Novel diterpene rearrangements: Dichotomous behavior of abietic type resin acids in strong acid medium


Kapoor, S.K.

Tetrahedron Letters: 85-2388

1973


We have been interested) in the study of unusual terpene rearrangements of synthetic and biogenetic interest via observable carbocations in strong acid medium. In an earlier communicationib a facile and general rearrangement of cation (I) derived from abietic-type diterpene resin acids to the ring contracted abeoabietate cation (II) in 96% sulphuric acid was described.

Tetrahedron
Letters
No.
26,
pp
2385
-
2388,
1973.
Pergamon
Press.
Printed
in
Great
Britain.
NOVEL
DITERPENE
REARRANGEMENTS:
DICHOTOMOUS
BEHAVIOUR
OF
ABIETIC-TYPE
RESIN
ACIDS
IN
STRONG
ACID
MEDIUM
Goverdhan
Mehta
and
Surinder
K.
Kapoor
Department
of
Chemistry,
Indian
Institute
of
Technology,
Kanpur
-208016,
India
(Received
in
UK
26
April
1973;
accepted
for
publication
18
May
1973)
We
have
been
interested
)
in
the
study
of
unusual
terpene
rearrangements
of
synthetic
and
biogenetic
interest
via
observable
carbocations
in
strong
acid
medium.
In
an
earlier
communication
ib
a
facile
and
general
rearrangement
of
cation(I)
derived
from
abietic-type
diterpene
resin
acids
to
the
ring
contracted
abeoabietate
cation
(II)
in
96%
sulphuric
acid
was
described.
It
was
observed
therein
that
the
fate
of
H
0
O
,C
-
®
H
2
(i)
96°!.
H2SO4
R
T
ion
(I)
in
chlorosulphonic
acid
or
99%
sulphuric
acid
is
dramatically
altered.
We
wish
to
describe
here
a
novel
rearrangement
of
levopimaric
acid(III),
2
which
is
initiated
by
the
dehydration
and
decarbonylation
of
the
C
4
-carboxylic
acid
function
3
and
proceeds
through
the
formation
of
intermediate
dication
(IV).
H
4P
C—
OH
H
(Iv)
Rapid
dispersal
of
a
10%
CH
2
C1
2
solution
of
LPA
in
C1S0
3
H
at
-25
°
gave
a
burgundy
solution,
whose
NMR
spectrum
exhibited
signals
compatible
lb
with
structure
(I).
When
the
solution
of
(I)
was
allowed
to
warm
up
to
g/t
profuse
evolution
of
CO
ensued
and
new
signals
emerged
in
the
NMR
spectrum
of
the
ion.
These
signals
2385
2386
No.
26
consisted
of
an
isopropyl
doublet at
1.25 (J=6.5
Hz),
a
tertiary
methyl
singlet
at
1.28,
two
doublets
at 0.91
and
1.31
(J=6
Hz)
due
to
two
secondary
methyl
resonances
and
an
olefinic
proton singlet at
6.88 (cf. 7.61 in
ion
(I))
.
These
NMR
parameters
along with
the
high field shift
of
the olefinic proton
resonance
indicating
the
further dispersal
of
the
positive charge, suggested the
presence
of
rearranged
ions
(V)
&
(VI)
in
CIS°
3
H
solution.
On
quenching
the cations
(V)
&
(VI)
in iced
Na
2
CO
3,
H
MeO
a
mixture
of
C
19
-trienes
(VII
&
VIII,
UV:
A
300
nm)
was
recovered in
88%
yield.
MaX
R2
YR
1~/
R2
Ri
Ri=CH3
,
R2=11
(V)
Ri
=CH3
R2
=
H
(VIII)
R1
=
1-1
,
R
2
=
CH
3
(VI)
R1
=
H
,
R2
T.
CH
3
The
triene mixture
was
extremely
labile
&
complex
and
furnished disproportionation
products
(XI),
(XII)
and
styrene
(IX)
on
attempted
purification.
However,
the
structure
4
of
the
trienes
and
cations
(V)
&
(VI)
can
be
deduced
from
the
dehydro-
genation
and
aromatisation products
of
triene mixture.
The
results are depicted in
chart
1
and
the
physico-chemical characterisation
of
the
compounds
(IX)-,
(XIII) is
summarised
below:
Compound
(IX)
:
C19
H
26'
`max
Me0H
256
nm
(k
=9430)
;
v
max
(
neat)
:
1630,
885,
830
cm
1
.
NMR:S
1.23
(6H,
d,
J=7
Hz,
CH
-6-H), 0.87
(3H,
s,
CH
-
6
-
), 0.93
(3H,
d,
J=6.5
Hz,
3
6H
3
3
H--CH
3
