Strategies towards the synthesis of 6-N,N-diethylcarbamyloxy-1,4-dimethoxy- 7-naphthylboronic acid

Article abstract of DOI:10.1080/00397910701542962

After acetylation, condensation between Danishefsky's diene and benzoquinone afforded a stable methoxytriacetoxydihydronaphthalene intermediate, which was subsequently transformed by the Snieckus DOM protocol into the regiospecific 7-naphthylboronic acid.

Full text of DOI:10.1080/00397910701542962

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Strategies towards the Synthesis of
6
‐N,N‐Diethylcarbamyloxy‐1,4‐dimethoxy‐
‐naphthylboronic Acid
7
a
a
b
a
F. Ameer , Ivan R. Green , K. Krohn & M. Sitoza
a
Department of Chemistry , University of the Western Cape , Bellville,
South Africa
b
Department of Chemistry , University of Paderborn , Paderborn, Germany
Published online: 10 Sep 2007.
To cite this article: F. Ameer , Ivan R. Green , K. Krohn & M. Sitoza (2007) Strategies towards the Synthesis
of 6‐N,N‐Diethylcarbamyloxy‐1,4‐dimethoxy‐ 7‐naphthylboronic Acid, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:18, 3041-3057, DOI:
10.1080/00397910701542962
To link to this article: http://dx.doi.org/10.1080/00397910701542962
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Synthetic Communications , 37: 3041–3057, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701542962
Strategies towards the Synthesis of 6-N,N-
Diethylcarbamyloxy-1,4-dimethoxy-
7
-naphthylboronic Acid
F. Ameer and I. R. Green
Department of Chemistry, University of the Western Cape, Bellville,
South Africa
K. Krohn
Department of Chemistry, University of Paderborn, Paderborn, Germany
M. Sitoza
Department of Chemistry, University of the Western Cape, Bellville,
South Africa
Abstract: After acetylation, condensation between Danishefsky’s diene and benzoqui-
none afforded a stable methoxytriacetoxydihydronaphthalene intermediate, which was
subsequently transformed by the Snieckus DOM protocol into the regiospecific
7-naphthylboronic acid.
Keywords: boronic acids, carbamate esters, Danishefsky’s diene, Snieckus DOM
Numerous binaphthoquinones (viz., diospyrin 1 and isodiospyrin 2) have been
[1]
isolated from plant sources (viz., Diospyros piscatoria (Gurke),) Diospyros
[2,3]
[2]
montana, and Diospyros rotandifolia)
and have formed a common ingre-
dient in several medicinal decoctions. The importance of diospyrin 1 and some
Received in the USA January 31, 2007
Address correspondence to Ivan R. Green, Department of Chemistry, University
of the Western Cape, Private Bag X17, Bellville 7530, South Africa. E-mail:
igreen@uwc.ac.za
3
041

3
042
F. Ameer et al.
[
4]
of its analogs as an antimycobacterial agent has recently been published,
[5]
and it was previously synthesized by Mori and Yoshida (Fig. 1). In Asian
0
countries, other binaphthoquinones (viz., 3,3 -biplumbagin 3, elliptinone 4,
and maritinone 5) have been isolated from Plumbago zeylancia and also
[
6]
used in medicinal applications.
Within the limited examples in Fig. 1, the linkage between the two
naphthoquinones falls within one of three categories (viz., quinone–quinone
3
linkage has a profound effect on the degree and nature of the activity of
, quinone–nonquinone 1, and nonquinone–nonquinone 2, 4, and 5). This
[
1]
these systems, and it would appear that the hydroxy groups also have an
important role to play. To make a contribution to the range of binaphthoqui-
nones and their activity, the naphthylboronic acid 27 was required for Suzuki
cross-coupling reactions. This article describes some of the chemistry that led
to its synthesis.
The route envisaged for the synthesis of the key intermediate phenol 12
[7]
required the condensation between Danishefsky’s diene 6 and quinone 7.
Subjection of the crude reaction mixture to chromatographic purification
[8]
afforded the quinone 8 in a yield of 40%. Acetylation of the 6-OH group
Figure 1. Biologically active binaphthoquinones.

6-N,N-Diethylcarbamyloxy-1,4-dimethoxy-7-naphthylboronic Acid
3043
with acetic anhydride in pyridine afforded a quantitative yield of the
[
corresponding acetate 9. This compound was subsequently reductively
9]
[9]
methylated by reducing the quinone with aqueous sodium dithionite first,
followed by treatment with dimethylsulphate and potassium carbonate in
refluxing acetone. However, the major product isolated from the reaction
mixture was the trimethoxy naphthalene 10 (71%) with the desired product
1
1 being obtained in 22% yield. No doubt, under the basic reaction conditions
employed, the acetate group at C-6 was hydrolyzed to the corresponding
phenol, which in turn was methylated. This problem was overcome by
methylation under neutral conditions in benzene using methyl iodide and
[
10]
silver(I) oxide
yield from 9. Finally, gentle hydrolysis of acetate 11 using 1% KOH/
to afford the desired naphthalene acetate 11 in 90%
[11]
MeOH led to the isolation of the target molecule 12
Scheme 1).
To improve the overall yield of naphthol 12, due to the poor yield in the
formation of quinone 8, an alternative route was investigated. Contrary to
Danishefsky’s reported findings, in our hands, acetylation of the crude
Diels–Alder reaction mixture afforded only an 8% yield of the 1,4,6-triace-
in a yield of 88%
(
[
7]
[
7]
toxynaphthalene 13. The major product isolated in a 92% yield was the
fairly stable enol acetate 14 (Fig. 2). Assignment of the structure 14 to the
1
enol acetate is based on data from HRMS, H NMR, and C NMR spectra.
13
Our initial attempt to aromatize the adduct 14 into the desired triacetate
3 involved its treatment with acetic anhydride and pyridine under reflux.
1
However, careful chromatographic purification of the reaction mixture
afforded three products [viz., the triacetoxynaphthalene 13 (42%), the tetraa-
cetoxybinaphthyl ether 15 (11%), and surprisingly, the diacetoxynaphthol 16
(21%) (Fig. 3)]. Assignment of the structure 15 to the binaphthyl ether was
based on HRMS, IR, and H and C spectral data.
1
13
Heating adduct 14 in acetic acid under reflux effected a quantitative con-
version into the desired triacetoxynaphthalene 13. Finally, treatment of 13
with cerium ammonium nitrate in aqueous acetonitrile afforded a quantitative
yield of acetoxyquinone 9. Thus the latter route (viz., 14 . 13 . 9 . 11 .
1
2), which involved five steps with an overall yield of 73%, was preferred
Scheme 1.

