Synthesis of an azide-tethered 4H-furo[3,4-b]indole

Article abstract of DOI:10.24820/ark.5550190.p011.099

We describe the synthesis of a novel azide-tethered 4H-furo[3,4-b]indole as a putative intermediate for a projected route to the 2-acylindole class of indole alkaloids. O N₃ Me N N N H N H H O

Full text of DOI:10.24820/ark.5550190.p011.099

The Free Internet Journal
for Organic Chemistry
Paper
Archive for
Arkivoc 2019, part vi, 0-0
Organic Chemistry
to be inserted by editorial office
Synthesis of an azide-tethered 4H-furo[3,4-b]indole
Daniel J. Keavy, Yanbing Liu, and Gordon W. Gribble*
Department of Chemistry, Dartmouth College, Hanover, NH 03755, USA
Email: ggribble@dartmouth.edu
This paper is dedicated to the memory of Robert J. Milligan
Received 10-29-2019
Abstract
Accepted 01-05-2020
Published on line 01-15-2020
We describe the synthesis of a novel azide-tethered 4H-furo[3,4-b]indole as a putative intermediate for a
projected route to the 2-acylindole class of indole alkaloids.
O
Me
N3
N
N
N
N
H
H
H O
Keywords: Indole, 4H-furo[3,4-b]indole, azide, lithiation
DOI: https://doi.org/10.24820/ark.5550190.p011.099
Page 1
©AUTHOR(S)

Arkivoc 2019, vi, 0-0
Keavy, D. J. et al.
Introduction
Synthetic routes to the family of 2-acylindole alkaloids (e.g., burnamicine (1), vobasine (2), dregamine (3), and
tabernaemontanine (4)) (Figure 1), are relatively few.1 In contrast, the biogenetically related sarpagine,
macroline, and ajmaline collection of alkaloids has received intense interest over many decades.3
,2
–6
MeO C
MeO C
2
2
CH3
CH3
Me
N
N
N
N
N
N
H O
H O
H O
vobasine (2)
a-Et (dregamine) (3)
b-Et (tabernaemontanine) (4)
OH
burnamicine (1)
Figure 1
In continuation of our interest in the synthesis and utility of the 4H-furo[3,4-b]indole ring system for the
7
construction of carbazoles, pyridocarbazoles, and related ring systems, we now report the synthesis of azide-
tethered 4H-furo[3,4b]indole 5 for a projected synthesis of the 2-acylindole alkaloid skeleton 6 (Scheme 1).
O
O
N3
N
N
N
Me
N
H
PhO S
2
PhO S
2
5
O
Me
Me
Me
N
N
N
N
N
N
H O
H O
PhO S
2
6
Scheme 1
Results and Discussion
Our synthesis of azido furo[3,4-b]indole 5 began with an improved preparation of 5-azidopentanol (11)
(
Scheme 2). Treatment of tetrahydropyran (7) with freshly fused ZnCl
2
and acetyl chloride gave 5-chloropentyl
9
acetate (8) in excellent yield. Displacement of the chloride with sodium azide under phase transfer conditions
Page 2
©AUTHOR(S)

Arkivoc 2019, vi, 0-0
Keavy, D. J. et al.
afforded 5-azidopentyl acetate (9) in high yield. Saponification with K
2
CO
3
MeOH yielded 5-azidopentanol
(10). Both azides 9 and 10 were distilled (not to dryness) and fully characterized. A Swern oxidation of 10 to
5
-azidopentanol (11) proceeded in 48% yield. However, PCC oxidation of 10 afforded 11 in 89% yield. This
aldehyde is relatively unstable but could be distilled without incident.
ZnCl2
CH COCl
NaN3
H O
3
2
O
AcO
Cl
AcO
N3
n-Bu NBr
87%
4
7
8
9
87%
K CO
2
3
PCC
MeOH
rt
CH Cl2
HO
N3
O
N3
2
10
89%
11
98%
Scheme 2
Our first approach to the target furo[2,4-b]indole 5 is shown in Scheme 3. Lithiation of 3-methyl-1-
phenylsulfonyl)indole (12) with sec-BuLi and quenching with 5-azidopentanal (11) gave 13 in 51% yield (80%
(
based on recovered starting material). Oxidation of 13 with activated MnO afforded ketone 14 in high yield.
