Synthesis and bioactivity of the gibberellin, 18-hydroxy-GA1 (GA132)

Article abstract of DOI:10.1039/b800728d

As part of a study to confirm putative structural assignments to new gibberellins and to furnish sufficient quantities for biological investigations, a twenty step synthesis of 18-hydroxy GA₁ from gibberellic acid (GA₃) is described, allowing the confirmation of structure for a new gibberellin, GA₁₃₂, that occurs in developing grains of barley (Hordeum vulgare). The early part of the sequence involved cleavage of the C(3)-C(4) bond in the A-ring of a 3-oxo intermediate. The ring was then reformed as part of a domino process involving the conjugate addition of alkoxide to an α-methylene lactone moiety followed by an intramolecular aldol reaction. The bioactivities of the new GA, and its 18-hydroxy-GA ₄ relative, have been confirmed in dwarf barley growth and α-amylase induction assays. The Royal Society of Chemistry.

Full text of DOI:10.1039/b800728d

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Synthesis and bioactivity of the gibberellin, 18-hydroxy-GA1
(GA132)†
Ellen Morrison,a Peter M. Chandler,b Regan J. Thomsona and Lewis N. Mander*a
Received 15th January 2008, Accepted 1st February 2008
First published as an Advance Article on the web 26th February 2008
DOI: 10.1039/b800728d
As part of a study to confirm putative structural assignments to new gibberellins and to furnish
sufficient quantities for biological investigations, a twenty step synthesis of 18-hydroxy GA1 from
gibberellic acid (GA3) is described, allowing the confirmation of structure for a new gibberellin, GA132
that occurs in developing grains of barley (Hordeum vulgare). The early part of the sequence involved
cleavage of the C(3)–C(4) bond in the A-ring of a 3-oxo intermediate. The ring was then reformed as
part of a “domino” process involving the conjugate addition of alkoxide to an a-methylene lactone
moiety followed by an intramolecular aldol reaction. The bioactivities of the new GA, and its
,
18-hydroxy-GA4 relative, have been confirmed in dwarf barley growth and a-amylase induction assays.
Introduction
Gibberellins (“GAs”) in which the 18-methyl group has under-
gone oxidation have been isolated from immature seeds of the
sword bean (Canavalia gladiata)1 and from both immature and
germinated barley grain (Hordeum vulgare). GAs from the latter
species were identified as putative 18-OH derivatives of GA1 (1),
GA4 (2), GA34 (3) and GA48 (4)2,3 on the basis of GC-MS spectral
comparisons and KRIs4 of reference compounds that were derived
from metabolic feeds of 18-hydroxy GA12 to the fungus Gibberella
fujikuroi B1–41a.5 We have recently confirmed the identity of 18-
hydroxy GA4 by synthesis6 and have now embarked upon the
preparation of 18-hydroxy GA1 with a view to confirming both
the identity of the endogenous material and providing sufficient
material to assess its bioactivity.
Scheme 1 Reagents: i, NaOH, H2O–THF; ii, EtOCOCl, Et3N; NaBH4;
iii, LDA; CBr4.
We suspected that the problems arose from our choice of acetate
for the protection of the 13-hydroxyl and accordingly employed a
MOM ether group instead. The preparation of an advanced 18-
substituted intermediate then proceeded uneventfully and mostly
in excellent yield as outlined in Scheme 2, although, as in the
GA4 series, reduction of acid 14 proved to be problematical. Thus,
the A-ring double bond in 12 was removed by conjugate hydride
addition following Hanson’s protocol8 and after reconstitution of
the 3-oxo function to give ketone 13, cleavage of the A-ring was
readily achieved by means of a retro-Claisen reaction induced by
brief treatment (3 minutes) with NaOH in aqueous THF.9 Under
these conditions, a 5 : 1 mixture of C4 epimers was obtained
with the endo-methyl isomer predominating.10 Attempts to reduce
the 3-carboxyl by the standard treatment with NaBH4 of the
mixed anhydride formed from ethyl chloroformate11 that had been
Results and discussion
Our first attempt to prepare 18-hydroxy-GA1 followed the same
strategy that we had employed in the synthesis of 18-hydroxy-
6
GA4 and is outlined in Scheme 1. Initially, we employed ketone
5,7 and although the preparation of 6 proceeded smoothly, yields
for subsequent steps en route to the ene-lactone 9 were less than
satisfactory (for details see experimental section).
aResearch School of Chemistry, Institute of Advanced Studies, The Australian
National University, Canberra, ACT 0200, Australia
bCSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia.
E-mail: mander@rsc.anu.edu.au; Fax: 61-2-61258114
† Electronic supplementary information (ESI) available: Selected NMR
data for 18-alkoxy gibberellin derivatives. See DOI: 10.1039/b800728d
1416 | Org. Biomol. Chem., 2008, 6, 1416–1424
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Scheme 3 Products from alkoxide additions to aldehyde 15.
even though the 3b : 3a ratio was unfavourable and the 16-ene
function would complicate removal of the benzyl group.
We dealt with the first issue through base catalysed equilibration
of the 3a-epimer which afforded a 2 : 1 mixture favouring
the 3a-epimer; separation, and recycling was effected with 92%
recovery for each cycle.16 Then, as outlined in Scheme 4, 23b was
17
oxidised first to the 17-nor-ketone with OsO4–NaIO4 to allow
hydrogenolytic removal of the benzyl group,18 resulting in diol 27.
Scheme 2 Reagents: i, Ac2O, Et3N; ii, MOMCl, iPr2NEt; iii, K2CO3,
MeOH; iv, H2CrO4; v, NaBH4, CuCl; vi, NaOH, H2O–THF; vii, BOP,
iPr2NEt; NaBH4; viii, TBSCl, imidazole, Et3N; ix, LDA; CBr4; x, TBAF;
xi, Dess–Martin periodinane; xii, ROH, DBU.
reasonably effective in the GA4 series proved to be unsatisfactory,
as did numerous other methods; only a procedure utilizing
benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluo-
rophosphate (“BOP”)12 proved to be acceptable.
To prepare 16, it was thought to be necessary to protect the free
C-3 hydroxyl, and accordingly the tert-butyldimethylsilyl ether 15
was formed in quantitative yield using standard conditions. Sub-
sequent treatment with LDA followed by tetrabromomethane13
afforded a bromolactone in excellent yield (99%) which, when
treated with TBAF, afforded carbinol 16 in 62% yield, the reagent
effecting both elimination of HBr and liberation of the 3-hydroxyl.
Then, formation of aldehyde 17 was smoothly achieved by Dess–
Martin periodinane oxidation14 as a prelude to carrying out the
Scheme 4 Reagents: i, OsO4, NaIO4; ii, H2, Pd(OH)2-C; iii, CH2Br2, Zn,
TiCl4; iv, DHP, Py.HOTs; v, nBuSLi, HMPA; vi, Dowex (H+).
The D16 double bond was restored by means of the Lom-
bardo procedure,19 the product protected as bis-tetrahydropyranyl
adduct 28, and then the ester function demethylated with lithium
propanethiolate.20 Finally, the target GA (1) was liberated by
acidic hydrolysis with Dowex resin. Confirmation of structure
for the putative 18-hydroxy-GA1 was then established by GCMS
=
desired domino transformation to 18 (R = CH2CH CH2) by
means of the conjugate addition of allyl alcohol followed by an
aldol reaction. This process resulted in a 62% yield of a 2 : 1
mixture of the 3a-epimer with the 3b-diastereomer. Unfortunately,
we could not remove the allyl group15 from the 3b-epimer and
achieved only a 40% yield with the 3a-epimer. We therefore
examined a number of other alcohols that might generate a C-
18 protecting group that would be more amenable to removal
(Scheme 3).
comparison of the silylated methyl ester with that of the synthetic
21
material and has been assigned as GA132
.
