SENSE OF SEQUENTIAL 1,5-SIGMATROPIC REARRANGEMENTS OF DIMETHYL-3,3-DIALKYL-3H-PYRAZOLE-4,5-DICARBOXYLATES - CRYSTAL AND MOLECULAR-STRUCTURES OF 2 DIMETHYL-4,5-DIALKYL-1H-PYRAZOLE-1,3-DICARBOXYLATES

3,3-Dialkyl-3H-pyrazole-4,5-dicarboxylic acid dimethyl esters (4), obtained by cycloaddition of R1R2C = N+ = N- (R1 = R2 = CH3; R1 = CH3, R2 = CH2CH3) to CH3O2CC = CCO2CH3, rearrange thermally by 1,5-sigmatropic alkyl shifts to both N and C. The latter rearrangement is followed by two successive 1,5-sigmatropic shifts of a methoxycarbonyl group. Final products of the threefold rearrangement were shown to be 4,5-dialkyl-1H-pyrazole-1,3-dicarboxylic acid dimethyl esters (6), rather than the isomeric 3,4-dialkyl-1H-pyrazole-1,5-dicarboxylic acid dimethyl esters (7), by means of single crystal X-ray diffraction. Those products therefore result from alkyl migration to C-4 of 4, followed by sequential migration of the methoxycarbonyl group, initially at C-4, to C-3 and then to N-2 of 4. In the initial alkyl migration step, ethyl migrates in preference to methyl, and in subsequent migration steps the methoxycarbonyl group migrates faster than the ethyl or methyl group. Crystals of 4-ethyl-5-methyl-1H-pyrazole-1,3-dicarboxylic acid dimethyl ester (6b) are monoclinic, of space group P2(1)/n, with a = 7.907(1) angstrom, b = 11.087(2) angstrom, c = 13.199(3) angstrom, V = 1124.9(4) angstrom 3, D(c) = 1.34 g cm-3, D(m) = 1.33 g cm-3 for Z = 4, and R1 = 0.0772 (R2 = 0.0626) for 1474 reflections (R1 = 0.0428, R2 = 0.0422 for 903 reflections with I > 3-sigma(I). The structure of 6a is similar.