3
044
F. Ameer et al.
Figure 2. Acetylation products of the Diels–Alder adduct.
over the earlier route (8 . 9 . 11. 12), which involved four steps but had an
overall yield of 32%.
BROMINATIONS
The original route envisaged for the synthesis of the target naphthalene
[
12]
boronic acid 27 required the regiospecific bromination
phenol 12 (see numbering in Scheme 1) followed by protection of the C-6
OH group, metal–halogen exchange, and boronation.
at position 7 in
[
13]
Thus treatment of
naphthol 12 with 1 mol equivalent of bromine in acetic acid produced two
products [viz., the dibromophenol 17 (13%) and the monobromophenol 18
(
72%) (Scheme 2)]. This result demonstrated two features: (a) that the two
methoxy groups activated the ring of their attachment to a greater extent
than the 6-OH and (b) that a-activation of the 6-OH was more strongly
directed at C-5 than at C-7. To evaluate the influence that reversing the
electron-donating character at C-1 and C-4 of phenol 12 would have on the
bromination products, diacetoxy phenol 16 was brominated in a similar
way. However, only the monobrominated diacetoxyphenol 19 was isolated
(
78%). Thus although bromination occurred exclusively in the phenol ring
in this case, a-naphthalene activation controlled attack at the sterically
more congested C-5 site to override b-naphthalene activation at the sterically
more accessible C-7 position.
Figure 3. Conversion products of 14.

6-N,N-Diethylcarbamyloxy-1,4-dimethoxy-7-naphthylboronic Acid
3045
Scheme 2.
Attention was then focused on bromination of adduct 14. With its olefinic
bond between C-6 and C-7, it was hoped to eventually afford the C-7 haloge-
nated products. Thus treatment of adduct 14 with 1 mol equivalent of bromine
in acetic acid at 248C for 3 h afforded the 2,3-dibromonaphthoquinone 20
(11%) and triacetoxynaphthalene 13 (65%) as the only two identifiable
products (Scheme 3).
On the other hand, treatment of adduct 14 with 1 mol equivalent of
bromine in acetic anhydride at 108C afforded three products [viz., the bis-
naphthoquinone 21 (4%), the triacetoxynaphthalene 13 (68%), and the bromo-
phenol 19 (10%)].
Changing the solvent to dichloromethane did not alter the product distri-
bution nor the yields to any extent. Thus at 108C the products isolated were 21
(
4%), 13 (69%), and 19 (10%), whereas at 48C, the isolated products were 21
3%), 13 (64%), and 19 (9%). Finally, treatment of adduct 14 with 5 mol
(
equivalents of NBS in hot acetic acid produced the dibromophenol 22 in a
yield of 71% (Scheme 3).
BORATIONS
As a consequence of the bromination strategy not being able to regioselec-
tively brominate C-7 of naphthol 12, an entirely new approach was adopted
[viz., employing the well-proven directed ortho metallations (DOM)
developed over the years by Snieckus.
[13,14]
Thus to establish the method-
[15]
ology, phenol was converted to the carbamate 23 (94%)
and subsequently
(Scheme 4). Similarly, phenol 12 was
[
13]
to the ortho-boronic acid 24 (82%)

3
046
F. Ameer et al.
Scheme 3.
converted into the corresponding carbamate 25 (83%), which upon
treatment under a similar protocol consistently afforded three products,
which were chromatographically separated. The first product to elute was
assigned the structure 26 (26%), and the next was the naphthol 12 (40%), a
product of hydrolysis. The last compound to elute was the target boronic
acid 27 (30%).
Assignment of the structure 26 to the first product to elute was based on
the following spectral evidence. The molecular ion at m/z 288.1346
confirmed a molecular formula of C H O (requires 288.1362). In
1
7 20 4
addition, a major fragmentation representing the loss of a CH CH CO-
3
2
group (viz., m/z 57) supports the assigned structure as well. Strong absorption
2
1
bands in the infrared spectrum at 3383 and 1680 cm supported the presence
of hydroxy and ketone functional groups respectively. A 3-proton triplet at

6-N,N-Diethylcarbamyloxy-1,4-dimethoxy-7-naphthylboronic Acid
3047
Scheme 4.
1
.97 ppm (J 7.4) in the H NMR spectrum was demonstrated via correlation
0
spectroscopy (COSY) coupling to two 1-proton dt’s at 1.60 and 1.90 ppm
0
0
(J 18.0 and 7.4) and represents H-5 and H-4 of the pentanoyl side chain.
A 3-proton doublet at 1.29 ppm (J 7.0) assigned to H-1 was also confirmed
0
via COSY spectroscopy to be coupled to a 1-proton quartet at 3.74 ppm
0
(J 7.0) assigned to H-2 of the side chain. In the aromatic region, two ortho-
coupled 1-proton doublets at 6.50 and 6.73 ppm (J 8.0) are assigned to H-2
and H-3 respectively, whereas two 1-proton singlets at 7.63 and 8.80 ppm
are assigned to H-5 and H-8 respectively. A D O exchangeable peak at
2
1
2.00 ppm is assigned to the C-6-OH group. Significant signals in the
0
1
3
0
C NMR spectrum include those at 11.9 and 17.6 ppm for C-5 and C-1 ,
0
0
0
2
7.3 and 41.9 ppm for C-4 and C-2 , and 211.5 for the C-3 C 55 O.
Assignment of the position of boronation for boronic acid 27 was based
1
primarily on the four 1-proton signals in the aromatic region of the H
NMR spectrum [viz., a doublet at 6.53 ppm (J 7.0) assigned to H-2, a
doublet at 6.70 ppm (J 7.0) assigned to H-3, a singlet at 7.67 ppm assigned
to H-5, and a singlet at 8.23 ppm assigned to H-8].
Thus, the target boronic acid 27 was synthesized in a overall yield of 18%
involving seven steps starting from Danishefsky’s diene 6 and quinone 7.
Attention is currently being focused on improving the yield of the sterically