2
However, subsequent attempts to brominate selectively the C-3 methyl group with NBS proceeded in low
yield at best, and often afforded mixtures of at least two other bromine-containing 2-acylindoles.
Me
Me
1
. sec-BuLi
. 11
1%
N3
2
N
N
5
SO Ph
PhO S
2
OH
2
12
13
Me
MnO2
NBS
Br
N3
CCl4
N3
CH Cl
2
2
N
N
reflux
PhO S
O
PhO S
O
2
2
86%
15
14
Scheme 3
Our successful synthesis of furo[2,4-b]indole 5 is shown in Scheme 4. Indole was converted in standard
fashion to 1-(phenylsulfonyl)indole-3-carbaldehyde (17) in excellent yield. Protection of the formyl group with
Page 3
©AUTHOR(S)

Arkivoc 2019, vi, 0-0
Keavy, D. J. et al.
ethylene glycol gave ketal 18. Lithiation of 18 and quenching with aldehyde 11 gave alcohol 19, which was
directly transformed into the desired azido-furo[3,4-b]indole 5 in 60% yield for the two steps.
CHO
CHO
PhSO Cl
POCl3
DMF
2
(CH OH)2
2
p-TsOH
NaOH
N
N
N
H
H
PhH
4
n-Bu NHSO4
SO Ph
2
96%
97%
16
96%
17
O
O
HOAc
O
1. sec-BuLi
. 11
1%
O
O
N3
OH
N3
2
KF
N
N
N
hydroquinone
5
PhO S
SO Ph
PhO S
2
2
2
60% from 18
5
18
19
Scheme 4
Conclusion
We have synthesized an azido-tethered furo[3,4-b]indole (5) preparatory for future efforts to effect the
proposed Scheme 1 as an entry into the 2-acylindole family of alkaloids.
Experimental Section
General. Thin layer chromatography was performed on precoated (0.2 mm) silica gel 60 F254 plastic sheets
visualized using a 254 nm UV lamp. Flash chromatography was performed with 230–400 mesh Silicycle gel 60.
Melting points were taken on a Laboratory Devices Mel Temp or a Buchi 510 melting point apparatus in open
1
13
capillaries and are uncorrected. H and C NMR spectra were recorded on a Varian EM-300A Fourier
transform spectrometer. The chemical shifts noted from these spectra are reported in parts per million (ppm,

) using the signal of chloroform-d1 ( 7.27) or acetone-d
6
( 1.94) or Me Si as an internal standard. Infrared
4
spectra were recorded on a Perkin-Elmer 599 spectrophotometer and were obtained either neat or using solid
potassium bromide pellets. Both low-resolution and high-resolution mass spectra (MS and HRMS) were
performed at the National Institutes Regional Facility at Rockefeller University, New York, NY., or were
measured at 35 eV and 70 eV on a Finnigan EI-CI 4023 gas chromatograph-mass spectrometer. Elemental
analyses were performed by Atlantic Microlab, Inc., Atlanta, GA, or Desert Analytics, Tucson, AZ.
5
-Chloropentyl acetate (8). Tetrahydropyran 7 (105 g, 1.22 mol) was treated with acetyl chloride (87.4 g, 1.12
8
(17.55 g, 0.129 mol) according to the method of Synerholm.9 An
mol) and freshly fused (60 Torr) ZnCl
2
exothermic reaction ensued and the mixture was heated on a steam bath for 1.25 h. The mixture was cooled
Page 4
©AUTHOR(S)

Arkivoc 2019, vi, 0-0
Keavy, D. J. et al.
to room temperature, diluted with benzene (200 mL), washed with water (2 x 400 mL), saturated sodium
bicarbonate (400 mL), and saturated brine (400 mL), dried (Na
2
SO
4
), filtered, and concentrated in vacuo.
