First total synthesis of ampullosine, a unique isoquinoline alkaloid isolated from Sepedonium ampullosporum, and of the related permethylampullosine

Article abstract of DOI:10.1039/c9ra06839b

A straightforward and convenient approach toward the first total synthesis of ampullosine, a structurally unique 3-methylisoquinoline alkaloid isolated from Sepedonium ampullosporum, is reported. Access to the related O-methyl ampullosine methyl ester from a common intermediate is also disclosed. The synthetic sequence toward the natural product comprised a Kolbe-type carboxylation of 3,5-dihydroxybenzoic acid and further esterification of the diacid, followed by masking of one of the phenols through selective ester reduction and subsequent acetonide formation. Installation of the three-carbon atom required for the 3-methylpyridine ring was performed by triflation of the remaining free phenol and a Pd-catalyzed Suzuki-Miyaura reaction with potassium E-propenyltrifluoroborate. Deprotection of the acetonide, followed by partial oxidation of the benzylic alcohol to the salicylaldehyde, O-methylation of the free phenol and hydrazonation of the resulting ortho-anisaldehyde derivative gave a hydrazone-based 1-azatriene. This was further subjected to 6π-azaelectrocyclization to afford permethylampullosine (11 steps, 14% overall yield), whereas exhaustive demethylation with AlI₃ generated in situ gave ampullosine (12 steps, 3.2% global yield).

Full text of DOI:10.1039/c9ra06839b

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First total synthesis of ampullosine, a unique
isoquinoline alkaloid isolated from Sepedonium
ampullosporum, and of the related
permethylampullosine†
Cite this: RSC Adv., 2019, 9, 33096
*
and Teodoro S. Kaufman
*
Didier F. Vargas,
Enrique L. Larghi
A straightforward and convenient approach toward the first total synthesis of ampullosine, a structurally
unique 3-methylisoquinoline alkaloid isolated from Sepedonium ampullosporum, is reported. Access to
the related O-methyl ampullosine methyl ester from a common intermediate is also disclosed. The
synthetic sequence toward the natural product comprised
a Kolbe-type carboxylation of 3,5-
dihydroxybenzoic acid and further esterification of the diacid, followed by masking of one of the phenols
through selective ester reduction and subsequent acetonide formation. Installation of the three-carbon
atom required for the 3-methylpyridine ring was performed by triflation of the remaining free phenol
and a Pd-catalyzed Suzuki–Miyaura reaction with potassium E-propenyltrifluoroborate. Deprotection of
the acetonide, followed by partial oxidation of the benzylic alcohol to the salicylaldehyde, O-methylation
of the free phenol and hydrazonation of the resulting ortho-anisaldehyde derivative gave a hydrazone-
based 1-azatriene. This was further subjected to 6p-azaelectrocyclization to afford
permethylampullosine (11 steps, 14% overall yield), whereas exhaustive demethylation with AlI3 generated
in situ gave ampullosine (12 steps, 3.2% global yield).
Received 28th August 2019
Accepted 7th October 2019
DOI: 10.1039/c9ra06839b
rsc.li/rsc-advances
Chemical investigations of Sepedonium spp. revealed that this
species is an interesting source of various natural products with
Introduction
potential biopharmaceutical applications (Fig. 1), including
peptaibols, uncommon peptides with a broad spectrum of
biological activities,4 the unusual cyclic peptide chrysosporide,5
the azaphilone chrisodin (1),6 as well as some mono- and bis-
anthraquinones (skyrin, rugulosin, chrysophanol)1,7 and tropo-
lone derivatives (2–3),8 among others.5
Infected organisms are rarely colonized by secondary fungal
parasites, to compete with Sepedonium for host nutrients. This
observation supports the speculation that Sepedonium species
produce small defense molecules that prevent further infections.
Recently, the group of Arnold examined the culture extract of
S. ampullosporum and isolated ampullosine (4).9 This alkaloid,
responsible for the deep yellow color of its culture uid, is the
rst and only isoquinoline isolated from the genus Sepedonium.
Its structure was established by spectroscopic means.
The genus Sepedonium (Ascomycetes) is saproparasitic and
comprises asexual states of fungi that parasitize on other fungi;
more specically basidiocarps of the mushroom order Bole-
tales.1 They operate by rst settling on their eshy living hosts,
which they kill to employ the organic matter as nutrients to
grow and develop. These fungicolous species have a mould-like
aspect, with thick-walled gold-yellow colored aleurioconidia
and colorless thin-walled phialoconidia.
The genus was established at the beginning of the XIX
century. However, the seminal works of Petch and Pieschel2
described variations of S. chrysospermum, namely S. ampullo-
sporum,3a S. chalcipori,3b and S. microspermum,3c which along
with S. laevigatum ended up conforming autonomic species on
the basis of sequence analysis of the internal transcribed spacer
region of the nuclear ribosomal RNA genes.1
In addition, methylation of the heterocycle afforded O-
methyl ampullosine methyl ester (5), which contributed to the
identication of the natural product. Employing LC/ESI, the
authors were able to detect ampullosine in ten out of twelve
strains of Sepedonium, belonging to eight different species.
However, they observed that the two most phylogenetically
distant species lacked the alkaloid.
Fungi of the genus Sepedonium have been studied as
producers of antibiotics and pigments, as well as for their
properties as antagonist agents of various plant pathogens.
´
Instituto de Quımica Rosario (IQUIR, CONICET-UNR), Facultad de Ciencias
´
´
Bioquımicas y Farmaceuticas, Universidad Nacional de Rosario, Suipacha 531,
S2002LRK Rosario, Argentina. E-mail: Kaufman@iquir-conicet.gov.ar; larghi@
iquir-conicet.gov.ar
The structure of ampullosine is unique. Despite some 3-meth-
ylisoquinolines have been synthesized, the combination of this
† Electronic supplementary information (ESI) available: NMR spectra of the
intermediates and nal products. See DOI: 10.1039/c9ra06839b
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Scheme 1 Retrosynthetic analyses of ampullosine (4) and its derivative
5.
phenolic moiety, according to the nal targets and an ortho-
disubstituted carbonyl condensed with an ammonia derivative
which carries a suitable leaving group.
A couple of additional disconnections were carried out on 10
to uncover the salicylaldehyde derivative 11. The reasoning was
that condensation with an ammonia derivative (oxime, hydra-
zone, etc.) could provide the required activated nitrogen atom
for the pyridine ring of the target; in addition, it was con-
jectured that a suitable cross-coupling reaction with an allyl/
propenyl derivative15 could perform the substitution of a prop-
erly located halogen or activated phenol, enabling the installa-
tion of the three carbon atoms chain, required to build the
heterocyclic ring.
Further analysis of 11 determined that its substituent
pattern could be advantageously simplied towards a suitable
starting material, by considering an oxidation state adjustment
of the aldehyde. This placed terephthalic acid derivative 12 in
the path toward the natural product. In turn, the ortho-disub-
stituted carboxylic acid was disconnected, to reveal compound
13 as an apt starting material, considering the availability of
protocols to access salicylic acid derivatives from phenols.16
Interestingly, despite different 3-halobenzoic acids have
been reported in the literature,17 they exhibited some accessi-
bility drawbacks; among them, lack of suitable precursors or
efficient transformations from commercial materials. There-
fore, it was concluded that among the possible candidates to
full the odd structural requirements imposed by the retro-
synthetic analysis, only 3,5-dihydroxybenzoic (14) is inexpen-
sive, easily available and displays the most ideal substitution
pattern.
