The esterification of gibberellins with dimethylformamide dimethyl acetal (DMFDMA)

Article abstract of DOI:10.1515/znb-1997-1015

The key step is to prepare rare gibberellins by converting the more readily available gibberellins (GA₃, GA₄, GA₇ and GA₁₃) into their methyl esters. The DMFDMA is employed as an esterifying reagent to avoid the diazomethane due to its toxic, explosive and hazardous nature. It is observed that DMFDMA apart from esterification can also be used as acetylating and epimerizing reagent.

Full text of DOI:10.1515/znb-1997-1015

The Estenfication of Gibberellins with Dimethylformamide Dimethyl Acetal
(DMFDMA)
M uhammad Shaiq Ali*-a, James R. H ansonb, Viqar Uddin A hm ada
a H. E. J. Research Institute of Chemistry, University of Karachi, Karachi-75270, Pakistan
b The School of M olecular Sciences, University of Sussex, Brighton, Sussex BN1 9QJ, U.K.
Z. Naturforsch. 52b, 1237-1244 (1997); received July 7, 1997
Gibberellins, Esterification, Dim ethylform am ide Dim ethyl A cetal
The key step is to prepare rare gibberellins by converting the more readily available gib-
berellins (G A 3, G A 4, G A 7 and G A 13) into their methyl esters. The D M F D M A is em ployed
as an esterifying reagent to avoid the diazom ethane due to its toxic, explosive and hazardous
nature. It is observed that D M FD M A apart from esterification can also be used as acetylating
and epimerizing reagent.
Introduction
quire the conversion of the hydroxyl groups at C-
3 and C-13 to chloro and other derivatives by rea-
gents which would also react with the carboxyl
OH. The methyl esters are commonly and easily
prepared with diazomethane. This reagent suffers
from the disadvantages of toxicity and it is explo-
sive presenting a hazard when the reaction is car-
ried out on a large scale. O ther methods of esteri-
fication for example the Fischer-Speier m ethod
using acid involve conditions that would lead to
the rearrangem ent of the gibberellins. Thus ring A
of gibberellic acid (1) undergoes aromatization to
form allogibberic acid (5) in the presence of acid
at room tem perature whilst under slightly more
vigorous conditions the C/D ring system un-
The gibberellins are a a widespread group of
plant horm ones [1], They were first recognized as
m etabolites of the fungus, Gibberella fujikuroi.
Later on they were also found to present in higher
plants [1-4]. The main interest in the gibberellins
is due to their role as natural plant growth regulat-
ing horm ones e.g. gibberellic acid (1) [5]. Nearly
80 gibberellin plant hormones are known and
num bered gibberellin A! •••An [6]. They are char-
acterized by a common carbon skeleton but differ
from one another in the position and the number
of their functional groups. They can be divided
into two families: A C 19 series examplified by gib-
berellic acid (1) and a C2o series examplified by
gibberellin A ]3 (2). They were originally discov-
ered as the phytotoxic metabolites of a rice patho-
gen, G. fujikuroi [7-8].
dergoes
a W agner-Meerwein rearrangem ent to
form the 8:13-isogibberellins, e.g. gibberic acid (6).
Utility o f DMFDMA
Hence, a mild, non-toxic reagent was required.
Dimethyl formamide dimethyl acetal (7) has been
recom m ended as a mild esterifying reagent. In
1965 Eschenm oser et al. [9] described the reaction
of formamide acetals with carboxylic acids. It was
found to be a facile m ethod for the preparation
of ester. The key step in the proposed mechanism
involves the cyclic displacement of dimethylfor-
mamide as shown in Scheme 1. It is the object of
this work to establish suitable conditions for the
esterification of the gibberellins and to see the be-
haviour of this reagent (D M FD M A ) on gibberel-
lins at low (room tem perature) and at high tem -
peratures (reflux).
