Synthesis and Diels-Alder reactions of 9-(4-benzyloxazolin-2-yl) anthracene

Article abstract of DOI:10.1016/j.tet.2004.01.070

The synthesis of 9-(4-benzyloxazolin-2-yl)anthracene is described employing a new approach for the cyclisation of β-hydroxy amides to oxazolines. Thermal Diels-Alder reactions with N-methyl maleimide were found to be considerably slower than those previously observed. Essentially no diastereoselectivity was observed in these reactions as the benzyl stereodirecting group is remote from the reactive site. Minor rate enhancements were noticeable in the presence of some added Lewis acids, but with no diastereoselection.

Full text of DOI:10.1016/j.tet.2004.01.070

Tetrahedron 60 (2004) 2765–2770
Synthesis and Diels–Alder reactions of
9-(4-benzyloxazolin-2-yl) anthracene
*
Ramadan A. Bawa and Simon Jones
Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield S3 7HF, UK
Received 20 November 2003; revised 22 December 2003; accepted 22 January 2004
Abstract—The synthesis of 9-(4-benzyloxazolin-2-yl)anthracene is described employing a new approach for the cyclisation of b-hydroxy
amides to oxazolines. Thermal Diels–Alder reactions with N-methyl maleimide were found to be considerably slower than those previously
observed. Essentially no diastereoselectivity was observed in these reactions as the benzyl stereodirecting group is remote from the reactive
site. Minor rate enhancements were noticeable in the presence of some added Lewis acids, but with no diastereoselection.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
and have been successfully employed in many asymmetric
transformations.6
Chiral auxiliaries now form part of the routine set of tools
available to the synthetic chemist and can be used in a great
variety of stereoselective transformations.1 As part of an
ongoing research programme we have been developing
chiral anthracene derived auxiliaries such as the methyl
ether 1 (Fig. 1) that make use of a highly diastereoselective
Diels–Alder reaction with alkenes in the addition step.2–5
2. Results and discussion
2.1. Synthesis of oxazoline 2
Synthesis of the target oxazoline 2 started from com-
mercially available anthracene 9-carboxylic acid 3. Heating
in an excess of thionyl chloride gave the acid chloride 4 in
excellent yield (98%), followed by treatment with (S)-
phenylalaninol7 to give the key b-hydroxy amide 5.
Cyclisation of this amide to the oxazoline proved to be
troublesome. Classical reaction with thionyl chloride8
surprisingly returned starting material, as did direct
treatment with the more reactive TiCl4. Use of triethyl-
orthoformate as a dehydrating agent also returned starting
material (Scheme 1).
Figure 1.
Although the synthesis of ether 1 in enantiomerically pure
form is relatively straightforward, this still requires use of
catalytic asymmetric reduction to introduce the chirality. In
contrast, more classical auxiliaries, such as Evans’ oxa-
zolidinone, employ stereogenic elements installed directly
from the chiral pool. We have been working towards the
goal of introducing stereogenic elements from the chiral
pool directly into the anthracene framework. This work
details our approach to the synthesis and evaluation of
Diels–Alder reactions of 9-(4-benzyloxazolin-2-yl)-anthra-
cene 2 (Fig. 1). This target was chosen since oxazolines can
easily be prepared from naturally occurring a-amino acids
However, using diethylaminosulfur trifluoride (DAST) the
reaction was more successful.9 Addition of 1.1 equiv. of
DAST at low temperature gave the desired oxazoline 2
cleanly. Although this reagent gave the desired product, its
high cost is prohibitive of performing this reaction on a
larger scale. Research from this group has recently reported
a titanium catalysed phosphorylation procedure10 that could
be used to prepare the phosphate 6 which could then be
induced to cyclise upon treatment with a suitable base.
However, using the standard conditions for this reaction
only the chloride 7 was obtained in excellent isolated yield.
This is surprising, since in the phosphorylation of all
primary alcohols previously studied, no trace of the
corresponding chloride was ever observed. In any event,
reaction of the chloride 7 with t-BuOK led to deprotonation
Keywords: Oxazoline; Anthracene; Diels–Alder.
*
Corresponding author. Tel.: þ44-114-222-9483; fax: þ44-114-222-9346;
e-mail address: simon.jones@sheffield.ac.uk
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.01.070

