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20.5.12.1.3 Method 3: Synthesis from Alk-2-enoic Acid Esters by Bromination–Dehydrobromination

DOI: 10.1055/sos-SD-020-01178

Evano, G.Science of Synthesis, (2007201264.

One of the earliest reported methods for the synthesis of alk-2-ynoic acid esters relies on a brominationdehydrobromination sequence starting from alk-2-enoic acid esters; the overall equation of the process corresponds to a formal dehydrogenation. The bromination of the alk-2-enoic acid ester 41 gives the vicinal dibromide 42, which is then reacted with a strong base for the dehydrobromination. The latter occurs through two consecutive E2 steps: formation of the bromoalkene 43 and a second elimination yielding the alk-2-ynoic acid ester 44 (Scheme 20). Because the last dehydrobromination is the slower step of the process, the use of a strong base such as potassium tert-butoxide is required otherwise the main product isolated from the reaction is the bromoalkene 43. The use of an aqueous base is prohibited because the ester would be saponified to the carboxylic acid in the process.[‌139‌]

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