Determination of the geometry of acetoxyendiynes and acetoxyenynes by NMR heteronuclear 13C1H scalar couplings and 13C NMR chemical shifts. Structural assignment of the oxylipin natural products peyssonenynes A and B

Abstract

Solution NMR methods were used for the structural characterization of the acetoxyendiyne E/Z configuration of the marine natural products peyssonenynes A and B and their synthetic analogs derived from palmitic acid. The scarcity of protons in the proximity of the olefin precluded the determination of the double bond geometry using (1)H NMR methods that rely on proton-proton scalar couplings or experiments such as NOESY or ROESY. Long range (1)H-(13)C heteronuclear scalar couplings, (n)J(CH), measured with the 2D excitation sculptured indirect detection experiment (EXSIDE) proved useful and highly reliable for the analysis of the enol acetate geometry. In addition, it was found that the chemical shift of some carbon atoms in the proximity of the olefin was also sensitive to the double bond configuration of these molecules providing an even simpler way to determine their geometry. This protocol showed its robustness by similar analysis of simpler silyl-protected acetoxyenynes derived from fatty acids. These NMR experimental results and stereochemical predictions were rationalized by DFT calculations.