Carbonyl ylides, cycloaddition with epoxide

Epoxide 96 was prepared such that photolytic conversion to the carbonyl ylide could be followed by an intramolecular cycloaddition with the tethered pendant olefin. However, photolysis of epoxide 96 led only to the formation of the regio-isomer 97 and the aldehyde 98 with no evidence of the corresponding cycloadduct. It was presumed that 97 arose from the ylide by thermal recyclization to the epoxide while 98 could form through the loss of a carbene from the ylide. The failure of the tethered alkene to undergo cycloaddition may have resulted from a poor trajectory for the cycloaddition. An extended analogue (99) allowed greater flexibility for the dipolarophile to adopt any number of conformations. Photolysis of epoxide 99 did lead to formation of the macrocyclic adduct 100, albeit in modest yields. 

The 2,4,6-triphenylpyrylium terafluoroborate (TPT)-sensitized electron transfer of aryl-substituted epoxides such as 250 leads to ring opening via selective C—O bond cleavage, while subsequent [3 + 2]-cycloaddition of the resultant carbonyl ylide with electron-rich olefins 251 leads to the synthesis of substituted THF derivatives 252 and 253 (Scheme 8.69) [109]. 

The mechanism for the addition of diazoalkanes to a C=—O double bond is generally written along lines similar to those discussed so far (Scheme 1 X" = N2+). The initial adduct is also the progenitor of the various rearrangement pathways. However, the subject of mechanism is by no means settled, with 1,3-dipolar cycloadditions and carbonyl ylide formation" considered to be prominent alternatives. In general, successful epoxidation of carbonyl compounds improves with increasing electron-poor character of the C—O bond. When the diazoalkane is electron poor, yields of epoxide diminish. 

Tetrakis(l,T-binaphthyl-2,2 -diyl phosphate) complexes (119) are reported to be much more effective catalysts than the more commonly used carboxylate complexes for enantioselective intramolecular, tandem, carbonyl ylide forma-tion/cycloaddition of a-diazo- -keto esters. The ring-opening reactions of epoxides with diphenyl phosphorazidate (120) have been investigated. A wide range of epoxide substrates have been studied and the products, (121) or (122), depend on the substrate structure. The microbial hydroxylation of novel phos-... 

The l,3-thiazole-5-thione (36) undergoes a regioselective 1,3-dipolar cycloaddition reaction with a carbonyl ylide. The ylide is thermally generated by the electrocyclic ring opening of the epoxide (37) to give the spirocyclic adducts (38a and 38b) <97HCA1190>. 

S.2.2.6. Tetrahydrofurans from Carbonyl Ylides. Carbonyl ylides are unusual species that must be generated as needed. One method is to break the C-C bond of certain epoxides thermally or photochemically, which yields an open chain. Thereby one carbon is positively charged and the other is negatively charged, which is the structural feature of carbonyl ylides. This is shown in Scheme 5.33 A. Cycloaddition with an alkene follows immediately to give tetrahydrofuran derivatives (Scheme 5.33). 

Common reactions of the ylide include (i) [2,3]-sigmatropic rearrangement of allylic, propargylic, and allenic ylides (ii) [l,2]-shift (Stevens rearrangement) (iii) 1,3-dipolar cycloaddition of the ylide generated from carbonyl compounds or imines with dipolarophiles, usually G=G or C=C bonds and (iv) nucleophilic addition/elimination, leading to the formation of epoxides or cyclopropanes (Figure 2). 

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