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

• Published in: Helvetica Chimica Acta
• Main Authors: Guella, Graziano, Mancini, Ines, Pietra, Francesco
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1987
• Subjects: North East Atlantic
• Online Access: http://dx.doi.org/10.1002/hlca.19870700521; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fhlca.19870700521; https://onlinelibrary.wiley.com/doi/pdf/10.1002/hlca.19870700521

Abstract The raspailynes (novel long‐chain enol ethers of glycerol having the enol ethers 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 Bl (=(−))‐3‐[(1 Z ,5 Z )‐(tetradeca‐1,5‐dien‐3‐ynyl)oxy]‐1,2‐propanediol;(−‐ 2 ) rapidly react with aerial O 2 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‐2ynal( 4 ) from (−)‐ 2 ). This reaction must be caused by triplet O 2 , since thermally generated singlet O 2 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 ‐tetra‐hydropyranyl derivatives 6b and 6c , respectively. Resistance to both hydroytic conditions and singlet O 2 of these compounds is thought to arise from electron depletion at the enol‐ether C(beta;) atom by the acetylenic group. Plausible reaction pathways for enol‐ether bond rupture in these compounds by aerial O 2 are outlined.