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R. A. Bawa, S. Jones / Tetrahedron 60 (2004) 2765–2770
Scheme 1. Reagents and conditions: (i) SOCl2 (excess), K; (ii) (S)-phenylalaninol, Et3N, THF, 0 8C; (iii) DAST, CH2Cl2, 278 8C; (iv) 5 mol. % TiCl4,
(PhO)2P(O)Cl, Et3N, CH2Cl2, rt; (v) t-BuOK, THF.
of the N–H proton of the amide and subsequent ring closure
to give the target oxazoline 2. Although this route comprises
two steps, the overall yield of 63% is comparable to that of
the DAST reaction, but at a fraction of the cost of the
reagents. Efforts are ongoing to further elaborate this
method as an effective alternative route for the synthesis
of oxazolines.
change in hybridisation from sp3 to sp2 adjacent to C-9
leading to a reduction of allylic strain. This was confirmed
by calculation of the rotational barrier of oxazoline 2 using
molecular modeling11 giving an estimated energy barrier to
rotation of 3.43 kcal mol21. Such a small value would
permit free rotation at room temperature. The minimum
energy conformer was found to be that with the oxazoline
lying perpendicular to the anthracene ring system (Fig. 3).
It is interesting to note that if the oxazoline 2 was left for
prolonged periods of time during the crystallization process,
the dimer 8 was formed (Fig. 2). Attempts to replicate this
using photochemical dimerisation in acetonitrile returned
starting material. Thermal dimerisation by heating at reflux
in toluene returned starting material even after 5 days,
although changing solvent to dichloromethane gave a small
quantity of dimer (15%). The dimer could be cleaved back
to the anthracene adduct by heating at reflux in toluene for
24 h.
Figure 3. Minimum energy conformer of oxazoline 2.
2.2. Thermal Diels–Alder reactions
Figure 2.
Trial Diels–Alder reactions were performed by heating
oxazoline 2 in toluene at reflux for 2 h with maleic
anhydride and N-methylmaleimide. Surprisingly, no
reaction was observed with maleic anhydride, however
some product (73%) was observed with N-methyl
maleimide giving the addition adduct as a 50:50 mixture
of diastereoisomers 9 and 10 as observed from the signals in
1
The H NMR spectrum of the oxazoline 2 is interesting
compared to the methyl ether 1. Many 9-substituted
anthracenes suffer from restricted rotation around the C-9
bond due to steric interactions with the proximal peri
hydrogen atoms (H-4 and H-5). For the ether 1 this
manifests itself as broad signals for the peri protons at d
8.71 ppm leading to a rotational barrier of approximately
12.2 kcal mol21 at 281 K.4 However the H-4 and H-5
protons of the oxazoline 2 appear as sharp signals in the
1
the H NMR spectrum (Scheme 2). Partial separation of
these two diastereoisomers allowed the relative assignment
1
of a number of signals in the H and 13C NMR spectrum,
although absolute assignment was not possible.
1
aromatic region of the H NMR spectrum implying free
rotation about the C-9 bond. This is a consequence of a
The yield observed in this reaction is significantly lower

