Syntheses of arnottin i and arnottin II

Article abstract of DOI:10.1021/acs.joc.5b00107

Short total syntheses of arnottin I and II were accomplished in 5 and 6 steps, respectively. A sesamol-benzyne cycloaddition with a 3-furyl-benzoate followed by regiospecific lactonization provided rapid, large-scale access to the core of arnottin I. Saponification of arnottin I and hypervalent iodide mediated spirocyclization provided an efficient and direct preparation of racemic arnottin II.

Full text of DOI:10.1021/acs.joc.5b00107

Note
pubs.acs.org/joc
Syntheses of Arnottin I and Arnottin II
Matthew J. Moschitto, David R. Anthony, and Chad A. Lewis*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States
S
* Supporting Information
ABSTRACT: Short total syntheses of arnottin I and II were accomplished in 5 and 6 steps, respectively. A sesamol-benzyne
cycloaddition with a 3-furyl-benzoate followed by regiospecific lactonization provided rapid, large-scale access to the core of
arnottin I. Saponification of arnottin I and hypervalent iodide mediated spirocyclization provided an efficient and direct
preparation of racemic arnottin II.
Scheme 1. Preparation of Arnottin I
he arnottins (I and II) are coumarin-type natural products
Tisolated from the bark of the Xanthoxylum arnottianum
Maxim (rutaceae) and are surmised to possess antibiotic
properties.1 Syntheses of these interesting metabolites have
been reported after the initial isolation and structural
determination.2 The absolute stereochemical configuration of
spirocyclic arnottin II was synthetically determined via
asymmetric dihydroxylation of a reduced isomer of arnottin I
by Yamaguchi and co-workers.2c
Tandem oxidative dearomatization and spirocyclization3 is
capable of converting simple benzocoumarins to chiral
spirocyclic lactones that are found in several natural product
families. The direct conversion of arnottin I to arnottin II using
this method has not yet been reported. Hypervalent iodide
mediated oxidative dearomatization has been employed in the
synthesis of various natural products including galanthamine
and other amaryllidaceae alkaloids.4,5 Recent advances have
allowed for asymmetric hypervalent iodide oxidative dearoma-
tization;6,7 however, few examples exist within total synthesis.
To study and expand upon current methods for hypervalent
iodide mediated spirocyclization, a short and concise synthesis
of the arnottins was pursued. The preparation of arnottin I
utilized a benzyne cycloaddition, hydrolysis, and oxidative
spirocyclization to arnottin II to provide large quantities of the
metabolites for testing.
prepared from readily available dimethoxybenzoic acid and
converted to either the methyl (5a) or tert-butyl bromoester
(5b) in 2 steps.2c The bromoester (5) was cross-coupled under
Suzuki’s condition with commercially available 3-furylboronic
acid in 82 and 96% yield for the methyl and tert-butyl ester,
respectively. The benzyne-mediated cycloaddition proved
capricious and required the sesamol silyltriflate to be used in
excess (1.4 equiv) with slow generation of the benzyne using
weakly soluble CsF in acetonitrile. The in situ cycloaddition
produced the arnottin I progenitors (rac-7a, rac-7b) in 66 and
64% yield, respectively.
