Incorporation of an allene unit into alpha-pinene via beta-elimination
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The two double-bond isomers 3-iodo-2,6,6-trimethylbicyclo[3.1.1]hept-2-ene (6b) and 3-iodo-4,6,6-trimethylbicyclo[3.1.1]hept-2-ene (11) were synthesized by reacting 2,6,6-trimethylbicyclo[3.1.1]heptan-3-one (9) with hydrazine, followed by treatment with 12 in the presence of Et3N. Treatment of 11 with t-BuOK as base in diglyme at 220 degrees resulted in the formation of 9 and 6,6-dimethyl-4-methylidene-bicyclo[3.1.1]hept-2-ene (12). For the formation of 9, the cyclic allene 7 is proposed as an intermediate. Treatment of the second isomer, 6b, with t-BuOK at 170 degrees gave rise to the diene 12 and the dimerization product 17. The underlying mechanism of this transformation is discussed. On the basis of density-functional-theory (DFT) calculations on the allene 7 and the alkyne 15, the formation of the latter as the intermediate was excluded.