Synthesis of the Bretonins, Polyolefinic Esterified Glyceryl Ethers of an Unidentified Sponge from the North‐Brittany Sea: Absolute Configuration and Novel Structure Assignment

The absolute configurations of acetylated bretonin A (= (+}‐( R )‐ 1‐[(acetoxy)methyl]‐2 ‐{[(4E, 6 E, 8 E)‐ dodeca ‐4,6,8‐ trienyl ] oxy } ethyl 4‐ acetoxybenzoate ; (−)‐ 1b ) and isobretonin A (= (+)‐(S)‐3‐{[( 4 E, 6 E, 8 E)‐ do‐deca‐4,6,8‐trienyl]oxy}‐2‐hydroxypropyl 4‐hydroxybenzoate ; (+)‐2), previously isolated from an undetermined sponge of the North Brittany sea, were established by comparison with synthetic (+)‐ lb and (+)‐ 2 , obtained from the condensation of commerical (−)‐(R)‐2,2‐dimethyl‐1,3‐dioxolan‐4‐yl p ‐toluenesuifonate ((−)‐( R )‐ 15 ) with a mixture of (4 E ,6 E ,8 E )‐ ( 14e ) and (4 E ,6 Z ,8 E )‐dodeca‐4,6,8‐trien‐1‐ol ( 14z ). This also allowed confirming the structure and configuration of bretonin B (= (S)‐ 2 ‐{[(4 E ,6 Z ,8 E )‐ dodeca‐4,6,8‐trienyl]oxy}‐1‐(hydroxy‐methyl)ethyl 4‐hydroxybenzoate ; 3 ) which was also isolated from the same sponge, albeit in a too small amount for a complete study. As concerns the glyceryl ethers precursors of the bretonins, co‐occurrence of the usual ( S )‐con‐figuration (from 1a ) with the unusual ( R )‐configuration (from (+)‐ 2 )) poses intriguing biogenetic problems.