Effects ofbis homoallylic and homoallylic hydroxyl substitution on the olefinic13C resonance shifts in fatty acid methyl esters

Substitution of a hydroxyl group at thebis homoallylic position (OH group located three carbons away from the olefinic carbon) in C₁₈ unsaturated fatty acid esters (FAE) induces a 0.73±0.05 ppm upfield and a 0.73±0.06 ppm downfield shift on the δ and ε olefinic¹³C resonances relative to the unsubstituted FAE, respectively. If the hydroxyl group is located on the carboxyl side of the double bond of thebis homoallylic hydroxy fatty acid esters (BHAHFA), the olefinic resonances are uniformly shifted apart by |1.46+|Δδdb_u‖] where Δδdb_u represents the absolute value of the double bond resonance separation in the unsubstituted FAE and 1.46 ppm is the sum of the absolute values of the δ and ε shift parameters. With hydroxyl substitution on the terminal methyl side of the double bond, the olefinic shift separation is equal to |1.46−|Δδdb_u‖]. In homoallylic (OH group located two carbons away from the olefinic carbon) substituted FAE the γ and δ induced hydroxyl shifts for thecis double bond resonances are +3.08 and −4.63 ppm, respectively while thetrans double bond parameters are +4.06 and −4.18 ppm, respectively. The double bond resonance separation in homoallylic hydroxy fatty acid esters (HAHFA) can be calculated from the formula |7.71−|Δδdb_u‖] forcis and |8.24−|Δδdb_u‖] for thetrans case when the OH substitution is on the carboxyl side of the double bond. Conversely, when the OH resides on the terminal methyl side, the double bond shift separations forcis andtrans isomers are |7.71+|Δδdb_u‖] and |8.24+|Δδdb_u‖], respectively. The derived shift parameters can verify the positions of both the double bond and hydroxyl substitution from the olefinic resonance separation in long-chain fatty acid derivatives, obviating the need for destructive analytical methods. Reference to brand or firm name does not constitute endorsement by the United States Department of Agriculture over others of a similar nature not mentioned.