Dihydrofurans from carbonyl ylides

C-H Insertion. Catalytic C-H functionalization is a powerful approach for C-C bond formation. As described in the previous update, [Cu(acac)2]-catalyzed cycloaddition of a,j8-enones with dimethyl diazomalonate would afford dihydrofurans through carbonyl ylide formation (eq 24). Yet, when enaminone was employed as substrate in the same reaction, naphthalen-l(4//)-one was obtained as major product (eq 55). The authors suggested that the product formation may arise from the unusual carbenoid C-H insertion followed by aromatic nucleophilic substitution. 

Hosami and co-workers (12,173,174) prepared alternative forms of nonstabilized ylides (Scheme 4.87). They generated tailor-made carbonyl ylides from substituted 1,3-dichloroethers through a 1,3-elimination pathway mediated with a samarium reagent. These simple carbonyl ylide intermediates are valuable for preparing tetrahydro- and dihydrofurans. To a mixture of bis(chloromethyl) ethers and... 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or alkynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound120 (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield120. In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate121, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

A type of 1,3-dipole that has received considerable recent interest is the carbonyl ylide. One method for its formation makes use of carbenoid chemistry (see Section 4.2). Cyclization of an electrophiUc rhodium carbenoid onto a nearby carbonyl group provides access to the carbonyl ylide. Cycloaddition with an alkyne or alkene dipolarophile then gives the dihydro- or tetrahydrofuran product. For example, the carbonyl ylide 235, formed from the diazo compound 234 and rhodium(II) acetate, reacts with dimethyl acetylenedicarboxylate to give the bridged dihydrofuran 236 (3.148). 

Heterocycles.—The phosphonium salt (59) is an effective three-carbon synthon, as demonstrated by its reaction with enolates of /9-keto-esters (Scheme 20) to give cyclopentenyl sulphides via an intramolecular Wittig reaction.63 Ylides are also intermediates in the synthesis of dihydrofurans (60) from the cyclopropylphos-phonium salt (61) and sodium carboxylates (Scheme 21).64 Cumulated ylides are very useful for the synthesis of heterocyclic compounds, e.g. (62), from molecules which contain both an acidic Y—H bond and a carbonyl or nitroso-function, as shown in Scheme 22.65... 

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