Cycloaddition reactions Wolff rearrangement

The reaction of cyclohexene with the diazopyruvate 25 gives unexpectedly ethyl 3-cyclohexenyl malonate (26), involving Wolff rearrangement. No cyclo-propanation takes place[28]. 1,3-Dipolar cycloaddition takes place by the reaction of acrylonitrile with diazoacetate to afford the oxazole derivative 27[29]. Bis(trimethylstannyl)diazomethane (28) undergoes Pd(0)-catalyzed rearrangement to give the A -stannylcarbodiimide 29 under mild conditions[30]. 

Carboalkoxymethylenes, like acylmethylenes, undergo rearrangement to ketenes as well as the olefin addition and C—H insertion reactions characteristic of methylenes.<37> Thus the photolysis of ethyl diazoacetate in olefinic solvents leads to substantial yields of products, which can be rationalized in terms of a Wolff rearrangement of the carboethoxymethylene followed by cycloaddition of the resulting ethoxyketene to the olefin ... 

The cycloaddition reactions of the unsymmetrical a-diazo-/3-diketone, 2-diazo-l-phenyl-l,3-butanedione 330, with diaryl imines 331 took place with high regioselectivity, affording exclusively the 6-methyl-5-phenyl-substituted 477-l,3-oxazin-4-ones 332 via the acetylphenylketene, generated by the thermal Wolff rearrangement of 330 (Equation 32) <2002HAC165>. 

Cyclobutanones. Irradiation of 1 in the presence of cyclohexene produces the cyclobutanone 2 rather than the cyclopropane 3, as originally believed. This reaction involves a Wolff rearrangement of the initially formed carbene (a) to a ketene (b), which undergoes regio- and stereoselective cycloaddition to the olefin. 

The requisite a-diazo thiol esters are conveniently prepared by using the "detrifluoroacetylative" diazo transfer strategy previously developed in our laboratory. Cycloadditions are best carried out by using as little as 0.006 equiv of rhodium(II) acetate to promote the thia-Wolff rearrangement. Reactions involving the more nucleophilic ketenophiles proceed smoothly in refluxing dichloromethane (40°C), while cycloadditions with less reactive partners are best accomplished in 1,2-dichloroethane (83°C). As is standard for ketene cycloadditions, the optimal protocol involves slowly adding a solution of the diazo thiol ester to a solution of the ketenophile and catalyst in order to minimize competitive ketene dimerization. 

R.L. Danheiser and co-workers generated a key vinylketene intermediate via tandem Wolff rearrangement-ketene-alkyne cycloaddition to utilize it in a photochemical aromatic annulation reaction Danheiser benzannulation) for the total synthesis of the phenalenone diterpene salvilenone. ... 

Occasionally the ketene (14) is sufficiently stable to allow its isolation in the absence of the coreactant RXH (e.g. R = r2 = aryl). In hydrocarbon solvents, however, the ketene is usually lost through selfdimerization or through inter- or intra-molecular cycloaddition reaction with alkenic double bonds. This aspect of the Wolff rearrangement is covered in Section 3.9.3.3. Extensions to the Wolff rearrangement strategy are covered in the concluding sections. 

The ketene-alkene cycloaddition gives cyclobutanones in a thermal reaction. Most ketenes are not kinetically stable, so they are usually generated in situ, either by E2 elimination of HC1 from an acyl chloride or by a Wolff rearrangement of an a-diazoketone (see Chapter 2). 

Photolysis or thermolysis of 1 in the presence of A -benzylideneaniline (R = H) and A -(diphenyl-methylene)aniline (R = Ph) afforded 5,6-diphenyl- and 5,6,6-triphenyl-l,2-bis(diphenylme-thylene)-5-azaspiro[2.3]hexan-4-one (5), respectively. These reactions can be interpreted in terms of the cycloaddition of the dimethyleneketene (resulting from the Wolff rearrangement) with azobenzene and imines." " ... 

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