A convergent synthetic path to arnottin I was planned using a
benzyne cycloaddition with a 3-furyl benzoate, followed by
lactone formation (Scheme 1). Bis-lithiation of 6-bromosesa-
mol,8 followed by trimethylsilyl chloride quench and triflation
using under Mori’s procedure9 afforded benzyne precursor 4 in
74% overall yield on 5 g scale. The furyl coupling partner was
Received: January 15, 2015
Published: March 9, 2015
© 2015 American Chemical Society
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DOI: 10.1021/acs.joc.5b00107
J. Org. Chem. 2015, 80, 3339−3342

The Journal of Organic Chemistry
Note
Unsaturated bicyclic ethers, such as 7, can be converted to
naphthols under a variety of conditions including Bronsted
acids,10 ruthenium,11 rhodium,12 and aluminum complexes.13
The naphthol regioisomer produced is less predictable and was
unknown for structure 7 at the onset of these studies. For the
transformation of esters rac-7a and rac-7b to 1, several
conditions were attempted: Lewis acids such as BF3-etherate
or Sc(OTf)3 gave complex mixtures and nucleophiles such as
NaI and NaBr resulted in degradation. Bronsted acids were
most effectual with HCl and TsOH both providing arnottin I
for rac-7a and rac-7b in differing yield. Exposure of methyl ester
rac-7a to TsOH in methanol led to nonregiospecific naphthol
formation resulting in both arnottin I (15%) and the 4-
naphthol derivative 8.14 Identical reaction conditions were
attempted with tert-butyl ester rac-7b and surprisingly led to
clean conversion of arnottin I in 92% yield with no 4-naphthol
derivative observed. The synthetically prepared arnottin I
displayed spectroscopic and physical properties identical to the
natural product.1 The disparity in reaction yield between rac-7a
and rac-7b to arnottin I and the absent 4-naphthol derivative in
the rac-7b reaction suggests the mechanism follows different
paths for each ester employed. The methyl ester (rac-7a)
produced both arnottin I and the 4-naphthol derivative
suggesting naphthol formation occurred with little regiospeci-
ficity. The resultant 1-naphthol produced arnottin I whereas the
4-naphthol was incapable of cyclization. The tert-butyl ester
(rac-7b) cleanly afforded arnottin I, suggesting the ester is
deprotected prior to naphthol formation. One plausible
mechanism is the weakening of a tert-butyl ester proton due
to carbonyl lone-pair donation into the C−O σ* of the bicycle,
resulting in loss of isobutylene and carboxylate promoted
rupture of the allylic ether as shown by intermediate 9 (Figure
1). The resultant benzylic alcohol (10) is quickly dehydrated to
which reduced the conversion to arnottin I and retained
solubility for the spirocyclization studies.
The saponified phenol acid was immediately exposed to
different hypervalent iodide conditions to effect the spirocyc-
lization. The highly reactive bis(trifluoroacetoxy)iodobenzene
(PIFA) was necessary to form racemic arnottin II in DCM with
cesium carbonate as an additive in 23% yield (Table 1, entry 1).
Table 1. Screen of hypervalent iodide conditions
temp
yield
a
entry
solvent
DCM
additive
CsCO3
time (h)
(°C)
(%)
1
2
3
4
5
4
4
−40
−20
0
23
25
40
28
35
25
56
TFE
HFIP
12
12
12
12
12
DCM/HFIP
HFIP
0
Et3N, DMAP
MgCIO4
0
b
6
b
HFIP/MeCN
HFIP
0
7
0
a
Yield refers to isolated yields following silica gel chromatography.
Syringe pump addition of PIFA over 1 h.
b
Kita noted substantial increases in yield using fluorinated
solvents10 that were also investigated. Trifluoroethanol (TFE,
entry 2, 25%) was inferior as compared to hexafluoroisopro-
panol (HFIP, entry 3, 40%) as solvent. Blends of DCM and
HFIP (entry 4, 28%) did not offer enhanced yields. An
intensely colored intermediate was formed over the first 20 min
of the reaction and persisted throughout the duration of the
reaction. Two possible intermediates could exist; a ligand
exchanged trapped γ3-iodane 11, or a dissociated cationic
complex 12.4 We reasoned the breakdown of the mixed
b
hypervalent intermediate could occur through liberation of the
weaker carboxylate donor using a nucleophilic donor. DMAP
(entry 5) was added to the reaction, but did not increase the
yield substantially as compared to its absence (entry 3).
Perchlorate salts, known to increase yields involving
nucleophilic addition to phenoxenium radical cations,15 also
did not improve the reaction and had noted degradation (entry
6). Slow addition of PIFA over 1 h provided a boost in yield
and generated arnottin II in 56% overall yield over the two
steps (entry 7). Attempts at asymmetric spirocyclization using
chiral hypervalent iodide sources are planned.
In conclusion, we developed an efficient route from readily
available materials to the arnottins using a benzyne cyclo-
addition and regiospecific ether fragmentation−lactonization
cascade. Several other coumarin based natural products can
now be accessed more efficiently using this tactic.
Figure 1. Proposed carboxylate-assisted lactonization cascade.
1
arnottin I. Furthermore, in situ H NMR, showed no tert-
butanol formation during the course of the reaction
discouraging the phenol formation and lactonization scenario.
Isobutylene was not observed; however, volatility would make
detection difficult. The fortuitous sequence afforded large
quantities of arnottin I and allowed the study of conversion to
arnottin II.
