[696-68-4] · C8H11N · 6-Dimethylaminofulvene · (MW 121.18)
(synthesis of azulenes2)
Physical Data: mp 67-68 °C.
Form Supplied in: yellow crystals; commercially available.
Preparative Methods: may be conveniently prepared by the reaction of cyclopentadienylsodium with N,N-Dimethylformamide-Dimethyl Sulfate.3,4 Alternatively, cyclopentadiene has been condensed with N,N-Dimethylformamide Diethyl Acetal.5,6
Purification: recrystallization from petroleum ether or cyclohexane.
Handling, Storage, and Precautions: should be protected from light during storage.
6-Dimethylaminofulvene undergoes [6 + 4] cycloadditions with thiophene 1,1-dioxides (1) to yield substituted azulenes (3). The reaction is believed to proceed via the indicated intermediate (2), although it has not been directly observed. Subsequent extrusion of sulfur dioxide and elimination of dimethylamine yields the azulene product (eq 1). 3,4-Dichlorothiophene 1,1-dioxide is significantly more reactive than the unsubstituted heterocycle.7,8 Azulenes have also been obtained in low yield from related reactions of 6-dimethylaminofulvene with 5-alkoxycarbonyl-2-pyrones.9
6-Dimethylaminofulvene has been used in short synthetic routes to the hydroazulene sesquiterpene degradation products guaiazulene and chamazulene. The [6 + 4] cycloaddition reaction of 6-dimethylaminofulvene with 2-ethyl-5-methylthiophene 1,1-dioxide (4) produced a mixture of regioisomers favoring the desired product (eq 2). The major isomer (5) was converted into chamazulene in two additional steps.2
Charles G. Caldwell
Merck Research Laboratories, Rahway, NJ, USA