Fig. 1 Chemical structures of some polyketides (1–3) and ampullosine
(4), isolated from Sepedonium species, as well as the semisynthetic
ampullosine derivative (5) and structurally related, synthetic 3-methyl
6-carboxymethyl isoquinolines (6–9).
motif with a 6-carboxylic acid moiety is rare; further, although they
are seldom found in patents,10 a few and scattered examples started
to appear in the literature just recently, favored by the ourishment
of research into C–H activation reactions based on ruthenium (6)11
and rhodium (7–9)12 catalysis. In many cases, however, the current
limitations of this novel chemistry still turn unfavorable its use to
approach ampullosine.
We have been interested in the synthesis of structurally
unique heterocyclic natural products13 and have used the 6p-
electrocyclization of 1-azatrienes as an efficient and atom-
economic strategy toward the synthesis of natural products or
their analogs, containing the 3-methyl isoquinoline motif.14
In pursuit of these interests, herein we wish to report the rst
total synthesis of ampullosine (4) and the synthesis of O-methyl
ampullosine methyl ester (5), from a common intermediate, using
the easily available 3,5-dihydroxybenzoic acid and employing a 6p-
azaelectrocyclization reaction as the key transformation, according
to the retrosynthetic analysis depicted in Scheme 1.
On the other hand, despite various methodologies are
available for the ortho formylation of phenols, some protocols
such as Duff, Skattebøl, Reimer–Tiemann, Vilsmeier–Haack
and Gattermann–Koch require activated substrates, while
others (ortho metalation) demand protection of the sensitive
groups, such as the carboxyl moiety, and all of them may afford
mixtures of isomeric products.18 Furthermore, some literature
Results and discussion
The initial N–C3 disconnection of the analysis was performed
on 4 and 5 and unveiled the polysubstituted benzenoid 10 as
a t key intermediate for the proposed 6p-azaelectrocyclization
based ring closure toward both targets. The main features of 10
are a properly masked carboxyl group, a differentially protected
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precedents suggested that formylation of 13 may result in the catalyst, furnished 96% of the triate 18.22b,c Interestingly
undesired formyl derivative, and in low yield.19
chromatographic elution with hexane:CH2Cl2 mixtures proved
Therefore, the synthesis commenced with 14 (Scheme 2), critical to obtain the product free from N-phenyltriamide, a by-
which was submitted to a Kolbe–Schmitt type carboxylation product of the transformation.
with KHCO3 at 180 C, under a CO2 atmosphere (balloon).20 It
In continuation of the sequence, the triate was submitted
ꢀ
n
was observed that the use of glycerol as reaction medium to a Stille cross-coupling reaction with Bu3SnCH2CH]CH2 in
resulted in a deep red coloured gummy mass, which difficulted diglyme, in the presence of LiCl and under PdCl2(PPh3)2 catal-
extraction and product purication, affording 15 in poorly ysis. This afforded the expected allyl-substituted compound 19
reproducible yields, below 60%. Luckily, however, employing in mixture (95 : 5) with the propenyl derivative E-20 in
a minimum amount of propylene glycol (nal concentration of combined 70% yield, as observed in its 1H NMR spectrum.
14 was ꢁ4 mol Lꢂ1) as solvent provided 15 as an easily recov- However, despite our experience, it was found comparatively
erable pale yellow solid in better yield (80%).
difficult to obtain clean samples of 19, devoid of undesirable
Furthermore, similar performances were observed when the contamination with tin residues, which somehow hindered
reaction was carried out in a closed vessel, purged with CO2, a proper development of the projected isomerization step
a requirement of the reaction mechanism.21 Without purica- toward 20.
tion, the resulting solid mass containing the diacid product 15
was submitted to selective esterication with Me2SO4 providing needs, due to their improved air-stability, reduced cost and
the dimethyl ester 16 (70% yield for the two steps). improved nucleophilicity, the use of organotriuoroborate salts
Therefore, an alternative was sought. Fortunately for our
In turn, the diester was selectively reduced with NaBH4, has emerged23 as an excellent alternative to the boronates and
under anchimeric assistance provided by the neighbouring boronic acids, expensive conventional substrates for the
phenol moieties to the ortho-disubstituted C4 carboxylate, and Suzuki–Miyaura reactions.24 Their advantages are more evident
the resulting benzyl alcohol was converted to the acetonide 17 when it is also considered that vinylboronic acids oen require
in 75% overall yield.
In order to install the projected three carbon atom chain for issues, tending to polymerize.26
excess reagent to achieve better yields25 and suffer from stability
the heterocyclic ring, compound 17 was next activated toward
Therefore, the modication of the Suzuki–Miyaura reaction
a Stille cross-coupling reaction, by triate ester formation with pioneered by Molander was implemented, and the cross-
PhNTf2. The latter reagent was elected over the more reactive coupling of 18 was performed with potassium propenyl tri-
Tf2O aer considering previous failures of the anhydride in uoroborate (21)27 and Cs2CO3 as base. The reaction required
similar transformations.22a
optimization (Table 1), since the initial results with Pd(OAc)2 as
Luckily, the reaction of 17 with N-phenyl triimide, the metal source and DavePhos as ligand afforded only
employing Et3N as base and DMAP as nucleophilic substitution moderate yields of 20 (53–58%) when the reaction was run in
a toluene-water system (4 : 1, v/v) at 90–100 ꢀC during 16–24 h,
respectively (entries 1 and 2).
Furthermore, under these conditions, the required product
was observed in mixture with minor amounts of the Z-isomer 22
(3–4%) and with the allyl derivative 19 (9–13%). This was
somewhat surprising, since alkenyltriuoroborates are known
to retain the double bond geometry with a high degree of
delity.24b
Changing the palladium source to PdCl2(PPh3)2 did not met
with much better success (entries 3 and 4) when DPPF was
employed as ligand (36% yield) or in the presence of DavePhos
in dioxane/H2O (10 : 1, v/v). Similar moderate results (53–56%
yield) were obtained with the use of Pd(OAc)2 and DavePhos in
THF/H2O (4 : 1, v/v) at 70 ꢀC for 15 h (entry 5), where some
unreacted starting material (10%) was concomitantly isolated,
along with the proto-deoxygenated product 23 (12%), recog-
nized through the diagnostic signals in its 1H NMR spectrum [d
7.55 (dd, J ¼ 1.5 and 7.8, H-2), 7.50 (d, J ¼ 1.5, H-6) and 7.03 (d, J
¼ 7.8, H-3)].
Employing a toluene/water solvent in association with
Scheme 2 Reagents and conditions: (a) CO2 (sealed tube), KHCO3, PdCl2(PPh3)2 was found to afford signicantly improved results,
1,2-propanediol, 180 ꢀC, 6 h; (b) Me2SO4, KHCO3, acetone, reflux, 10 h,
attaining 85% yield of 20 (entry 6). This performance could not
be surpassed by the use of BuPhos as ligand (entry 7) nor by
microwave irradiation (entry 8), which gave 20 in 68–69% yield.
In all these cases, mixtures of 19, 20 and 22 were observed.