Saponification of arnottin I required immediate exposure to
spirocyclization conditions due to competitive recyclization and
recovery of arnottin I in acid. Careful hydrolysis of arnottin I
was followed by protonation of the carboxylate to a pH of 2−3,
EXPERIMENTAL SECTION
■
tert-Butyl 6-(Furan-3-yl)-2,3-dimethoxybenzoate (6b). A total
of 1.43 g (4.50 mmol, 1.0 equiv) of tert-butyl 6-bromo-2,3-
dimethoxybenzoate (5b),2c 750.0 mg (6.76 mmol, 1.5 equiv) of
furan-3-boronic acid, and 1.865 g (13.50 mmol, 3.0 equiv) of K2CO3
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DOI: 10.1021/acs.joc.5b00107
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The Journal of Organic Chemistry
Note
were dissolved in DMF (30.0 mL). The resulting solution was
degassed under argon for 10 min. Then, 253.0 mg of Pd(PPh3)2Cl2
(1.06 mmol, 8 mol %) and H2O (10.0 mL) were added to the reaction
flask, and this solution was stirred at 90 °C for 4 h. The reaction
mixture was run through a Celite plug, diluted with toluene (10.0 mL),
and concentrated en vacuo. This product was purified by column
chromatography (3:1 hexanes/EtOAc, v/v) and dried under vacuum
to yield 1.32 g of tert-butyl 6-(furan-3-yl)-2,3-dimethoxybenzoate (6b,
Arnottin I (1). A total of 150.0 mg (0.354 mmol, 1.0 equiv) of tert-
butyl 6-(5,8-dihydro-5,8-epoxynaphtho[2,3-d][1,3]dioxol-6-yl)-2,3-di-
methoxybenzoate (7b) was added to a flame-dried round-bottomed
flask and dissolved in anhydrous MeOH (5 mL). Next 15.2 mg
(0.0884 mmol, 0.25 equiv) of p-toluenesulfonic acid was added to this
solution and the mixture stirred at 50 °C under nitrogen atmosphere
for 24 h. The reaction mixture was then cooled to 0 °C and filtered,
and the off-white solid was washed with cold MeOH. This product was
dried under vacuum to yield 113.4 mg of arnottin I (1, 92% yield). Mp
1
96% yield) as a white solid. Mp 46−48 °C; H NMR (400 MHz,
1
>250 °C; H NMR (400 MHz, CDCl3) δ 7.89 (d, J = 8.9 Hz, 1H),
CDCl3) δ 7.55 (dd, J = 1.6, 0.9 Hz, 1H), 7.42 (t, J = 1.7 Hz, 1H), 7.07
(d, J = 8.5 Hz, 1H), 6.92 (d, J = 8.5 Hz, 1H), 6.56 (dd, J = 1.8, 0.9 Hz,
1H), 3.89 (s, 3H), 3.88 (s, 3H), 1.51 (s, 9H); 13C NMR (100 MHz,
CDCl3) δ 167.0, 151.8, 145.8, 142.8*, 142.7*, 139.7*, 139.6*, 130.4,
124.8, 123.7, 122.5, 113.1, 111.2*, 111.1*, 82.3, 61.5, 56.0, 28.1
(*denotes rotamers); IR (neat, cm−1) 2973, 2935, 1715, 1482, 1291,
1265, 1059, 1029, 795; TLC Rf = 0.60 (3:1 hexanes/EtOAc, v/v);
HRMS (DART) m/z calcd for C17H21O5 (M + H)+: 305.1389, found
305.1375.
7.85 (s, 1H), 7.84 (d, J = 8.8 Hz, 1H), 7.54 (d, J = 8.8 Hz, 1H), 7.45
(d, J = 8.8 Hz, 1H), 7.14 (s, 1H), 6.10 (s, 2H), 4.03 (s, 3H), 3.99 (s,
3H); 1H NMR (600 MHz, DMSO-d6) δ 8.08 (d, J = 8.7 Hz, 1H), 8.03
(d, J = 8.7 Hz, 1H), 7.66 (app t, J = 9.4 Hz, 2H), 7.59 (s, 1H), 7.34 (s,
1H), 6.17 (s, 2H), 3.97 (s, 3H), 3.92 (s, 3H); 13C NMR (151 MHz,
DMSO-d6) δ 155.7, 152.5, 150.8, 148.0, 147.9, 144.8, 130.5, 128.7,
122.5, 120.8, 118.8, 117.7, 117.7, 114.1, 111.5, 103.5, 101.0, 96.9, 60.2,
56.3; IR (neat, cm−1) 3010, 2943, 1734, 1489, 1463, 1276, 1122, 1039,
821; TLC Rf = 0.50 (1:1 hexanes/EtOAc, v/v); HRMS (DART) m/z
calcd for C20H15O6 (M + H)+: 351.0869, found 351.0855.
