Dihydrofuran derivatives, carbonyl ylide

Photolytic decomposition of diazirine (25) in the presence of ethyl acetate led to the formation of the ester derived carbonyl ylide (26). Trapping of the ylide with diethyl fumarate led to the formation of the dihydrofuran 28, generated by dehydrochlorination of the initial cycloadduct 27. It occurred with a rate constant... 

For clarification of the reaction mechanism, the rearrangements of cis- and trans-phenylvinyloxiranes have been investigated, to avoid the formation of dihydrooxepine. c/s-Dihydrofuran derivatives are formed by conrotational opening of the oxiranes through a carbonyl-ylide intermediate. 

Electrocyclic ring opening of oxiranes - either thermally or photochemically - yields carbonyl ylides, which can be trapped with alkynes to give dihydrofurans. If l-buten-3-ynyl-oxirans are used as starting materials, the corresponding ethenylfurans are obtained (Equation (49)). This sequence, which has been studied quite intensively by Eberbach and co-workers, can also be used to prepare annulated derivatives <93CB975>. 

Carbonyl ylides undergo 1,3-dipolar cycloaddition with alkenes and al-kynes to yield tetrahydrofuran and 2,5-dihydrofuran derivatives, respectively (Scheme 5.14). 

Arnold first discovered that PET reactions of cis- and rrans-2,3-diphenyloxiranes (1 and 2) with electron-accepting sensitizers such as DCN, 1,4-dicyanobenzene, dimethyl terephthalate, and methyl 4-cyanoben-zoate produce the isomeric tetrahydrofuran derivatives 7 and 8 (Scheme 2). The proposed mechanism involves the CC bond cleavage of the oxirane radical cations to give ylide radical cations 3 and 4, followed by back electron transfer (BET) to generate carbonyl ylides 5 and 6. The resulting carbonyl ylides are trapped with dipolarophiles, such as acrylonitrile, maleonitrile, and fumaronitrile, to produce the observed products. In the absence of dipolarophiles, C)S-/ir s-isomerization (1/2 = 0.19-0.28) was observed, which is consistent with the formation of carbonyl ylides 5 and 6, as well as their radical cations 3 and 4. This method was successfully apphed to the synthesis of various tetrahydrofurans (Scheme 3) and dihydrofurans (Scheme 4). ... 

The reaction of dimedone (39) with tetraphenylbismuthonium derivatives and BTMG gave the a,a-diphenyl derivative (40). But when dimedone (39) was treated with triphenylbismuth carbonate, an ylide (41) was obtainedP This ylide was later isolated as a stable crystalline compound. This ylide (41) can also be prepared by reaction of the sodium salt of dimedone either with triphenylbismuth dichloride or with triphenylbismuth oxide. Similarly, Meldrum s acid gave the corresponding bismuthonium ylide with triphenylbismuth carbonate and with triphenylbismuth dichloride.3 36 uch ylides can also be made by decomposition of the appropriate dicarbonyl diazonium derivative in the presence of triphenylbismuthane catalysed by bis(hexafluoroacetylacetonato)copper (II) 37 These ylides react with aldehydes to give cyclopropanes, dihydrofurans and a,p-unsaturated carbonyl... 

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