Organic Reactions Volume 4 - Sciencemadness Dot Org

24 ORGANIC REACTIONS
what elevated temperatures. The crystalline products obtained under
these "forced" conditions are regarded as terpenylsuccinic anhydrides.
If maleic acid is employed instead of the anhydride, a-pinene, limonene,
terpinolene, and carene are all first isomerized to a-terpinene, which
then yields the normal adduct (XXXVIII).
An interesting feature in the fulvene series is the presence of a third
double bond which does not take part in the Diels-Alder reaction, but
which vitally influences the steric course of the addition process. Addition
of maleic anhydride to 6,6-dimethylfulvene (XL) no longer takes
place selectively; endo and exo adducts (XLI and XLII respectively)
are produced concurrently in nearly equal quantities. 127 ' 151 6,6-Pentamethylenefulvene
(XLIII) with maleic anhydride at room temperature
yields first the endo adduct and, upon longer standing, the exo
isomer. Elevated reaction temperatures favor formation of the exo adduct.
103 The presence of the third double bond in 6,6-dimethylfulvene
and 6,6-pentamethylenefulvene has the effect of neutralizing the accumulation
of double bonds which normally results in exclusive production of
the endo configuration. With a fulvene carrying an aromatic or unsaturated
substituent the effect is even more striking. In 6,6-diphenylfulvene
(XLIV) the accumulation of double bonds actually favors the
exo configuration (XLV) and no endo isomer is produced. 127,151
/
^CH3
CC
^CH3
XL
H2 CH2
CH2-CH2
XLIII
Fulvene adducts generally dissociate with remarkable facility. The
endo adduct of maleic anhydride and 6,6-pentamethylenefulvene dissociates
readily, whereas the exo isomer is stable. 103,199
199 Kohlor and Kable, J. Am. Chcm,. $oc, 57, 017 (1935).