Ketenes, cycloaddition with diazo compounds

The reaction of (trialkylsilyl)vinylketenes with nucleophilic carbenoid reagents, such as sulfur ylides and diazo compounds, has been used for synthesis of substituted cyclopentenones by stereoselective 4 + 1-annulation (Scheme 12). The strategy relies on the remarkable ability of silyl substituents to stabilize ketenes and suppress their tendency to undergo dimerization and 2 - - 2-cycloaddition. 

Ketenes rarely produce [3+ 2]-cycloaddition products with diazo compounds. The reaction possibilities are complex, and nitrogen-free products are often obtained (5). Formation of a cyclopropanone represents one possibihty. Along these lines, the synthesis of (Z)-2,3-bis(trialkylsilyl)cyclopropanones and (Z)-2-trialkylsilyl-3-(triethylgermyl)cyclopropanones from diazo(trialkylsilyl)methanes and appropriate silyl- or germylketenes has been reported (256,257). It was found that subsequent reaction of the cyclopropanone with the diazoalkane was not a problem, in contrast to the reaction of diazomethane with the same ketenes. The high cycloaddition reactivity of diazomethylenephosphoranes also extends to heterocumulenes. The compound R2P(C1)=C=N2 (R = N(/-Pr)2) reacts with CS2, PhNCO and PhNCS to give the corresponding 1,2,3-triazole derivative (60). 

Diazetidinones are prepared by the thermal [2 + 2] cycloaddition of ketene with diazo compounds (Scheme 3) (83HC(42)443). Use of ketenimines instead of ketene furnished imino derivatives (67JHC155). 

Oxazin-4-ones are obtained by cycloadditions between ketenes and isocyanates (Scheme 167). The benzoylphe-nylketene generated from diazo compound 359 forms [4 + 2] DielsAlder adducts 361 with the C=N group of aldehyde imines 360 (Scheme 168) <2001J(P1)2266, CHEC-III(8.05.9.2)417>. 

An entirely different reaction occurred when 2-(4-diazo-3-oxobutyl)-benzoxazole (239) was treated with Rh(II) octanoate. In addition to undergoing dipolar cycloaddition to produce cycloadduct 241 (20%), the highly stabilized dipole (i.e., 240) formed from the benzoxazole loses a proton to produce the cyclic ketene N,O-acetal 242. This compound reacts further with the activated 7t-bond of DMAD to give zwitterion 243. The anionic portion of 243 then adds to the adjacent carbonyl group, producing a new zwitterionic intermediate 244. In the presence of water, this species is converted to the observed phenolic lactam 245. 

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