The bioactivities of the new GA and the previously prepared
18-hydroxy-GA4,6 as well as their parents, GA1 and GA4, were
compared in dwarf barley growth assays22 and are shown in
Table 2. GA4 was about 10-fold less active than GA1 in this
bioassay, and the 18-hydroxy derivatives each had about one
quarter of the activity of the parent compound.
18-OH GA1 and 18-OH-GA4 were tested at two concentrations
in the a-amylase induction bioassay: 10−8 M and 10−9 M for GA1
and 18-OH GA1, and at 10−7 M and 10−8 M for the less active GA4
We also hoped to improve the total yield and proportion of the
3b-epimer. The outcomes for small scale experiments are displayed
in Table 1. Once reproducibility and yields on a larger scale were
taken into account, we decided in favour of the benzyloxy adduct,
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Table 1 Yields of adducts from treatment of aldehyde 17 with various alkoxides
Alcohol
Yield (%)
63
3a : b epimer ratio
Alcohol
Yield (%)
72
3a : b epimer ratio
2 : 1
2 : 1
72
3 : 1
24
58
2 : 1
1 : 1
57
31
1 : 1
10 : 1
—
—
25
>10 : 1
Table 2 Estimates of relative activities of different GAs in a dwarf barley
of 18-OH GA1, associated with the production of a-amylase in
the mutant grains. The probability that 18-OH GA1 might have
biological activity has now been confirmed in both leaf growth
and a-amylase induction bioassays. It was not possible to make
an accurate estimate of the relative amounts of GA1 and of 18-
OH GA1 in the original study3 because a deuterated standard is
not yet available for 18-OH GA1. However, based on its relative
abundance to 2H GA8, 18-OH GA1 may be present at up to 6 times
the content of GA1, and even though it is not as active as GA1 in
a-amylase induction, it might still constitute the major bioactive
GA in such grains. GA4 and 18-OH GA4 were not examined in
the original study, but these GAs are considerably less active than
their 13-hydroxylated counterparts in both the growth assay and
the a-amylase assay.
leaf growth assay
a
Compound
[H]50 (M)
GA1
GA4
18-OH GA1
18-OH GA4
3.1 × 10−7
3.1 × 10−6
1.3 × 10−6
1.2 × 10−5
a [H]50 is the concentration of GA required for half-maximal stimulation
of leaf elongation rate.
and 18-OH GA4 (Fig. 1). In this assay, a-amylase is produced in the
interval 2–4 days following the commencement of GA treatment.23
At 2 d the highest a-amylase activities occurred at the highest
concentrations of GA1 or GA4, with the 18-OH derivatives of
these GAs being less active than the parent GA. There was a
difference of about 10-fold in the relative activities of GA1 and
GA4, with the former being the more active. In both cases the
18-OH derivatives have less activity than the parent compounds.
At later times the 18-OH GAs had either lower or approximately
equivalent activity to the parent GAs. The a-amylase induction
assay is less suited than the growth assay to determining relative
activities of different GAs because it requires destructive sampling
and is not a linear response.
Experimental section
Bioassays
The dwarf barley growth assay is based on the maximal elongation
rate of the first leaf of a GA-deficient dwarf mutant (M489) of
Himalaya barley.22 This mutant contains a single amino acid sub-
stitution in the GA 3-oxidase enzyme predominant in vegetative
parts of the plant. Grains were placed in filter paper envelopes, and
imbibed and germinated in 1 mM potassium phosphate buffer,
pH 5.5, either without GA (control), or containing GA3 (10 lM,
which gives a near-maximal growth response), or the GAs to
be tested (a range of concentrations). The concentration of GA
required for half-maximal stimulation of elongation rate ([H]50)
was estimated by re-arrangement of equation [3] of Weyers et al.,24
assuming that p = 1:
Conclusion
The methodology for introducing the 18-hydroxyl into the gib-
berellin skeleton has been more thoroughly explored and has
allowed the structure of putative 1 to be confirmed. Also, sufficient
material has been obtained to allow a preliminary examination of
its biological activity. In the barley grain development mutant
described by Green et al.3 there were elevated levels of GA1 and
Ramp × [H] − [H]
[H]50
=
(1)
R − Rmin
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Fig. 1 Production of a-amylase by endosperm half-grains incubated with GAs at different concentrations. A. GA1 and 18-OH GA1. B. GA4 and 18-OH
GA4. At the indicated time the half-grains were homogenised with the medium, and a-amylase activity assayed as described in Methods.
where Ramp is the amplitude of the response (response in the
presence of saturating GA3 minus the response in the absence
of GA3), R is the response for the compound being assessed for
activity, Rmin is the response on control medium, and [H] is the
concentration of the compound being assessed.
(EI): 420.1784 (M+, 38% C22H28O8 requires 420.1784), 378 (85),
360 (100), 346 (33), 332 (66), 305 (78), 290 (51), 272 (42), 227 (44),
199 (27), 171 (31), 129 (34), 105 (34), 91 (60), 79 (30).
The a-amylase assay using barley endosperm half-grains was
utilized as previously described.23
ent-13-Acetoxy-3-hydroxy-3,4-seco-20-norgibberell-16-en-7-oic
acid 7-methyl ester 19,10-lactone 4-epimers 7
Synthetic procedures
To a stirred solution of 6 (150 mg, 0.36 mmol) in THF (4.0 mL) at
0 ◦C, was added Et3N (109 lL, 2.2 equiv.) and ethyl chloroformate
(88 lL, 2.0 equiv.) dropwise. After 1.5 h, NaBH4 (54 mg, 4.0 equiv.)
was added and the mixture stirred a further 3 h after which NaBH4
(92 mg, 6.85 equiv.) was added. After stirring a further 2 h, the
mixture was acidified with HCl (1 M) and the reaction mixture
poured into a bilayer of HCl (1 M) and EtOAc. The aqueous
layer was separated and the organic layer washed several times
with 1.0 M HCl. The aqueous layers were back extracted into
EtOAc, and the combined organic layers washed several times
with aqueous sodium bicarbonate. The alkaline aqueous layers
were back extracted into EtOAc, the combined organic layers
washed with brine and dried (Na2SO4). Concentration in vacuo
and column chromatography on silica (50% EtOAc 50% light
petroleum, increasing to 100% EtOAc) gave a 7 : 1 (exo-H : endo-
H) ratio of the C-4 epimers 7 (44 mg, 30%); mmax (cm−1): 3443,
3058, 2949, 1764, 1732, 1664, 1437, 1369, 1311, 1241, 1202, 1173,
1100, 1050, 964, 894. dH (CDCl3) 1.19 (2.6 H, d, J 7.3, H-18 major
epimer), 1.38 (0.4 H, d, J 7.6, H-18 minor epimer), 1.99 (3 H, s,
OAc), 2.79 (1 H, d, J 2.3, H-6), 2.98 (1 H, m, H-4), 3.08 (1 H, dd, J
2.4, J 10.9, H-5), 3.66 (2 H, t, J 6.3, H-3), 3.67 (3 H, s, CO2CH3),
4.92 (1 H, d, J 1.9, H-17), 4.97 (1 H, s, H-17). m/z (EI): 406.1988
(M+, 38% C22H30O7 requires 406.1992, M+, 65%), 388 (10), 364
For general directions see ref. 6.
ent-13-Acetoxy-3,4-seco-20-norgibberell-16-ene-3,7-dioic acid
7-methyl ester 19,10-lactone 4-epimers (6)
To a stirred solution of 5 (2.0 g, 5.0 mmol) in THF (40.0 mL) and
water (10.0 mL), was added NaOH (1.2 g, 6 equiv.). After vigorous
stirring for 3 min, HCl (1 M) was added and the reaction mixture
poured into a bilayer of water and EtOAc. The aqueous layer was
separated, back extracted in EtOAc and the combined organic
layers extracted several times with aqueous sodium bicarbonate.
