Acetals, propargylic

The unsaturated c.vo-enol lactone 17 is obtained by the coupling of propargylic acetate with 4-pentynoic acid in the presence of KBr using tri(2-furyl)-phosphine (TFP) as a ligand. The reaction is explained by the oxypalladation of the triple bond of 4-pentynoic acid with the ailenyipailadium and the carbox-ylate as shown by 16, followed by reductive elimination to afford the lactone 17. The ( -alkene bond is formed because the oxypalladation is tnins addition[8]. 

No reaction of soft carbon nucleophiles takes place with propargylic acet-ates[37], but soft carbon nucleophiles, such as / -keto esters and malonates, react with propargylic carbonates under neutral conditions using dppe as a ligand. The carbon nucleophile attacks the central carbon of the cr-allenylpal-ladium complex 81 to form the rr-allylpalladium complex 82, which reacts further with the carbon nucleophile to give the alkene 83. Thus two molecules of the a-monosubstituted /3-keto ester 84, which has one active proton, are... 

Reaction of the chloro-substituted propargyl acetate 132 with aniline gave the pyrrolidine derivative 133 that was cyclized through treatment with... 

A rather more complex amino alcohol side chain is accessible by a variation of the Mannich reaction. Taking advantage of the acidic proton in acetylenes, propargyl acetate (62) is condensed with formaldehyde and dimethylamine to give the acetylated amino... 

The intramolecular hydroarylation/cyclisation of aryl propargylic acetates catalysed by the system [AuCl(IPr)]/AgBF (1 1, 2 mol%, 72-92%, rt, 5 min) was developed as a versatile and efficient method leading to indene derivatives 110 (Scheme 2.20). Analogous catalytic systems, where the IPr was substituted by PPh, gave lower conversions and chemo-Zregio-selectivity. 

Scheme 2.21 Postulated mechanism for the synthesis of substituted indenes by intramolecular hydroarylation of propargylic acetates...

Scheme 5.36 Golden catalysed cycloisomerisation of 1,5-enynes bearing a propargylic acetate...

Propargylic acetates, halides, and sulfonates usually react with a double-bond shift to give allenes.34 Some direct substitution product can be formed as well. A high ratio of allenic product is usually found with CH3Cu-LiBr-MgBrI, which is prepared by addition of methylmagnesium bromide to a 1 1 LiBr-Cul mixture.35... 

In addition to the reactions discussed above, there are still other alkyne reactions carried out in aqueous media. Examples include the Pseudomonas cepacia lipase-catalyzed hydrolysis of propargylic acetate in an acetone-water solvent system,137 the ruthenium-catalyzed cycloisomerization-oxidation of propargyl alcohols in DMF-water,138 an intramolecular allylindination of terminal alkyne in THF-water,139 and alkyne polymerization catalyzed by late-transition metals.140... 

Catalyst Reactivation Using Propargyl Acetate. The Wiped-Film Evaporator/02 reactivation procedure and the Capture of Active Catalyst Using Solid Acidic Support with FI2 Elution procedure (see above) both involve the separation of uncomplexed phosphine from rhodium complex. Since the value of the uncomplexed phosphine is significant, technology that does not require separation of phosphine during catalyst reactivation is desirable. 

Dienes can be obtained from silylallenes by protodesilylation using boron trifluo-ride-acetic acid complex (equation 29)62. Since silylallenes can be obtained by the reaction of propargyl acetate with cuprous reagent derived from chloromethyltrimethylsilane, this reaction sequence constitutes conversion of propargylic acetate to butadiene through one carbon homologation. 

Allenic alcohols,3 In the presence of Sml2 and Pd[P(C6H3),]4, sec- and tert-propargylic acetates add to ketones to give allenic alcohols as the only or major product. A mixture of allenic and homopropargylic alcohols is formed from reactions of primary propargylic acetates. 

Fluka, and propargyl acetate and propargyl methacrylate were from Lancaster. All other terminal olefins were synthesized according to established standard procedures. 

Related competitions between a propargylic ether on one side and a propargylic acetal on the other always delivered the product of an isomerization in the direction of the ether. Compound 96 with stoichiometric amounts of potassium hexamethyl-disilazide serves as a recent example [228] (Scheme 1.42) other references describe the same reactivity [229-231],... 

The first examples of allene syntheses using copper-mediated SN2 substitution processes are documented for the reaction of propargylic acetates 7 with lithium dialkyl-cuprates, which led to the formation of allenes 8 with moderate to good chemical yields (Scheme 2.2) [2]. 

Scheme 2.2 Allene synthesis via SN2 substitution of propargylic acetates 7 with cuprates.

In the area of allenic non-natural product chemistry, the synthesis of the [34]alle-nophane 14 (Scheme 2.4) is particularly noteworthy, with all four of its allenic bridges being formed through subsequent SN2 substitution reactions of propargylic acetates with a methyl magnesium cuprate [14] (see Section 2.5 for an alternative synthesis of macrocyclic allenes). 

Scheme 2.14 Synthesis of allenes 38 and 40 by reduction of propargyl acetates with lithium dimethylcuprate. THP = tetrahydropyranyl.

Scheme 2.15 Reduction of propargyl acetates to terminal allenes with Stryker s reagent.

Scheme 2.58 Palladium-catalyzed reduction of propargylic acetates with samarium diiodide.

Palladium-catalyzed reduction of propargyl acetates is possible with Sml2 in the presence of a proton source (Scheme 3.17) [51]. The allene/alkyne selectivity is greatly influenced by the choice of the proton source. Propargyl phosphates were also converted into hydridoallenes by Pd-catalyzed reduction with Sml2 [52],... 

Propargyl halides [56] and propargyl acetates [57] have also been employed for Pd-catalyzed carbonylation (Scheme 3.22). From the latter substrates, allenylcarboxylic acids were obtained under phase-transfer conditions (with nBu4NBr in aqueous NaOH and 4-methyl-2-pentanone) [56],... 

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