Aqueous conditions alkyne synthesis

Unsaturated lactones lacking substitution at C-4 are the simi est ones available via this general type of cycloaddition. Several syntheses of these lactones are of practical value, including two Pd-based meth-ods. However, the considerable utility of metal carbonyl anions in lactone synthesis is illustrated by a rhodium carbonyl anion catalyst system which gives very high yields upon reaction with a variety of internal alkynes under weakly basic aqueous conditions, essentially water-gas shift conditions. These conditions were established to maximize chemoselectivity with respect to other possible alkyne carbonylation products. Regioselectivity is modest in this process, but was not examined systematic ly (equation 13). ... 

Removal of the tri-wo-propylsilyl (TIPS) and tm-butyldimethylsilyl (TBS) protecting groups could be accomplished concomitantly with TBAF in tetrahydrofuran at 0 °C, but here competing elimination of the secondary bromide was observed. Better overall yields and cleaner conversion was observed when TBS ether was cleaved with 5 % aqueous HF in acetonitrile at 0 °C followed by removal of the acetylenic TIPS with TBAF under milder conditions of -78 °C.10 The diastereomers are not separated before the desilylation process therefore even a 3 1 mixture of E- and Z-enyne is obtained. Prelaureatin 4 and its F-isomer 17 are likewise goals in natural product synthesis. Crimmins and co-workers developed an own synthetic route to 4. The reaction sequence is similar up to aldehyde 55. Afterwards a Z-vinyl-iodide is selectively formed and the alkyne is introduced via a Sonogashira reaction. 

Only rarely is proton loss observed from the intermediate cation derived from acylation of a terminal alkyne. In contrast, acylation of alkynylsilanes provides an excellent strategy for the synthesis of alkynyl ketones. " Here, it is apparent that elimination of the silyl substituent competes very effectively with capture of a nucleophile. Acylation of bis(trimethylsilyl)ethyne" provides a convenient route to terminal alkynic ketones, particularly since desilylation can be carried out using bases as weak as dilute aqueous borax, conditions mild enough to prevent the ready hydration of the triple bond (Scheme 25)." Silylated diynes are also excellent substrates for Friedel-Crafts acylation. ... 

Recently, Chen et al. described a synthesis of isoquinoline, furopyridine, and thienopyridinederivatives (58) in moderate to excellent yields (up to 86 %) under Pd-catalyzed MW-assistedone-pot conditions (Scheme 7.40) [109]. Sequential coupling, elimination with ammonium acetate, and annulation of ortho-hromoai-ylaldehydes (57) and terminal alkynes occur. Ammonium acetate is used as the elimination reagent and gives superior results to its aqueous ammonia counterpart. It is noteworthy that no copper catalyst is necessary in this reaction. 

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