591234-54-7Relevant articles and documents
Sense of sequential 1,5-sigmatropic rearrangements of dimethyl-3,3-dialkyl-3H-pyrazole-4,5-dicarboxylates. Crystal and molecular structures of two dimethyl-4,5-dialkyl-1H-pyrazole-1,3-dicarboxylates
Frampton, Christopher S.,Majchrzak, Michael W.,Warkentin, John
, p. 373 - 378 (2007/10/02)
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 CH3O2CCCCO2CH3, 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 P21/n, with a=7.907(1) Angstroem, b=11.087(2) Angstroem, c=13.199(3) Angstroem, V=1124.9(4) Angstroem3, Dc=1.34 g cm-3, Dm=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?(I).The structure of 6a is similar. Key words: 3,3-dialkyl-3H-pyrazoles, 1,5-sigmatropic rearrangements of; 4,5-dialkyl-1H-pyrazoles, crystal and molecular structures; 1,5-sigmatropic rearrangements of pyrazoles, sense of.