3
048
F. Ameer et al.
preferred ortho-directed boronation of carbamate 25 and its subsequent
reactions in Suzuki–Mayori cross-coupling protocols.
EXPERIMENTAL
1
13
H and C NMR spectra were recorded using a Varian 200-MHz spec-
trometer at 208C in deuterochloroform and J values are given in Hertz.
Infrared spectra were measured as Nujol mulls on a Perkin Elmer FT-IR
1000PC spectrometer. Melting points were recorded using a Fisher-Johns
melting-point apparatus. Mass spectra were recorded on a Finnigan-Matt
GCQ spectrometer. Column chromatography was carried out using Merck
Kieselgel 60 (70–230 mesh) as dry columns. The residue obtained upon
workup refers to material obtained from the dried (magnesium sulphate)
organic extract after filtration and solvent removal. Hexane refers to that
1
3
fraction of bp 70–758C. In C-spectra, assignments with the same superscript
may be interchanged.
6
-Hydroxy-1,4-naphthoquinone (8)
Treatment of benzoquinone 7 (3.0 g, 27.8 mmol) in benzene (60 mL) with
1
[7]
-methoxy-3-trimethylsilyloxybutadiene 6 (5.0 g, 29.2 mmol) in benzene
10 mL) dropwise over 5 min followed by heating under reflux for 3 h and
(
evaporation of solvent afforded a residue that was chromatographed using
EtOAc–hexane (3:7) to afford quinone 8 (1.92 g, 40%) as yellow crystals,
[8]
mp 194–1968C (from EtOAc–hexane). (Lit. mp 193–1948C); nmax 3450
2
and 1666 cm ; d [(CD ) CO] 6.96 (2H, s, H-2 and H-3), 7.25 (1H, dd, J
1
H
3 2
8
1
.8 and 2.6, H-7), 7.42 (1H, d, J 2.6, H-5) and 7.94 (1H, d, J 8.8, H-8). d
C
a
12.8 (C-7), 121.7 (C-5) , 125.9 (C-4a) , 129.9 (C-8) , 135.3 (C-8a) , 139.2
b
a
b
c
c
C-2) , 140.0 (C-3) , 163.6 (C-6), 184.6 and 185.8 (C 55 O). (Found: C,
(
69.20; H, 3.3%; M, 174. Calc. for C H O : C, 69.0; H, 3.5%; M, 174.)
10 6 3
6-Acetoxy-1,4-naphthoquinone (9)
Hydroxyquinone 8 (348 mg, 2.0 mmol) in acetic anhydride (15 mL) and
pyridine (2 mL) was stirred under nitrogen at 248C for 18 h after which the
reaction solution was poured into ice/water to give a precipitate of the acet-
oxyquinone 9 (430 mg, 100%) as yellow needles, mp 98–998C (from
[
9]
21
EtOAc–hexane) (Lit. mp 96–978C) vmax 1727 and 1668 cm ; dH 2.36
3H, s, CH CO), 6.99 (2H, s, H-2 and H-3), 7.48 (1H, dd, J 8.4 and 2.5,
H-7), 7.80 (1H, d, J 2.5, H-5) and 8.13 (1H, d, J 8.4, H-8). d 21.1
(
3
C
a
a
b
(
CH CO ), 119.6 (C-7), 127.3 (C-5) , 128.6 (C-8) , 129.6 (C-8a) , 133.7
3 2
b
C-4a) , 138.7 (C-2) , 138.9 (C-3) , 155.2 (C-6), 168.5 (C 55 O ester), 184.1
c
c
(

6-N,N-Diethylcarbamyloxy-1,4-dimethoxy-7-naphthylboronic Acid
3049
and 184.2 (C 55 O of quinone). (Found: C, 66.6; H, 3.5%; M, 216. Calc. for
C H O : C, 66.7; H, 3.7%; M, 216.)
1
2 8 4
1,4,6-Trimethoxynaphthalene (10) and 6-Acetoxy-1,4-
dimethoxynaphthalene (11)
6-Acetoxy-1,4-naphthoquinone 9 (300 mg, 1.39 mmol) in ether (200 mL) was
vigorously shaken with sodium dithionite solution (10 g/100 mL), which
resulted in the ethereal solution losing its yellow color. The ether solution
was dried (MgSO ) and evaporated to yield a residue, which was immediately
4
dissolved in acetone (20 mL) and treated with Me SO (700 mg, 5.56 mmol)
2
4
in the presence of K CO (770 mg, 5.56 mmol) and then vigorously stirred
3
2
and heated under reflux under an N atmosphere for 12 h. The filtrate was con-
2
centrated under reduced pressure and purified by column chromatography
using EtOAc–hexane (1:4) as eluent to afford 10 (213 mg; 71%) as a
yellow-brown oil; d 3.99 (9H, s, 3x OCH ), 6.57 (1H, d, J 8.4, H-2), 6.70
H
3
(1H, d, J 8.4, H-3), 7.17 (1H, dd, J 8.4 and 2.6, H-7), 7.51 (1H, d, J 2.6,
H-5), 8.12 (1H, d, J 8.4, H-8). d 55.4, 55.7 and 55.8 (OCH ), 100.5 (C-2) ,
a
C
3
a
01.2 (C-3) , 104.2 (C-7) , 118.1 (C-5) , 121.6 (C-4a) , 123.7 (C-8) , 127.6
d
C-8a) , 148.8 (C-1) , 149.9 (C-4) , and 158.1 (C-6). (Found: C, 71.3; H,
a
b
c
b
1
c
d
(
6
.6%; M, 218. Calc. for C H O : C, 71.5; H, 6.5%; M 218.)
13 14 3
Further elution afforded 6-acetoxy-1,4-dimethoxynaphthalene 11 (66 mg,
2%) as a light yellow oil spectroscopically identical to the material syn-
2
thesized as described next.
6
-Acetoxy-1,4-dimethoxynaphthalene (11)
6
-Acetoxyquinone 9 (500 mg, 2.3 mmol) in ether (200 mL) was similarly
reduced as just described to afford a residue, which was immediately
treated with methyl iodide (1.63 g, 11.5 mmol) in the presence of silver(II)
oxide (1.45 g, 11.69 mmol) in dry benzene (40 mL). This was then stirred
and heated under reflux under a nitrogen atmosphere for 12 h and filtered.
The filtrate was concentrated under reduced pressure and purified by
column chromatography using EtOAc–hexane (1:4) as eluent, to afford the
acetoxynaphthalene 11 (509 mg, 90%) as a light yellow oil; n
2
max
1
1726 cm ; d 2.35 (3H, s, OCOCH ), 3.93 (6H, s, 2x OCH ), 6.66 (1H, d,
H 3 3
J 9.2, H-2), 6.68 (1H, d, J 9.2, H-3), 7.30 (1H, dd, J 9.2 and 2.6, H-7), 7.95
(
1H, d, J 2.6, H-5), and 8.25 (1H, d, J 9.2, H-8). d 21.1 (CH CO ),
C 3 2
a
5.7 (OCH ), 55.8 (OCH ), 103.2 (C-2) , 104.2 (C-3) , 113.3 (C-7),
a
5
1
1
3 3
b
b
c
c
d
20.8 (C-5) , 123.7 (C-8) , 124.4 (C-4a) , 127.1 (C-8a) , 148.8 (C-4) ,
d
49.2 (C-1) , 149.6 (C-6), 169.6 (C 55 O). (Found: C, 68.6; H, 5.5%; M, 246.
Calc. for C H O : C, 68.3, H, 5.7%; M, 246.)
14 14 4

