Acylation of Grape Leaves Polyprenols

Aims: The study is aimed to obtain acylated derivatives of polyprenols which were isolated from leaves of grape Vitis vinifera L.

Study Design: Modification of natural compounds.

Place and Duration of Study: Department of Organic Chemistry, Institute of the Chemistry of Plant Substances (ICPS). The study was carried out between January, 2011 and December, 2012.

Methodology: Polyprenols were acylated by anhydrides of mono- and dicarboxylic acids in absolute benzene in the presence of catalytic amounts of dry pyridine within 1-2 hours (method A). Polyprenols were acylated also by anhydrides of mono- and dicarboxylic acids in the presence of catalyst (dry pyridine) in absence of solvents at 70-80оС within 1 or 2 hours (method B). For the acylation of polyprenols the microwave method has been used in absence of the solvent and the catalyst (method C). The courses of reactions were controlled by TLC, HPLC, and HPTLC. Structure of acyl-products determined by IR-, 1H NMR and mass-spectra.

Results: In the method A with decreasing of acid’s strength and increasing of molecular weight the yield of acyl-products is decreased. Acylation by benzoyl anhydride yielded to acyl-products between acylation by acetic and propionic anhydrides. Decreasing of yields of acyl-products was observed at usage of anhydrides of dicarboxylic acids. In the method B decreasing of the yields was observed also with decreasing of acid’s strength and increasing of molecular weight of the acylating agent. Thus yields were a bit high (in addition 1-6 %). In the method C (MWA) duration of acylation reaction has sharply contracted (on 20 60 times in comparison with the method A, and 10-30 times in comparison with the method B), i.e. yields of acyl-products are enlarged by 13.5-24.6 % (in comparison with a method A) and 7.5-21.6 % (with a method B) at usage of anhydrides of monocarboxylic acids. At MWA acylation by anhydrides of dicarboxylic acids duration of reaction contracts at 20-40 times in comparison with the method A and 10-30 times in comparison with the method B. The yields are increased on 8.2 - 17.5 % (in comparison with the method A) and 5.2-16.5 % (in comparison with the method B).

Conclusion: Acylation reactions of polyprenols with anhydrides of mono- and dicarboxylic acids, organic acids have been studied in various conditions. Usage MWA allows performing esterifications with high yields (from 59 % to 89.6 % in the case of monocarboxylic acids and from 56.2 to 78 % in the case of dicarboxylic acids). Duration of MWA acylation reaction was 6-10 minutes in the case of polyprenol acylation by monocarboxylic acids, and 12-14 minutes in the case of dicarboxylic acids. Therefore MWA method, undoubtedly, is a cost effective method for the acylation.