Esterification of gibberellins
The conversion of the more readily available
gibberellins, gibberellic acid (1), gibberellin A 4
and A 7 (3 and 4) and gibberellin A 13 (2) to the
rare gibberellins is often accomplished via their
m ethyl esters. Indeed much of the Chemistry of
the gibberellin has been studied with their methyl
esters. The esters are easier to crystallize and chro-
m atograph. Furtherm ore, many of the reactions
involved in the conversion to other gibberellins re-
*
Reprint requests to Dr. M. S. Ali.
0932-0776/97/1000-1237 $06.00 © 1997 Verlag der Zeitschrift für Naturforschung. All rights reserved.
N
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M. S. Ali et al. ■The Esterification of Gibberellins
1238
H3,C\
/
O— CH33
N— CH
/
o — c h 3
h3c
7
der reflux for 1 h. The product 8 obtained, was
confirmed by its proton NMR and infra-red
spectroscopy. The 'H NMR spectrum showed an
additional methyl singlet at d 1.96 due to the
presence of acetate group at C-3 whereas the
normal Me-18 resonated at d 1.05 as a singlet.
The spectrum also exhibited a pair of broad sin-
glets at d 4.82 and 4.99 related to the olefinic
protons. The infra-red spectrum displayed four
carbonyl absorptions at 1801, 1763, 1750 and
1725 cm -1. The former two absorptions belong
to the anhydride function and the latter two ab-
sorptions correspond to the OAc and carboxyl
Results and Discussion
In order to establish the utility of DM FDM A
for compounds in the gibberellin series, acetate-
anhydride 8, keto-acid 9, G A 13 (2) and G A 3 (1)
were treated under reflux and at room tem per-
atures with this reagent. The products, thus ob-
tained, were identified spectroscopically.
Preparation o f acetate-anhydride 8 and treatment
with DMFDMA
The acetate-anhydride
treating the G A 13 (2) with acetic anhydride un-
8
was prepared by
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M. S. Ali et al. ■The Esterification of Gibberellins
1239
functions. Thus the above mentioned spectral
data confirmed the structure 8.
was isolated, the 'H NM R spectrum of which ex-
actly m atched with that of 10 previously obtained
from the same reaction when it was perform ed un-
der reflux. The teatm ent of DM FDM A with ace-
tate-anhydride and the products formed during
this reaction have been illustrated in Scheme 2.
CH
O—CH,
f
\ 1A
(CH3)2N CH (OCHj;
N — CH
H,C
i DMFDMA)
0
1
Preparation o f 19-nor, 3-oxo derivative o f GA I3 9
R—C —O
and treatment with DMFDMA
Oxidation of G A 13 (2) with 8 N chromium triox-
ide give an unstable /3-keto-acid 1 which was con-
firmed by the disappearance of the carbinylic pro-
ton at d 3.96 in the proton NM R spectrum. The ß-
keto-acid was easily decarboxylated on heating to
afford e/ir-3-oxo-19-nor-gibberell-16-en-7,20-dioic
acid 9 which was easily recognized by the appear-
ance of methyl doublet in ’H NM R spectrum.
CH,
\
O -jC H ,
/ ^ J
-
N — CH
(
H5C^
When 19-nor-keto-acid
9
was treated with
DM FDM A under reflux and at room tem perature,
the same product was isolated from both the reac-
tions which was assigned to the em-3-oxo-19-nor-
gibberell-16-en-7,20-dioic acid-7,20-dimethyl ester
13. The infra-red spectrum did not show the hy-
droxyl absorption while on the other hand the pro-
ton NM R spectrum exhibited two methoxyl sig-
nals at d 3.68 and 3.77 and confirmed the structure
13 (see Scheme 3).
CH,
OMc
o
N — CH
R - C = 0
Schcmc
W hen 8 was treated with DM FDM A under re-
flux, two compounds 10 and 11 were formed which
were separated by column chromatography. The
com pound 10 which was eluted first found to be
the 3-acetoxy derivative of G A 13 trimethyl ester
on the basis of its proton NMR spectroscopy. The
spectrum showed five methyl signals at d 1.11,
2.09, 3.60, 3.67 and 3.72 as singlets related to the
Me-18, OAc and three OMe, respectively. The two
doublets at d 2.52 and 3.88 with coupling constant
Action o f DMFDMA on gibberellin A I3 (2)
The fungal metabolite gibberellin A 13 (2) was
treated with DM FDM A at room tem perature and
the product, thus obtained, was assigned as ent-3/3-
hydroxy-gibberell-16-en-7,19,20-trioic acid 7,19-di-
12.6 Hz were assigned to H-5 and H-6, methyl ester, 20-^3 lactone 14 which had pre-
respectively.