3
050
F. Ameer et al.
6-Hydroxy-1,4-dimethoxynaphthalene (12)
6-Acetoxy-1,4-dimethoxynaphthalene 11 (130 mg, 0.53 mmol) in MeOH
(
10 mL) was treated with 1% KOH/MeOH (1.1 mL) with constant stirring
at 248C for 1 h. The reaction mixture was then poured into water (100 mL)
and acidified (litmus) with 0.1 M HCl, and the solution was extracted with
dichloromethane to give naphthol 12 (95 mg, 88%) as a greenish oil; n
max
2
368 cm ; dH 3.93 (6H, s, 2x CH O), 5.34 (1H, s, D O exchangeable,
1
3
6
8
5
1
3 2
-OH), 6.55 (1H, d, J 8.0, H-2), 6.70 (1H, d, J 8.0, H-3), 7.20 (1H, dd, J
.0 and 2.4, H-7), 7.51 (1H, d, J 2.4, H-5) and 8.25 (1H, d, J 8.0, H-8). d
C
a
5.8 (OCH ), 55.9 (OCH ), 101.1 (C-5) , 104.4 (C-2) , 104.5 (C-3) ,
a
a
3
3
a
b
b
c
17.2 (C-7) , 121.7 (C-8a) , 124.2 (C-8), 127.8 (C-4a) , 148.6 (C-1) , 150.0
c
(C-4) , 154.0 (C-6). (Found: C, 70.7; H, 5.7%; M, 204. Calc. for C H O :
12 12 3
C, 70.6; H, 5.9%; M, 204.)
1,4,7-Triacetoxy-5-methoxy-5,8-dihydronaphthalene (14)
Benzoquinone 7 (3.0 g, 27.8 mmol) in benzene (60 mL) was treated with
-methoxy-3-trimethylsilyloxybutadiene 6 (5.0 g, 29.2 mmol) in benzene
10 mL) at 248C, and the resulting solution was gently heated and stirred at
0–708C for 1 h under nitrogen. The residue obtained upon removal of the
solvent was treated with acetic anhydride (80 mL) and pyridine (40 mL) at
58C for 12 h and then poured into water (400 mL) and chilled. The
1
(
6
2
product was chromatographed using EtOAc–hexane (3:7) as eluent to
afford the triacetate 14 (8.54 g, 92%) as white crystals, mp 146–1478C
2
from EtOH); nmax 1728 cm ; dH 2.20, 2.32, and 2.33 (each 3H, each s,
1
(
CH CO -), 2.66 (1H, dd, J 17.8 and 2.1, pseudo H-8a), 2.91 (1H, ddd, J
3
2
17.8, 5.0, and 2.4, pseudo H-8e), 3.23 (3H, s, 5-CH O), 4.57 (1H, dd, J 5.0
and 2.2, H-5), 6.39 (1H, d, J 2.4, H-6), 6.99 (1H, d, J 8.8, H-2), and 7.10
3
(
1H, d, J 8.8, H-3). d 20.9, 21.0, and 21.2 (3x CH CO), 32.4 (C-8), 55.5
C 3
a
a
b
(
CH O), 69.9 (C-5), 106.8 (C-6), 121.3 (C-2) , 123.5 (C-3) , 123.9 (C-4a) ,
3
b
27.2 (C-8a) , 144.1 (C-1) , 146.4 (C-7) , 150.2 (C-4) , 168.5, 168.9,
c
c
c
1
and 169.0 (C 55 O). (Found: C, 61.3; H, 5.6%; M, 334. Calc. for C H O :
1
7 18 7
C, 61.1; H, 5.4%; M, 334.) Further elution afforded the 1,4,6-triacetoxy-
naphthalene 13 (67 mg, 8%) identical to material synthesized as
described next.
0 0 0
,4,6-Triacetoxynaphthalene (13), 1,1 4,4 -Tetraacetoxy-6,6 -
oxobisnaphthalene (15), and 1,4-Diacetoxy-6-hydroxynaphthalene (16)
1
Adduct 14 (480 mg, 1.44 mmol) in acetic anhydride (25 mL) and pyridine
(5 mL) was heated and stirred under reflux for 2 h; the reaction mixture
then poured into water (100 mL) and extracted with dichloromethane,