(70% overall); (c) (1) NaBH4, THF/H2O (4 : 1), 0.1 M phosphate buffer pH
7.5, 0 ꢀC/r.t., 1.5 h; (2) 2,2-DMP, TsOH, r.t., 5 h (75% overall); (d)
PhNTf2, DMAP, NEt3, CH2Cl2, r.t. 16 h, (96%); (e) nBu3SnCH2CH]CH2,
PdCl2(PPh3)2, PPh3, LiCl, diglyme, 125 ꢀC, 15 h (19 and 20, 95 : 5, 70%
global); (f) RuClH(CO)(PPh3)3 (cat.), PhMe, 80 ꢀC.
t
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Table 1 Optimization of the propenylation of the triflate 18a
Yield (%)
Run no.
Catalyst
Ligand
Solvent
Temp. (ꢀC)
Time (h)
18
19
20
22
23
1
2
3
4
5
6
7
8
9
Pd(OAc)2
Pd(OAc)2
DavePhos
DavePhos
DPPF
DavePhos
DavePhos
DavePhos
tBuPhos
PhMe/H2Ob
PhMe/H2Ob
PhMe/H2Ob
Dioxane/H2Oc
THF/H2Od
100
90
100
100
70
100
100
100
80
16
24
15
15
15
15
15
0.7
24
—
—
—
—
10
—
4
13
9
—
3
—
4
3
5
—
53
58
36
47
56
85
68
69
95
4
3
—
3
—
4
4
6
—
—
—
—
2
12
—
—
—
—
PdCl2(PPh3)2
PdCl2(PPh3)2
Pd(OAc)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PhMe/H2Ob
PhMe/H2Ob
PhMe/H2Ob,e
THF/H2Od,f
DavePhos
DavePhos
—
—
a
The experiments were carried out at a nal substrate concentration of 0.1 M in the mixture of solvents, employing 1.6 equiv. of the triuoroborate
b
c
salt, 3 equiv. Cs2CO3, 5 mol% of the Pd catalyst and 10 mol% of the ligand. A PhMe/H2O 4 : 1 (v/v) medium was employed. The reaction was
carried in a dioxane/H2O 10 : 1 (v/v) medium. A THF/H2O 4 : 1 (v/v) medium was used. The reaction was carried out under microwave
irradiation (250 W). f LiCl (0.4 equiv.) was added.
d
e
Examining the whole set of results, it was noticed that the hand, the tested oxidants were also able to oxidize the neigh-
deoxygenated product 23 was obtained only in ethereal solvents. bouring phenol moiety to the related quinones.
It was conjectured that this outcome may be the result of the
As shown in Table 2, a series of trial and error experiments
lack of reactivity or stability of some reaction intermediate. with PDC,29 IBX30 and activated MnO2 (ref. 31) were executed.
Thus, it was thought that addition of LiCl may improve the Aer initial discouraging results (entries 1–3) where 25 was
results.
obtained (20–34% yield) along with unidentiable products,
It has been shown that LiCl is a required additive when the presumable resulting from overoxidation,32 it was found that
transformation is run in ethereal solvents, because it may act as MnO2 provided a signicantly better yield when the solvent was
a source of chloride ligand, stabilizing the Pd intermediates and changed from CH2Cl2 to EtOAc (entry 4).
making the cross-coupling stage more efficient.28 It has also
Additionally, it was observed that warming to 50 ꢀC
been demonstrated that the lithium salt turns the Pd catalyst improved the performance to 54% yield (entry 5), also short-
more active towards transmetallation and increases its prone- ening the reaction time from 16 h to 5 h, whereas the best
ness to undergo oxidative addition, resulting in a powerful results (55% yield) were achieved in boiling EtOAc for 3 h
accelerant of the reaction rate. Furthermore, LiCl enhances (entry 6), when it was observed complete consumption of the
both the polarity of the solvent, and the leaving ability of starting material.
anionic ligands.
Therefore, LiCl was added to a THF/H2O mixture of the
starting triate, Cs2CO3, PdCl2(PPh3)2 and DavePhos. Reward-
ingly, the LiCl supplemented reaction cleanly afforded 95%
yield of the expected derivative 20, free of isomers and under
ꢀ
milder conditions (80 C), as shown in entry 9. Next, the ace-
tonide moiety of 20 was smoothly hydrolysed by exposure to p-
toluenesulfonic acid in THF/H2O, affording the benzylic alcohol
24 in almost quantitative yield (Scheme 3).
However, oxidation of the latter to the corresponding alde-
hyde 25, which is a key step of the synthesis, proved to be tricky.
The starting alcohol demonstrated to exhibit some instability,
probably favoured by an easy benzylic protonation, followed by
Scheme 3 Reagents and conditions: (a) TsOH (cat.), THF/H2O, 60 ꢀC,
48 h, (98%) or 1 M HCl, THF, 45 ꢀC, 18 h, (90%); (b) see Table 2.
dehydration and further quinomethide formation; on the other
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Aer having secured a satisfactory access to the key propenyl
salicylaldehyde 25, the latter was submitted to our recently
developed one-pot hydrazonation/6p-azaelectrocyclization
protocol.33 Thus, 25 was exposed to 1,1-dimethylhydrazine in
PhCF3 with the addition of AcOH as a promoter, to afford the
hydrazone intermediate 26 (Scheme 4). Without purication,
the hydrazone was heated at 160 ꢀC for 45 min in the same
medium, under microwave irradiation. Unexpectedly, however,
the process resulted in recovery of 50% of the starting aldehyde
25, and no signs of the expected product 27 could be detected.
Under the hypothesis that the presence of a free phenolic
hydroxyl group could have caused the observed outcome, 25 was
protected with a readily removable group. Therefore, 25 was
treated with MeSO2Cl and Et3N and DMAP in CH2Cl2, to afford
55% yield of the unstable mesylate ester 28. The latter exhibited
diagnostic signals of the methanesulfonate group, as a singlet
resonating at dH 3.32 ppm and a resonance at dC 38.6 ppm.
Unfortunately, however, application of the one-pot hydra-
zonation/6p-azaelectrocyclization protocol also met with
failure; neither the expected mesylate 30 nor the isoquinoline
27 were found.33
In view of these discouraging results, an alternate protecting
group was sought. Considering our previous results with the
benzyl ether and MOM as protecting groups,33 and taking into
account our objective, to synthetically access per-
methylampullosine (5), the intermediate 25 was O-methylated
with MeI in DMF at 0 ꢀC, employing K2CO3 as base, to give 31 in
85% yield (Scheme 5). Then, the latter was submitted to our
hydrazonation/6p-azaelectrocyclization/elimination protocol,33
affording 5 in 64% overall yield, through the intermediacy of
32a, which was not isolated because of its poor hydrolytic
stability.
Scheme 4 Reagents and conditions: (a) Me2N–NH2, AcOH, PhCF3,
r.t., 1.5 h; (b) MW, 160 ꢀC, 45 min (25, 50% recovered); (c) MsCl, Et3N,
CH2Cl2, 0 ꢀC/r.t., 24 h (55%); (d) MW, 160 ꢀC, 45 min.
was converted into the methoxime intermediate 32b by reaction
with methoxylamine in absolute EtOH. Aer replacing the
solvent with PhCF3, compound 32b was heated under micro-
ꢀ
wave irradiation at 180 C for 45 min to undergo the 6p-azae-
lectrocyclization, furnishing 5 in 50% yield, accompanied by the
related 3,4-dihydroisoquinoline 33 (15% yield).
