route from (2Z)-5-[((4S)-4-hydroxy-2,6,6-trimethyl-3-oxo-1-cyclohexenyl)-3- methyl-2-penten-4-ynyl]triphenylphosphonium bromide, (7-formyl-2-methyl-2,4,6-octatrienyl)triphenylphosphonium bromide and (2E)-3-methyl-4-oxo-2-pentenal, 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 C15 + C16 approach. A hypothetical structure for hopkinsiaxanthin is discussed, based on present and previously reported spectroscopic and chemical data for (all-E)-(3S)- and (9Z)-(3S)-7'-apohopkinsiaxanthin and on data previously reported for hopkinsiaxanthin itself.