Abstract
The mechanism and the final outcome of the Knoevenagel-Doebner reaction are discussed. The condensation reaction between different hydroxy-substituted aromatic aldehydes and malonic acid is performed using piperidine as organocatalyst. The key role of the catalyst is clearly pointed out during the decarboxylation of ferulic acid, without the use of a strong decarboxylating agent, leading to a 4-vinylphenol derivative. Based on the results obtained, the studied pathway may be important in the understanding of vinylphenol production during malting and brewing of wheat and barley grains. Finally, changing the solvent of the reaction from pyridine to water in the Knoevenagel-Doebner reaction of 4-hydroxybenzaldehydes, dimerization of resulting styrene derivatives is observed. These results can be of interest also in the field of food chemistry, since cinnamic acids are frequently found in fruits and vegetables used for human consumption.
Highlights
Β-Unsaturated carboxylic acids are versatile compounds which allow the access to a variety of synthetic intermediates as well as natural products, such as lignin related structures (Figure 1) [1]
Within a research program devoted to the use of,βunsaturated carboxylic acids to prepare lignans as well as high added value building blocks, the synthesis of cinnamic acid derivatives through a Knoevenagel-Doebner reaction was performed [3,4]
Even though the Knoevenagel reaction has been employed to obtain cinnamic acid and derivatives, both the reaction and work-up conditions previously reported are different than those described in this work
Summary
Β-Unsaturated carboxylic acids are versatile compounds which allow the access to a variety of synthetic intermediates as well as natural products, such as lignin related structures (Figure 1) [1]. Organocatalysis has become a field of great importance within organic synthesis, allowing the development of widely applicable reactions while avoiding the use of metal catalysts [6,7,8,9]. In this context, we condensed different aromatic aldehydes with malonic acid, using piperidine as organocatalyst and pyridine as solvent [10,11]. When vanillin (4-hydroxy-3-methoxybenzaldehyde) was employed as electrophile, the formation of 2-methoxy-4-vinylphenol 1a, instead of ferulic acid ((E)3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid) 2a, was observed (Figure 2)
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