Thermal Intramolecular Alder-Ene Cycloisomerization of 1,6-Allenynes

Abstract

Alder-ene cycloisomerizations of 1,6-allenynes occur without catalysts under refluxing conditions. Depending on the substrate, THF, toluene, or xylenes can be used as solvents to provide the corresponding 6-methylene-1-vinylcyclohexene in up to 98% yield.

Key words - allenes - alkynes - cyclizations - ene reactions - trienes

References and Notes

• For gold-, platinum-, and gallium-catalyzed cycloisomerization of 1,6-allenynes, see:
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• 4 We have also observed a few thermal Alder-ene cycloisomerizations of hydroxylated 1,5-allenynes silylated at the triple bond, see: Zriba R. Gandon V. Aubert C. Fensterbank L. Malacria M. Chem. Eur. J.  2008,  14:  1482 
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3

The configuration of 5b, as well as 5c,f,g,i (see Table [1] ), was ascertained by NOE experiment between the vinyl proton at R⁴ and the R³ substituent.

6

The vinyl carbanionic fragment in compounds of type C can be cis or trans. The isomer depicted in Scheme [3] was found to be formed preferably in all cases by ca 2 kcal/mol of enthalpy of activation.