Barton-Zard pyrrole annulation

Pyrrolyl)-4,5-dihydroisoxazole derivatives 402 have been synthesized (Scheme 1.48) in good yields (66%-78%) by regioselective 1,3-dipolar cycloaddition of nitrile oxides to 1-phenylsulfony 1-1,3-dienes, followed by Barton-Zard pyrrole annulation using ethyl isocyanoacetate anion (444). 

The copper and nickel nitroporphyrins 6 (M = Cu, Ni) undergo pyrrole ring annulation with isocyanoacetates according to the Barton-Zard condensation originally designed for the preparation of simple monopyrroles112 (cf. Houben-Weyl, Vol.E6a, p650). 

The Barton Zard condensation is one more important and marvelous SnH heterocyclization leading to pyrrole ring annulation to nitroalkenes, nitroarenes or nitrohetarenes on being treated with alkyl isocyanoacetates in the presence of a base (85CC1098, 90T7587). The reaction starts with nucleophilic attack of alkyl isocyanoacetate carbanion 160 ortho to the NCT group of substrate 161. The... 

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