With these results it was concluded that the hydrazone
pathway should be preferred to the methoxime alternative. The
1H and 13C NMR spectroscopic data of the synthetic compound
5 fully matched those reported by the group of Arnold for their
semisynthetic O-methyl ampullosine methyl ester.9
It was also found that treatment of 5 with aluminium and
iodine (which generates in situ the powerful Lewis acid AlI3),35
Interestingly, the 3,4-dihydroisoquinoline 33 was isolated
concomitantly in 26% yield. The latter was recognized by its
pattern of signals in the 1H NMR spectrum, related to H-3 (m),
H-4 (dd) and Me-3 (d). A reaction mechanism to explain the
generation of 3,4-dihydroisoquinolines as side products of 6p-
azaelectrocyclization of similar heterocyclic systems has been
proposed.33
It has been shown that, in aqueous solution, aliphatic
hydrazones are 102–103 times more sensitive to hydrolysis than
the analogous oximes.34 Therefore, conjecturing that the use of
a more stable ammonia derivative for condensation with 31 may
improve the performance of the 6p-azaelectrocyclization, 31
Table 2 Optimization of the oxidation of benzyl alcohol 24 to 25
Oxidizing
Run no. agent
Temperature
Solvent (ꢀC)
Time (h) Yield (%)
1
2
3
4
5
6
IBX
PDC
MnO2
MnO2
MnO2
MnO2
EtOAc
CH2Cl2 r.t.
CH2Cl2 r.t.
EtOAc
EtOAc
EtOAc
50
1.5
2
16
16
5
23
34
20
50
54
55
Scheme 5 Reagents and conditions: (a) MeI, K2CO3, DMF, 0/t.a.,
1.5 h (85%); (b) Me2N–NH2, AcOH, PhCF3, r.t., 1.5 h or MeO–NH2,
EtOH, r.t., 1 h; (c) MW, 160 ꢀC (via 32a: 1 h, 5, 64%; 33, 26%; via 32b;
45 min, 5, 50%; 33, 15%); (d) Al0, I2, DMSO, MeCN, 80 ꢀC, 20 h (23%).
r.t.
50
78
3
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effected the simultaneous removal of both methyl groups, peaks of CDCl3 (dH ¼ 7.26 ppm, dC ¼ 77.16 ppm), DMSO-d6 (dH
affording ampullosine (4) in 23% yield.
¼ 2.50 ppm, dC ¼ 39.52 ppm) and CD3OD (dH ¼ 3.31 ppm, dC ¼
49.00 ppm) as well as the signal of CFCl3 for (dF ¼ 0.00 ppm),
were used as internal reference. The magnitudes of the coupling
constant (J) values are given in hertz. To complete the elucida-
tion, NOE and 2D NMR experiments (HSQC and HMBC) were
employed. HRMS was obtained with a Bruker MicroTOF-Q II
instrument from UMyMFOR (Buenos Aires, Argentina). The
microwave-assisted reactions were carried out in a CEM
Discover microwave reactor. The UV-Vis spectrum of ampullo-
sine (4) was acquired in an Agilent model 8453 spectropho-
tometer equipped with a diode-array detector, in the range 205–
700 nm and matched quartz cells (10 mm optical path). The
sample was dissolved in MeOH, and the spectrum was taken
against a blank of the solvent.
Dimethyl 2,6-dihydroxyterephthalate (16). A mixture of 3,5-
dihydroxybenzoic acid (14) (380 mg, 2.5 mmol) in 1,2-pro-
panediol (700 mL) was heated at 100 ꢀC in a pressure tube until
the mixture was completely homogenized. Aer cooling, KHCO3
(1.25 g, 12.5 mmol) was added, and the resulting suspension
was purged with CO2, sealed and heated at 180 ꢀC for 6 h. Aer
cooling, water (50 mL) and concentrated HCl (50 mL) were
added to the resulting whitish residue, the mixture was extrac-
ted with Et2O (6 ꢃ 50 mL). The combined extracts were dried
over MgSO4 and the solvent was evaporated in vacuo on a rotary
evaporator, affording the product dicarboxylic acid product 15
(381 mg, 80%) as a pale yellow solid. Without further purica-
tion, this solid was dissolved in acetone (5 mL), KHCO3 (500 mg,
5 mmol) was added and the mixture was stirred for 15 min at
room temperature under an argon atmosphere. Then, Me2SO4
(470 mL, 5 mmol) was added dropwise, and the suspension was
heated at reux for 10 h. Aer completion of the reaction, the
volatiles were evaporated, saturated NaHCO3 (50 mL) was added
to the residue and the products were extracted with EtOAc (3 ꢃ
45 mL). The combined organic phases were dried over MgSO4,
ltered, and concentrated under reduced pressure. The residue
was puried by column chromatography (hexane:EtOAc, 1 : 0
/ 2 : 8 v/v) to afford the desired product 16 (394 mg, 70%, two
steps), as a white solid, Rf: 0.53 (hexane : EtOAc, 6 : 4 v/v), mp:
146–148 ꢀC (lit. 20a and 20d 146–149 ꢀC); 1H NMR d: 9.67 (br s,
w1/2 ¼ 10.2, 2H, ArOH), 7.11 (s, 2H, H-3 and H-5), 4.11 (s, 3H, 1-
CO2Me) and 3.90 (s, 3H, 4-CO2Me); 13C NMR d: 169.6 (1-CO2Me),
165.8 (4-CO2Me), 160.8 (C-2 and C-6), 137.3 (C-4), 109.2 (C-3 and
C-5), 102.9 (C-1), 53.4 (1-CO2Me) and 52.6 (4-CO2Me).
Experimental
General information
All the reactions were carried out under dry argon atmospheres,
using oven-dried glassware and freshly distilled anhydrous
solvents. Anhydrous CH2Cl2 was obtained from an M. Braun
solvent purication and dispenser system. Anhydrous EtOAc
was obtained by a 3 h reux of the PA grade product over P2O5
and further distillation of the liquid. Anhydrous DMF was ob-
tained by heating the PA grade product over BaO for 4 h, fol-
lowed by distillation under reduced pressure. Anhydrous Et3N
was prepared by distillation of the commercial product from
CaH2. The anhydrous solvents were stored in dry Young
ampoules. All other solvents and reagents were used as
received.
The reactions were monitored by TLC (silica gel 60 GF254) run
in different hexanes:EtOAc mixtures. The chromatographic
spots were revealed by exposure to UV light (254 and 365 nm)
and spraying with ethanolic p-anisaldehyde/sulfuric acid
reagent, followed by careful heating to improve selectivity; to
detect the isoquinolines, the Dragendorff reagent (Munier and
Macheboeuf modication)36a was used. The ash column
chromatographies were performed under conditions that
entailed a modication of known procedures.36b–d They were run
under positive pressure with slurry-packed silica gel 60 H for
thin layer chromatography (particle size < 55 mm), employing
gradient of solvent polarity techniques with hexane : EtOAc,
hexane : CH2Cl2 and EtOAc : EtOH, as required. The column
size (height and diameter), amount of silica gel, solvent polarity
variation (gradient) step, solvent volume per step and fraction
volume were adjusted for each case (compound and reaction),
taking into account mainly the amount of sample, its polarity
and the presence of impurities, especially if their Rf value in the
corresponding TLC analysis was close to that of the product.
The chromatographic variables were adjusted aiming to elute
the product approximately aer the 15th tube, when the solvent
polarity was equivalent to that yielding a Rf z 0.50 in the TLC
plate. The chromatographies were continued aer elution of the
expected product, in order to ensure recovery of all the relevant
compounds.
