Synthesis and conformational analysis of furfuryl vinyl ethers

Abstract

Five furfuryl vinyl ethers were synthesized from the corresponding furfuryl alcohols and acetylene at high and atmospheric pressures in superbasic medium KOH-DMSO. On the example of 2 vinyloxymethylfuran and 2 vinyloxymethyltetrahydrofuran, their conformational structures have been studied by quantum chemistry methods, NMR and IR spectroscopy. In particular, from the results of the quantum chemical calculations, as well as on the basis of experimental measurement and nonempirical calculations of the high level spin spin coupling constants 13 C-13 C and 13 C-1 H, a conclusion has been drawn on the s cis orientation of the vinyl groups with respect to the C α -O bond in the predominant conformers of 2 vinyloxy methylfuran and 2 vinyloxymethyltetrahydrofuran.Vinyl ethers are of wide practical use in organic synthesis 1-4 and in chemistry of polymers. 5-8 A great advance in the development of vinylation reaction of alcohols with acetylenes took place due to the superbasic catalytic systems. 9-13 Vinyl ethers of available natural compounds such as steroids, 14 alkaloids, 15 carbohydrates have been synthesized using this methodology. 16 These compounds are of undoubted interest from the point of view of their stereochemical structure. Recently, we have performed a highly selective synthesis of tetravinyl ether of methyl α D glucoside 16 using superbasic catalysts and studied its structure in details. 17 Development of technologically acceptable methods for the synthesis of vinyl ethers on the basis of furfuryl alcohols is no less important task. The latter are compo nents of biomasses enriched with pentoses (including waste of agriculture and wood industry). 18 Furan and hydrofuran rings are important structural fragments in various bio logically active natural and synthetic compounds. 19,20 Introduction of highly reactive double bond into the side chain allows one to considerably broaden synthetic poten tial of these compounds.By the time of the present study, it has been be lieved 21,22 that lowering the temperature of the reaction of 2 hydroxymethylfuran with acetylene to 125-135 °C at high pressure provides an increase in the yield of the vinyl ether to 68%, however, the vinyl ether is not formed below this range of temperatures. 2 Hydroxymethyltetra hydrofuran, on the contrary, reacts with acetylene under more drastic conditions: for successful realization of the reaction, raising the temperature to 180 °C (catalyst, KOH) 23 or the use of metal potassium 24 and sodium 25 for the generation of corresponding alkoxide is required.Recently, 26 starting from alcohol 1a and acetylene we have developed an efficient method for the synthesis of the corresponding 2 vinyloxymethylfuran (3a) at high pressure in the system MOH-DMSO (M = Na, K) (Scheme 1). Processing at 75-85 °C (by 50 °C lower in comparison with the optimum data reported in the litera ture 22 ) allowed us to largely suppress resinification and reach the 80% yield of product 3a.Scheme 1 X = CH 2 (a), CH 2 OCH 2 CH 2 (b), CH-Pr (c) The present work is aimed a...