R. A. Bawa, S. Jones / Tetrahedron 60 (2004) 2765–2770
2767
anthracene ring and hence no discrimination (Fig. 4).
Invoking a rationale based upon thermodynamic stability of
the diastereoisomeric products, the predicted heats of
formation of 9 and 10 are 28.66 and 28.50 kcal mol21
,
respectively, leading to a calculated Keq of 1.23 at 110 8C.14
This equates to a 55:45 ratio of diastereisomers which is in
good agreement with the observed selectivity.
Scheme 2. Reagents and conditions: (i) N-methyl maleimide, C6H5CH3, K,
2 h.
Figure 4.
when compared to the ether 1 (73% for 2 vs 96% for 1 under
otherwise identical reaction conditions). This was attributed
to the electron withdrawing nature of the oxazoline group
and to confirm this, the Diels–Alder reaction with N-methyl
maleimide was performed with a series of electron rich and
poor 9-substituted anthracene derivatives (Scheme 3, Table
1). These results predictably indicate that electron with-
drawing groups do retard the rate of the Diels–Alder
reaction and that the oxazoline, as suspected, is a good
electron withdrawing group.
2.3. Diels–Alder reactions in the presence of Lewis
acidic metal triflates
Lewis acids, especially metal triflates, have been used to
successfully catalyse the room temperature Diels–Alder
reaction of anthracene derivatives with dienophiles.15
Oxazolines have also been shown to act as efficient
templates for cation co-ordination. Thus, oxazoline 2 was
treated with N-methyl maleimide at room temperature in the
presence of the metal triflates Mg(OTf)2, Cu(OTf)2,
Y(OTf)3 and Sc(OTf)3. Unfortunately, essentially no rate
enhancement was observed, with any improvement in the
diastereomeric ratio. The latter is not surprising since
addition of a Lewis acid is unlikely to bring the benzyl
stereodirecting group into closer proximity to the reactive
site.
Scheme 3. Reagents and conditions: (i) N-methyl maleimide, C6H5CH3, K,
2 h.
3. Conclusions
Synthesis of 9-(4-benzyloxazolin-2-yl) anthracene has been
achieved and a new synthetic route to such compounds
disclosed. All attempted stereoselective Diels–Alder
reactions proved to be unsuccessful resulting from poor
reaction rates and no selectivity. However this work does
indicate the need for an electron-rich auxiliary to increase
reaction rates, in addition to ensuring the close proximity of
the stereodirecting group to the reaction centre for high
selectivity.
Table 1. Diels–Alder additions of anthracene derivatives 2, 11–13 with
N-methyl maleimide in toluene at reflux for 2 h
Starting material
X
Product
Conversion (%)a
11
12
13
2
Me
H
Br
2-Oxazolinyl
14
15
16
9/10
98
81
82
73
a
Calculated from the ratio of integrals of the signals corresponding to
1
starting material and addition product in the H NMR spectrum.
4. Experimental
4.1. General
The absence of diastereoselectivity observed in this reaction
is in retrospect perhaps not so surprising. Using a rationale
based upon kinetic arguments,12,13 the approach of a
dienophile will occur where electrostatic interactions can
be minimized. The minimum energy conformation of the
oxazoline is likely to have the oxazoline ring orientated
orthogonally to the anthracene ring system to minimize peri
interactions, a premise supported by the modeling studies
discussed earlier. However in this conformation, the
stereogenic centre of the oxazoline ring is located remote
from the reactive centre, resulting in no interaction with the
carbonyl group of the dienophile on either face of the
THF and toluene were freshly dried over sodium, while
CH2Cl2 was dried over lithium aluminium hydride.
Anhydrous DMSO was obtained by distillation in vacuuo.
Glassware was flame dried and cooled under vacuum before
use and all reactions were carried out under nitrogen unless
otherwise stated. TLC was carried out using Merck
aluminium TLC sheets (silica gel 60 F254). Visualisation
of the TLC plates was carried out using a UV lamp or by
dipping in KMnO4 then exposure by heating. Flash column

