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Abstract
A regio‐ and chemoselective preparation of bicyclic alkoxyoxazolium salts from amide derivatives of proline and pipecolic acid by electrophilic amide activation is reported. Mechanistic NMR experiments suggest an unusual role for the base and highlight the effect of substitution pattern of the substrates.
Highlights
The high abundance of carboxamides[1] in combination with their distinct, mild nucleophilic properties[2] makes them an interesting target for the investigation of organic reactions
Already in the 19th century it was observed that primary amides react readily with dehydrating agents such as PCl5 or concentrated sulfuric acid to give nitriles.[3]. This resulted in an early recognition that the poor electrophilicity of the carbonylcarbon of carboxamides can be readily enhanced by electrophilic activation.[2]
The electrophilic activation of tertiary amides has been used for the synthesis of certain N-aryloxazolium salts (Scheme 1c).[8]
Summary
The high abundance of carboxamides[1] in combination with their distinct, mild nucleophilic properties[2] makes them an interesting target for the investigation of organic reactions. We would like to report the synthesis of bicyclic oxazolium salts based on proline- and pipecolic acid-derived amides (Scheme 1d). The products display interesting reactivity and can be converted to complex structures by formal cycloaddition.[7a,9]
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