), 2.7-3.1
(3H,
m,
benzyTiC),
6.17
(1H,
t,
J=4
Hz,
Ar-
-H),
7.0541H,
d,
:
J=9
Hz,
Ar),
6.95
(1H,
s,
Ar),
7.51
(1H,
d,
J=9
Hz,
Ar)
Compound
(X):
C
19
H
24
,
Xmax
Me0H
318
nm
(€=7200)
and
306
nm
(E=6900);
v
max
(neat):
1660,
1635,
1610,
830,
795
cm
-1
.
NMR:S1.21
(6H,
d,
J=7Hz,
cH
3
-y-H),
1.19
(3H,
s,
CH
3
-C-),
CH
3
1.71
(3H,
s,
CH
3
-6=6-),
2.5-3.1
(3H,
m,
benzylic),
6.33
(2H,
ABq,
J=9.5
Hz,
-6=C-C=6-)
A
A
6.83
(1H,
s,
Ar),
6.9
(11-1,
d,
J=9
Hz,
Ar),
7.13
(1H,
d,
J=9
Hz,
Ar)
Compound
(XI):
C
19
H
28
,
?max
Me0H
280
nm
(E=1900)
and
272
nm
(E=1900);
v
izi
(neat): ]612,
890,
830,
640
cm
1
.
NMR:S
1.21
(6H,
d,
J=7
Hz,
CH
3
-c-F)
,
1.08
(3H,
s,
CH
3
- ? -
), 0.93
611
3
(3H,
d,
J=6
Hz,
CH
3
+H), 2.6-3.1
(4H,
m,
benzylic),
6.93
(1H,
s,
Ar),
7.01
(1H,
d,
J=8
Hz,
Ar),
7.28
(1H,
d,
J=8
Hz,
Ar).
No.
26
2387
Compound
(XII):
C
19
H
28
,
x
hi
maxe0H
280
nm
(f=1400)
and
272
nm
(f=1260);
v
max
(neat):
1615,
895,
830
cm
-1
.
NMR:g
1.22
(6H,
d,
J=7
Hz,
CH
3
1-),
0.93
(3H,
s,
CH
-C-)
'
_lb
0.7
8
(3H,
d,
J=7
Hz,
0
3
-?-H),
2.4-3.0
(4H,
m,
benzylic),
6.80
(1H,
s,
Ar),
6.88
(1H,
d,
J=8
Hz,
Ar),
7.06
(1H,
d,
J=8
Hz,
Ar)
.
CHART
1
Pd/C
9
4)H
A
Disprop
3-
ortionation
(VIDFL
(VIII)
AgNO
Si
0
...
Column
N
B
S
CCI4,11
Traces
(IX)
N
2
/PtO
2
Et0Ac
(XI)
H
2
/1
3
t0
2
Et
OAc
12%
(XIII)
Traces
+(XI)+(XII)
(X)
112/Pt02
Et0Ac
(m)
+
(IX)
20%
+
(X)
+
(XI)
+
(XII)
24%
7%
7%
Compound
(XIII):
C19
H26'
XV
le0H
266
nm
(6=7770);
vmax
(neat):
1662,
1610,
830,
730,
max
700
cm
.
NMR:j
1.2
(6H,
d,
J=7
Hz,
CH
3
-
-H),
0.95
(3H,
S.
CH
3
-6-),
0.96
(3H,
d,
J=6
Hz,
CH
3
+H),
2.6-3.1
(3H,
m,
benzyliar?
centre
6.21
(2H,
ABg,
J=8
Hz,
-c=c-),
H
H
centre
6.98
(3H,
m,
Ar).
The
mechanism
of
formation
of
carbocations
(V)
&
(VI)
and
derived
products
(VII)
-
>
(XIII)
is
straight
forward
and
outlined
in
chart
2.
The
novel
rearrangement
2388
No.
26
CHART
2
+
1
(vi)
-
H
2
0
(1)
(IV)
-
CO
(V)
of
abietic
type
acids
involving
C
10
-methyl
migration
and
their
dual
behaviour
in
strong
acids
provides
a
new
variant
to
the
synthetic
utility
of
stable
carbocations
in
highly
ionising
medium.
We
intend
to
make
use
of
this
methyl
migration
sequence
to
synthesize
rearranged
terpenes
employing
model
systems.
Acknowledgement:
The
authors
wish
to
thank
Dr.
Nityanand,
CDRI,
Lucknow
and
Dr.
P.T.
Manoharan,
IIT-Madras
for
the
NMR
spectra
of
our
compounds.
One
of
the
authors
(5.K.K)
also
wish
to
thank
CSIR,
New
Delhi,
for
the
award
of
fellowship.
REFERENCES
1.
(a)
D.G.
Farnum
and
G.
Mehta,
Chem.
Comm.,
1643
f1968);
(b)
G.
Mehta
and
S.K.
Kapoor,
Tetrahedron
Letters,
4947
(1972);
(c)
G.
Mehta
and
B.P.
Singh,
Unpublished
results.
2.
Most
of
the
present
work
was
done
employing
(III)
but
it
has
been
shown
that
other
resin
acids
of
abietic
family
also
give
the
same
ionic
species.
3.
For
examples
of
decarbonylation
of
aliphatic
carboxylic
acids
via
acylium
ions,
see,
G.A.
Olah
and
A.M.
White,
J.
Amer.
Chem.
Soc.,
B9,
3591
(1967).
4.
Similar
rearrangement
of
abietic
acid
in
FSO,H
has
been
independently
investi-
gated
by
Dr.
D.G.
Farnum,
Michigan
State
Uni0ersity,
U.S.A.
The
gross
carbo-
cyclic
structure
of
the
compounds
(v)—
(XIII)
has
been
further
confirmed
by
comparison
with
some
of
his
synthetic
compounds
and
our
own
degradative
work.
We
wish
to
thank
Dr.
Farnum
for
sending
us
a
pre-print
5
of
his
paper
prior
to
publication.
5,
D.G.
Farnum
and
R.A.
Mader,
J.
Amer.
Chem.
Soc.,
95,
000
(1973).