Butenolide Formation and Related Reactions

Using a y-hydroxyalkynoate combines two effects - the unusual contra-electronic regioselectivity leading to a-attack and a cyclization evolving from the juxtaposition of the hydroxyl and ester functionalities. Thus, as shown in Equation 1.41, the initial adduct 35 between hydroxyalkynoate 33 and alkene 34 spontaneously lactonizes upon its formation to form butenolide 36. The presence of a second monosubstituted double bond proceeds equally facilely in like fashion such that the bis-annulated product 37 is directly formed [38]. Chemoselective addition of the less sterically 

Replacing the hydroxyl group with an amido group provides access to the corresponding nitrogen heterocydes, the 3-pyrrolinones [40]. The example in Equation 1.42 highlights the chemoselectivity of the process since normally the diene moiety of substrate 39 is considered the more reactive functionality. 

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