Sequential substitution of 1,2-dichloro-ethene: a convenient stereoselective route to (9Z,11E)-, (10E,12Z)- and (10Z,12Z)-[1-C-14] conjugated linoleic acid isomers

Conjugated linoleic acid (CLA) isomers are present in human foods derived f rom milk or ruminant meat. To study their metabolism, (9Z,11E)-, (10E,12Z)- and (10Z,12Z)-[1-C-14]-octadecadienoic acids with high radiochemical and i someric purities (> 98%) were prepared by stereoselective multi-step synthe ses involving sequential substitution of 1,2-dichloro-ethene. In the case o f the (9Z,11E) isomer, a first metal-catalyzed cross-coupling reaction betw een (E)-1,2-dichloro-ethene and 2-non-8-ynyloxy-tetrahydro-pyran, obtained from 7-bromo-heptan-1-ol, gave a conjugated chloroenyne. A second coupling reaction with hexylmagnesium bromide provided a heptadecenynyl derivative. Stereoselective reduction of the triple bond and bromination afforded (7E,9 Z)-17-bromo-heptadeca-7,9-diene. Formation of the Grignard reagent and carb onation with (CO2)-C-14 gave (9Z,11E)-[1-C-14]-octadeca-9,11-dienoic acid ( overall yield from 7-bromo-heptan-1-ol, 14.4%). (10E,12Z)- and (10Z,12Z)-[1 -C-14]-octadeca-10,12-dienoic acids were synthesized by the same methodolog y using 1-heptyne, 8-bromo-octan-1-ol and, respectively, (E)-1,2-dichloro-e thene and its (Z) isomer (overall yield from 8-bromo-octan-1-ol, 13.1% (10E ,12Z); 17.2% (10Z,12Z)). Impurities ( < 2% if present) were identified as b eing (E,E) CLA isomers and were removed by RP-HPLC. Metabolism studies in a nimal are in progress. <(c)> 2001 Elsevier Science Ireland Ltd. All rights reserved.