earliest examples of carbonyl ylide synthesis involves photocatalysis. Photolysis of diazotetrakis(trifluoromethyl)cyclopentadiene* (DTTC) in the presence of tetramethylurea can generate the carbonyl ylide by an intermolecular nucleophilic attack and subsequent aromatization of the DTTC moiety. This was isolated and characterized by X-ray crystallography due to the stability imparted by aromaticity, electron withdrawing trifluoromethyl groups, and the electron donating dimethylamine groups. Stable carbonyl ylide dipoles can then be used in [3+2] cycloaddition reactions with dipolarophiles. Another early example of carbonyl ylide synthesis by photocatalysis was reported by Olah et al. Dideuteriodiazomethane was photolysed in the presence of formaldehyde to generate the dideuterioformaldehyde carbonyl ylide. Synthesis of carbonyl ylides from hydroxypyrones by proton transfer Carbonyl ylides can be synthesized by acid catalysis of hydroxy-3-pyrones in the absence of a metal catalyst. An initial tautomerization occurs, followed by elimination of the leaving group to aromatize the pyrone ring and to generate the carbonyl ylide. A cycloaddition reaction with a dipolarophile lastly forms the oxacycle. This approach is less widely employed due to its limited utility and requirement for pyrone skeletons. 5-hydroxy-4-pyrones can also be used to synthesize carbonyl ylides by an intramolecular hydrogen transfer. After hydrogen transfer, the carbonyl ylide can then react with dipolarophiles to form oxygen-containing rings. Synthesis of α-halocarbonyl ylides from dihalocarbenes Dihalocarbenes have also been employed to generate carbonyl ylides, exploiting the electron withdrawing nature of dihalocarbenes. Both phenyl(trichloromethyl)mercury and phenyl(tribromomethyl)mercury are sources dichlorocarbenes and dibromocarbenes, respectively. The carbonyl ylide can be generated upon reaction of the dihalocarbenes with ketones or aldehydes. However, the synthesis of α-halocarbonyl ylides can also undesirably lead to the loss of carbon monoxide and the generation of the deoxygenation product. Synthesis of carbonyl ylides from diazomethane derivatives by metal catalysis A universal approach for generating carbonyl ylides involves metal catalysis of α-diazocarbonyl compounds, generally in the presence of dicopper or dirhodium catalysts. After release of nitrogen gas and conversion to the metallocarbene, an intermolecular reaction with a carbonyl group can generate the carbonyl ylide. Subsequent cycloaddition reaction with an alkene or alkyne dipolarophile can afford oxygen-containing five-membered rings. Popular catalysts that give modest yields towards synthesizing oxacycles include Rh2(OAc)4 and Cu(acac)2. Mechanism of the 1,3-dipolar cycloaddition reaction mediated by metal catalysis of diazocarbonyl compounds The universality and extensive use of 1,3-dipolar cycloaddition reactions mediated by metal catalysis of diazocarbonyl molecules, for synthesizing oxygen-containing five-membered rings, has spurred significant interest into its mechanism. Several groups have