9
Distillation at reduced pressure provided 161 g (87%) of 8 as a colorless liquid: bp 117–122 °C (29 Torr) (lit.
1
bp 104 °C (18 Torr)); H NMR (CDCl
3
)  4.05 (t, 2H, J 6.6 Hz), 3.55 (t, 2H, J 6.5 Hz), 2.05 (s, 3H), 1.85-1.45 (m,
-
1
6
5
H); IR (neat) 2954, 2868, 1739, 1453, 1387, 1364, 1233, 1043, 735, 648 cm .
-Azidopentyl acetate (9). A magnetically stirred solution of sodium azide (47.9 g, 737 mmol) in water (150
mL) was treated at reflux with tetra-n-butylammonium bromide (8.27 g, 25.6 mmol) and 8 (59.8 g, 352 mmol).
The mixture was heated at reflux for 1 h and a second portion of sodium azide (54.2 g, 806 mmol) was added.
The mixture was heated at reflux for 12 h, cooled to room temperature, diluted with water (100 mL), and
separated the layers. The aqueous layer was extracted with fresh CH
extracts were washed with saturated brine (300 mL), dried (K CO ), filtered, and concentrated in vacuo to
provide a colorless liquid. Distillation at reduced pressure provided 34.1 g (87%) of 9 as a colorless liquid (bp
29–137 °C (29 Torr)). Redistillation at reduced pressure provided the analytical sample: bp 133–135 °C (29
2
Cl
2
(3 x 150 mL), the combined organic
2
3
1
1
13
Torr): H NMR (CDCl
CDCl )  170.3, 63.7, 50.8, 28.1, 27.8, 22.8, 20.4; IR (neat) 2944, 2868, 2099, 1739, 1456, 1388, 1363, 1239,
066, 1044 cm . Anal. Calcd for C
Azide 9 appears to be a new compound.
-Azidopentan-1-ol (10). A solution of 9 (115 g, 669 mmol) in 95% methanol (225 mL) was treated with 5.90 g
3
)  4.12 (t, J 6.0 Hz, 2H), 3.34 (t, J 6.0 Hz, 2H), 2.04 (s, 3H), 1.81–1.37 (m, 6H); C NMR
(
1
3
-
1
7
13
H N
3
O : C, 49.11; H, 7.65; N, 24.54. Found: C, 49.18; H, 7.67; N, 24.56.
2
5
(
42.7 mmol) of K
2
CO
3
and magnetically stirred for 24 h. The suspension was treated with an additional 3.94 g
and stirred for 24 h. The mixture was then poured into saturated sodium chloride
O (6 x 200 mL), and the combined extracts were dried (K CO ), filtered,
and concentrated in vacuo. Distillation at reduced pressure provided 84.8 g (98%) of 10 as a colorless liquid:
(28.5 mmol) of K
2
CO
3
solution (350 mL), extracted with Et
2
2
3
1
13
bp 127–129 °C (28 Torr); H NMR (CDCl
CDCl ) 61.6, 50.9, 31.6, 28.2, 22.6; IR (neat) 3335, 2935, 2864, 2092, 1456, 1349, 1261, 1051, 906, 877 cm .
Anal. Calcd for C O: C, 46.50; H, 8.58; N, 32.53. Found: C, 46.30; H, 8.51; N, 32.39. This alcohol is
known but has not been previously characterized.
