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Keywords:

  • Cyclization;
  • Linoleic acid;
  • Maleic anhydride;
  • Renewable resources;
  • Rhodium catalysis

Abstract

Fatty acid-based 3,6-disubstituted-1,2,3,6-tetrahydro-phthalic acid anhydride derivatives were synthesized by Rhodium catalyzed addition of maleic anhydride to linoleic and oleic acid. The maleinization of linoleic acid leads to cyclohexenoic anhydrides having an allylic double bond. The cis-configuration of this double bond was confirmed by synthesis of the corresponding anhydrides via Diels–Alder reaction of calendic acid, α-eleostearic acid and β-eleostearic acid with maleic anhydride, which was reported to be highly regio- and stereoselective. These compounds were co-injected with the products of the Rh-catalyzed maleinated linoleic acid into HPLC column so that the peaks of the resulting chromatogram could be allocated to the corresponding cyclohexenoic anhydrides. Besides the activity of the used catalysts RhCl3(H2O)3 and Rh(OAc)2, their selectivity towards the cyclohexenoic anhydrides was investigated. Furthermore, the influence of reaction temperature, equivalents of maleic anhydride and reaction time was analyzed to determine the optimum reaction conditions for the highest overall yields of the resulting 3,6-disubstituted-1,2,3,6-tetrahydro-phthalic acid anhydride derivatives.

Practical applications: Fatty acid-based 3,6-disubstituted-1,2,3,6-tetrahydro-phthalic acid anhydride derivatives which can be used during polycondensation processes can be synthesized via Diels–Alder reaction of conjugated fatty acids and maleic anhydride. In this context, the substructure of the six-membered carbon ring is of great relevance for the hardness of the resulting polyesters. The main disadvantage of this synthesis route is caused by the non-conjugation of the most natural fatty acids, e.g. linoleic acid, which requires a further step and increase costs. The Rh-catalyzed addition of maleic anhydride to unsaturated fatty acids enables the synthesis of such rings using MUFA, e.g. oleic acid or non-conjugated PUFA, e.g. linoleic acid. Especially, the addition to linoleic acid leads to monomers of high interest having an allylic double bond. The synthesized cyclohexenoic anhydrides are considered to be a promising opportunity for bio-based substitutes of, e.g. phthalic acid anhydride, trimellitic acid, and their di-, tetra-, or hexahydro derivatives being commonly used in polycondensation processes, which may open the door towards advanced bio-based products, e.g. for lacquers and coatings.