The combined alkaline layers were acidified with HCl (1 M) and
extracted several times with EtOAc. The combined organic layers
were washed with brine, dried (Na2SO4) and the solvent removed
in vacuo to afford, without further purification, a 7 : 1 (exo-H :
endo-H) mixture of the C-4 epimers of 6 (1.7 g, 85%); mmax (cm−1):
3058, 2951, 1767, 1731, 1665, 1437, 1369, 1311, 1265, 1241, 1173,
1099, 1043, 962, 893. dH (CDCl3) 1.21 (2.5 H, d, J 7.0, H-18 major
epimer), 1.40 (0.5 H, d, J 7.6, H-18 minor epimer), 2.00 (3 H, s,
OAc), 2.81 (1 H, d, J 2.1, H-6), 2.98 (1 H, dq, J 11.0, J4,5 11.0,
J 7.0, H-4), 3.07 (1 H, dd, J5,4 11.0, J5,6 2.1, H-5), 3.69 (3 H, s,
CO2CH3), 4.93 (1 H, d, J 1.8, H-17), 4.99 (1 H, s, H-17); m/z
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(52), 346 (93), 332 (62), 318 (88), 305 (100), 286 (67), 259 (67), 231
(67), 213 (45), 171 (53), 129 (52), 105 (57), 91 (92), 69 (70).
(58 mL, 0.33 mol, 3 eq) was added followed by dropwise addition
of MOMCl (25.2 mL, 0.33 mol, 3eq). The reaction was allowed
to warm to room temp and then DMAP (100 mg) was added and
the reaction was stirred for 2 days at room temp. After cooling
to 0 ◦C, sat. NaHCO3 (20 mL) was added and the mixture was
stirred for 15 min. The organic phase was then washed with water,
cold 2 M HCl, sat. NaHCO3 and brine, dried (MgSO4) and the
solvent removed in vacuo to furnish a colourless oil. No further
purification was carried out. mmax (cm−1): 2940, 2884, 1780, 1736,
1449, 1437, 1372, 1332, 1269, 1227, 1158, 1101, 1039, 976, 895;
dH (CDCl3) 1.15 (3 H, s, H-18), 1.72–2.21 (9 H, m), 2.13 (3 H, s,
OAc), 2.78 (1 H, d, J 10.7, H-6), 3.32 (1 H, d, J 10.7, H-5), 3.38
(3 H, s, MOM), 3.75 (3 H, s, CO2CH3), 4.55, 4.77 (2 H, 2 × ABd, J
7.1, MOM), 5.04 (1 H, m, H-17), 5.18 (1 H, dd, J 1.5, 2.7, H-17),
5.34 (1 H, dd, J 0.69, 3.7, H-3), 5.87 (1 H, dd, J 3.8, 9.3, H-2),
6.39 (1 H, dd, J 0.69, 9.3, H-1); dC NMR (CDCl3) 14.5, 16.9, 21.1,
38.0, 41.2, 43.8, 50.4, 50.8, 51.0, 52.3, 52.5, 53.7, 55.6, 70.5, 83.4,
90.5, 92.2, 108.8, 129.3, 134.4, 153.4, 170.3, 172.7, 177.4; m/z (EI):
446.1943 (M+ C24H30O8 requires 446.1941).
ent-13-Acetoxy-4a-bromo-3-hydroxy-3,4-seco-norgibberell-16-
en-7-oic acid 7-methyl ester 19,10-lactone 8
To a stirred solution of the alcohol mixture prepared above
(50 mg, 0.12 mmol) in THF (2.0 mL), at −78 ◦C, was added
LDA (0.25 M, 1.25 mL, 2.5 equiv.). The solution was stirred
for 15 min before adding a THF solution (0.5 mL) of CBr4
(102 mg, 2.5 equiv.), the reaction mixture was then stirred a further
40 min before quenching with aqueous ammonium chloride. The
solution was allowed to warm to room temp and poured into
a bilayer of EtOAc and water. The organic layer was separated
and washed several times with water. The aqueous layers were
back extracted into EtOAc, and the combined organic layers
washed with brine and dried (Na2SO4). Concentration in vacuo
and column chromatography on silica (20% EtOAc and 80% light
petroleum, increasing to 80% EtOAc) gave the bromide as a brown
oil (16 mg, 27%); mmax (cm−1): 3380, 2952, 2870, 1772, 1733, 1665,
1592, 1452, 1437, 1171, 1045, 984, 894. dH (CDCl3): 1.92 (3 H, s,
H-18), 2.00 (3 H, s, OAc), 2.81 (1 H, t, J 2.2, Jgem 15.5, H-15),
2.87 (1 H, d, J 1.9, H-6), 3.61 (1 H, d, J 2.0, H-5), 3.70 (2 H,
t, J 6.3, H-3), 3.72 (3 H, s, CO2CH3), 4.92 (m, 1 H, H-17), 4.99
(1 H,s, H-17); m/z (EI): 484.1097 (M+, 38% C22H29O779Br requires
406.1992, M+, 4%), 424/426 (21), 405 (26), 387 (48), 373 (44), 359
(28), 345 (90), 331 (48), 303/301 (70), 285 (92), 273 (100), 231 (50),
213 (49), 171/169 (42), 129 (57), 105 (48), 91 (97), 79 (49), 69 (62).
ent-3a-Hydroxy-13-methoxymethoxy-gibberella-1,16-dien-7-oic
acid 7-methyl ester 19,10-lactone
To a solution of the acetate prepared above (0.11 mol) in methanol
(400 mL) was added a K2CO3–KHCO3 solution (1 : 1, 110 mL,
0.22 mol, 2 eq) in 3 portions—7 min apart—and then the reaction
was stirred until complete (1 h). The reaction was quenched and
brought to pH 5 with cold 2 M HCl. The solvent was removed and
the residue was taken up in EtOAc and water. The aqueous phase
was extracted with EtOAc (×5). The combined organic phase was
then washed with 10% K2CO3 solution and brine, dried (MgSO4)
and the solvent removed to furnish a white solid (43.21 g, 97% over
two steps); mp 124–126 ◦C (Found: C, 65.20, H, 6.94%. C22H28O7
requires C, 65.33; H, 6.98%); mmax (cm−1): 3457, 2949, 2884, 1775,
1734, 1451, 1377, 1331, 1269, 1251, 1197, 1160, 1102, 1039, 894,
747; dH (CDCl3) 1.24 (3 H, s, H-18), 1.60–2.24 (9 H, m), 2.44 (1 H,
br, OH), 2.80 (1 H, d, J 10.6, H-6), 3.22 (1 H, d, J 10.6, H-5),
3.37 (3 H, s, MOM), 3.74 (3 H, s, CO2CH3), 4.16 (1 H, d, J 3.7,
H-3), 4.55, 4.77 (2 H, 2 × ABd, J 7.1, MOM), 5.04 (1 H, m,
H-17), 5.17 (1 H, m, Hꢀ-17), 5.91 (1 H, dd, J 3.7, 9.3, H-2), 6.32
(1 H, dd, J 0.69, 9.3, H-1); dC (CDCl3) 14.6, 17.0, 38.0, 41.3, 43.8,
50.6, 50.9, 51.0, 52.5, 53.1, 53.7, 55.6, 70.0, 83.5, 90.8, 92.2, 108.8,
132.7, 133.0, 153.4, 173.0, 178.8; m/z (EI): 404.1833 (M+ C22H28O7
requires 404.1835).