Methyl 6-(Furan-3-yl)-2,3-dimethoxybenzoate (6a). A total of
2.00 g (18.0 mmol) of methyl 6-bromo-2,3-dimethoxybenzoate (5a)
yielded 3.53 g (12.5 mmol, 82% yield) of methyl 6-(furan-3-yl)-2,3-
dimethoxybenzoate (6a) according to the above procedure. Mp 43−
45 °C; 1H NMR (400 MHz, CDCl3) δ 7.49 (dd, J = 1.6, 0.9 Hz, 1H),
7.42 (t, J = 1.7 Hz, 1H), 7.10 (d, J = 8.5 Hz, 1H), 6.96 (d, J = 8.5 Hz,
1H), 6.50 (dd, J = 1.8, 0.9 Hz, 1H), 3.89 (s, 3H), 3.89 (s, 3H), 3.83 (s,
3H); 13C NMR (126 MHz, CDCl3) δ 168.3, 151.6, 145.9, 143.0,
139.1, 128.3, 124.5, 123.8, 122.6, 113.6, 110.3, 61.5, 55.8, 52.3; IR
(neat, cm−1) 2950, 1723, 1500, 1258, 905, 804; TLC Rf = 0.40 (2:1
hexanes/EtOAc, v/v); HRMS (DART) m/z calcd for C14H14O5 (M +
H)+: 262.0841, found 262.0838
Arnottin II (2). Arnottin I (1, 12.0 mg, 0.035 mmol) was
suspended in 0.5 mL of THF:MeOH:H2O (3:1:1, v/v/v) and 7.0 mg
(0.17 mmol, 5.0 equiv) of LiOH·H2O was added. The reaction was
heated at 50 °C for 2 h over which time the solution became red. The
flask was cooled to 0 °C and 1 M HCl was added dropwise until the
solution turned yellow-orange indicating a pH of 2−3. The solution
was extracted with Et2O (2 × 1.0 mL), dried over Na2SO4 and
concentrated at 23 °C yielding an unstable yellow-orange solid. The
flask was cooled to 0 °C and HFIP (0.50 mL) was added. PIFA (17.0
mg, 0.041 mmol, 1.1 equiv) in 0.50 mL of HFIP was added over 1 h at
0 °C via syringe pump and then the reaction was stirred at 23 °C. After
12 h the reaction was quenched with 1 M HCl (1.0 mL), extracted
with CH2Cl2 (2 × 1.0 mL), dried over Na2SO4 and concentrated.
Flash chromatography (2:1 hexanes/EtOAc, v/v) yielded arnottin II
(2, 7.0 mg, 0.02 mmol, 56% yield) as an off yellow solid. Mp 206−208
°C; 1H NMR (400 MHz, CDCl3) δ 7.37 (s, 1H), 7.05 (d, J = 8.3 Hz,
1H), 6.78 (m, 2H), 6.68 (d, J = 9.9 Hz, 1H), 6.09 (m, 4H), 4.17 (d, J =
0.8 Hz, 3H), 3.86 (s, 4H); 13C NMR (126 MHz, CDCl3) δ 191.1,
167.6, 154.1, 153.6, 149.1, 148.6, 139.2, 134.6, 130.3, 128.4, 123.2,
119.2, 117.4, 115.6, 108.1, 107.7, 102.6, 84.5, 62.8, 57.0; IR (neat,
cm−1) 1682, 1770, 2853, 2921; TLC Rf = 0.45 (1:1 hexanes/EtOAc v/
v); HRMS (DART) m/z calcd for C20H15O7 (M + H)+: 367.0818,
found 367.0811.