6-N,N-Diethylcarbamyloxy-1,4-dimethoxy-7-naphthylboronic Acid
3051
which was backwashed with water, 0.5 M hydrochloric acid, and finally
0% aqueous sodium hydrogen carbonate. The residue obtained upon
workup was very carefully chromatographed using EtOAc–hexane (3:7)
1
as eluent to afford in order of elution the 1,4,6-triacetoxynaphthalene 13
(
(
183 mg, 42%) as cream crystals, mp 106–1078C (from ethanol).
[15] 21
Lit.
mp 94–958C); nmax 1726 cm ; dH 2.34 and 2.4(ꢀ2) (9H, s,
2
CH CO ), 7.24 (1H, d, J 8.4, H-2), 7.25 (1H, d, J 8.4, H-3), 7.32 (1H,
3
dd, J 9.2 and 2.2, H-7), 7.61 (1H, dd, J 2.2 and 0.8, H-5), and
7
1
1
1
.91 (1H, dd, J 9.2 and 0.2, H-8). d 21.1(ꢀ2), 21.3 (CH CO ),
C
3
2
a
12.9 (C-7) , 117.6 (C-5) , 118.7 (C-8) , 122.5 (C-2) , 123.5 (C-3) ,
a
a
a
a
b
b
c
c
c
25.7 (C-4a) , 128.4 (C-8a) , 144.1 (C-1) , 144.4 (C-4) , 149.6 (C-1) ,
69.1, 169.2, and 169.3 (C 55 O). (Found: C, 63.4; H, 4.5%; M, 302.
Calc. for C H O : C, 63.4; H, 4.5%; M, 302.)
1
6 14 6
Further elution afforded the binaphthalene ether 15 (80 mg, 11%) as light
yellow crystals, mp 123–1248C (from EtOAc–hexane); n 1740–
max
2
750 cm ; dH 2.36 and 2.44 (each 6H, s, CH CO ), 6.40 (1H, d, J 8.5,
1
1
H-2 ) , 6.44 (1H, d, J 8.5, H-3 ) , 6.85 (d, J 8.2, H-2) , 6.94 (1H, d, J 8.2,
3 2
0
a
0
a
a
a
H-3) , 7.16 (1H, dd, J 9.0 and 2.2, H-7) , 7.22 (1H, dd, J 8.8 and
b
0
b
c
0
d
.4, H-7 ) , 7.42 (1H, d, J 2.2, H-5) , 7.74 (1H, d, J 8.8, H-8 ) , 7.75 (1H, d,
2
J 2.4, H-5 ) , and 8.02 (1H, d, J 9.0, H-8) . d 21.0, 21.1, 21.2, and 21.3
0
c
d
C
(4x CH CO ), 107.8, 108.6, 112.1, 114.1, 117.9, 119.1, 120.7, 122.0, 122.7,
and 123.3 (C-H aryl), 124.6, 125.5, 125.9, and 128.1 (C-4a, C-8a, C-4a ,
3 2
0
0
0
and C-8a ), 139.4 and 139.7 (C-6 and C-6 ), 148.2, 149.5, 149.8, and 150.1
0
0
(
C-1, C-4, C-1 , and C-4 ), 169.9, 170.2, 170.6, and 170.7 (C 55 O). (HRMS:
02.1260. Calc. for C H O : 502:1264.)
5
2
8 22 9
Further elution afforded a thick oil, which solidified on standing to
give 1,4-diacetoxy-6-hydroxynaphthalene 16 (80 mg, 21%) as white
needles, mp 141–1428C (from EtOAc–hexane); nmax 3350 and
2
729 cm ; dH 2.41 and 2.45 (each 3H, s, CH CO ), 5.60 (1H, br s,
1
1
D O exchangeable, 6-OH,), 7.02 (1H, dd, J 8.2 and 2.6, H-7), 7.06 (1H,
3 2
2
dd, J 2.6 and 0.6, H-5), 7.06 (1H, d, J 8.2, H-2), 7.19 (1H, d, J 8.2,
H-3), and 7.71 (1H, dd, J 8.4 and 0.6, H-8). d 21.0 and 21.1
C
a
a
a
(
(
(
CH CO ), 104.0 (C-7), 115.2 (C-5) , 118.5 (C-8) , 119.1 (C-2) , 123.1
3 2
c
C-4a) , 123.9 (C-3) , 129.2 (C-8a) , 143.3 (C-1) , 144.7 (C-4) , 154.7
b
a
b
c
C-6), 169.6 and 169.7 (C 55 O). (Found: C, 64.6; H, 4.5%; M, 260.
Calc. for C H O : C, 64.4; H, 4.65%; M, 260.)
14 12 5
1
,4,6-Triacetoxynaphthalene (13)
Adduct 14 (500 mg, 1.5 mmol) in acetic acid (20 mL) was heated under reflux
for 2 h, after which the mixture was poured into water (200 mL) and cooled to
08C for 12 h. The precipitate that formed was filtered off to afford 1,4,6-tria-
cetoxynaphthalene 13 (453 mg, 100%), spectroscopically identical to the
material synthesized earlier.