ent-3a-Acetoxy-13-hydroxy-gibberella-1,16-dien-7-oic acid
7-methyl ester 19,10-lactone
To a solution of GA3 methyl ester (11) (41.6 g, 0.12 mol) in CH2Cl2
(250 mL) cooled to 0 ◦C was added Et3N (80.5 mL, 0.58 mol, 5 eq)
and acetic anhydride (54.5 mL, 0.58 mol, 5 eq). The reaction was
stirred overnight at room temp and then cooled to 0 ◦C. Water
(5 mL) was added and the mixture was stirred for 15 min at
0
◦C. An additional 20 mL of water was added, the ice bath
removed and the reaction was stirred for 20 min. The layers were
then separated and the aqueous phase was extracted with CH2Cl2
(×3). The organic phases were combined and washed with 1 M
HCl, water, K2CO3 solution and brine, dried (MgSO4), filtered,
and the solvent evaporated. Purification by recrystallisation from
ether and hexane furnished a white solid (40.4 g, 86%); mp 175–
◦
177 C (Found: C, 65.35, H, 6.54%. C22H26O7 requires C, 65.66;
H, 6.51%); mmax (cm−1): 3513, 2939, 2873, 1777, 1735, 1450, 1437,
1373, 1331, 1228, 1159, 1100, 1022, 974, 896, 735; dH (CDCl3): 1.15
(3 H, s, H-18), 1.69–2.21 (9 H, m), 2.13 (3 H, s, OAc), 2.78 (1 H,
d, J 10.9, H-6), 3.33 (1 H, d, J 10.9, H-5), 3.75 (3 H, s, CO2CH3),
4.98 (1 H, m, H-17), 5.29 (1 H, d, J 1.9, H-17), 5.34 (1 H, d, J 3.7,
H-3), 5.88 (1 H, dd, J 3.7, 9.3, H-2), 6.39 (1 H, dd, J 0.69, 9.3,
H-1); dC (CDCl3): 14.5, 17.2, 21.1, 38.4, 43.3, 44.9, 50.6, 50.7, 51.2,
52.3, 52.4, 53.6, 70.4, 78.4, 90.4, 108.0, 129.4, 134.4, 157.1, 170.3,
172.6, 177.4. m/z (EI) 402.1680 (M+ C22H26O7 requires 406.1679).
ent-13-Methoxymethoxy-3-oxo-gibberella-1,16-dien-7-oic acid
7-methyl ester 19,10-lactone (12)
To a cooled solution of alcohol prepared above (25.7 g, 0.063 mol)
in acetone (300 mL) was added Jones reagent until an orange
colour persisted. The excess Jones reagent was then quenched
with propan-2-ol and the reaction was diluted with EtOAc. The
reaction was washed with water, sat. NaHCO3 (×3) and brine,
dried (MgSO4) and the solvent removed to furnish the ketone 12
as a white solid (24.71 g, 97%); mp 134–136 ◦C (Found: C, 65.28;
H, 6.53%. C22H26O7 requires C, 65.66; H, 6.51%); mmax (cm−1): 2950,
2883, 1782, 1733, 1696, 1439, 1380, 1318, 1269, 1260, 1197, 1149,
1093, 1037, 916, 713; dH (CDCl3): 1.28 (3 H, s, H-18), 1.77–2.26
(9 H, m), 2.90 (1 H, d, J 10.3, H-6), 3.38 (3 H, s, MOM), 3.52
ent-3a-Acetoxy-13-methoxymethoxy-gibberella-1,16-dien-7-oic
acid 7-methyl ester 19,10-lactone
A solution of GA3 methyl ester 3-acetate (44.55 g, 0.11 mol) in
CH2Cl2 (300 mL) was cooled to 0 ◦C. Diisopropylethylamine
1420 | Org. Biomol. Chem., 2008, 6, 1416–1424
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(1 H, d, J 10.3, H-5), 3.74 (3 H, s, CO2CH3), 4.56, 4.78 (2 H, 2 ×
ABd, J 7.1, MOM), 5.06 (1 H, m, H-17), 5.19 (1 H, dd, J 1.7,
3.0, H-17), 6.05 (1 H, d, J 9.5, H-2), 7.26 (1 H, d, J 9.5, H-1); dC
(CDCl3): 11.8, 17.1, 38.0, 41.7, 43.9, 50.2, 51.4, 51.8, 52.8, 55.7,
62.5, 65.2, 83.3, 89.8, 92.3, 109.1, 129.4, 147.3, 152.6, 171.8, 173.1,
191.7; m/z (EI) 402.1682 (M+ C22H26O7 requires 402.1679).
4.78 (2 H, 2 × ABd, J 7.2, MOM), 5.05 (1 H, br s, J 2.1, H-17),
5.15 (1 H, t, J 2.2, H-17); dC (CDCl3): 10.7, 17.7, 28.0, 31.0, 35.0,
38.0, 42.7, 43.7, 51.6, 52.3, 52.7, 55.8, 57.8, 63.3, 83.4, 92.2, 92.8,
108.8, 152.3, 172.4, 174.0, 200.2; m/z (EI) 404.1838 (C22H28O7 M+
requires 406.1835).
ent-13-Methoxymethoxy-3,4-seco-20-norgibberell-16-ene-3,7-
dioic acid 7-methyl ester 19,10-lactone (14)
ent-3-Hydroxy-13-methoxymethoxy-gibberell-16-en-7-oic acid
7-methyl ester 19,10-lactone 3-epimers
To a solution of ketone 13 (0.281 g, 0.69 mmol) in THF (12 mL)
was added NaOH (0.083 g, 2.08 mmol, 3 eq) in water (3 mL). The
reaction was stirred vigorously for 3 min and then quenched with
1 M HCl. The reaction mixture was poured into EtOAc and water
and the layers separated. The aqueous phase was extracted with
EtOAc (×3) and then the combined organic phase was extracted
with sat. NaHCO3 (×4). The aqueous extract was acidified with
1 M HCl and back-extracted with EtOAc (×5). The organic
phase was then washed with brine, dried (MgSO4) and the solvent
removed to furnish the carboxylic acid 14 as a white solid (0.293 g,
100%). No further purification was necessary; mp 139–141 ◦C;
Found: C, 62.35, H, 7.01%. C22H30O8 requires C, 62.55; H, 7.16%;
mmax (cm−1): 3518, 3188, 2950, 1767, 1729, 1439, 1367, 1310, 1244,
1172, 1117, 1099, 1039, 960, 895, 800; dH (CDCl3): 1.24 (3 H, d, J
7.1, H-18), 1.60–2.68 (13 H, m), 2.78 (1 H, d, J6,5, H-6), 2.99 (1 H,
dq, J4,5 11.0, J 7.1, H-4), 3.08 (1 H, dd, J5,4 11.0, J5,6 2.1, H-5),
3.35 (3 H, s, MOM), 3.69 (3 H, s, CO2CH3), 4.56, 4.80 (2 H, 2 ×
ABd, J 7.3, MOM), 5.06 (2 H br t, H-17); dC (CDCl3): 12.3, 19.0,
28.0, 29.2, 32.5, 38.1, 38.3, 41.5, 44.4, 49.3, 49.4, 51.0, 51.1, 52.3,
55.7, 84.2, 91.9, 95.3, 107.9, 149.7, 174.8, 178.1, 179.0; m/z (EI)
422.1937 (C22H30O8 M+ requires 422.1941).
To a cooled solution of enone 12 (20.5 g, 0.051 mol) in methanol
(1.2 L) was added CuCl (25.2 g, 0.25 mol, 5 eq) and then NaBH4
(19.2 g, 0.51 mol, 10 eq) carefully in several portions (bubbles
vigorously!). After NaBH4 addition was complete the reaction
was stirred for 20 min more and then quenched with acetic acid
and filtered through Celite. The solvent was then removed in vacuo
and the residue was taken up in EtOAc and water. The aqueous
phase was extracted with EtOAc (×3) and the combined organic
phase was washed with water, sat. NaHCO3 (×3) and brine, dried
(MgSO4) and the solvent removed to furnish the alcohol as a
white solid (19.96 g, 97%) which was used directly in the next step.
