The methylation of sodium 1,2,3,4-tetramethylcyclopentadienyl in liquid ammonia gives a mixture that analyzes 88% of 1,2,4,5,5-pentamethylcyclopentadiene and 12% of 1,2,3,4,5-pentamethylcyclopentadiene. The methylation of the latter leads to the formation of hexamethylcyclopentadiene in good yield.
We were the first to discover the 1,3-migration of a methyl group in the cyclopentadiene ring. At 425–500°, 1,2,4,5,5-pentamethylcyclopentadiene is smoothly isomerized to 1,2,3,4,5-pentamethylcyclopentadiene. The coupling of two consecutive intramolecular acts of 1,2-migration of the methyl group is considered to be the mechanism of such isomerization. The pyrolysis of the readily available 1,2,4,5,5-pentamethylcyclopentadiene is a convenient preparative method for the synthesis of 1,2,3,4,5-pentamethylcyclopentadiene.
The obtained dienes were characterized by the adducts with maleic anhydride and their derivatives. A study was made of the stereochemistry of the condensation products of 1,2,3,4,5-pentamethylcyclopentadiene with maleic anhydride. The 7-syn isomer (with respect to the double bond) predominates in the mixture of epimers formed at the C7 center of the bicyclo[2.2.1]heptene system.
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See  for Communication 28 in the given series.
Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 609–621, March, 1968.
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Mironov, V.A., Pashegorova, V.S., Fadeeva, T.M. et al. Cyclic unsaturated compounds. Russ Chem Bull 17, 588–597 (1968). https://doi.org/10.1007/BF00911617
- Double Bond