tert-Butyl 6-(5,8-Dihydro-5,8-epoxynaphtho[2,3-d][1,3]-
dioxol-6-yl)-2,3-dimethoxybenzoate (7b). A total of 335.0 mg
(1.036 mmol, 1.4 equiv) of 6-(trimethylsilyl)benzo[d][1,3]dioxol-5-yl-
trifluoromethanesulfonate (4)9 and 225.0 mg (0.74 mmol, 1.0 equiv)
of tert-butyl 6-(furan-3-yl)-2,3-dimethoxybenzoate (6b) were dissolved
with acetonitrile (10.0 mL) in a flame-dried flask. The solution was
charged with argon; then 337.0 mg (2.22 mmol, 3.0 equiv) of CsF was
added, and the reaction mixture was let to stir at 23 °C for 24 h. The
solution was diluted with water (30.0 mL) and extracted with ethyl
acetate (3 × 15.0 mL). The combined organic layers were washed with
water (30.0 mL) and brine (30.0 mL), dried over MgSO4, and
concentrated under reduced pressure. The product was purified by
column chromatography (3:1 hexanes/EtOAc, v/v) to yield 201.6 mg
of tert-butyl 6-(5,8-dihydro-5,8-epoxynaphtho[2,3-d][1,3]dioxol-6-yl)-
2,3-dimethoxybenzoate (7b, 64% yield) as an off-white solid. Mp 150
ASSOCIATED CONTENT
■
1
°C (decomp.); H NMR (400 MHz, CDCl3) δ 6.98 (d, J = 8.5 Hz,
S
* Supporting Information
1H), 6.94 (m, 2H), 6.89 (d, J = 8.6 Hz, 1H), 6.83 (t, J = 0.5 Hz, 1H),
5.94 (d, J = 1.4 Hz, 1H), 5.86 (d, J = 1.4 Hz, 1H), 5.79 (t, J = 0.8 Hz,
1H), 5.73 (dt, J = 1.8, 0.8 Hz, 1H), 3.87 (s, 3H), 3.84 (s, 3H), 1.49 (s,
9H); 13C NMR (126 MHz, CDCl3) δ 166.7, 153.3, 152.6, 146.2,
144.8, 144.4, 143.4, 143.0, 136.7, 129.7, 122.9, 122.1, 112.6, 104.0,
103.7, 101.3, 85.0, 84.0, 82.4, 61.5, 56.1, 28.1; IR (neat, cm−1) 2973,
2935, 1715, 1482, 1292, 1265, 1142, 1059, 1029, 795; TLC Rf = 0.31
(3:1 hexanes/EtOAc, v/v); HRMS (DART) m/z calcd for C20H17O7
(M − C4H8)+: 369.0974, found 369.0960.
1H and 13C NMR spectra for all new compounds is provided.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: chad.lewis@cornell.edu
■
Notes
Methyl 6-(5,8-Dihydro-5,8-epoxynaphtho[2,3-d][1,3]dioxol-
6-yl)-2,3-dimethoxybenzoate (7a). A total of 780.0 mg (2.23
mmol) of methyl 6-(furan-3-yl)-2,3-dimethoxybenzoate (6a) yielded
375.0 mg (0.98 mmol, 66% yield) of methyl 6-(5,8-dihydro-5,8-
epoxynaphtho[2,3-d][1,3]dioxol-6-yl)-2,3-dimethoxybenzoate (7a) as
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The project described was supported by Award Number
T32GM008500 from the National Institute of General Medical
Sciences.
1
per the above procedure. Mp 56−58 °C (decomp); H NMR (500
MHz, CDCl3) δ 6.98 (d, J = 8.6 Hz, 1H), 6.93 (m, 2H), 6.81 (m, 2H),
5.93 (d, J = 1.4 Hz, 1H), 5.87 (d, J = 1.4 Hz, 1H), 5.75 (t, J = 0.8 Hz,
1H), 5.71 (dt, J = 1.8, 0.8 Hz, 1H), 3.87 (s, 3H), 3.83 (s, 3H), 3.77 (s,
3H); 13C NMR (126 MHz, CDCl3) δ 167.9, 153.5, 152.4, 146.3,
144.7, 144.3, 143.1, 142.6, 136.5, 127.6, 123.4, 122.0, 113.1, 103.9,
103.5, 101.2, 84.9, 83.9, 61.6, 56.0, 52.5; IR (neat, cm−1) 2941, 1724,
1460, 1254, 1035; TLC Rf = 0.5 (2:1 hexanes/EtOAc v/v); HRMS
(DART) m/z calcd for C21H19O7 (M + H)+: 383.1131, found
383.1129.
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■
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The Journal of Organic Chemistry
Note
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