3
052
-Acetoxy-1,4-naphthoquinone (9)
F. Ameer et al.
6
To the triacetoxynaphthalene 13 (520 mg, 1.72 mmol) in CH CN (30 mL) and
3
water (6 mL), cerium(IV) ammonium nitrate (4.71 g; 5 mmol) in water (6 mL)
was added with rapid stirring at 248C. Stirring was continued for 3 h, after
which the reaction mixture was poured into water (100 mL) to give the
quinone 9 (372 mg, 100%), identical to material prepared earlier.
2,5-Dibromo-6-hydroxy-1,4-dimethoxynaphthalene (17) and 2-Bromo-6-
hydroxy-1,4-dimethoxynaphthalene (18)
6-Hydroxy-1,4-dimethoxynaphthalene 12 (110 mg, 0.54 mmol) in AcOH
(
10 mL) was treated with bromine (100 mg, 0.54 mmol) in AcOH (2 mL)
with constant stirring for 15 min, after which the reaction mixture was
poured into water (100 mL), extracted with dichloromethane, and backwashed
with 10% aqueous sodium hydrogen carbonate to afford a residue, which was
purified by column chromatography using EtOAc–hexane (3:7) as eluent to
give dibromonaphthol 17 (25 mg, 13%) as yellow crystals, mp 79–808C
2
1
(
from EtOAc–hexane); nmax 3350 cm ; dH 3.90 (6H, s, CH O), 6.51
3
(
H-7), 8.00 (1H, d, J 9.2, H-8). d 56.3 and 61.6 (CH O), 102.2 (C-2),
1H, s, D O exchangeable, 6-OH), 6.96 (1H, s, H-3), 7.30 (1H, d, J 9.2,
2
C
3
a
a
b
b
c
09.8 (C-5) , 112.3 (C-3) , 117.9 (C-7) , 123.9 (C-8) , 124.2 (C-8a) , 126.5
1
c
d
d
(
C-4a) , 147.3 (C-6), 151.4 (C-1) , 151.8 (C-4) . [Found: C, 40.0; H, 2.8%;
M, 360/362/364 (1:2:1). Calc. for C H Br O : C, 39.8; H, 2.8%; M, 360/
1
2
10
2 3
3
62/364 (1:2:1).]
Further elution afforded the 2-bromonaphthalene 18 (112 mg, 72%) as
light yellow needles, mp 108–1098C (from EtOAc–hexane); n
max
2
1
3350 cm ; d 3.93 (6H, s, CH O), 5.25 (1H, s, D O exchangeable, 6-OH),
H 3 2
6.84 (1H, s, H-3), 7.15 (1H, dd, J 8.4 and 2.4, H-7), 7.48 (1H, d, J 2.4,
H-5), 7.99 (1H, d, J 8.4, H-8). d 56.0 (OCH ), 61.6 (OCH ), 105.3 (C-2),
C
3
3
a
08.8 (C-7) , 109.1 (C-3) , 118.8 (C-5) , 124.3 (C-8) , 124.4 (C-4a) , 127.3
a
a
a
b
1
b
C-8a) , 147.1 (C-6), 151.3 (C-1) , 153.9 (C-4) . [Found: C, 50.7; H, 3.6%;
c
c
(
M, 282/284 (1:1). Calc. for C H BrO : C, 50.9; H, 3.9%; M, 282/284 (1:1).]
1
2
11
3
1,4-Diacetoxy-5-bromo-6-hydroxynaphthalene (19)
Diacetoxynaphthol 16 (260 mg, 1.0 mmol) in AcOH (15 mL) was treated with
bromine (160 mg, 1.0 mmol) in AcOH (4 mL) with stirring at 248C, and
stirring was continued for 20 min, after which time the reaction mixture
was poured into ice/water (100 mL) to produce 5-bromophenol 19 (264 mg,
7
8%) as slightly greyish crystals, mp 127–1288C (from EtOAc–hexane);
21
vmax 3350 and 1740 cm ; dH 2.46 (6-H, s, COCH ), 6.48 (1H, s, D O
exchangeable, 6-OH), 7.15 (1H, d, J 8.2, H-2), 7.21 (1H, d, J 8.2, H-3),
3
2

6-N,N-Diethylcarbamyloxy-1,4-dimethoxy-7-naphthylboronic Acid
3053
7
2
1
1
.28 (1H, d, J 9.2, H-7), and 7.83 (1H, d, J 9.2, H-8). d 21.1 (OCOCH ),
C
3
a
2.0 (OCOCH ), 100.4 (C-7), 116.5 (C-3) , 118.0 (C-5) , 122.1 (C-2) ,
a
a
3
a
b
b
c
c
23.6 (C-8) , 124.8 (C-4a) , 126.1 (C-8a) , 142.3 (C-1) , 144.8 (C-4) ,
52.4 (C-6), 169.1 (C 55 O), and 170.2 (C 55 O). [Found: C, 49.7; H, 3.3%;
M, 338/340 (1:1). Calc. for C H BrO : C, 49.6, H, 3.3%; M, 338/340 (1:1).]
1
4
11
5
6
-Acetoxy-2,3-dibromo-1,4-naphthoquinone (20)
Bromine (268 mg, 1.68 mmol) in acetic acid (2 mL) was dripped into a
solution of adduct 14 (560 mg, 1.68 mmol) in acetic acid (8 mL) at 248C
over 3 min in a nitrogen atmosphere and thereafter stirring was continued
for 3 h. The reaction mixture was poured into water (200 mL), and the
organic material extracted into dichloromethane, which was backwashed
with saturated sodium hydrogen carbonate to yield a residue, which was chro-
matographed using EtOAc–hexane (3:7) as eluent to yield the dibromo-
naphthoquinone 20 (70 mg, 11%) as bright yellow crystals, mp 181–1828C
2
1
(
from EtOAc–hexane); nmax 1754 and 1665 cm
;
3H, s, OCOCH ), 7.52 (1H, dd, J 8.8 and 2.2, H-7), 7.90 (1H, d, J 2.2,
dH 2.37
(
H-5), and 8.23 (1H, d, J 8.8, H-8). d 21.1 (OCOCH ), 121.3 (C-7) ,
3
a
C
3
a
a
a
a
b
27.9 (C-5) , 128.3 (C-2) , 130.3 (C-8) , 132.5 (C-3) , 142.4 (C-4a) , 143.0
1
b
C-8a) , 155.6 (C-6), 168.3 (C 55 O of ester), 175.0 and 175.2 (C 55 O).
(
þ
(
Found: C, 38.3; H, 1.5%; M , 372/374/376. Calc. for C H Br O : C,
1
2
6
2 4
38.5; H, 1.6%; M 372/374/376.) Further elution afforded 1,4,6-triacetoxy-
naphthalene 13 (330 mg, 65%), which had identical spectroscopic properties
to the material synthesized previously.
0
0
6
,6 -Diacetoxy-2,2 -binaphthoquinone (21), 1,4,6-Triacetoxynaphthalene
(13), and 1,4-Diacetoxy-5-bromo-6-hydroxynaphthalene (19)
A solution of bromine (321 mg, 2.01 mmol) in acetic anhydride (2 mL) was
dripped into a solution of the adduct 14 (670 mg, 2.01 mmol) in acetic
anhydride (15 mL) under a nitrogen atmosphere at 108C over 5 min, and
stirring continued for 2 h. The reaction mixture was then poured into ice/
water and extracted into dichloromethane. The residue obtained upon
workup was chromatographed using EtOAc–hexane (3:7) to afford the
dimer 21 (37 mg, 4%) as yellow crystals, mp 112–1148C (from EtOAc–
2
1
hexane); n
m, H-3, H-3 , H-7, and H-7), 7.79 and 7.87 (each 1H, each d, J 2.2, H-5
1728 and 1660 cm ; dH 2.36 (6H, s, OCOCH ), 7.50 (4H,
max
0
3
0
0
and H-5 ), 8.11 and 8.20 (each 1H, each d, J 8.4, H-8 and H-8 ). d 21.1
0 0
2x OCOCH ), 120.0 (C-3) , 121.0 (C-3 ) , 127.4 (C-7) , 127.4 (C-7 ) ,
3
C
a
a
b
b
(
c
d
0
c
0
d
e
1
1
28.5 (C-2) , 129.0 (C-5) , 129.3 (C-2 ) , 130.0 (C-5 ) , 132.6 (C-4a) ,
0
e
e
0
e
0
e
33.5 (C-4a ) , 139.9 (C-8a) , 140.4 (C-8a ) , 140.5 (C-8 and C-8 ) , 155.3
f
C-6) , 155.6 (C-6 ) , 168.3 and 168.4 (C 55 O of the two ester groups),
0
f
(