Column chromatography on silica (EtOAc–light petroleum, 1 : 1)
of a small portion (170 mg) gave the 3b-alcohol (12 mg) as a gum
followed by the crystalline 3a-epimer (145 mg).
3b-epimer: dH (CDCl3): 1.15 (3 H, s, H-18), 1.60–2.23 (13 H, m),
2.69 (1 H, d, J 10.1, H-6), 3.20 (1 H, d, J 10.1, H-5), 3.37 (3 H, s,
MOM), 3.71 (3 H, s, CO2CH3), 3.84 (1 H, m, H-3), 4.55, 4.77
(2 H, 2 × ABd, J 7.2, MOM), 5.02 (1 H, br s, H-17), 5.14 (1 H, t, J
2.5, H-17); dC (CDCl3): 15.0, 17.4, 27.5, 28.4, 38.2, 42.0, 43.9, 49.8,
51.6, 52.1, 52.5, 53.0, 54.8, 55.7, 70.5, 83.7, 92.2, 94.0, 108.5, 153.3,
173.2, 178.2; m/z (EI) 406.1991 (C22H30O7 M+ requires 406.1992).
3a-epimer: mp 120–122 ◦C; Found: C, 64.75, H, 7.59%.
C22H30O7 requires C, 65.01; H, 7.44%; mmax (cm−1): 3462, 2948,
2885, 1772, 1733, 1439, 1380, 1345, 1285, 1271, 1255, 1194, 1148,
1077, 1040, 899, 735; dH (CDCl3): 1.19 (3 H, s, H-18), 1.40–2.30
(13 H, m,), 2.54 (1 H, d, J 9.5, H-6), 2.76 (1 H, d, J 9.5, H-5), 3.37
(3 H, s, MOM), 3.66 (1 H, m, H-3), 3.73 (3 H, s, CO2CH3), 4.54,
4.77 (2 H, 2 × ABd, J 7.2, MOM), 5.02 (1 H, br s, H-17), 5.13 (1 H,
t, J 2.3, H-17); dC (CDCl3): 13.1, 17.5, 28.0, 29.9, 30.26, 38.1, 42.3,
43.9, 51.0, 51.5, 52.6, 54.7, 55.7, 57.6, 73.4, 83.6, 92.2, 92.8, 108.5,
153.0, 173.2, 177.3;%. m/z (EI) 406.1995 (C22H30O7 M+ requires
406.1992).
ent-3-Hydroxy-13-methoxymethoxy-3,4-seco-20-norgibberell-16-
en-7-oic acid 7-methyl ester 19,10-lactone
Diisopropylethylamine (0.32 mL, 1.8 mmol, 1.2 eq) was added
to a mixture of the carboxylic acid (0.65 g, 1.5 mmol) and BOP
reagent (0.75 g, 1.7 mmol, 1.1 eq) in THF (15 mL) and stirred
at room temp for 45 min. NaBH4 (0.102 g, 2.7 mmol, 1.75 eq)
was added and the reaction was stirred for 30 min at room
temp. A second portion of NaBH4 (0.102 g, 2.7 mmol, 1.75 eq)
was added and the reaction was again stirred for 30 min. The
reaction was then quenched with 1 M HCl (5 mL), stirred for
20 min and then poured into EtOAc–5% 2-butanol and water.
The organic phase was washed with water (×2) and the combined
aqueous phases were extracted with EtOAc–5% 2-butanol (×3).
The combined organic phase was then washed with brine, dried
(MgSO4) and the solvent removed in vacuo. Purification by silica
gel chromatography (EtOAc) furnished the alcohol (0.529 g, 84%)
as a colourless oil; mmax (cm−1): 3452, 2945, 2879, 1764, 1729, 1448,
1436, 1369, 1310, 1239, 1191, 1169, 1040, 960, 916; dH (CDCl3):
1.23 (3 H, d, J 7.3, H-18), 1.58–2.05 (12 H, m), 2.58 (1 H, dt, J 15.8,
3.1, H-15), 2.78 (1 H, d, J 2.2, H-6), 2.94–3.12 (2 H, m, H-4, H-5),
3.35 (3 H, s, MOM), 3.69 (3 H, s, CO2CH3), 3.69 (2 H, overlapped,
H-3), 4.56, 4.81 (2 H, 2 × ABd, J 7.0, MOM), 5.06 (2 H, m, H-17);
dC (CDCl3): 12.3, 19.0, 27.4, 34.2, 38.2, 38.6, 41.6, 44.4, 49.5, 51.0,
51.1 (2 × C), 52.2, 55.7, 62.8, 84.2, 91.9, 96.3, 107.7, 149.9, 174.9,
179.4; m/z (EI) 408.2150 (C22H32O7 M+ requires 408.2148).
ent-13-Methoxymethoxy-3-oxo-gibberell-16-en-7-oic acid
7-methyl ester 19,10-lactone (13)
To a cooled solution of alcohols prepared above (17.9 g, 0.044 mol)
in acetone (200 mL) was added Jones reagent until an orange
colour persisted. The excess Jones reagent was then quenched
with propan-2-ol and the reaction was diluted with EtOAc and
water. The reaction was washed with water, sat. NaHCO3 (×3)
and brine, dried (MgSO4) and the solvent removed to furnish the
ketone 13 as a white solid (13.34 g, 75%). mp 79–80 ◦C; Found: C,
64.86, H, 6.77%. C22H28O7 requires C, 65.33; H, 6.98%; mmax (cm−1):
2941, 2890, 1781, 1726, 1448, 1382, 1325, 1289, 1199, 1148, 1038,
958, 935, 916, 898, 732; dH (CDCl3): 1.20 (3 H, s, H-18), 1.70–2.21
(10 H, m), 2.46–2.66 (3 H, m), 2.81 (1 H, d, J 9.1, H-6), 3.06 (1 H,
d, J 9.1, H-5), 3.38 (3 H, s, MOM), 3.72 (3 H, s, CO2CH3), 4.56,
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ent-3-tert-Butyldimethylsilyloxy-13-methoxymethoxy-3,4-seco-
20-norgibberell-16-en-7-oic acid 7-methyl ester 19,10-lactone (15)
The reaction was poured into EtOAc–5% 2-butanol and water.
The organic phase was washed with water (×2) and the combined
aqueous phases were extracted with EtOAc–5% 2-butanol (×3).
The combined organic phase was then washed with brine, dried
(MgSO4) and the solvent removed in vacuo. Purification by silica
gel chromatography (2 : 1 EtOAc–hexane) furnished 16 (0.017 g,
74%) as a colourless oil; mmax (cm−1): 3494, 2947, 1759, 1732, 1659,
1438, 1402, 1351, 1329, 1274, 1196, 1169, 1115, 1041, 975, 895,
818; dH (CDCl3) : 1.50–2.10 (12 H, m), 2.61 (1 H, dt, J 16.1, 3.1,
H-15), 2.65 (1 H, d, J 0.88, H-6), 3.33 (3 H, s, MOM), 3.51 (1 H,
d, J 1.3, H-5), 3.65 (2 H, t, J 6.4, H-3), 3.72 (3 H, s, CO2CH3),
4.54, 4.78 (2 H, 2 × ABd, J 7.3, MOM), 5.06 (2 H, m, H-17), 5.72
(1 H, d, J 2.2, H-18), 6.31 (1 H, d, J 2.6, H-18); dC (CDCl3): 19.0,
27.2, 35.2, 38.2, 41.2, 44.6, 48.9, 50.2, 51.8, 52.3, 55.8, 60.5, 62.8,
84.2, 91.9, 95.2, 107.8, 124.3, 140.6, 149.4, 170.2, 174.1; m/z (EI)
406.1992 (C22H30O7 M+ requires 406.1992).
