Reversible 1,3-dipolar cycloaddition of dimethyl 2-thiono-1,3-dithiole-4,5-dicarboxylate with dimethyl acetylenedicarboxylate

It was shown that dimethyl 2-thiono-1,3-dithiole-4,5-dicarboxylate (2) and dimethyl acetylenedicarboxylate (DMAD) undergo a 1,3-dipolar cycloaddition to produce a short-lived ylide intermediate (3). The 1,3-dipolar cycloaddit ion took place even at room temperature, although sluggishly, but took plac e much more rapidly under application of a high pressure of 500 MPa. The 1, 3-dipolar cycloaddition is reversible and the ylide 3 immediately splits in to 2 and DMAD. When the reaction of 2 with DMAD was carried out at room tem perature without solvent, a spiro-1,3-dithiole (11) was performed in 11% yi eld, whereas the reaction at 150 degrees C provided a thiophene derivative (13) in 41% yield. It was found that 11 undergoes a thermal rearrangement t o 13. Results of attempted chemical trapping of the ylide 3 are also report ed. (C) 2000 John Wiley & Sons, Inc.