viously been obtained by the reduction of the 3-
keto-ester 12 with sodium borohydride [10]. The
infra-red spectrum showed absence of hydroxyl
absorption while on the other hand NMR
spectrum showed the presence of two methoxyl
groups in the molecule which appeared at d 3.63
and 3.69. The absence of hydroxyl group in the
molecule clearly explains the formation of lactone
ring across 20-^3. A plausible explanation may be
that the rate of methylation of the carboxylic acid
groups at positions C-7 and C-19 was faster than
of the group at C-20 and also faster than the rate
of formylation of the 3-hydroxyl group; hence the
7,19-dimethyl ester was formed first. Once the 3-
hydroxyl group has been formylated, the carboxyl-
The second product 11 eluted from the same
column afforded trimethyl ester of G A 13 which
showed three methoxyl signals at <3 3.59, 3.66 and
3.71 in the proton NM R spectrum. The signal at d
3.96 (t, 7 = 6.0, 3.0 Hz) was assigned to H-3. The
same H-3 signal in 10 was observed at d 5.21. The
downfield shift of H-3 signal in 10 is due to the
presence of acetyl group. The structures of the
compounds 10 and 11 were further confirmed by
treating G A 13 (2) with etherreal diazomethane to
afford a com pound identical to 11 which on ace-
tylation yielded 10
W hen acetate-anhydride was treated with
DM FDM A at room tem perature only one product
.
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1240
M. S. Ali et al. ■The Esterification of Gibberellins
diazomethane
Scheme
2
2 (GA3)
atom due to the formyl group was confirmed by
microanalysis which showed 21 carbons in the
molecule. The proton NMR spectrum also dis-
played a signal at d 3.72 as a singlet assigned for
methoxyl group. The exomethylene protons reso-
ate anion at C-20 would then act as an internal
nucleophile to displace the formimido group at C-
3 and hence, form the ent- 20—^3/3 lactone. The
epimerization at C-3 can be explained by
Scheme 4. The same compound 14 was form ed
when the G A L3 was treated with DM FDM A un- nated at
d 4.94 (s) and 5.21 (br, s) while the other
protons associated with endocyclic double bond
showed their resonances at d 5.87 (dd, J - 9.3, 3.6
Hz, H-2) and 6.36 (dd. /H = 9.3, 0.9 Hz, H -l).
der reflux.
Action of DMFDMA on gibberellic acid (1)
3-Formyl, 7-methyl ester 15 was obtained when
gibberellic acid 1 was treated with DM FDM A at
room tem perature. The same product was also ob-
served when this reaction was perform ed under
reflux. The 'H NMR spectrum of 15 exhibited a
downfield singlet at d 7.89 which was assigned to
formyl proton. The presence of an extra carbon
Experimental
Preparation o f acetate-anhydride 8 and treatment
with DMFDMA
The gibberellin A 13 (2) (1.57 g) isolated from
the fungus Gibberella fujikuroi was refluxed with
acetic anhydride (10 ml) for 1 h. Then diluted with
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M. S. Ali et al. ■The Esterification of Gibberellins
1241
water, and the product was recovered in ethyl ace- (1 H, s, H-17), 4.89 (1 H, s, H-17') and 5.21 (1 H, t,
tate. The ethyl acetate layer was washed with
water and dried over sodium sulphate. The solvent
7 = 4.9, 2.5 Hz, H-3).
Further elution with 40% ethyl acetate in light
was removed by evaporation under reduced pres- petroleum gave G A 13 trimethyl ester 11 (142 mg),
sure which afforded a gum. This was chrom ato- which was crystallized from chloroform as plates.
graphed on silica using ethyl acetate and light pe-
troleum as a mobile phase. Elution with 55% ethyl
acetate in light petroleum gave acetate-anhydride
of G A 13 (8) (1.11 g) as a white powder.