Methyl 5-hydroxy-2,2-dimethyl-4H-benzo[d][1,3]dioxine-7-
carboxylate (17).20a,20d A mixture of NaBH4 (195 mg, 5.16
Equipment
The melting points (uncorrected) were measured on an Ernst mmol) in 0.05 M phosphate buffer, pH 7.5 (1.5 mL) was added
Leitz Wetzlar model 350 hot-stage microscope. The FT-IR dropwise to a stirred solution of 16 (390 mg, 1.72 mmol) in THF
spectra were recorded on a Shimadzu Prestige 21 spectropho- (9 mL) cooled to 0 ꢀC in an ice-water bath. The resulting solution
tometer, as solid dispersions in KBr disks (solid samples). The was stirred for 1.5 h, when the reaction was quenched with 10%
NMR spectra were recorded in CDCl3 unless informed otherwise citric acid solution (5 mL) at 0 ꢀC. Then, brine (20 mL) was
with a Bruker Avance 300 NMR spectrometer (300.13 MHz for added and the organic products were extracted with EtOAc (3 ꢃ
1H, 75.48 MHz for 13C and 282.38 MHz for 19F). In addition, 20 mL). The combined organic layers were dried over MgSO4
some spectra were acquired with a Bruker Avance 400 MHz and concentrated in vacuo. The crude residue was puried by
spectrometer (400.13 MHz for 1H, 100.61 MHz for 13C). The column chromatography to give methyl 3,5-dihydroxy-4-
chemical shis are reported in ppm on the d scale, and TMS was (hydroxymethyl)benzoate (286 mg), as a whitish solid. The
used as the internal standard (d ¼ 0.0 ppm); the residual solvent product was ltered through a short silica gel column and
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without further purication, this solid was dissolved in 2,2- compound 19 as a 95 : 5 mixture with the related b-propenyl
dimethoxypropane (5 mL), p-TsOH$H2O (32 mg, 0.17 mmol) derivative E-20 (14.6 mg, 70%), as a colour5 0.47 (hex-
was added at 0 C, the ice bath was removed, and the mixture ane : EtOAc, 9 : 1 v/v); 1H NMR d: 7.43 (d, J ¼ 1.4, 1H, H-6), 7.39
ꢀ
was stirred for 5 h at room temperature. The reaction mixture (d, J ¼ 1.4, 1H, H-8), 5.90 (ddt, J ¼ 17.0, 10.1 and 6.2, 1H,
was placed again in an ice bath and NaHCO3 (20 mg) was added. ArCH2CH]CH2), 5.10 (ddt, J ¼ 10.1, 1.4 and 1.4, 1H, ArCH2-
Aer stirring for 5 min, the resulting solution was puried by CH]CH2cis), 5.00 (ddt, J ¼ 17.0, 1.6 and 1.6, 1H, ArCH2CH]
silica gel column chromatography (hexane : EtOAc, 1 : 0 / 1 : 1 CH2trans), 4.83 (s, 2H, H-4), 3.88 (s, 3H, CO2Me), 3.25 (dt, J ¼ 6.2
v/v), furnishing 17 (307 mg, 75%, two steps), as a white solid, Rf: and 1.6, 2H, ArCH2CH]CH2) and 1.52 (s, 6H, Me-2); 13C NMR d:
0.50 (hexane : EtOAc 7 : 3 v/v), mp: 141–142 ꢀC; IR (KBr, n^): 3327, 166.9 (CO2Me), 151.5 (C-8a), 136.2 (C-5), 135.1 (ArCH2CH]
2995, 2846, 1697, 1595, 1431, 1365, 1276, 1149, 1060, 950, 862, CH2), 130.0 (C-7), 123.2 (C-4a), 122.3 (C-6), 116.8 (ArCH2CH]
763 and 642 cmꢂ1; 1H NMR d: 7.15 (d, J ¼ 1.2, 1H, H-8), 7.11 (d, J CH2), 116.7 (C-8), 99.3 (C-2), 59.7 (C-4), 52.2 (CO2Me), 36.2
¼ 1.2, 1H, H-6), 5.92 (s, 1H, ArOH), 4.86 (s, 2H, H-4), 3.89 (s, 3H, (ArCH2CH]CH2) and 24.7 (Me-2).
CO2Me) and 1.55 (s, 6H, Me-2); 13C NMR d: 167.2 (CO2Me), 152.5
Methyl
(E)-2,2-dimethyl-5-(propen-1-yl)-4H-benzo[d][1,3]
(C-8a), 152.1 (C-5), 130.0 (C-7), 112.7 (C-4a), 111.0 (C-6), 107.7 (C- dioxine-7-carboxylate (20). A mixture of the triate 18 (100 mg,
8), 99.8 (C-2), 58.1 (C-4), 52.5 (CO2Me) and 24.7 (Me-2); HRMS 0.27 mmol), potassium trans-1-propenyltriuoroborate (68 mg,
(ESI-TOF): found m/z: 261.0729; C12H14NaO5 ([M + Na]+) 0.45 mmol), Cs2CO3 (264 mg, 0.81 mmol), LiCl (5 mg, 0.12
requires m/z: 261.0733.
mmol), DavePhos (10 mg, 10 mol%), and PdCl2(PPh3)2 (9 mg,
Methyl 2,2-dimethyl-5-{[(triuoromethyl)sulfonyl]oxy}-4H- 5 mol%), was placed in a pressure tube capped with septum.
benzo[d][1,3]dioxine-7-carboxylate (18). A stirred suspension of The solids were treated with THF (2 mL) and degassed water (0.5
phenol 27 (303 mg, 1.27 mmol) in anhydrous CH2Cl2 (4 mL) was mL) with stirring and then were purged with dry argon for 5
cooled to 0 ꢀC under argon and successively treated with DMAP minutes. The tube was sealed and the reaction mixture was
ꢀ
(152 mg, 0.12 mmol), Et3N (322 mg, 445 mL, 3.18 mmol) and heated at 80 C for 24 h in an oil bath. Aer cooling to room
PhNTf2 (635 mg, 1.77 mmol). The reaction mixture was le to temperature, the reaction mixture was diluted with water (6 mL)
acquire room temperature and further stirred overnight. Then, and extracted with CH2Cl2 (3 ꢃ 10 mL). The combined organic
the reaction was diluted with CH2Cl2 (50 mL), the organic phase phases were dried over MgSO4, ltered, and concentrated under
was washed with saturated NaHCO3 (2 ꢃ 50 mL), dried over reduced pressure. The residue was puried by column chro-
MgSO4, ltered, and concentrated under reduced pressure. The matography (hexane : EtOAc, 1 : 0 / 8 : 2 v/v), to afford the
residue was chromatographed (hexane : CH2Cl2, 1 : 0 / 3 : 2 v/ desired product 20 (67 mg, 95%), as a white solid, Rf: 0.47
v), furnishing 18 (451 mg, 96%), as a white solid, Rf: 0.20 (hexane : EtOAc, 9 : 1 v/v), mp: 70–71 ꢀC; IR (KBr, n^): 3030, 2999,
(hexane : CH2Cl2, 7 : 3 v/v), mp: 63–64 ꢀC; IR (KBr, n^): 3086, 2850, 1722, 1577, 1431, 1338, 1224, 1141, 1012, 904, 819 and
2997, 2962, 1724, 1629, 1583, 1423, 1328, 1253, 1138, 1029, 925, 767 cmꢂ1; 1H NMR d: 7.67 (d, J ¼ 1.4, 1H, H-6), 7.36 (d, J ¼ 1.4,
829, 767 and 638 cmꢂ1; 1H NMR d: 7.55 (d, J ¼ 1.4, 1H, H-6), 7.51 1H, H-8), 6.20–6.33 (m, 2H, ArCH]CHMe and ArCH]CHMe),
(d, J ¼ 1.4, 1H, H-8), 4.93 (s, 2H, H-4), 3.92 (s, 3H, CO2Me) and 4.88 (s, 2H, H-4), 3.89 (s, 3H, CO2Me), 1.91 (d, J ¼ 4.7, 3H,
1.56 (s, 6H, Me-2); 13C NMR d: 165.1 (CO2Me), 153.1 (C-8a), 145.1 ArCH]CHMe) and 1.53 (s, 6H, Me-2); 13C NMR d: 167.0
(C-5), 131.3 (C-7), 118.7 (q, J ¼ 320, CF3), 118.6 (C-6), 118.0 (C- (CO2Me), 151.4 (C-8a), 135.2 (C-5), 130.2 (ArCH]CHMe), 130.0
4a), 113.7 (C-8), 100.8 (C-2), 57.4 (C-4), 52.8 (CO2Me) and 24.6 (C-7), 125.6 (ArCH]CHMe), 121.2 (C-4a), 118.8 (C-8), 116.6 (C-
(Me-2); 19F NMR d: ꢂ73.3 (s, CF3SO3Ar); HRMS (ESI-TOF): found 6), 99.2 (C-2), 60.2 (C-4), 52.2 (CO2Me), 24.7 (Me-2) and 18.9
m/z: 393.0226; C13H13F3NaO7S ([M + Na]+) requires m/z: (ArCH]CHMe); HRMS (ESI-TOF): found m/z: 285.1098;
393.0226.
