Abstract
A simple and efficient method for the preparation of 1-methyl-3-oxo-7-oxabicyclo[2.2.1]hept-5-en-2-carboxylic acid methyl ester (1) is described. The first step is a highly regioselective Diels-Alder reaction between 2-methylfuran and methyl-3-bromo-propiolate. A remarkably difficult ketal hydrolysis reaction was effected by treatment with HCl, a simple reagent that was shown to be more efficient, in this case, than commonly used more elaborate methods.
Highlights
7-Oxabicyclo[2.2.1]hept-5-ene derivatives are important intermediates and the base-induced opening of the oxygen bridge of these types of compounds has been used in the stereoselective syntheses of a number of shikimic acid derivatives and other natural products [1,2,3]
In this paper we describe a short and efficient synthesis of methyl 1-methyl-3-oxo-7oxabicyclo[2.2.1]hept-5-en-2-carboxylic acid methyl ester (1, Figure 1), starting with a Diels-Alder reaction between 2-methylfuran (2) and methyl 3-bromopropiolate (3) (Scheme 1)
The Diels-Alder reaction between the diene 2 and dienophile 3 resulted in a 15.7:1 mixture of the regioisomers 4a and 4b, respectively, which were separated by column chromatography and analyzed by nuclear magnetic resonance (NMR), with the assignment of all hydrogens, carbons and coupling constant values being accomplished using a combination of 1H-NMR, 13C-NMR, gsHMQC and gsHMBC
Summary
7-Oxabicyclo[2.2.1]hept-5-ene derivatives are important intermediates and the base-induced opening of the oxygen bridge of these types of compounds has been used in the stereoselective syntheses of a number of shikimic acid derivatives and other natural products [1,2,3].
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