To a cooled solution of alcohol prepared above (0.429 g, 1.1 mmol)
and imidazole (14 mg, 0.21 mmol, 0.2 eq) in DMF (15 mL)
was added Et3N (0.29 mL, 2.1 mmol, 2 eq) and then tert-
butyldimethylsilyl chloride (0.32 g, 2.1 mmol, 2 eq). The reaction
was stirred overnight at room temp and then poured into EtOAc
and water. The aqueous phase was extracted with EtOAc (×3).
The combined organic phase was washed with water and brine,
dried (MgSO4) and the solvent evaporated. Purification by silica
gel chromatography (4 : 1 hexane–EtOAc–2 : 1 hexane–EtOAc)
furnished 15 (0.494 g, 90%) as a colourless oil; mmax (cm−1): 2951,
2857, 1769, 1733, 1436, 1361, 1251, 1195, 1150, 1099, 1042, 961,
836, 776; dH (CDCl3): 0.04 (6 H, s, Si-Me2), 0.88 (9 H, s, Si-tBu),
1.23 (3 H, d, J 7.2, H-18), 1.50–2.10 (12 H, m), 2.55 (1 H, dt, J
15.8, 2.9, H-15), 2.77 (1 H, d, J 2.2, H-6), 2.95–3.10 (2 H, m, H-4,
H-5), 3.35 (3 H, s, MOM), 3.62 (2 H, t, J 5.1, H-3), 3.68 (3 H, s,
CO2CH3), 4.56, 4.80 (2 H, 2 × ABd, J 7.2, MOM), 5.05 (2 H, m,
H-17); dC (CDCl3) : −4.9, 12.3, 18.7, 19.0, 26.3, 27.6, 28.0, 34.1,
38.2, 38.6, 41.6, 44.4, 49.5, 51.00, 51.04, 52.2, 55.7, 63.1, 84.2, 91.9,
96.4, 107.6, 150.1, 174.8, 179.3; m/z (EI) 522.3013 (C28H46O7Si M+
requires 522.3013).
ent-13-Methoxymethoxy-3-oxo-3,4-seco-20-norgibberell-4(18),
16-dien-7-oic acid 7-methyl ester 19,10-lactone (17)
Dess–Martin periodinane (0.060 g, 0.16 mmol, 1.5 eq) was added
to 14 (0.043 g, 0.11 mmol) in CH2Cl2 (5 mL) and stirred at room
temp for 2 h. Na2S2O3 (2.5 mL) and sat. NaHCO3 (2.5 mL) were
then added and the reaction was stirred vigorously just until both
layers were clear. The layers were separated and the aqueous phase
was extracted with CH2Cl2 (×3). The combined organic phase
was washed with water and brine, dried (MgSO4) and the solvent
removed in vacuo. Purification by silica gel chromatography (1 :
1 EtOAc–hexane) furnished aldehyde 15 (0.039 g, 91%) as a
colourless oil. mmax (cm−1): 2948, 1761, 1725, 1659, 1439, 1274,
1169, 1114, 1041, 895; dH (CDCl3): 1.50–2.09 (9 H, m), 2.26–2.64
(4 H, m), 2.65 (1 H, d, J 1.1, H-6), 3.33 (3 H, s, MOM), 3.46 (1 H.
d, J 1.1, H-5), 3.72 (3 H, s, CO2CH3), 4.54, 4.78 (2 H, 2 × ABd, J
7.3, MOM), 5.06 (2 H, m, H-17), 5.74 (1 H, d, J 2.2, H-18), 6.34
(1 H, d, J 2.6, H-18), 9.76 (1 H, t, J 1.1, CHO); dH (CDCl3): 19.0,
30.9, 38.2, 38.8, 41.2, 44.6, 48.7, 50.2, 51.9, 52.4, 55.8, 60.5, 84.1,
91.9, 94.2, 107.9, 125.0, 140.0, 149.2, 169.8, 174.0, 200.5; m/z (EI)
404.1834 (C22H28O7 M+ requires 406.1835).
ent-4a-Bromo-3-tert-butyldimethylsilyloxy-13-methoxymethoxy-
3,4-seco-20-norgibberell-16-en-7-oic acid 7-methyl ester 19,10-
lactone
A solution of n-butyllithium (1.6 M in hexanes, 0.25 mL,
0.40 mmol, 1.5 eq) was added to a solution of diisopropylamine
(0.056 mL, 0.40 mmol, 1.5 eq) in THF (3 mL) at 0 ◦C and the
reaction was stirred for 20 min. After cooling to −78 ◦C, 15
(0.14 g, 0.27 mmol) in THF (3 mL) was added and the reaction
was stirred for 25 min at −78 ◦C. CBr4 (0.22 g, 0.66 mmol, 2.5 eq)
in THF (3 mL) was added and the reaction was stirred for 40 min
at −78 ◦C. The reaction was then quenched with NH4Cl and
allowedtowarmtoroomtemp(60min)before pouring intoEtOAc
and water. The organic phase was washed with water (×2). The
aqueous phase was extracted with EtOAc (×2). The combined
organic phase was then washed with brine, dried (MgSO4) and
the solvent removed. Purification by silica gel chromatography
(4 : 1 hexane–EtOAc) furnished the bromide (0.143 g, 88%) as a
colourless oil; mmax (cm−1): 2951, 2857, 1775, 1734, 1436, 1359, 1280,
1246, 1213, 1169, 1149, 1107, 1040, 960, 835, 775; dH (CDCl3) :
0.05 (6 H, s, SiMe2), 0.89 (9 H, s, Si-tBu), 1.50–2.24 (12 H, m),
1.95 (3 H, s, H-18), 2.57 (1 H, dt, J 15.8, 3.1, H-15), 2.83 (1 H, d,
J 2.1, H-6), 3.33 (3 H, s, 3H, MOM), 3.60 (1 H, d, J 2.1, H-5),
3.65 (2 H, t, J 6.3, H-3), 3.72 (3 H, s, CO2CH3), 4.54, 4.79 (2 H,
2 × ABd, J 7.3, MOM), 5.06 (2 H, m, H-17); dC (CDCl3) : −4.9,
18.6, 18.7, 26.1, 26.3, 28.2, 33.4, 38.1, 41.3, 44.5, 47.0, 50.9, 52.4,
53.3, 55.6, 55.7, 59.2, 63.0, 84.1, 91.9, 97.2, 107.8, 149.3, 173.4,
174.6; m/z (EI) 600.2120 (C28H45O7Si79Br M+ requires 600.2118);
602.2104 (C28H45O7Si81Br M+ requires 602.2097).
ent-18-Benzyloxy-3-hydroxy-13-methoxymethoxy-gibberell-
16-en-7-oic acid 7-methyl ester 19,10-lactone 3-epimers (18, R =
CH2Ph)
DBU (4.5 mL, 30 mmol, 10 eq) was added to a solution of aldehyde
17 (1.22 g, 3.0 mmol) in 10 mL of benzyl alcohol and the reaction
was stirred at room temp for 18 h. After quenching with 1 M
HCl, the reaction mixture was poured into EtOAc and water.
The organic phase was washed with water (×2). The combined
aqueous phase was extracted with EtOAc (×3). The combined
organic phase was washed with brine, dried over MgSO4 and the
solvent removed. Purification by silica gel chromatography (5 : 1
hexane–EtOAc–1 : 1 EtOAc–hexane) furnished in sequence, the
3b-epimer (0.717 g) followed by the 3a-isomer (0.396 g), 72% total
yield, both as colourless oils.
ent-3-Hydroxy-13-methoxymethoxy-3,4-seco-20-norgibberella-
4(18),16-dien-7-oic acid 7-methyl ester 19,10-lactone (16)
3b-epimer: mmax (cm−1): 3480, 2947, 2879, 1772, 1734, 1439, 1270,
1196, 1158, 1111, 1073, 1040, 918, 893, 863, 737, 699; dH (CDCl3):
1.65–2.35 (13 H, m), 2.76 (1 H, d, J 9.7, H-6), 3.36 (3 H, s, OCH3-
MOM), 3.51 (1 H, d, J 1.3, 3-OH), 3.51 (1 H, d, J 9.7, H-5), 3.60,
To a solution of the bromide prepared above (0.034 g, 0.057 mmol)
in THF (3 mL) was added 1 M TBAF solution in THF (0.17 mL,
0.17 mmol, 3eq) and the reaction stirred for 4 h at room temp.