C
15H18NaO4 ([M + Na]+) requires m/z: 285.1097.
Methyl (E)-3-hydroxy-4-(hydroxymethyl)-5-(propen-1-yl)
Methyl
5-allyl-2,2-dimethyl-4H-benzo[d][1,3]dioxine-7-
carboxylate (19). A mixture of triate 28 (29 mg, 0.08 mmol), benzoate (24)
LiCl (3 mg, 0.07 mmol), Ph3P (10 mg, 50 mol%), and PdCl2(-
Method A. A stirred solution of the acetonide 20 (187 mg, 0.71
PPh3)2 (6 mg, 10 mol%), was placed in a tube, capped with mmol) and p-TsOH.H2O (26 mg, 0.14 mmol) in a THF : H2O
a septum, purged with dry argon for 5 minutes and then diluted mixture (1 : 1 v/v, 14 mL) was heated at 60 ꢀC for 48 h. The
ꢀ
with anhydrous diglyme (0.5 mL). Aer stirring for 5 min, the reaction was cooled to 0 C, quenched with saturated aqueous
resulting mixture was treated with Bu3SnCH2CH]CH2 (31 mL, NaHCO3 (30 mL), and the resulting mixture was extracted with
0.10 mmol), the reaction tube was sealed and the mixture was EtOAc (3 ꢃ 50 mL). The combined organic layers were dried
heated at 125 ꢀC for 15 h. Aer cooling to room temperature, the over MgSO4, and concentrated in vacuo. The crude residue was
reaction mixture was diluted with Et2O (15 mL), a saturated puried by column chromatography (hexane : EtOAc, 1 : 0 /
solution of KF (10 mL) was added and the mixture was stirred 4 : 6 v/v), to give 24 (154 mg, 98%), as a white solid, Rf: 0.25
for 20 min in order to quench the organotin-derivatives. The (hexane : EtOAc, 7 : 3 v/v), mp: 95–97 ꢀC; IR (KBr, n^): 3502, 3385,
aqueous phase was separated and the organic layer was washed 3169, 2953, 2850, 1689, 1581, 1427, 1340, 1247, 1099, 1001, 964,
1
with brine (3 ꢃ 8 mL). The organic solvent was ltered through 883 and 761 cmꢂ1; H NMR d: 8.12 (br s, 1H, W1/2 ¼ 15.3 Hz,
a short pad of Florisil and Celite (1 : 1 w/w) and the ltrate was ArOH), 7.54 (d, J ¼ 1.6, 1H, H-6), 7.35 (d, J ¼ 1.6, 1H, H-2), 6.48
dried over MgSO4 prior to concentration under reduced pres- (dq, J ¼ 15.5 and 1.7, 1H, ArCH]CHMe), 6.08 (dq, J ¼ 15.5 and
sure. Column chromatography (hexane : EtOAc, 1 : 0 / 8 : 2 v/ 6.6, 1H, ArCH]CHMe), 4.96 (s, 2H, ArCH2OH), 3.88 (s, 3H,
v) of the residue gave the corresponding allyl substituted CO2Me), 3.04 (br s, 1H, W1/2 ¼ 21.8 Hz, ArCH2OH) and 1.87 (dd, J
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¼ 6.6 and 1.7, 3H, ArCH]CHMe); 13C NMR d: 167.3 (CO2Me),
156.4 (C-3), 138.2 (C-5), 130.7 (ArCH]CHMe), 130.3 (C-4), 127.2
(ArCH]CHMe), 126.5 (C-1), 120.0 (C-6), 115.8 (C-2), 60.1
(ArCH2OH), 52.4 (CO2Me) and 18.8 (ArCH]CHMe); HRMS (ESI-
TOF): found m/z: 245.0782; C12H14NaO4 ([M + Na]+) requires m/
z: 245.0784.
Methyl (E)-4-formyl-3-methoxy-5-(propen-1-yl)benzoate (31).
A mixture of the methyl 3-hydroxybenzoate 25 (53 mg, 0.24
mmol) and K2CO3 (46 mg, 0.33 mmol) in anhydrous DMF (1 mL)
was stirred for 15 min at 0 ꢀC. The system was treated with
iodomethane (20 mL, 0.31 mmol) and the resulting reaction
mixture was stirred at room temperature for 1.5 h. Then, the
mixture was poured into cold water (10 mL), and the products
were extracted EtOAc (3 ꢃ 15 mL). The combined organic pha-
ses were washed with brine (25 mL), dried over MgSO4, ltered
and concentrated in vacuo. The residue was puried by column
chromatography (hexane : EtOAc, 1 : 0 / 7 : 3 v/v) to afford 31
(48 mg, 85%), as a yellowish-white solid Rf: 0.52 (hex-
ane : EtOAc, 8 : 2, v/v), mp: 72–73 ꢀC; IR (KBr, n^): 3086, 2954,
2873, 2785, 1718, 1681, 1566, 1404, 1330, 1238, 1103, 1010, 964,
819 and 765 cmꢂ1; 1H NMR d: 10.61 (d, J ¼ 0.5, CHO), 7.77 (ddd,
J ¼ 1.4, 0.7 and 0.7, 1H, H-6), 7.48 (d, J ¼ 1.4, 1H, H-2), 7.20 (dq, J
¼ 15.5 and 1.7, 1H, ArCH]CHMe), 6.30 (dq, J ¼ 15.5 and 6.6,
1H, ArCH]CHMe), 3.95 (s, 3H, ArOMe), 3.95 (s, 3H, CO2Me) and
1.93 (dd, J ¼ 6.6 and 1.7, 3H, ArCH]CHMe); 13C NMR d: 192.3
(CHO), 166.3 (CO2Me), 162.3 (C-3), 141.2 (C-5), 134.8 (C-1), 131.7
(ArCH]CHMe), 128.5 (ArCH]CHMe), 124.6 (C-4), 120.8 (C-6),
110.1 (C-2), 56.3 (ArOMe), 52.7 (CO2Me) and 18.9 (ArCH]
CHMe); HRMS (ESI-TOF): found m/z: 235.0960; C13H15O4 ([M +
H]+) requires m/z: 235.0965.