3
054
F. Ameer et al.
177.0, 177.3, 181.4, and 181.6 (C 55 O of quinones). (Found: C, 66.9; H, 3.1%;
HRMS 430.0687. Calc. for C H O : C, 67.0; H, 3.3%; HRMS 430.0689.)
2
4 14 8
Further elution afforded 1,4,6-triacetoxynaphthalene 13 (413 mg, 68%),
which had identical spectroscopic properties to the material synthesized
earlier. Finally, the third compound to elute was the bromophenol 19
(
68 mg, 10%) with identical spectral properties to material synthesized earlier.
1,4-Diacetoxy-5,7-dibromo-6-hydroxy-8-methoxy-naphthalene (22)
A solution of adduct 14 (500 mg, 1.50 mmol) dissolved in hot AcOH (30 mL)
was added over 5 min to N-bromosuccinimide (2200 mg, 7.48 mmol) in
AcOH (30 mL) and water (60 mL) at 55–608C. The resulting solution was
stirred at 55–608C for 30–45 min, after which it was poured into ice/
water, and the resulting precipitate was filtered to afford the dibromonaphtha-
lene 22 (445 mg, 71%) as yellow crystals, mp 202–2038C (from EtOAc–
2
1
hexane); nmax 3400 and 1738 cm ; dH 2.44 and 2.50 (each 3H, s,
OCOCH ), 3.33 (3H, s, OCH ), 5.09 (1H, s, D O exchangeable, 6-OH),
3
3
2
7
.29 (1H, d, J 8.8, H-2), and 7.35 (1H, d, J 8.8, H-3). d 21.1 and
C
a
1.7 (CH CO), 58.7 (MeO), 124.3 (C-5) , 125.1 (C-2) , 126.5 (C-7) , 126.9
a
a
2
3
a
b
b
c
(
C-3) , 127.7 (C-4a) , 131.0 (C-8a) , 144.6 (2 ꢀ ) (C-1 and C-4) , 147.3
c
x2) (C-6 and C-8) , 168.0 and 168.4 (C 55 O). (Found: C, 40.0; H, 2.5%; M,
(
4
46/448/450. Calc. for C H Br O : C, 40.2; H, 2.7%; M 446/448/450.)
15 12 2 6
N,N-Diethylphenylcarbamate (23)
Phenol (5 g, 53.19 mmol) in pyridine (10 mL) was treated with N,N-diethyl-
carbamylchloride (7.2 g, 53.10 mmol) in a pressure-capped glass bottle. This
was then heated for 6 h in an oil bath at 1108C. The reaction mixture was then
poured onto ice/water (200 mL) and extracted with ether, which was washed
with 10% HCl followed by aqueous sodium hydrogen carbonate to afford the
[
15]
phenylcarbamate 23 (9.14 g, 89%) as light brown oil; dH 1.22 (6H, m,
CH CH -), 3.40 (4H, m, -CH CH ), 7.0 to 7.5 (5H, m, Aryl-H).
3
2
2
3
2-N,N-Diethylcarbamyloxyphenyl Boronic Acid (24)
Phenylcarbamate 23 (1 g, 5.2 mmol) in THF (20 mL) was treated
with TMEDA (600 mg, 5.2 mmol) followed by sec-butyllithium (5.5 mmol)
at –788C under a nitrogen atmosphere for 1 h and then allowed to warm to
room temperature for 2 h. The reaction mixture was cooled again to –788C
for 40 min, treated with trimethylborate (540 mg, 5.2 mmol), stirred for a
further 3 h, treated with saturated ammonium chloride (20 mL), and
1
4
extracted with dichloromethane to afford the boronic acid 24 (1.01 g,

