Alkyne Fatty Acids

Whilst alkene fatty acids occur abundantly in nature, acetylene fatty acids are rare. The most important ones are tariric acid (18:1 6a) and crepenynic acid (18:2 9c12a). Furthermore acids with conjugated acetylene groups are known.[1] Alkyne fatty acids are furthermore prepared and used as intermediates in fatty acid synthesis, because they allow the easy assembly of building blocks and the stereospecific hydrogenation to cis- or trans-double bonds.[2].
Alkynes can be prepared from alkenes by bromination/debromination. In this way the one-pot conversion of oleic acid to stearolic acid (1) and of 11-undecenoic acid to 11-undecynoic acid (2) was achieved in 85% and 78%, respectively.[3, 4]
The triple bond allows a number of reactions that are not possible with the double bond. Some of these have been applied here:
Triple bond isomerization offers the possibility to isomerize internal alkyne groups into terminal ones by using the acetylene zipper. This way stearolic acid (1) could be isomerised to 90% methyl 17-octadecynoate (3) and converted to 93% 17-octadecyn-1-ol (4).[4]
Substitution of the acetylenic-CH-bond: 2-Alkyne fatty acids are biologically active. By deprotonation and carboxylation 19-hydroxy-nonadeca-2-ynoate was obtained in 85% yield.[5]
The ester 3 could be dimerized with CuCl, DBU, pyridine and dioxygen to 96% dimethyl hexatriaconta-17,19-diyn-1,36-dioate.[4] The hydrogen in the acetylenic CH-bond in 2 - 4 can be substituted by aryl in nearly 90% yield, when reacted with iodobenzene and CuI, PdCl2 as catalyst.[5]
Addition to the triple bond: Hydratisation of 3 with Hg(II) as catalyst yielded 98 % methyl 17-oxooctadecanoate. With PdCl2, CuCl2, dioxygen and carbon monoxide in methanol the internal triple bonds in 1 could be converted in a methoxycarbonylation to 58% methyl 9(10)-methoxycarbonyloctadeca-9-enoate. Terminal triple bonds in 2 - 4 afforded the bismethoxycarbonyl adducts in 76-83% yield.[5,6]
Cyclotrimerization: With Pd/C, TMSCl as catalyst the octadecynol 4 could be converted into 96% of the cyclotrimers.[5-7]. Terminal alkynes could not be dimerized with this catalyst, however, a Co(I)-catalyst afforded the cyclotrimer of 2 in 85% yield. From 2 - 4 and different alkylnitriles with another cobalt-catalyst pyridine derivatives were obtained in 62 - 93% yield. Phenylisocyanate the ester 1 or the corresponding alcohol could be cyclotrimerized with a Ni-catalyst to pyridones in 98% and 75% yield, respectively.[5,6] The cyclotrimers of 1 and 4 were converted into surfactants by ethoxylation, sulfatation and N-methyl-glucamidation. They show interesting Langmuir-isotherms with similarities to the lung surfactant and exhibit low surface tensions.[8]
Support by Henkel KGaA, Düsseldorf and the Fachagentur “Nachwachsende Rohstoffe” is gratefully acknowledged

[1] F.D.Gunstone in The Lipid Handbook ( F.D. Gunstone, J.L.Harwood, F.B. Padley, eds.) Chapman and Hall, London, 1986, page 13.
[2] F.D. Gunstone, J.L. Harwood and N. Krog in The Lipid Handbook (F.D. Gunstone, J.L.Harwood, F.B. Padley, eds.) Chapman and Hall, London, 1986, page 287.
[3] L.S. Silbert, J. Am. Oil Chem. Soc. 1984, 61,1090.
[4] K.E. Augustin, Hans J. Schäfer Liebigs Ann. Chem. 1991, 1037-1040.
[5] Kim Augustin, Ph. D. thesis, Universität Münster 1991.
[6] U. Biermann, W. Friedt, S. Lang, W. Lühs, G. Machmüller, J.O. Metzger, M. Rüsch gen. Klaas, H.J. Schäfer, M.P. Schneider, Angew. Chem. 2000, 39, 2206-2224.
[7] J.M. Renga, A. G. Olivero, M. Bosse, US Pat. 4959 488, 1990.
[8] C. Rüdiger, Ph.D. thesis, Universität Münster, 1999.