Furans alkynyl ketones

Gevorgyan and co-workers demonstrated that allenyl imines can be formed in situ by treating alkynylimines with a base (see Section 15.8, compound 185) [71, 72]. The same principle also works for the in situ formation of allenyl ketones from alkynyl ketones and their conversion to furans with a copper(I) catalyst [71, 72]. That Cu(I) would catalyze the isomerization of an allenyl ketone was known from work of Hashmi et al. [57, 58],... 

The reaction of alkynyl ketones with strong potassium base (KOrBu, KHMDS, KH) yields substituted furans in moderate to good yields <99T2847>. 

A related synthesis of highly substituted furans was reported by Schmalz and Zhang from cyclopropyl alkynyl ketones. The reaction scope included a great variety of nucleophiles such as several alcohols, indole or even acetic acid [140]. 

Dihydroxyalkynes (110) may be considered as masked 1,4-dicarbonyl compounds. Their isomerization-dehydration to furans (111) can be accomplished by Pd catalysts (93CC764). Alkynyl ketones of type (112) and (113) can be cyclized by Pd(0) and Pd(II) catalysts (86TL4893, 87S1022,... 

Cascade migration of acyloxy and phosphatyloxy group/cyclization reaction of alkynyl ketones catalyzed by copper or silver provided trisubstituted furans in high yields. A study of the reaction mechanism was also reported <07JA9868>... 

Similarly, 1,5-diketones give pyrans. Conjugated 1,4-diketones, such as 1,4-diphenylbut-2-en-l,4-dione is converted to 2,5-diphenylfuran with formic acid, 5% Pd/C, PEG-200, and a sulfuric acid catalyst with microwave irradiation. Formic acid reacts with alcohols to give orthoformates. Note that alkynyl ketones are converted to furans with palladium (II) acetate. ... 

Allenyl ketones presynthesised, or generated in situ by acylation of silyl-allenes, or by palladium(0)-catalysed isomerisation of conjugated or non-conjugated alkynyl ketones, can be cyclised to furans. The ring closure has been effected with silver or palladium catalysis acylation of silylallenes leads to the furan directly. In the presence of water, palladium(II)-catalysed closure of / ,7-alkynyl ketones is believed to proceed via the enol, not the allene. l,2,3-Trienyl-4-ols have also been shown to cyclise to give furans. ... 

Wipf and co-workers developed a base-catalyzed cyclization of alkynyl ketones into a general synthesis of furans. The authors then extended the... 

Gevorgyan et al. demonstrated that somewhat unstable and not simply accessible allenones could be substituted with far more easily available conjugated alkynyl ketones [83,134,135] in furan syntheses. Thus, it was shown that cycloisomerization of alkynyl ketones 46 proceeded smoothly in the presence of Cu(I)-catalyst providing furans 47 in high yields (Scheme 8.20) [116]. This protocol allowed a highly efficient... 

An alternative to the existing protocols for furan syntheses, Zn(II)-catalyzed transformation of skipped alkynyl ketones was recently elaborated by Dembinski et al. Accordingly, 5-endo-dig-cycloisomerizationofalkynyl ketones 89, in the presence of the Zn(II)-catalyst, at room temperature provided 2,5-di- and 2,3,5-trisubstituted furans 90 in high yields (Scheme 8.35) [145, 146]. This route tolerates sensitive... 

In a similar way, iminoalkynes containing aryl, alkenyl, and alkyl substituents undergoes Cul-catalyzed cyclization in excellent yields and short reaction times to give isoquinolines, pyridines, and pyrroles via a cycloisomerization (eq 18)4 Copper chloride is preferred, however, for the conversion of cyclic alkynyl imines to pyrrole-containing heterocycles. Copper iodide catalyzes the formation of furans from alkynyl ketones. ... 

Cycloisomerization of alkynylated ketones in a 5-exo-dig manner is an atom-economical approach to furans. In 1999, Picquet et al. reported the PdCla-catalyzed cycloisomerization of an alk-4-yn-l-one derivative 116a to the corresponding furan... 

The add-catalyzed cyclodehydration of (Z)- and ( )-6-hydroxy-a,p-unsaturared ketones offers a mild synthesis of substituted furans. In the case of ( )-olefins, photochemical isomerisation was found to accelerate the reaction <96TL6065>. Reaction of alkynyl(phenyl)iodonium tetrafluoroborates with tropolone in the presence of a base yields 2-substituted furotropones (Scheme 16, <96TL5539>). 

Another example of Au-catalyzed was reported using alkynyl cyclopropyl ketones as a starting material. Trisubstituted furans were given in high yields under mild condition via a domino reaction process and an example is given below <06AG(I)6704>. 

The AuCl-catalysed 4 + 2-cycloaddition of benzyne with o-alkynyl(oxo)benzenes produced anthracene derivatives having a ketone in the 9-position, in good to high yields under mild conditions.118 Hypervalent iodine compounds, [5-acyl-2-(trimethyl-silyl)]iodonium triflates, readily yielded acylbenzynes which could be trapped with furan.119 Both DMAD and benzyne reacted with borabenzene to yield substituted borabarrelenes and borabenzobarrelenes, respectively.120... 

A synthesis of [3]-fused furans involving a ring enlargement can be effected by the treatment of a-alkynyl-a-cyclopropylcycloalkanones with an electron-rich Au(l) catalyst in the presence of a suitable nucleophile (Equation 18) <2006AGE6704>. The nucleophiles that can be used include alcohols, phenols, carboxylic acids, indole, and 2-pyrrolidone. Open-chain ketones as well as other ring sizes react with comparable yields. Silver and lanthanide triflates are also effective catalysts for this transformation. 

It has been shown by Marshall and Sehon that in the presence of AgNOs absorbed on silica gel, /3-alkynyl allylic alcohols, which are more accessible than allenyl ketones, also undergo cycloisomerization to furans (Equation 19) <1995JOC5966>. Similarly, 3-trifluoromethylfurans can be prepared utilizing a Pd(ii) catalyst <2000JOC2003>. 

Furans are formed from a-propargyl ketones. Alkynyl benzyl sulfides cyclize to give 2-aryl-2,3-dihydrothiophenes. Deprotonation at the benzylic position initiates the cyclization. 

Using this remarkably effective catalyst alkynyl allylic alcohol (e.g. 265) are converted into furans 266 very efficiently. Similarly, allenic alcohols (e.g. 267) can be converted into 2,5-dihydrofurans (e.g. 268) by this relative of a 5-e rfo-dig process <93JOC7180 94JOC324>. At a higher oxidation level, conjugated allenic ketones 269 can be similarly cyclized, effectively by carbonyl hydration and a final dehydration to the furans 270 <92JOC3387 94JOC7169>. 

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