-Azidopentanal (11) (Swern). A mechanically stirred solution of oxalyl chloride (4.0 mL, 46 mmol) in CH
100 mL) at -60 °C under argon was treated dropwise with a solution of dimethyl sulfoxide (7.4 g, 95 mmol) in
CH Cl (18 mL). The solution was stirred for 2 min and treated with a solution of 10 (5.2 g, 40 mmol) in CH Cl
40 mL) over 4 min. The solution was stirred for 15 min, and treated with triethylamine (28.0 ml, 201 mmol)
3
)  3.63 (bs, 3H), 3.34 (t, J 5.1 Hz, 2H), 1.87–1.54 (m, 6H); C NMR
-
1
(
3
5
H
11 3
N
1
0-12
5
(
2
Cl
2
2
2
2
2
(
via syringe, and allowed to warm to room temperature (1 h). The solution was stirred for 1 h at room
temperature, treated with water (200 mL), the layers were separated, and the aqueous layer was extracted
with CH
5
2
2
Cl
00 mL), dried (K
.5 g (48%) of 11 as a colorless liquid: bp 108–114 °C (25 Torr) (Lit. bp 60 °C (2 Torr)); H NMR (CDCl
2
(4 x 75 mL). The combined organic extracts were washed with water (6 x 500 mL) and brine (3 x
2
CO ), filtered, and concentrated in vacuo to give 11 as a yellow liquid. Distillation afforded
3
13
1
3
) 9.79
)  201.3, 50.8,
1
3
(t, J 2.7 Hz, 1H), 3.30 (t, J 6.4 Hz, 2H), 2.67–2.35 (m, 2H), 1.80–1.55 (m, 4H); C NMR (CDCl
3
-1
4
3.0, 28.1, 19.0; IR (neat) 2940, 2868, 2722, 2095, 1724, 1455, 1411, 1390, 1350, 1268 cm . Spectral data
1
3,14
agreed with 11 as prepared below and with that reported in the literature.
5
-Azidopentanal (11) (PCC). To a suspension of pyridinium chlorochromate (12.5 g, 0.0581 mol) in methylene
chloride (130 mL) was added dropwise a solution of 5-azidopentanol (10) (5.00 g, 0.0387 mol) in methylene
chloride (20 mL). The mixture was stirred at room temperature for 10 min and diluted with ethyl ether (100
Page 5
©AUTHOR(S)

Arkivoc 2019, vi, 0-0
Keavy, D. J. et al.
mL). The solids were removed by filtration and the filtrate was subjected to flash chromatography (ethyl
acetate/hexane 20/80) to give 11 as an oil (4.39 g, 89%), which was used directly as described below.
6
1
-Azido-1-hydroxyl-1-[3-methyl-1-(phenylsulfonyl)indol-2-yl]pentane (13). A magnetically stirred solution of
2 (15.00 g, 55.18 mmol) in dry THF (250 mL) at -70 °C under argon was treated with sec-butyllithium (1.11 M
in cyclohexane, 59.7 mL, 66.3 mmol) via syringe. The solution was stirred for 0.5 h, allowed to slowly warm to
room temperature (1 h), stirred for 3 h, recooled to -70 °C, and treated with aldehyde 11 (9.06 g, 71.3 mmol).
The solution was allowed to warm to room temperature (1 h), stirred for 12 h, treated with saturated NH
4
Cl
(4 x 500 mL). The combined
), filtered, and concentrated in
(
100 mL), poured into saturated NH
organic extracts were washed with water (500 mL), brine (500 mL), dried (K
vacuo. Flash chromatography (CH Cl
material) as a colorless oil: IR (neat) 3538, 2928, 2860, 2086, 1448, 1357, 1170, 956, 744, 722, 682 cm ; H
NMR (CDCl )  8.05 (d, J = 8.5 Hz, 1H), 7.77 (d, J = 8.2 Hz, 2H) 7.47–7.19 (m, 6H), 5.22 (q, J = 8.1 Hz, 1H), 3.75
d, J = 8.8 Hz, 1H), 3.22 (t, J = 6.6 Hz, 2H), 2.30 (s, 3H), 2.21–2.09 (m, 1H), 2.08–1.96 (m, 1H), 1.65–1.54 (m, 2H),
4
Cl (300 mL), and extracted with CH
2
Cl
CO
2
2
3
2
2
elution) gave 13 (11.28 g, 51%; 80% based on recovered starting
-
1 1
3
(
1
3
1
1
5
6
1
.54–1.44 (m, 1H), 1.35–1.22 (m, 1H); C NMR (CDCl )  137.8, 137.76, 136.7, 133.6, 131.1, 128.9, 126.3,
3
25.2, 123.8, 120.4, 119.2, 115.1, 67.3, 51.1, 36.5, 28.4, 23.3, 9.6. Anal. Calcd for C20
H
22
N
4
3
O S: C, 60.28; H,
.57; N, 14.06; S, 8.05. Found: C, 59.96; H, 5.60; N, 14.14; S, 7.70.