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3.96 (2 H, 2 × ABd, J 9.8, H-18), 3.62 (3 H, s, CO2CH3), 4.16
(1 H, br d, J 2.3, H-3), 4.45, 4.47 (2 H, 2 × ABd, J 11.6, PhCH2),
4.54, 4.76 (2 H, 2 × ABd, J 7.2, –MOM), 5.03 (1 H, br s, H-17),
5.13 (1 H, t, J 1.9, H-17), 7.31 (5 H, m, C6H5); dC (CDCl3): 17.5,
27.3, 27.5, 38.1, 42.8, 44.0, 50.3, 50.8, 51.3, 52.2, 52.4, 55.7, 57.9,
70.8, 71.9, 74.4, 83.7, 92.1, 95.7, 108.3, 127.8, 128.3, 128.8, 136.8,
152.9, 173.0, 175.0.
3a epimer: Found: C, 67.78, H, 6.77%; C29H36O8 requires C,
67.95, H, 7.08%; mmax (cm−1): 3435, 2948, 2879, 1771, 1733, 1439,
1286, 1254, 1196, 1148, 1115, 1075, 1040, 914, 899, 735, 698; dH
(CDCl3): 1.45–2.30 (13 H, m), 2.69 (1 H, d, J 9.1, H-6), 2.90 (1 H,
d, J 9.1, H-5), 3.36 (3 H, s, MOM), 3.55 (3 H, s, CO2CH3), 3.58,
3.97 (2 H, 2 × ABd, J 10.6, H-18), 4.01 (1 H, m, H-3), 4.42, 4.47
(2 H, 2 × ABd, J 11.9, PhCH2), 4.54, 4.77 (2 H, 2 × ABd, J 7.0,
MOM), 5.01 (1 H, br s, H-17), 5.11 (1 H, m, H-17), 7.31 (5 H, m,
C6H5); dC (CDCl3): 17.7, 29.2, 30.0, 38.1, 43.1, 43.8, 51.26, 51.30,
51.9, 52.2, 55.7, 56.1, 58.7, 66.4, 70.1, 73.8, 83.6, 92.2, 94.1, 108.3,
127.6, 128.0, 128.6, 137.8, 152.6, 173.2, 175.0; m/z (FAB) 535.2308
(C29H36O8Na [M + Na]+ requires 535.2308).
DBU (1.1 mL, 7.2 mmol, 10 eq) was added to a solution of the
3a-epimer (0.367 g, 0.72 mmol) in 5 mL of benzyl alcohol and the
reaction was stirred at room temp for 2 h. After quenching with
1 M HCl, the reaction mixture was poured into EtOAc and water.
The organic phase was washed with water (×2). The aqueous
phase was extracted with EtOAc (×3). The combined organic
phase was washed with brine, dried over MgSO4 and the solvent
removed. Purification by silica gel chromatography (5 : 1 hexane–
EtOAc–1 : 1 EtOAc–hexane) furnished the products as a mixture
of the 3a-epimer (0.230 g) with the 3b isomer (0.108 g); 92% total
recovery, both as colourless oils.
solid after removal of the solvent (0.052 g, 90% yield); mp 170–
172 ◦C; Found C, 59.13; H, 6.60%; C21H28O9: C, 59.43; H, 6.65%;
mmax (cm−1): 3480, 2951, 1752, 1439, 1284, 1203, 1154, 1118, 1034,
989, 918, 733; dH (CDCl3): 1.60–2.45 (13 H, m), 2.91 (1 H, d, J
11.0, H-6), 2.98 (1 H, dd, J 4.8, 8.8, 18-OH), 3.35 (3 H, s, MOM),
3.42 (1 H, d, J 10.8, H-5), 3.66 (1 H, m, 3-OH), 3.71 (1 H dd, J
4.8, 12.5, H-18), 3.80 (3 H, s, CO2CH3), 4.03 (1 H, dd, J 8.8, 12.5,
H-18), 4.30 (1 H, m, H-3), 4.64, 4.81 (2 H, 2 × ABd, J 7.5, MOM);
dC (CDCl3): 17.5, 27.5, 27.6, 31.8, 38.4, 46.6, 48.8, 49.3, 52.3, 53.3,
53.8, 56.3, 58.7, 64.2, 70.3, 82.9, 93.0, 94.1, 173.8, 175.0, 215.7;
m/z (EI) 424.1738 (C21H28O9 M+ requires 424.1733).
ent-3a,18-Dihydroxy-13-methoxymethoxy-gibberell-16-en-7-oic
acid 7-methyl ester 19,10-lactone
Lombardo reagent prepared from CH2Br2 (0.75 mL)19 was added
as a suspension dropwise via a pipette to a solution of ketone
(0.046 g, 0.11 mmol) in CH2Cl2 (2 mL) at room temp. The reaction
was monitored by TLC and when complete was poured into a
slurry of NaHCO3 (solid) and water (2 : 1) and ether. The mixture
was shaken until a clear organic layer was obtained and then the
layers were separated. The organic layer was dried over MgSO4
and the solvent removed. The crude 16-alkene (0.020 g, 43%) was
obtained as an oil and carried through to the next reaction; mmax
(cm−1): 3440, 2930, 1764, 1733, 1455, 1435, 1283, 1247, 1199, 1159,
1112, 1039, 918, 892; dH (CDCl3) : 1.59–2.22 (13 H, m), 2.85 (1 H,
d, J 11.0, H-6), 3.01 (1 H, dd, J 4.8, 9.2, 18-OH), 3.38 (1 H, d, J
11.0, H-5), 3.38 (3 H, s, MOM), 3.67 (1 H, dd, J 4.8, 12.5, H-18),
3.71 (1 H, m, 3-OH), 3.78 (3 H, s, CO2CH3), 4.00 (1 H, dd, J 9.1,
12.6, H-18), 4.28 (1 H, m, H-3), 4.56, 4.76 (2 H, 2 × ABd, J 7.2,
MOM), 5.05 (1 H, m, H-17), 5.20 (1 H, m, H-17); dC (CDCl3):
17.5, 27.67, 27.73, 38.3, 41.0, 43.4, 49.0, 49.3, 50.9, 53.0, 53.5,
55.7, 58.5, 64.6, 70.7, 83.4, 92.3, 94.3, 109.2, 153.7, 174.8, 175.2;
m/z (EI) 378.1678 (M+ 50%), HRMS: C20H26O7 (M+) requires
378.1684.
ent-3a,18-Dihydroxy-13-methoxymethoxy-16-oxo-17-norgibbe-
rellan-7-oic acid 7-methyl ester 19,10-lactone (27)
To a solution of 23b (0.037 g, 0.072 mmol) in dioxane and water (3 :
1, 5 mL) was added osmium tetraoxide (1 mol%) and the reaction
was stirred for 5 min at room temp. Then sodium periodate
(0.046 g, 0.22 mmol, 3 eq) was added and the reaction was stirred
overnight at room temp. The following day, more sodium periodate
(0.046 g, 0.22 mmol, 3 eq) was added and the reaction was again
stirred overnight at room temp. Upon completion, the reaction
was diluted with EtOAc and then washed with 1 M Na2S2O3 (×2),
water and brine, dried over MgSO4, and the solvent removed in
vacuo. Purification by silica gel chromatography (2 : 1 EtOAc–
hexane) furnished ketone 27 (0.022 g, 59% yield) as a colourless
oil; mmax (cm −1): 3492, 2950, 2893, 1753, 1453, 1366, 1284, 1203,
1153, 1113, 1049, 1032, 990, 916, 735; dH (CDCl3): 1.66–2.40 (13 H,
m), 2.85 (1 H, d, J 10.0, H-6), 3.34 (3 H, s, MOM), 3.46 (1 H, d,
J 0.9, C-3 OH), 3.55 (1 H, d, J 10.0, H-5), 3.62 (3 H, s, CO2CH3),
3.62, 3.97 (2 H, 2 × ABd, J 9.8, H-18), 4.16 (1 H, d, J 2.9, H-3),
4.46 (2H, s, PhCH2), 4.62, 4.78 (2 H, 2 × ABd, J 7.5, MOM), 7.31
(5 H, m, C6H5); dC (CDCl3): 17.4, 27.2, 27.4, 32.0, 39.4, 47.4, 49.4,
50.9, 52.5, 52.8, 53.0, 56.2, 58.2, 70.7, 71.8, 74.5, 83.2, 92.9, 95.3,
127.8, 128.4, 128.9, 136.7, 172.3, 174.6, 215.5.