Method B. A stirred solution of 20 (98 mg, 0.37 mmol) in THF
(4.5 mL) was ꢀtreated with 1 M HCl (3 mL) and the system was
ꢀ
heated at 45 C for 18 h. The reaction was cooled to 0 C and
quenched with saturated NaHCO3 (10 mL); the organic products
were extracted with EtOAc (3 ꢃ 15 mL). The combined organic
layers were dried over MgSO4, and concentrated in vacuo. The
crude residue was puried by column chromatography to give
24 (74 mg, 90%). The spectroscopic data of the product were in
full agreement with those of the product synthesized according
to Method A.
Methyl (E)-4-formyl-3-hydroxy-5-(propen-1-yl)benzoate (25).
A mixture of the benzylic alcohol 24 (50 mg, 0.22 mmol) and
freshly activated MnO2 (287 mg, 3.30 mmol) was slurred in
anhydrous EtOAc (3 mL) and the system was heated to reux for
3 h. Aer cooling, the reaction mixture was ltered through
a Celite pad, and the ltrate was concentrated in vacuo. The
resulting yellow residue was puried by column chromatog-
raphy (hexane : EtOAc, 1 : 0 / 7 : 3 v/v), to afford 25 (27 mg,
55%), as a yellow solid, Rf: 0.50 (hexane : EtOAc, 8 : 2 v/v), mp:
140–141 ꢀC; IR (KBr, n^): 3043, 2990, 2848, 1716, 1647, 1558,
1417, 1348, 1247, 1174, 1097, 999, 887 and 766 cmꢂ1; 1H NMR d:
11.74 (s, 1H, ArOH), 10.36 (s, 1H, CHO), 7.53 (d, J ¼ 1.4, 1H, H-6),
7.46 (d, J ¼ 1.4, 1H, H-2), 6.88 (dq, J ¼ 15.5 and 1.7, 1H, ArCH]
CHMe), 6.21 (dq, J ¼ 15.5 and 6.6, 1H, ArCH]CHMe), 3.93 (s,
3H, CO2Me) and 1.97 (dd, J ¼ 6.6 and 1.7, 3H, ArCH]CHMe);
13C NMR d: 195.8 (CHO), 165.9 (CO2Me), 162.5 (C-3), 143.8 (C-5),
137.3 (C-1), 135.0 (ArCH]CHMe), 125.2 (ArCH]CHMe), 119.5
(C-4), 119.4 (C-6), 117.4 (C-2), 52.7 (CO2Me) and 19.1 (ArCH]
CHMe); HRMS (ESI-TOF): found m/z: 243.0633; C12H12NaO4 ([M
+ Na]+) requires m/z: 243.0628.
Methyl (E)-4-formyl-3-((methylsulfonyl)oxy)-5-(prop-1-en-1-
yl) benzoate (28). A stirred solution of 25 (35 mg, 0.16 mmol)
in CH2Cl2 (1 mL) was cooled in an ice-water bath and succes-
sively treated with treated with Et3N (34 mL, 0.24 mmol) and
MeSO2Cl (15 mL, 0.19 mmol). The reaction mixture was stirred at
room temperature for 16 h. Then, brine (5 mL) was added to the
reaction mixture and extracted with EtOAc (3 ꢃ 20 mL). The
combined organic phases were washed successively with 0.2 M
HCl (10 mL), saturated NaHCO3 (10 mL) and then dried over
anhydrous MgSO4. The solvent was removed under reduced
pressure and the residue was chromatographed (hex-
ane : EtOAc, 1 : 0 / 2 : 8 v/v) to afford 28 (27 mg, 55%; 77% br s
m), as an unstable colorless oil Rf: 0.56 (hexane : EtOAc, 1 : 1 v/
v); 1H NMR d: 10.43 (s, CHO), 8.13 (d, J ¼ 1.4, 1H, H-2), 7.88 (d, J
¼ 1.4, 1H, H-6), 7.06 (dd, J ¼ 15.6 and 1.7, 1H, ArCH]CHMe),
6.30 (dq, J ¼ 15.6 and 6.6, 1H, ArCH]CHMe), 3.96 (s, 3H,
CO2Me), 3.32 (s, 3H, MeSO3ꢂ) and 1.93 (dd, J ¼ 6.6 and 1.7, 3H,
ArCH]CHMe); 13C NMR d: 189.7 (CHO), 164.9 (CO2Me), 149.3
(C-3), 142.5 (C-5), 134.8 (C-1), 134.4 (ArCH]CHMe), 129.0
(ArCH]CHMe), 127.6 (C-4), 126.6 (C-6), 122.1 (C-2), 52.8
(CO2Me), 38.6 (MeSO3ꢂ) and 18.9 (ArCH]CHMe).
Methyl 8-methoxy-3-methylisoquinoline-6-carboxylate (per-
methyl ampullosine, 5)9 and methyl 8-methoxy-3-methyl-3,4-
dihydroisoquinoline-6-carboxylate (33)
Method A. Methyl 4-formylbenzoate 31 (22 mg, 0.09 mmol),
glacial AcOH (5 mL, 0.09 mmol) and 1,1-dimethylhydrazine (7
mL, 0.09 mmol) were added to a microwave tube and diluted
with PhCF3 (1 mL). The mixture was purged with argon and
stirred at room temperature until TLC analysis indicated
complete consumption of the starting aldehyde (ꢁ1.5 h). Then,
the vessel was capped and irradiated in the microwave reactor
(160 ꢀC, ca. 250 W, 1 h). Aer cooling to room temperature, the
solvent was recovered by careful distillation under atmospheric
pressure, and the residue was puried by column chromatog-
raphy (hexane/EtOAc, 1 : 0 / 0 : 1 v/v), to afford 5 (13.3 mg,
64%), as a yellowish solid, Rf: 0.60 (hexane/EtOAc, 6 : 4 v/v), mp:
126–129 ꢀC; IR (KBr, n^): 3068, 2947, 2848, 1716, 1633, 1570,
1
1433, 1354, 1238, 1122, 1035, 927, 825 and 762 cmꢂ1; H NMR
(300 MHz, CDCl3) d: 9.56 (s, 1H, H-1), 8.03 (s, 1H, H-5), 7.50 (s,
1H, H-4), 7.37 (d, J ¼ 1.2, 1H, H-7), 4.07 (s, 3H, ArOMe), 3.99 (s,
3H, CO2Me) and 2.71 (s, 3H, Me-3); 13C NMR (75 MHz, CDCl3) d:
166.9 (CO2Me), 156.9 (C-8), 153.5 (C-3), 147.4 (C-1), 137.2 (C-4a),
132.2 (C-6), 121.2 (C-5), 120.6 (C-8a), 119.1 (C-4), 103.6 (C-7),
56.0 (ArOMe), 52.7 (CO2Me) and 24.3 (Me-3); 1H NMR (400
MHz, CD3OD) d: 9.31 (s, 1H, H-1), 7.94 (s, 1H, H-5), 7.58 (s, 1H,
H-4), 7.34 (s, 1H, H-7), 4.05 (s, 3H, ArOMe), 3.97 (s, 3H, CO2Me)
and 2.63 (s, 3H, Me-3); 13C NMR (100 MHz, CD3OD) d: 167.8
(CO2Me), 158.0 (C-8), 153.7 (C-3), 147.5 (C-1), 138.6 (C-4a), 134.0
(C-6), 121.8 (C-5), 121.5 (C-8a), 121.0 (C-4), 104.7 (C-7), 56.5
(ArOMe), 53.1 (CO2Me) and 23.5 (Me-3); HRMS (ESI-TOF): found
m/z: 254.0798; C13H13NNaO3 ([M + Na]+) requires m/z: 254.0788.