-Azido-1-oxo-1-[3-methyl-1-(phenylsulfonyl)indol-2-yl)]pentane (14). A magnetically stirred suspension of
3 (10.89 g, 27.3 mmol) and manganese (IV) oxide (60.88 g, 700.2 mmol) in CH Cl (300 mL) was heated at
2
2
reflux for 22 h under argon. The suspension was then cooled to room temperature, filtered, and the filtrate
concentrated in vacuo to give 9.34 g (86%) of a pale yellow oil that was homogeneous by TLC (CH
2
Cl ). Flash
2
1
chromatography (CH
2
Cl
2
elution) gave 7.95 g (73%) of 14 as a colorless oil: H NMR (CDCl
3
)  8.04 (d, J = 8.3
Hz, 1H), 7.60 (d, J = 7.9 Hz, 2H), 7.44–7.36 (m, 3H), 7.31–7.22 (m, 3H), 3.32 (t, J = 6.8 Hz, 2H), 3.05 (t, J = 7.3 Hz,
2
9
1
H), 2.20 (s, 3H), 1.89–1.78 (m, 2H), 1.76–1.65 (m, 2H); IR (neat) 2090, 1693, 1600, 1578, 1454, 1375, 1191,
-
1 13
60, 759, 731 cm ; C NMR (CDCl
24.8, 120.7, 116.0, 51.2, 43.9, 28.2, 21.6, 9.3. Anal. Calcd for C20
3
)  197.8, 137.1, 136.0, 135.1, 133.8, 131.7, 128.6, 127.3, 127.1, 126.4,
H
20
N O
4 3
S: C, 60.59; H, 5.08; N, 14.13; S, 8.09.
Found: C, 60.20; H, 4.82; N, 14.00; S, 7.98.
Indole-3-carbaldehyde (16). To ice cooled DMF (200 mL) was added phosphorus oxychloride (187.5 g, 1.22
mol) via an additional funnel. The resulting yellow-orange solution was stirred for 20 min and then a solution
of indole (100.0 g, 0.854 mol) in DMF (150 mL0 was added via an additional funnel. The reaction was stirred at
room temperature for 2 days, then poured into ice. An aqueous sodium hydroxide (230 g, 5.75 mol) solution
was added slowly frequently along with ice to keep the reaction cool. The resulting precipitate was collected
by filtration and washed thoroughly with water to afford 16 as a white solid (119.5 g, 96%) after drying; mp
1
5
1
1
1
1
90–192 °C (Lit. mp 196–197 °C); H NMR (DMSO-d )  11.80 (br, s, 1H), 9.94 (s, 1H), 8.18 (m, 1H), 7.95 (s,
6
H), 7.48 (m, 1H), 7.23 (m, 2H).
-(Phenylsulfonyl)indole-3-carbaldehyde (17). A mixture of 3-formylindole (16) (11.95 g, 0.0823 mol),
crushed sodium hydroxide (9.65 g, 0.241 mol) and tetra-n-butylammonium hydrogen sulfate (1.09 g, 3.21
mmol) in methylene chloride (100 mL) was stirred in an ice bath. Benzenesulfonyl chloride (17.16 g, 0.0972
mol) was added via an additional funnel over 10 min. The resulting mixture was stirred at room temperature
for 2 h. Water was added and the organic layer was washed several times with water and dried over
16
magnesium sulfate to afford 17 as a white solid (22.6 g, 96%) after removal of solvent; mp 156–157 °C (lit.
1
mp 158–158.5 °C). H NMR (CDCl
3
)  10.06 (s, 1H), 8.22–8.17 (m, 2H), 7.95–7.89 (m, 3H), 7.58–7.32 (m, 5H).