ent-3a,13,18-Trihydroxy-gibberell-16-en-7-oic acid 19,10-lactone
(1)
Dihydropyran (0.1 mL, 1.1 mmol, 10 eq) was added to a solution
of the diol (0.11 mmol from the previous reaction) and PPTS
(5 mg) in 4 mL CH2Cl2 and stirred at room temp overnight. The
reaction was then washed with water, sat. NaHCO3, and brine,
dried over MgSO4 and the solvent removed in vacuo. No further
purification was carried out. The crude mixture of diastereomers
28 was obtained as an oil and carried through to the next reaction.
dH (crude, CDCl3): 1.0–2.30 (25 H, m), 2.75–2.82 (1 H, d × 2, J
9.5, H-6), 3.36 (3 H, s, MOM), 3.36–3.75 (5 H, m), 3.67 (3 H, s,
CO2CH3), 3.69–3.93 (2 H, 2 × ABd, H-18), 4.21 [1 H, m, OCHO
(THP)], 4.53–4.78 (2 H, 2 × ABd, MOM), 5.03, 5.13 (2 H, m,
H-17).
A
solution of lithium n-butanethiolate in HMPA
(0.26 mL,1.5 M, 0.39 mmol) was added to the methyl ester
(0.11 mmol) in HMPA (0.5 mL) and stirred overnight at room
temp. The reaction mixture was then diluted with water, and
acidified with cold 2 M HCl to pH 3. The mixture was extracted
with EtOAc (×3) and the combined organic phase was washed
with KH2PO4 (to pH 4.5), CuCl2 solution, and brine. The organic
layer was dried over MgSO4 and the solvent removed in vacuo.
Benzyl ether (0.070 g, 0.14 mmol) and palladium hydroxide on
carbon (10 mg) were combined in EtOAc (5 mL) and hydrogen
gas was introduced to the flask via a balloon. The reaction was
stirred at room temp for 2 h and then filtered through Celite and
the solvent removed in vacuo. Ketone 27 was obtained as a white
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The crude product was dissolved in methanol (1 mL) and water
(0.2 mL), Dowex resin (H+) added, and the mixture heated for
3 h. The reaction was then cooled to room temp and filtered. The
resin was rinsed with methanol and the solvent was evaporated.
Purification on reverse-phase HPLC furnished a colourless gum,
(0.003 g, 8% over 4 steps); dH (MeOH-d4): 1.64–2.01 (11 H, m),
2.19 (1 H, d, J 15.6, H-15), 2.39 (1 H, dt, J 2.89, 15.6, H-15), 2.93
(1 H, d, J 10.3, H-6), 3.30 (1 H, d, J 10.3, H-5), 3.80, 3.84 (2 H, 2 ×
ABd, J 11.2, H-18), 3.97 (1 H, d, J 3.5, H-3), 4.94 (1 H, m, H-17),
5.20 (1 H, m, H-17); dH (MeOH-d4): 17.1, 27.3, 27.8, 38.7, 42.9,
44.8, 49.4, 50.1, 51.4, 52.4, 59.1, 61.4, 67.2, 77.5, 94.7, 106.2, 157.3,
175.0, 177.4; m/z (EI) 364 (5%), 346.1415 (55%, M+-18. C19H22O6
requires 346.1416), 328 (40), 300 (35), 284 (30), 270 (32), 255 (25),
242 (30), 227 (19), 213 (24), 199 (48), 121 (48), 105 (80), 91 (96),
69 (100); m/z (Me-TMS, EI) 594 (M+,%), 579 (21), 562 (10), 536
(9), 429 (15), 401 (21), 385 (37), 374 (46), 355 (44); KRI 2745.
Notes and references
1 N. Murofushi, N. Takahashi, T. Yokata and S. Tamura, Agric. Biol.
Chem., 1969, 33, 592; N. Murofushi, N. Takahashi, T. Yokata and S.
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4 E. Kovats, Helv. Chim. Acta, 1958, 41, 1951.
5 S. J. Gilmour, P. Gaskin, V. M. Sponsel and J. MacMillan, Planta, 1984,
161, 186; L. J. Beeley, PhD Thesis, University of Bristol, 1975.
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9 An analogous retro-Claisen reaction had previously been carried out
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10 In 1H-NMR spectra, 3,4-seco-gibberellins typically show a doublet
at ca. 1.2 ppm for the endo-methyl epimers (as determined by NOE
experiments in the GA4 series), while the exo-epimers afford a doublet
at ca. 1.4 ppm.
Preparation of 18-alkoxy derivatives 19a–26a and 19b–25b
General procedure. DBU (0.18 mL,) was added to a solution
of aldehyde 17 (50 mg) in 3 mL of alcohol and the reaction was
stirred at room temp for 18 h. After quenching with 1 M HCl, the
reaction mixture was poured into EtOAc and water. The organic
phase was washed with water (×2). The combined aqueous phase
was extracted with EtOAc (×3). The combined organic phase was
washed with brine, dried over MgSO4 and the solvent removed.
Purification by silica gel chromatography (5 : 1 hexane–EtOAc–1 :
1 EtOAc–hexane) furnished in sequence, the 3b-epimer followed
by the 3a-isomer, both as colourless oils. Yields and ratios are
summarized in Table 1. Selected NMR data are available in the
ESI (Table 3).†
11 J. S. Bindra, A. Grodski, T. K. Schaaf and E. J. Corey, J. Am. Chem.
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12 R. P. McGeary, Tetrahedron Lett., 1998, 39, 3319.
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16 Cf.J. MacMillan and R. J. Pryce, J. Chem. Soc., 1967, 740.
17 R. Pappo, D. S. Allen, R. U. Lemieux and W. S. Johnson, J. Org. Chem.,
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18 W. M. Pearlman, Tetrahedron Lett., 1967, 8, 1663.
19 L. Lombardo, Tetrahedron Lett., 1982, 23, 4293.
20 P. A. Bartlett and W. S. Johnson, Tetrahedron Lett., 1970, 21, 4459.
21 P. Gaskin and J. MacMillan, GC-MS of Gibberellins and Re-
lated Compounds: Methodology and a Library of Reference Spec-
tra, Cantock Enterprises, Bristol, U.K., 1992; http://www.plant-
hormones.info/gibberellin_nomenclature.htm.
22 P. M. Chandler and M. Robertson, Plant Physiol., 1999, 120, 623.
23 P. M. Chandler, A. Marion-Poll, M. Ellis and F. Gubler, Plant Physiol.,
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24 J. D. B. Weyers, N. W. Paterson, R. A’Brook and Z.-Y. Peng, Physiol.
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Acknowledgements
The authors wish to thank Bruce Twitchin (RSC) and Carol
Harding (CSIRO) for technical assistance.
1424 | Org. Biomol. Chem., 2008, 6, 1416–1424
This journal is
The Royal Society of Chemistry 2008
©