The 1H and 13C NMR spectral data of compound 5 were in
agreement with the literature.9
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Increasing solvent polarity afforded 33 (5.5 mg, 26%), as an
amber solid, mp: 99–101 ꢀC; IR (KBr, n^): 3005, 2951, 2837, 1724,
1618, 1570, 1446, 1319, 1222, 1112, 1001, 968, 864 and
767 cmꢂ1; 1H NMR (300 MHz, CDCl3) d: 8.72 (d, J ¼ 2.6, 1H, H-
1), 7.45 (s, 1H, H-5), 7.42 (s, 1H, H-7), 3.93 (s, 6H, ArOMe and
CO2Me), 3.60–3.70 (m, 1H, H-3), 2.78 (dd, J ¼ 16.2 and 5.6, 1H,
HB-4), 2.49 (dd, J ¼ 16.2 and 12.1, 1H, HB-4) and 1.39 (d, J ¼ 6.9,
3H, Me-3); 13C NMR (75 MHz, CDCl3) d: 166.6 (CO2Me), 156.9 (C-
8), 154.1 (C-1), 138.3 (C-4a), 133.1 (C-6), 121.2 (C-7), 120.2 (C-8a),
110.6 (C-5), 55.9 (ArOMe), 52.5 (CO2Me), 52.0 (C-3), 32.5 (C-4)
and 21.6 (Me-3); HRMS (ESI-TOF): found m/z: 256.0949;
Conclusion
We have developed a straightforward and convenient approach
toward the rst total synthesis of ampullosine, a structurally
unique 6-carboxy-3-methylisoquinoline. The heterocycle is the
rst and only alkaloid isolated from the saproparasitic fungus
Sepedonium ampullosporum. The synthesis of O-methyl ampul-
losine methyl ester, from a common intermediate, is also
reported.
The synthesis of the natural product used 3,5-dihydrox-
ybenzoic acid as starting material and its key steps included
a Kolbe–Schmitt type carboxylation for introduction of C1 of the
heterocyclic ring, a Molander cross-coupling with potassium
propenyl triuoroborate to install the remaining three carbon
atoms, a carbonyl hydrazonation to add the heterocyclic
nitrogen and a nal 6p-azaelectrocyclization to build the iso-
quinoline system.
The total synthesis of ampullosine was achieved in 12 steps
and 3.2% overall yield, whereas that of its permethyl derivative
was accomplished in 14% overall yield, aer 11 steps. Hydro-
lytic demethylation of O-methyl ampullosine methyl ester also
afforded ampullosine. To the best of our knowledge, this is the
rst report of an electrocyclization reaction toward isoquino-
lines bearing a 6-carboxylic acid derivative.
C
13H15NNaO3 ([M + Na]+) requires m/z: 256.0944.
Method B. A mixture of O-methylhydroxylamine hydro-
chloride (16 mg, 0.19 mmol) and NaOAc (17 mg, 0.21 mmoL)
in absolute EtOH (1.5 mL) was stirred at room temperature
for 30 min under argon, when the resulting suspension was
allowed to settle. An aliquot of the solution (1 mL) was
transferred to a microwave tube containing the methyl 4-
formylbenzoate 31 (23 mg, 0.1 mmol) and the resulting
mixture was stirred at room temperature for 1 h under
argon. Then, the solvent was removed under reduced pres-
sure and the residue was dissolved in PhCF3 (1 mL). The
tube was purged with Argon and the mixture was irradiated
ꢀ
in the microwave reactor (180 C, ca. 250 W, 45 min). Aer
cooling to room temperature, the solvent was recovered by
careful distillation under atmospheric pressure, and the
residue was puried through a short column chromatog-
raphy, to afford a separable mixture of 5 (11.5 mg, 50%) and
33 (3.5 mg, 15%). Their spectroscopic data were in full
agreement with those of the corresponding products,
synthesized employing Method A.
Conflicts of interest
There are no conicts of interest to declare.
Acknowledgements
Ampullosine (4). Iodine (91 mg, 0.36 mmol) was added in
one portion to a stirred mixture of permethylampullosine 5
(20 mg, 0.08 mmol), small pieces of aluminium foil (12 mg,
0.44 mmol) and dry DMSO (20 mL, 0.28 mmol) in acetonitrile (1
mL). The mixture was further stirred at 80 ꢀC for 20 h and
monitored by TLC until the completion of conversion. Aer
cooling to room temperature, the reaction was acidied to pH
3 with solution of citric acid (0.2 M, 5 mL) and was treated with
Na2SO3 (15 mg) before extraction with EtOAc (5 ꢃ 10 mL). The
organic phases were combined, and dried over MgSO4. Aer
removal of organic solvents under reduced pressure, the
residue was puried by column chromatography
(EtOAc : EtOH, 1 : 0 / 2 : 8 v/v), furnishing 4 (3.7 mg, 23%), as
a yellow solid, Rf: 0.40 (EtOAc : EtOH, 1 : 1 v/v). UV-Vis
(MeOH): 227 nm (log 3 ¼ 4.3) and 356 nm (log 3 ¼ 3.5). 1H
NMR (300 MHz, DMSO-d6) d: 10.88 (bs, 1H, OH), 9.37 (s, 1H, H-
1), 7.85 (s, 1H, H-5), 7.64 (s, 1H, H-4), 7.45 (s, 1H, H-7), 3.35 (s,
1H, OH) and 2.59 (s, 3H, Me-3); 13C NMR (100 MHz, DMSO-d6)
d 168.3 (6-CO2H), 154.8 (C-8), 152.2 (C-3), 147.2 (C-1), 137.3
(2C, C-4a and C-6), 119.5 (C-8a), 119.1 (C-4), 118.3 (C-5), 109.2
(C-7) and 24.2 (3-Me). HRMS (ESI-TOF): found m/z: 204.0659;
The authors gratefully acknowledge Consejo Nacional de
´
´
Investigaciones Cientıcas y Tecnicas (CONICET, PUE IQUIR
´
´
2016) and Agencia Nacional de Promocion Cientıca y Tec-
´
nologica (ANPCyT, PICT 2017-0149) for nancial support. D.
F. V. also thanks CONICET for his Doctoral fellowship. We are
also indebted to the ASaCTeI for an institutional grant (AC 2015-
0005) to acquire a 400 MHz NMR spectrometer, and dedicate
´
the paper to the memories of Prof. Dr Edmundo A. Ruveda
(1934–2018), founder and rst Director of IQUIR and of our
´
´
friend BSc. Marıa V. Mendez (1987–2019).
Notes and references
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