Page 6
©AUTHOR(S)

Arkivoc 2019, vi, 0-0
Keavy, D. J. et al.
1
-(Phenylsulfonyl)-3-formaldehyde ethylene glycol acetal (18).17 A mixture of 1-(phenylsulfonyl)-3-
formylindole (17) (20.0 g, 70.1 mmol), ethylene glycol (50.0 g, 80.6 mmol) and p-toluenesulfonic acid (1.00 g,
.26 mmol) in 200 mL of benzene was refluxed under nitrogen with a Dean-Stark trap for overnight. The
5
mixture was cooled and washed with water. The organic layer was dried over magnesium sulfate. A dark
yellow solid was obtained after removal of solvent. A white solid (19.6 g, 85%) was obtained after treatment of
1
the crude product with a mixture of ether and hexane, mp 87–90 °C; H NMR (CDCl
3
)  7.95–7.83 (m, 3H),
7
.63–7.17 (m, 7H), 6.03 (s, 1H), 4.11–3.98 (m, 4H). A sample for elemental analysis was prepared by
recrystallization from ether and hexane, mp 95–97 °C. Anal. Calcd for C17
H
15NO S: C, 61.99; H, 4.59; N, 4.25;
4
S, 9.74. Found: C, 61.75; H, 4.63; N, 4.36; S, 9.85.
1
-(Phenylsulfonyl)-2-(5’-azido-1’-hydroxypentyl)-3-formylindole ethylene glycol acetal (19). A solution of 1-
(phenylsulfonyl)-3-formylindole ethylene glycol (18) (2.20 g, 6.69 mmol) in 15 mL of THF was cooled to –78 °C.
sec-BuLi (6.2 mL, 1.3 M solution in hexane) was added dropwise, the resulting solution was stirred for an
additional 10 min. A solution of 5-azidopentanal (11) (1.02 g, 8.03 mmol) in THF (10 mL) was then added and
the mixture was stirred at –78 °C for 2 h and then allowed to warm to room temperature. The residue after
removal of solvent was purified by flash chromatography (30/70 EtOAc/Hexane) to obtain 19 as an oil (2.05 g,
1
6
7
2
4
7%), which was used directly in the next step; H NMR (acetone-d
6
)  8.17 (d, 1H, J = 8.4 Hz), 7.90 (m, 2H),
.76–7.54 (m, 4H), 7.32–7.18 (m, 2H), 6.61 (s, 1H), 5.80–5.76 (m, 1H), 4.88 (d, 1H, J = 5.2 Hz), 4.23–4.19 (m,
H), 4.07–3.98 (m, 2H), 3.36 (t, 2H, J = 6.6 Hz), 1.90–1.85 (m, 2H), 1.70–1.60 (m, 4H).
-(Phenylsulfonyl)-3-(4’-azidobutyl)-furo[3,4-b]indole (5). A mixture of 19 (100 mg, 0.219 mmol), potassium
fluoride (5.2 mg, 0.090 mmol) and hydroquinone (1.7 mg, 0.016 mmol) in 7.0 mL of acetic acid was heated in a
0 °C oil bath for 30 min. The mixture was allowed to cool and poured slowly into a saturated solution of
9
sodium bicarbonate, and then extracted with methylene chloride. The organic layer was dried over
magnesium sulfate. The residue after removal of solvent was purified by flash chromatography (30/70
1
EtOAc/Hexane) to give 5 as a white solid (53 mg, 61%); mp 95–96.5 °C; H NMR (acetone-d
6
)  8.02 (d, 1H, J
8
6
3
6
.4 Hz), 7.71 (s, 1H), 7.67 (m, 2H), 7.60–7.53 (m, 2H), 7.44–7.35 (m, 3H), 7.26–7.21 (m, 1H), 3.41–3.34 (t, 2H, J
.6 Hz), 3.21–3.17 (t, 2H, J 7.3 Hz), 1.89–1.80 (m, 2H), 1.72–1.63 (m, 2H). HRMS MS m/z Calcd for C20
H
18
N
4
O S:
3
+
94.1100 (M ). Found: 394.1096. Anal. Calcd for C20
18
H N
4
3
O S: C, 60.91; H, 4.56; N, 14.21; S, 8.12. Found: C,
0.95; H, 4.71; N, 14.11; S, 8.01.
