Dihydrofurans dimethyl diazomalonate

Cycloaddition. [4+1] Cycloaddition of dimethyl diazomalonate with conjugated dienes and enones proceeds in the presence of catalytic amounts of Rh or Cu, affording cy-clopentenes or dihydrofurans in fair yields (eqs 23 and 24). 

Synthesis of highly functionalized 2,5-dihydrofurans could be accomplished in moderate yields by the [Rh2(OAc)4]-catalyzed reaction of benzaldehyde, dimethyl diazomalonate, and acetyl-enedicarboxylate at room temperature (eq 53). ... 

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. 

The substrate scope of the Cu(II)-catalysed reaction of ene-biscarbonyls (76) with two common diazo compounds (i.e., dimethyl diazomalonate and ethyl diazoacetate) as car-benoid sources has been broadened. Depending on the electronic/steric nature of both the ene-biscarbonyl and the diazo compound, moderate to excellent levels of chemose-lectivity have been achieved, the products usually formed being the dihydrofurans (77), dihydrobenzoxepines (78), and dihydrooxepines (79). 

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. 

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