Diazomalonates, reduction

The reaction of 2,3-diphenylazirine 613 with diazomalonate 614 in the presence of dirhodium tetraacetate afforded 2,3-diphenylazetine-4,4-dicarboxylate 615, the structure of which was acknowledged through reduction toward azetidine 616 and hydrolysis toward /3-amino ketone 617 (Scheme 82) <2004TL6003>. 

Reaction of the enol ether 58 with dimethyl diazomalonate provides the spiro compound 59 in high yield. Reduction and acid catalyzed cyclopropane cleavage gives the unsaturated y-lactol 60 which can be oxidized to p-methylene y-butyro-lactone 61 20). 

The particular reaction described in Scheme 2 using dimethyl diazomalonate produces oxazoles 5 that bear a methoxy group at C-5. If desired, this substituent may be removed in some cases by reductive cleavage using LiB(Et)3H to give the 5-unsubstituted oxazoles 6.3.15 Alternatively, the 5-unsubstituted derivatives 6 may be obtained directly through the use of diazo formylacetate (2) in place of dimethyl diazomalonate (1).3 15 Some additional, representative examples of the use of 1 and 2 are shown below in the Table. 

Alkenes are converted to epoxides by oxidation with peroxy acids, and thereby they are protected with regard to certain chemical transformations. Alkaline hydrogen peroxide selectively attacks enone double bonds in the presence of other alkenes. The epoxides can be transformed back to alkenes by reduction-dehydration sequences or using triphenylphosphine, chromous salts, zinc, or sodium iodide and acetic acid. A more advantageous and fairly general method consists, however, of the treatment of epoxides with dimethyl diazomalonate in the presence of catalytic amounts of binuclear rhodium(II) car-boxylate salts. This deoxygenation proceeds under neutral conditions and without isomerization or cy-clopropanation of the liberated alkene (Scheme 97). Furthermore, epoxides can be converted to alkenes with the aid of various metal carbonyl complexes. Thus, they may be nucleophilically opened with... 

Under the conditions of homogeneous catalysis, decomposition temperatures are normally significantly lower than with the heterogeneous catalysts mentioned above, and cyclopropane yields in general are higher. However, catalysts of type 2 must first be converted into the active form [presumably a copper(I) monochelate] by brief heating or by in situ reduction (see Table 10). Another soluble catalyst, copper(I) triflate, even decomposes diazoacetic esters and diazomalonic esters at temperatures below 0 °C and sterically more encumbered diazocarbonyl compounds (e.g. a-diazo-a-trialkylsilyl acetic esters " ) still at room temperature, and has shown its effectiveness in a number of cyclopropanation reactions. Since copper(I) triflate is... 

The redox potentials of one-electron oxidation and reduction of aliphatic diazo compounds are relatively small. A table published by Bethell and Parker (1988, p. 400) contains seven corrected oxidation potentials including those of diazomethane, ethyl diazoacetate, diazodiphenylmethane, 9-diazofluorene, and of compounds 9.56 and 9.59. They were obtained by various authors using a rotating platinum disk electrode in acetonitrile and cover the range E i/2(ox) 0.77-2.10 V. The reduction potentials E i/2(red) -1.12 to —1.71 V for four compounds (diethyl diazomalonate, diazodiphenylmethane, 9.56 and 9-diazofluorene) are not strictly comparable because the measurement conditions (cyclic voltammetry) were not exactly the same. 

The alkaloid ( + )-retronecine (883, Scheme 129) is structurally similar to ( + )-heliotridine (850), with the exception that the stereocenter at C-7 is of opposite configuration. The basic approach to its synthesis involves a carbenoid displacement similar to that in the previous scheme. The acetyl protecting group of the common intermediate 875 (R=SPh) is changed to a TBS group, and the benzoate is converted to pivalate. Carbenoid displacement with dibenzyl a-diazomalonate in the presence of rhodium acetate gives 879. Reductive desulfurization... 

Carbenes derived from diazomalonate add smoothly to the vinyl ether (91) to provide a spiro-cyclopropane derivative in 85% yield which is convertible into the spiro-/S-methylenebutyrolactone (92) by sequential reduction, hydrolysis, and oxidation (Scheme 23). Similarly, the enol ether of cyclohexanone is transformed... 

An alternative furan synthesis is based upon allylic C-H insertion upon reaction with a ketone-derived enol ether. Reduction and hydrolysis affords the furan (eq 6). With aldehyde-derived enol ethers, copper(I) induced reaction with dimethyl diazomalonate yields an alkoxycyclopropane diester, whose reduction, hydrolysis, and oxidation affords a spiro- -methylene-y-lactone (eq7). 

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