Oxidative Breaking of Long-Chain Acetylenic Enol Ethers of Glycerol of the Marine Sponges Raspailia pumila and R. ramosa and of Model Compounds with Aerial Oxygen

Article abstract of DOI:10.1002/hlca.19870700521

The raspailynes (novel long-chain enol ethers of glycerol having the enol ether double bond conjugated, in sequence, to an acetylenic and an olefinic bond, isolated from the North-East-Atlantic sponges Raspailia pumila and R. ramosa) are stable under normal hydrolytic conditions for enol ethers.In contrast, when their solutions are evaporated, these lipids such as raspailyne B1 (=(-)-3-<(1Z,5Z)-(tetradeca-1,5-dien-3-ynyl)oxy>-1,2-propanediol; (-)-2) rapidly react with aerial O2 under normal laboratory-daylight conditions, with rupture of the C=C enol ether bond to give 1-O-formylglycerol (3) and an aldehyde (such as tridec-4-en-3-ynal(4) from (-)-2).This reaction must be caused by triplet O2, since thermally generated singlet O2 has no effect on (-)-2 in solution.That the mere presence of an enol-ether moiety conjugated to an acetylenic group is responsible for such a behaviour is demonstrated with the model compounds 1-methoxypentadec-1-en-3-yn-5-ol (6a) and its 5-O-acetyl or 5-O-tetrahydropyranyl derivatives 6b and 6c, respectively.Resistance to both hydrolytic conditions and singlet O2 of these compounds is thought to arise from electron depletion at the enol-ether C(β) atom by the acetylenic group.Plausible reaction pathways for enol-ether bond rupture in these compounds by aerial O2 are outlined.