Methyl propiolate munchnone cycloaddition reactions

TABLE 10.1. 1,3-DIPOLAR CYCLOADDITION REACTIONS OF IN SITU GENERATED MUNCHNONES AND METHYL PROPIOLATE AND METHYL 3-PHENYLPROPIOLATE"... 

The l,3- dipolar cycloaddition of alkynes to anhydro-5-hydroxyoxazolium hydroxides leads to pyrroles (equation 66). Electron-withdrawing substituents in the alkyne are activating and DMAD reacts 48 times faster with compound (235) than does methyl propiolate, which in turn is 7000 times more reactive than oct-l-yne. The reaction has many ramifications and it is frequently used to trap unisolable munchnones with or without substituents on the nitrogen atom by an in situ reaction in which the betaine is generated in the presence of the dipolarophile (equation 67). It has become an important method for constructi ng the pyrrole ring of fused heterocyclic systems, as in the example of equation (68). 

Similarly, Toupet and co-workers determined the structures of the pyrrole adducts 150 from the cycloaddition of munchnones 149 with methyl phenylpro-pionate by X-ray crystallography (Fig. 4.53). These highly regioselective cycload ditions are in accord with FMO predictions. A detailed study by this same team involved cycloaddition reactions of mimchnones with methyl propiolate and methyl... 

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