6-N,N-Diethylcarbamyloxy-1,4-dimethoxy-7-naphthylboronic Acid
3055
8
2%) as a brown oil. d 1.15 (6H, m, CH CH -), 3.25 (4H, m, CH CH -), 7.10
H
3
2
3
2
to 7.50 (3H, m, H-4, H-5, H-6), and 7.85 (1H, dd, J 9.2 and 2.6, H-3). (Found:
C, 55.4; H, 6.5%; M, 237. Calc. for C H BNO : C, 55.7; H, 6.8%; M, 237.)
1
1
16
4
6
-N,N-Diethylcarbamyloxy-1,4-dimethoxynaphthalene (25)
6
-Hydroxy-1,4-dimethoxynaphthalene 12 (270 mg, 1.33 mmol) in pyridine
10 mL) was treated with N,N-diethylcarbamylchloride (180 mg,
(
1
.33 mmol) in a pressure-capped glass bottle heated in an oil bath at 1108C
for 3 h. The cooled reaction mixture was then poured into ice/water and
extracted with ether. The ether solution was washed with 10% aqueous hydro-
chloric acid (20 mL) and then aqueous sodium hydrogen carbonate, and the
residue obtained was purified by column chromatography using EtOAc–
hexane (3:7) as eluent to afford carbamate 25 as an oil (334 mg; 83%); n
2
max
1
1
716 cm ; dH 1.26 (6H, m, 2x CH CH -), 3.20 (4H, m, -CH CH ), 3.93
3 2 2 3
(
(
6H, s, 2x OCH ), 6.66 (1H, d, J 9.2, H-2), 6.68 (1H, d, J 9.2, H-3), 7.24
3
1H, dd, J 9.2 and 2.4, H-7), 7.91 (1H, d, J 2.4, H-5), 8.20 (1H, d, J 9.2,
H-8). d 11.5 and 13.0 (CH CH -), 41.2 (x2) (-CH CH ), 55.8 (x2, OCH ),
3
C
3
2
2
3
a
02.8 (C-2) , 104.0 (C-3) , 113.2 (C-7), 121.4 (C-8) , 123.4 (C-5) , 124.1
a
b
b
1
c
c
d
d
d
C-4a) , 127.2 (C-8a) , 149.3 (C-6) , 149.7 (C-4) , 149.8 (C-1) , 154
(
þ
(
C 55 O). (Found: C, 67.1, H, 7.2%; M , 303. Calc. C H NO : C, 67.3; H,
1
7
21
4
7
.0%; M, 303).
0
0
6
6
-Hydroxy-7-(2 -3 -oxapentyl)-1,4-dimethoxynaphthalene (26),
-Hydroxy-1,4-dimethoxynaphthalene (12), and 6-N,N-
Diethylcarbamyloxy-1,4-dimethoxy-7-naphthylboronic Acid (27)
To a stirred solution of sec-butyllithium (3 mol equiv.) and TMEDA (3M
equiv.) in tetrahydrofuran (15 mL) at 2788C under nitrogen, the carbamate
2
MgBrEt O (780 mg, 3.03 mmol) was added with vigorous stirring. The
5 (300 mg; 0.99 mmol) in tetrahydrofuran (3 mL) was added. After 1 h,
2
reaction mixture was allowed to warm to 248C and formed a clear solution,
which was then cooled to 2788C, treated with methyl borate (310 mg,
3
mol), stirred at this temperature for 10 min, and then allowed to reach
48C overnight. The resulting reaction mixture was poured into aqueous
2
ammonium chloride and extracted with dichloromethane to yield an oily
residue, which was chromatographed using EtOAc–hexane (3:7) as eluent
to give as the first fraction the ketonaphthalene 26 (74 mg, 26%) as orange
2
1
needles, mp 60–618C (from EtOAc); n
(
3383 and 1680 cm ; dH 0.97
0
max
0
0
3H, t, J 7.4, H-5 ), 1.29 (3H, d, J 7.0, H-1 ), 1.60 (1H, dt J 18.0, 7.4, H-4 ),
0 0
1
.90 (1H, dt J 18.0, 7.4, H-4 ), 3.74 (1H, q, J 7.0, H-2 ), 3.93 and 3.97
each 3H, s, CH O), 6.50 (1H, d, J 8.0, H-2), 6.73 (1H, d, J 8.0, H-3), 7.63
(
(
3
1H, s, H-5), 8.80 (1H, s, H-8), and 12.00 (1H, s, D O exchangeable,
2

3
056
F. Ameer et al.
a
-OH). d 11.9, 17.6, 27.3, 41.9 (2’-3’-oxapentyl side chain), 100.6 (C-2) ,
6
C
a
a
b
b
07.1 (C-3) , 107.8 (C-5) , 119.8 (C-4a) , 119.9 (C-8a) , 126.8 (C-8), 131.6
1
c
C-7), 148.2 (C-1) , 150.5 (C-4) , 158.5 (C-6), 211.5 (C 55 O of 2 -3 -
c
0 0
(
oxapentyl side chain). (Found: C, 70.5; H, 7.2%; HRMS: 288.1346. Calc.
for C H O : C, 70.8; H, 7.0%; HRMS, 288.1362.)
1
7 20 4
Further elution afforded naphthol 12 (81 mg; 40%) identical in all
respects to material synthesized earlier. Continued elution afforded boronic
acid 27 (99 mg; 29%) as yellow-brown crystals, mp 211–2138C (from
2
1
EtOAc–hexane); nmax 3411 and 1627 cm ; dH 1.33 (6H, t, J 7.0,
CH CH -), 3.58 (4H, q, J 7.0, CH CH -), 3.93 (6H, s, OCH ), 6.53 (1H, d,
J 7.0, H-2) 6.70 (1H, d, J 7.0, H-3), 7.50 [1H, s, D O exchangeable OH of
3
2
3
2
3
2
B(OH) ], 7.67 (1H, s, H-5), 8.23 (1H, s, H-8), and 9.30 [1H, s, D O exchange-
2
2
able OH of B(OH) ]. d 13.5 and 42.6 (CH CH - respectively), 55.8 and 56.0
2
2
C
3
a
a
b
(
OCH ), 101.0 (C-2) , 105.5 (C-3) , 107.2 (C-5), 119.7 (C-4a) , 120.0 (C-
3
c
a) , 122.5 (C-8) , 129.0 (C-7) , 148.5 (C-1) , 150.1 (C-4) , 155.6 (C-6),
b
c
d
d
8
171.5 (C 55 O of the carbamate). Found: C, 58.6; H, 6.2%; HRMS 347.1544.
Calc. for C H BNO : C, 58.8; H, 6.4%; HRMS: 347.1540.
1
7
22
6
ACKNOWLEDGMENTS
The authors acknowledge financial support provided by the National Research
Foundation and the Universities of the Western Cape and Paderborn.
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