R-Butylcyanoketene

Ketene dimerization is the principal synthetic route to 4-methylene-2-oxetanones. This reaction proceeds very satisfactorily for ketene and methylketene, but disubstituted ketenes dimerize only to cyclobutane-1,3-diones. The cycloaddition reaction of r-butylcyanoketene to ketene and to methylketene gives a-cyanoalkylidene-/3-lactones in about 40% yield in addition to the cyclobutane-1,3-dione dimer of f-butylcyanoketene. A mechanism has been proposed for the formation of both types of dimers from a common zwitterionic intermediate (equation 111), with the relative amount of each product determined by the configurational equilibrium of the intermediate (80JOC4483, 75JOC3417). 

Cydoaddilions. Moore and Weyler noted that r-butylcyanoketene (2) is unusually reactive in cycloaddition reactions. Thus it reacts with cyclohexcne (3) to form the cyclobutanonc (4) in 63 % isolated yield. Note that phenylcyanoketene does not react with cyclohexenc.- ... 

Isocyanides readily undergo cycloaddition reactions, and these are very valuable in the formation of heterocyclic rings. Reaction of j5-nitrostyrene with an alkyl isocyanide gives a hydroxy indole (146). Reaction proceeds even more readily between tosylmethyl isocyanide (147), in which the methylene is activated, and aryldiazonium compounds. With ketenes, isocyanides give imino lactones. However, with r-butylcyanoketene, the reaction follows a different pathway involving the carbonyl bond of the ketene, to yield 148. A [1 + 3] cycloaddition of an isocyanide to a 1,3-dipole has been used to prepare azetidines. The method has been used for synthesis of a number of azetidines . 

Thiazole adds t-butylcyanoketene and dichloroketene to give a zwitterion wherein A-thiazolium and enolate moieties are linked by carbon-nitrogen bond. This intermediate leads to the corresponding 2-acylthiazole (35). The reaction with r-butyldicyanoketene leads also to the 2 1 cyclic adduct (36) (Scheme 4) <84JOC590>. The same reactions have been observed with 4- and 5-methyl-thiazoles. 

Thiazolo[4,5-d]pyrans [CjNS-CsO]. 2-Dimethylamino-thiazoles will undergo cycloaddition with two moles of t-butylcyanoketen to give a mixture of two thiazolo[4,5-f/]tetrahydropyran-2-ones (153 = H or Me, = CN, R =... 

The zwitterion (588) has been implicated in the thermal rearrangements of the t-butylcyanoketen-l,l-dimethylallene adducts (587 R = H or CDj) and the same intermediate appears to be involved in the conversion of (587 R = Me) into (589) by a prototropic shift from the allenic methyl to the enolate system. By contrast, the four adducts formed by addition of t-butylcyanoketen to 1,3-dimethylallene isomerize by a different pathway, possibly via the dienol (590). ... 

The additions of t-butylcyanoketen to cis- and trans-cyclo-octene have been shown to be stereospecific, and the stereochemistry of the addition of dichloroketen to norbomadiene has been confirmed, by chemical degradation of the adduct, to be entirely consistent with a [ 2 -I- 2j] concerted mechanism. The use of lanthanide shift reagents makes the configurational assignment of such adducts by nm.r. spectroscopy less ambiguous. ... 

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