Sydnones lithiation

Palladium insertion into the phenyl ring of 3-(4-methoxyphenyl)sydnone by lithiation of compound 103, phosphi-nation at C4, then insertion of palladium at the ortho C-H bond to give product 17 has been achieved (Scheme 4)... 

The ortfe-directed lithiation protocol allows selective functionalization of 3-arylsydnones in the aryl ring, the C4 position of the sydnone ring, or both simultaneously. The method allows access to fused ring structures (Sections 5.03.5.2.4 and 5.03.7.1.1). Reliable routes to otherwise unknown oxadiazolidines have been established (Section 5.03.9.4). Regiospecificity can now be achieved by judicious choice of substituents when propargylic esters react with sydnones to form pyrazoles (Section 5.03.5.2.6). 

The sydnone ring has also been used as an ortho-director of lithiation. Thus, on reaction with Bu Li-TMEDA, 3-phenylsydnone has been found to form a dilithio species which can be regiospecifically acylated at the ort/io-position by a weak electrophile. 

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