M.p.. 153-154 °C; IR (CHC13) vmax: 3400, 1730
(br.), 1660, 880 cm "1; NM R (CDC13, 360
MHz): (3 1.23 (3H , s, H-18), 2.58 (1H , d, 7=12.7
Hz, H-5), 3.59 (3H , s, OMe), 3.66 (3H , s, OM e),
3.71 (3H , s, OM e), 3.87 (1H , d, 7=12.7 Hz, H-6),
3.96 (1H , t, 7=6.0, 3.0 Hz, H-3), 4.80 (1 H, s, H-
17) and 4.88 (1H , br. s, H-17').
Com pound 8 was treated with DM FDM A at
room tem perature overnight. DM FDM A was re-
moved under reduced pressure without heating.
The product was purified by column chrom atogra-
phy. Elution with 25-30% ethyl acetate in light
petroleum gave the same product 10 which was
form ed when the same reaction was perform ed
under reflux.
IR (CHCh) vmax: 3246 (br.), 1801, 1763, 1750,
1725 and 1678 c m '1;
NM R (CDC13, 60 MHz):
d 1.05 (3H, s, H-18), 1.96 (3H , s, OAc), 4.82 (1H ,
br. s, H-17), 4.99 (1H , br. s, H -17') and 5.11 (1H ,
t, 7 = 4.7, 2.7 Hz, H-3).
The compound 8 (500 mg) was refluxed with
DM FDM A (10 ml) for 2 h. TTie D M FDM A was
removed by evaporation under reduced pressure
and the gum thus obtained, was chrom atographed
on silica. Elution with 25-30% ethyl acetate in
light petroleum gave 3-acetoxy derivative of G A 13
trimethyl ester 10 (330 mg) as a gum.
The ’H NM R spectra of G A 13 trimethyl ester
and its derivative 3-acetoxy exactly matched with
the products 11 and 10 which were formed during
the reaction between acetate anhydride 8 and
DM FDM A.
*H NM R (CDCU, 360 MHz): <3 1.11 (3H , s, H-
18), 2.09 (3 H, s, OAc), 2.52 (1H, d, 7 = 12.6 Hz, H-
5), 3.60 (3H, s, OM e), 3.67 (3H , s, OM e), 3.72
(3H, s, OMe), 3.88 (1H , d, 7=12.6 Hz, H-6), 4.81
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1242
M. S. Ali et al. ■The Esterification of Gibberellins
2
^ O H
f c o
13
..
.
1
1i
I
c o 2h
1
Preparation of 19-norJ-oxo derivative o f GA 13 (9)
and treatment with DMFDMA
(Jone's reagent) dropwise until the yellow colour
persisted for '/: h. Then m ethanol was added in or-
der to destroy the extra amount of Jone’s reagent.
The solvent was removed under reduced pressure.
G A |3 (2) (1.0 g) was dissolved in acetone (20
ml) and treated with 8 N chromium trioxide
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M. S. Ali et al. • The Esterification of Gibberellins
1243
The crude material was diluted with water and the
product was recovered in ethyl acetate. On evapo-
ration it gave a gum which was redissolved in mini-
mum amount of acetone and then, water (50 ml)
was added. The mixture was refluxed for 1 h and
the product was again recovered in ethyl acetate.
A fter concentration, it gave a solid which was
chromatographed on silica gel column. Elution
with 30% methanol in ethyl acetate afforded ent-
3-oxo-19-nor-gibberell-16-en-7,20-dioic acid 9 (800
mg) as a white powder. M.p.: 248 °C; IR (Nujol)
vmax: 3410, 1710, 1660 cm “1.
(1H , d, 7=12.6 Hz, H-6), 3.63 (3H , s, OM e), 3.69
(3H , s, OM e), 4.75 (1H , s, H-17), 4.81 (1H , br. s,
H-3) and 4.88 (1H , br. s, H-17').
Gibberellin A 13 (2) (1.0 g) treated with
DM FDM A (10 ml) under reflux for 2 h.
