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24.4.1.1.8.3 Variation 3: Synthesis of 1-Chloroalk-1-ynes from Esters

DOI: 10.1055/sos-SD-024-00886

Witulski, B.; Alayrac, C.Science of Synthesis, (200624921.

Methyl esters can be converted in three steps into 2-aryl- or 2-alkyl-1-chloroacetylenes 34 in overall yields of 5659% (Scheme 21).[‌76‌] These transformations involve the coupling of the lithium derivative of chloromethyl 4-tolyl sulfoxide with the methyl esters in tetrahydrofuran at 70°C, and afford the corresponding α-chloro α-sulfinyl ketones 32. These ketones are then subjected to enol trifluoromethanesulfonation, which proceeds smoothly at room temperature using N-phenylbis(trifluoromethanesulfonimide) as the reagent in 1,2-dimethoxyethane solution containing hexamethylphosphoric triamide. The resulting enol trifluoromethanesulfonates 33 undergo desulfinylative β-elimination when treated with butyllithium for a very short time (2min) at 100°C. Attempts to apply this method to the syntheses of 1-fluoroalk-1-ynes failed.

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