The one-pot reaction of imines, acid chlorides, and phosphonites has been found to generate a new class of phosphorus-based 1,3-dipolar cycloaddition reagent. These substrates, which are isomeric forms of classic Wittig reagents, undergo cycloaddition reactions in an analogous fashion to 1,3-oxazolium-5-oxides (Munchnones). However, these 1,3-dipoles can be generated in one pot, in a modular fashion, and directly from available reagents, suggesting their potential general utility in heterocycle synthesis. The latter is illustrated in the design of a phosphonite-mediated synthesis of pyrroles from a variety of imines, acid chlorides, and alkynes.