DM FDM A was removed under reduced pressure
gave a gum. The gum was loaded on silica gel
column, which yielded the same (3-lactone 14
(800 mg).
Action o f DMFDMA on gibberellic acid (1)
Gibberellic acid (1) (500 mg) was dissolved in
TH F (20 ml) and then treated with DM FDM A
(15 ml) under reflux for 2 h. The DM FDM A was
removed under reduced pressure to give a gum,
which was chromatographed on silica gel column.
Elution with 5% methanol in ethyl acetate gave 3-
formyl 7-methyl ester derivative of gibberellic acid
(15) (300 mg), which was crystallized from acetone
as needles.
The keto-acid
9 (400 mg) was treated with
DM FDM A (10 ml) under reflux for 2 h. The
DM FDM A was removed under reduced pressure
and the resulted gum was subjected to column
chromatography. Elution with 50% ethyl acetate
in light petroleum yielded ent-3-oxo-19-nor-gibbe-
rell-16-en-7,20-dioic acid 7,20-dimethyl ester 13
(190 mg), which was crystallized from diethyl
ether as needles.
M.p.: 102 °C; JH NM R (CDC13, 90 MHz): 6 0.96
(3H, d, 7 = 6.6 Hz, H-18), 3.68 (3H , s, OMe), 3.77
(3H, s, OMe) and 4.88 (2H, br. d, H-17). The keto-
acid 9 (400 mg) was treated with DM FDM A
(10 ml) at room tem perature overnight. The
DM FDM A was removed under reduced pressure
without heating. A fter chromatography it gave the
same diester 13.
M.p.: 168-170 °C; IR (Nujol) vmax: 3309, 1769,
1733 and 1661 cm “1.
C2iH 240 7 (388.4136)
Calcd
C 64.93 H 6.23% ,
Found C 64.54 H 6.77%.
JH N M R (CD 3OD, 360 MHz): (5 1.16 (3H , s, H-
18), 2.73 (1 H, d, 7= 10.6 Hz, H-6), 3.24 (1 H, d, 7 =
10.6 Hz, H-5), 3.72 (3H , s, OM e), 3.92 (1H , br. s,
H-3), 4.94 (1H , s, H-17), 5.21 (1H , br. s, H-17'),
5.87 (1H , dd, 7 = 9.3, 3.6 Hz, H-2), 6.36 (1H , dd,
J - 9.3, 0.9 Hz, H -l) and 7.89 (1H, s, formyl group).
W hen gibberellic acid (500 mg) was treated with
DM FDM A (15 ml) in TH F (20 ml) at room tem -
perature overnight, the same product 15 (205 mg)
was obtained.
Action of DMFDMA on gibberellin A 13 (2)
Gibberellin A 13 (2) (1.0 g) was treated with
DM FDM A (10 ml) at room temperature. The re-
action was checked by taking TLC at 15, 30 min
and then, each 1 h intervals. After 3 h a major spot
appeared. The DM FDM A was removed under re-
duced pressure without heating. The gum ob-
tained, was subjected to column chromatography.
Elution with 50% ethyl acetate in light petroleum
yielded e/?/-3/3-hydroxygibberell-16-en-7,19,20-tri-
oic acid 20^-3 lactone 7,19-dimethyl ester 14 (65
mg), which was crystallized from ethyl acetate as
needles.
Acknowledgements
We thank the British Council for financial sup-
port and ICI Pharmaceuticals for a gift of G A 3
and G A 13. We also thank Prof. A tta-ur-Rahm an
and Dr. D. R. M. Walton for establishing the link
programme between H EJ Research Institute of
Chemistry, University of Karachi, Pakistan and
University of Sussex, U.K.
M.p.: 150 °C; IR (CHC13) vmax: 1730 (br.), 1660,
883 cm "1; JH NM R (CDC13, 360 MHz): (3 1.42
(3H, s, H-18), 2.32 (1H, d, 7=12.6 Hz, H-5), 2.95
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1244
M. S. Ali et al. ■The Esterification of Gibberellins
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