Acknowledgements
We acknowledge and thank Dartmouth College and the National Institutes of Health, NIH Grant GM-30761, for
support of this work.
References
1
2
. Kingston, D. G. I.; Ekundayo, O. J. Nat. Prod. 1981, 44, 509.
https://doi.org/10.1021/np50017a001
. Kutney, J. P. Heterocycles 1977, 8, 813.
https://doi.org/10.3987/S(S)-1977-01-0813
Page 7
©AUTHOR(S)

Arkivoc 2019, vi, 0-0
Keavy, D. J. et al.
3
4
5
. Lounasmaa, M.; Hanhinen, P.; Westersund, M.; Halonen, N. In The Alkaloids: Chemistry and Biology, Vol.
2; Cordell, G. A., Ed.; Academic Press: San Diego, 1999, 103.
https://doi.org/10.1016/S0099-9598(08)60026-7
. Lounasmaa, M.; Hanhinen, P. In The Alkaloids: Chemistry and Biology, Vol. 55; Cordell, G. A. Ed.; Academic
Press: San Diego, 2001, 1–90.
https://doi.org/10.1016/S0099-9598(01)55002-6
. Namjoshi, O. A.; Cook, J. M. In The Alkaloids: Chemistry and Biology, Vol. 76; Knölker, H.-J., Ed.; Academic
Press: San Diego, 2016, 63.
5
https://doi.org/10.1016/bs.alkal.2015.08.002
6
7
8
9
1
1
1
1
1
. Rahman, M. T.; Cook, J. M. Synthesis 2019, 51, 1980.
https://doi.org/10.1055/s-0037-1610687
. McGowan, M. A.; Perreault, D. M.; Thornton, N. B.; Garaas, S. D.; Gribble, G. W. Arkivoc 2018, v, 334.
https://doi.org/10.24820/ark.5550190.p010.618
. House, H. O.; Crumrine, D. S.; Teranishi, A. Y.; Olmstead, H. D. J. Am. Chem. Soc. 1973, 95, 3310.
https://doi.org/10.1021/ja00791a039
. Synerholm, M. E. J. Am. Chem. Soc. 1947, 69, 2582.
https://doi.org/10.1021/ja01203a004
0. van Seeventer, P. B.; Kamerling, J. P.; Vliegenthart, J. F. G. Carbohydrate Res. 1997, 299, 181.
https://doi.org/10.1016/S0008-6215(97)00013-X
1. Asano, K.; Matsubara, S. Org. Lett. 2010, 12, 4988.
https://doi.org/10.1021/ol101990d
2. Fer, M. J.; Olatunji, S.; Bouhss, A.; Calvet-Vitale, S.; Gravier-Pelletier, C. J. Org. Chem. 2013, 78, 10088.
https://doi.org/10.1021/jo4014035
3. Pearson, W. H.; Lin, K.-C.; Poon, Y.-F. J. Org. Chem. 1989, 54, 5814.
https://doi.org/10.1021/jo00285a031
4. Rubinshtein, M.; James, C. R.; Young, J. L.; Ma, Y. J.; Kobayashi, Y.; Gianneschi, N. C.; Yang, J. Org. Lett.
2
010, 12, 3560.
https://doi.org/10.1021/ol101433v
1
1
5. James, P. N.; Snyder, H. R. Org. Synth. Coll. Vol. IV, 1963, 539.
6. Saulnier, M. G., Gribble, G. W. J. Org. Chem. 1982, 47, 757.
https://doi.org/10.1021/jo00344a001
1
7. Gribble, G. W.; Jiang, J.; Liu, Y. J. Org. Chem. 2001, 67, 1001.
https://doi.org/10.1021/jo010938q
Page 8
©AUTHOR(S)