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

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Abstract
The absolute configurations of acetylated bretonin A (= (+}‐(R)‐1‐[(acetoxy)methyl]‐2‐{[(4E,6E,8E)‐dodeca‐4,6,8‐trienyl]oxy}ethyl 4‐acetoxybenzoate; (−)‐1b) and isobretonin A (= (+)‐(S)‐3‐{[(4E,6E,8E)‐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 (4E,6E,8E)‐ (14e) and (4E,6Z,8E)‐dodeca‐4,6,8‐trien‐1‐ol (14z). This also allowed confirming the structure and configuration of bretonin B (= (S)‐2‐{[(4E,6Z,8E)‐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.