3- -furanones, rearrangement

The proposed meehanism for this eyeloearbonylation was shown to involve the insertion of Pd(0) speeies into the C—O bond of the substrate followed by rearrangement to the allenylpalladium intermediate 29 (as proposed for alkyl systems). Insertion of CO and subsequent reduetive elimination may lead to the 2,3-dienoie aeid 30, whieh undergoes eyelization, eatalyzed by traee quantities of an aeid present in the solvent, to the 2(5//)-furanone 28 (Seheme 10) (93JOC1538). 

Oxo-4-oxaspiro[2.3]hexanes are rearranged in the presence of a copper catalyst to give 2(5H)-furanones and 2(3/f)-furanones [94]. 

The photo [2 + 2] reaction between 5-methyl-2,3-dihydro-3-furanone (80) and methyl P-acetoxyacrylate (81) afforded a mixture of four adducts, from which the lactone (82) was isolated. The lactone (82) was reduced to the alcohol (53), which would rearrange quantitatively to the lactone aldehyde (84) upon treatment with... 

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. 

Addition of bromine to bisketene (Mc3SiC=C=0)2 (39) has been shown to produce the fumaryl dibromide (ii)-(40), which rearranged upon warming to furanone... 

The photo-Fries rearrangement of aryl hydrogen (or methyl) succinates 267 leads to 4-(2-hydroxyaryl)-4-oxobutanoic acids (or methyl esters 268), which are readily cyclized to 5-(2-acetoxyaryl)-2(3//)-furanones (269) (Scheme 68). [189-191] Photolysis of 269 [191] or the analogous open-chain enol acetates [192,193] leads to chromones. 

Beckmann rearrangement of 2,2,5,5-tetramethyltetrahydro-3-furanone oxime (362) afforded 1,3-oxazine 363 in 64% yield (equation 158). 1,4-Oxazines 365 were obtained by acidic deprotection-spirocyclization of oxime ethers 364 (equation 159) . 

Appropriately substituted diazo ketones have been converted into oxetanes in two instances by Wolf rearrangement processes. The structure of compound (52) was established by X-ray crystallography (69MI51300, 81CSC345). Reaction of 4,4-dibromo-2,2,5,5-tetramethyltetrahydro-3-furanone with aqueous base is a good method of preparation for 3-hydroxy-2,2,4,4-tetramethyloxetane-3-carboxylic acid (equation 90) (66JA1242). 

Synthetic methods for 2(5jF/)-furanones have been developed in the preparation of cardenolides (65MI31200). The ketone (171) when reacted with lithium ethoxyacetylide gives the carbinol (172) which undergoes acid catalyzed rearrangement to the a,/3-unsaturated ester (173). Allylic oxidation of (173) with selenium dioxide under mild conditions gives digitoxigenin acetate (174) (Scheme 38). 

This rearrangement-cyclization cascade was applied to natural products synthesis. For example, DDQ oxidation of the appropriate dihydro-3 (27T)-furanone enol acetates (Table 3.5, entries 1 and 7) afforded bullatenone and the antitumor agent geiparvarin in a very rapid route (Fig. 3.1).69 70... 

Although not the best catalyst, AgSbF6 led to the rearranged product in 50% yield. This cascade reaction probably started with the well-known cyclization of the ketone to the alkyne on silver coordination, giving a cyclic oxonium intermediate that rearranged to furanone via an alkyl 1,2-migration (Scheme 3.54).82... 

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