Pyranones rearrangement

The oxidative cyclization of allenyl alcohol 135 with a small excess of dimethyl-dioxirane leads to an intermediate diepoxide that rearranges to hydroxyfuranone 136 in 55% yield (Eq. 13.44) [52]. If the oxidative cyclization is conducted in the presence of 0.5 equiv. of toluenesulfonic acid, the major product is the furanone lacking the a-hydroxy group of 136. Hydroxyfuranones or pyranones are available from the same kinds of reactions of 5-methylhexa-3,4-dien-l-ol. 

Two one-pot syntheses of highly substituted pyran-2-ones have been published. One involves the reaction between t-BuNC, dialkyl acetylenedicarboxylates and bromomalonates <99JCR368>. In the other, cyclobutenediones are treated with 0-silylated cyanohydrins to yield a 4-acylcyclobutenone by a 1,4-silyl migration and cyanide displacement which rearranges to the pyranone (Scheme 18) <99JOC2145>. 

A range of 2,2-bis(trifluoromethyl)-l,3-oxazepin-5-ones (334) has been prepared by the reaction of oxazolidin-5-ones with 1-diethylamino-l-propyne (75TL3223). l,3-Oxazepan-2-one has been prepared by the Beckmann rearrangement of tetrahydro-2-pyranone oxime. 

The cycloaddition of carbon dioxide to A,A-diethylaminophenylacetylene leads to aminopyran-4-ones (72TL1131). Initial cycloaddition to the ynamine probably forms (422) which rearranges to the ketene. A further cycloaddition, this time at the conjugated amide, leads to the pyran-4-one (Scheme 142). Supporting evidence for the proposed mechanism includes the formation of the pyranone from the ketene (423) (72TL1135). 

It has since been shown that the enol ester (451) is an intermediate in the synthesis (69T715). Indeed such esters readily form chromones on treatment with alkali and the ortho acyloxy group becomes part of the pyranone ring as a result of a Baker-Venkataraman rearrangement (Scheme 160) (69T707). 

An alternative route to the polyoxybenzophenone involves cyclization of the triketo acid (512) to the pyranone (513) and subsequent rearrangement on treatment with lithium hydride (77JA1631). 

Alkynyl( aryl) iodine (I II) compounds 89 can also be employed in hetero-Claisen rearrangements [151] after reaction with appropriate thioamides yielding thiazoles of type 90 in reasonable yields as shown in Scheme 38 [152]. Ring enlargement reactions of furan derivatives into pyranones by hypervalent iodine compounds were reported as well [153]. 

Cleavage of the hetero ring of dibenzofuran by Li followed by the addition of ketones offers a useful route to 6,6-disubstituted dibenzo[/>,<7]pyrans 13 <06JOC8291> and a photochemical rearrangement of naphtho[2,3-b]benzofuranones was used to prepare intensely fluorescent indeno[ 1,2-/>]benzo[4,5-e]pyranones 14 <060BC3406>. 

The Au-catalysed [3,3] sigmatropic rearrangement of propargyl propynoates 29 leads to 5-vinylpyran-2-ones probably by way of a cationic intermediate 30 (Scheme 23). This oxocarbenium ion can be intercepted by electron-rich arenes and heteroarenes with attack occurring at the vinylic double bond leading to more complex pyranones <07AG(E)8250>. 

O Phenylhydroxylamines (see also Section I.S) react at room temperature with DMAD to give an enamine which undergoes a Cope rearrangement this cleaves the N—O bond and a pyranone is obtained in high yield. When R = NO2, additional heating of an intermediate with ethanolic hydrochloric acid is necessary. 

Malonyl chloride enolizes and cyclizes to form a pyranone (325) which combines with dialkyl-cyanamides to yield pyrano[4,3- ][l,3]oxazinediones (328), as well as pyranoazetediones (329). Presumably, the first step is the formation of adducts (326), which undergo rearrangement to the ureas (327) and elimination of hydrogen chloride to yield ketenes. Alternate intramolecular cycloadditions of the ketenes afford the final products (Scheme 93) <85CB4707>. 

Having synthesized enol ester 51b and 51c, examination of the NHC-catalyzed rearrangement was undertaken. Previously, NHCs such as diaryl imidazolylidene 13 had proven optimal for the preparation of simple pyra-nones. Unfortunately, subjection of enol ester 51b to carbene 13, generated in situ from precatalyst 13 HC1, failed to bring about any reaction, even when the reaction was heated to reflux (Table 2, entry 1). Similar results were achieved when the reaction was conducted in tetrahydrofuran (THF), with only trace amounts of the desired pyranone observed by NMR analysis of the crude reaction mixture (Table 2, entry 2). Thus, the challenges of total synthesis had found our methodology wanting. 

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