Alkynyl carboxylates

Alkynyl carboxylates, RC=C02CR. A wide variety of these previously unknown esters can be prepared in a two-step procedure from 1, as formulated for the alkynyl benzoates (2).3... 

Silyl esters have also been transformed successfully [137] another example reports the O-acylation of serine with a /l-alkynyl carboxylic acid anhydride accompanied by an in situ isomerization of the alkyne to the allene [138],... 

Partially hydrogenated pyrrolopyridines have been prepared through a sequence that includes an alkynyl-substituted pyrimidine ring. An initial intramolecular inverse electron demand Diels-Alder reaction is followed by a cycloreversion to form a dihydropyrrolopyridine. The pyrimidine ring is generated in four steps starting with an alkynyl carboxylic acid (Scheme 7) <2004JOC9215>. 

Stable esters coming formally from alkynols and carboxylic, sulphonic or dialkyl phosphoric acids were prepared for the first time via alkynyliodonium salts with the corresponding anion. Several alkynyl carboxylates were obtained by anion exchange of alkynyl iodonium tosylates the initially formed salts, PhI+C=CR RCO2, were converted spontaneously into the esters on elution through a chromatographic column packed with an anion-exchange resin (Pol+ArCOj) ... 

An alternative approach for alkynyl carboxylates involved reaction between [bis(acyloxy)iodo]benzenes and lithium acetylides [59]. Alkynyl iodonium salts afforded with sodium carboxylates in the presence of water 1-acyloxyketones heating in an excess of acetic acid gave similarly a-acetoxy ketones [60], Alkynyl tosylates and mesylates were obtained from the thermal decomposition of isolable alkynyl iodonium sulphonates. 

A more general and efficient approach to alkynyl carboxylates, also thought to involve alkynyliodonium carboxylate intermediates, entails the treatment of bis(acyloxyiodo)-arenes with alkynyllithium reagents (equation 88)81. These reactions are best conducted in the presence of 2-nitroso-2-methylpropane in order to suppress oxidative coupling of the lithium acetylides by the acyloxyiodanes. 

Under particular experimental conditions, ligand coupling may be observed between two alkyne ligands simultaneously present on the intermediate iodane. Indeed, addition of lithium acetylide to phenyliodine(in) dicarboxylates (90) led to modest to moderate yields of alkynyl carboxylates (91). By contrast, inverse addition [that is addition of PhI(OCOR)2 to the acetylide] led to the conjugated diynes (92) as the major product. (Scheme 5.9)... 

Alkynyl carboxylate esters, 75, are obtained in the reaction of lithium acetylides with bis(acyloxy)iodobenzene, 73 [Eq. (32)] [59], These reactions are likely to proceed through the intermediacy of the respective alkynyliodonium carboxylates (74), although no such salts have been isolated to date as they spontaneously decompose, via loss of iodobenzene, to the alkynyl carboxylates, 75. Only benzoate esters (75 R = CgHj) are sufficiently stable to isolate and store pure for longer periods. Simple alkylcarboxylates such as acetates are not stable although the hindered pivaloate ester (65 R = t-Bu, R = t-Bu) has been isolated in low yield [59]. Among the reasons for the instability of these esters is their sensitivity to moisture they both readily add water and undergo subsequent hydrolyses [60]. Because of the sensitivity to moisture, the isolated yield [41] of bisalkynyl benzoates, 76, from the bisalkynyliodonium triflates, 35, is only 6-15% [Eq. (33)]. 

Acetylenic ethers and esters represent an important class of functionalized acetylene derivatives of the hypothetical alkynols, RC=COH. The chemistry of acetylenic ethers has been well developed since their first preparation about 100 years ago Several exhaustive reviews covering the literature on acetylenic ethers and their analogs up to 1985 have been published in the last 30 years. In contrast to acetylenic ethers, esters of alkynols were unknown until the mid-1980s when the first preparation of alkynyl tosylates was reported. In the following years a wide variety of alkynyl carboxylate, phosphate and sulfonate esters has been prepared from alkynyl iodonium salts. The chemistry of these novel derivatives of alkynols has been summarized in a recent review. In the last 10 years considerable interest and research activity has arisen toward alkynols themselves and such derivatives as alkynolate salts and silyl ynol ethers. The present chapter will cover the chemistry of acetylenic ethers and esters as well as related derivatives of ynols with emphasis on new developments in this subject during the last 5-10 years. 

Alkynyl(phenyl)iodonium carboxylates 99 are even less stable than phosphates 98 and decompose immediately upon generation from iodonium triflates 100 or benzoate 101 with the formation of the desired alkynyl carboxylates 96 (equation 65) k... 

More recently we have reported the preparation of alkynyl esters by the reaction of lithium alkynolates with the corresponding acid chlorides (equations 14 and 15, Section II.B.2). This method works well for the synthesis of alkynyl phosphates, however, it has only limited applicability in the preparation of alkynyl carboxylates. ... 

Treatment of 15 with sodium carboxylates (101) in aqueous THF gave keto esters (102) as products (equation 46). Carbonylation of 11 in alcohol solvents, catalyzed by Pd(OAc)2, results in alkynyl carboxylates (103) (equation 47). Coupling of alkynyl(phenyl)iodonium... 

Alkynyl carboxylic are similarly cyclized to give lactones via 5-endo,... 

In 2010, Xue and coworkers [41] reported the viability of a copper-catalyzed decarboxylative coupling of alkynyl carboxylic acids with aryl h llides under relatively mild reaction conditions (Scheme 3.24a). On the basis of computation2il investigations, the authors proposed that in the initial step of the catalytic cycle, the copper(I) precursor was oxidized to a copper(lll) complex. 

Scheme 3.24 Cu-catalyzed decarboxylative coupling of alkynyl carboxylic acids (a) and decarboxylative furan synthesis (b), as described by Xue and coworkers [41].

The authors reported also one example that demonstrated the suitability of the method also for the synthesis of enol lactones starting from alkynyl carboxylic acids. Also in this case, due to the mechanism involving a Rh-vinylidene complex, the approach is limited to terminal alkynes. 

The group of Messerle studied in depth the cationic Rh(I) catalyzed cyclization of alkynyl alcohols and alkynyl carboxylic acids. In 2000, they reported the cyclization of 4-pentynols to five-member cyclic acetals in which a molecule of alcohol (either from the solvent or from a second molecule of the starting alcohol) is incorporated into the products [31]. The reactions were performed in the presence of [Rh(bim)(CO)2l BPh4" (bim = bis(A-methylimidazol-2-yl)methane), in ace-iont-d at 60°C in a nuclear magnetic resonance (NMR) tube (Scheme 13). 

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