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R. A. Bawa, S. Jones / Tetrahedron 60 (2004) 2765–2770
chromatography was carried out with Fluorochem Limited
Silica Gel 40-63u 60A. Melting points were measured on a
C24H21NO2 requires 355.1572), 221 (31), 205 (100,
C15H9Oþ), 177 (38), 151 (5), 91 (6).
Gallenkamp apparatus and are uncorrected. 1H, 13C and 31
P
NMR spectra were recorded on a Bruker AC-250 or a
Bruker Avance 300 spectrometer or AMX-400 spectrometer
or JEOL 500 MHz spectrometer. Residual proton signals
from the deuteriated solvents were used as references
[chloroform (1H, 7.25 ppm; 13C, 77 ppm) and DMSO (1H,
2.50 ppm; 13C, 39.7 ppm)]. Coupling constants were
measured in Hz. All infrared spectra were recorded on
Perkin–Elmer Spectrum RX/FT-IR system with a Dura-
SamplIR II ATR accessory. Optical rotations were recorded
on a Perkin–Elmer 241 automatic polarimeter at 589 nm
(Na D line) with a path length of 1 dm with concentrations
quoted in gm 100 mL21. Mass spectra were recorded on a
Micromass Autospec M spectrometer.
4.1.3. 9-(4S-Benzyloxazolin-2-yl)anthracene 2 (DAST
method). The hydroxy-amide 5 (2.534 g, 7.140 mmol)
was dissolved in dry CH2Cl2 (140 cm3) and then cooled to
278 8C. DAST (2 cm3, 16.33 mmol) was added to the
cooled mixture and stirred for 2 h at 278 8C. The resulting
solution was quenched with NH4OH (25 cm3, 10% by vol.)
and the reaction mixture warmed to room temperature.
EtOAc (20 cm3) was added, followed by NaHCO3 (25 cm3)
and the organic layer separated. The aqueous layer was
extracted with EtOAc (2£20 cm3) and the combined organic
layers washed with brine (25 cm3) and dried over Na2SO4.
The solvent was removed under reduced pressure to afford a
dull yellow solid (2.348 g). The crude material was purified
using column chromatography (EtOAc/petrol 10:90) to
afford the title compound 2 as a thick yellow oil (1.532 g,
64%); [a]D¼212.7 (c 1, CHCl3); (Found: C, 85.80; H, 5.44;
N, 4.18. C24H19NO requires C, 85.43; H, 5.68; N, 4.15%);
4.1.1. 9-Anthranoyl chloride 4.16 9-Anthracene carboxylic
acid 3 (0.455 g, 2.05 mmol) and thionyl chloride (3.5 cm3)
were stirred at reflux for 3 h under nitrogen, then allowed to
cool to room temperature. The excess thionyl chloride was
removed under reduced pressure, the residue washed with
diethyl ether (2£2 cm3), and the diethyl ether evaporated to
afford a dull yellow solid of 9-anthranoyl chloride 4
(0.489 g, 98%) that required no further purification, mp
94–96 8C (lit.15 96–97 8C); dH (300 MHz; CDCl3) 8.62
(1H, s, 10H), 8.14 (2H, d, J¼8.7 Hz, ArCH), 8.08 (2H, d,
J¼8.4 Hz, ArCH), 7.67 (2H, dd, J¼8.7, 6.6 Hz, ArCH),
7.56 (2H, dd, J¼8.4, 6.6 Hz, ArCH). Spectroscopic data was
in agreement to that in the literature.
n
max (film)/cm21 3056, 3028, 1659; dH (300 MHz; CDCl3)
8.56 (1H, s, 10H), 8.04–7.97 (4H, m, ArCH), 7.53–7.43
(4H, m, ArCH), 7.38–7.29 (5H, m, ArCH), 5.04 (1H, dtd,
J¼9.6, 7.8, 4.9 Hz, CH), 4.66 (1H, dd, J¼8.6, 7.8 Hz,
CHHO), 4.48 (1H, dd, J¼8.6, 9.6 Hz, CHHO), 3.46 (1H, dd,
J¼13.8, 4.9 Hz, PhCHH), 3.21 (1H, dd, J¼13.8, 7.8 Hz,
PhCHH); dC (75 MHz; CDCl3) 163.8 (CvN), 138.0, (ArC),
131.4 (ArCH), 130.5 (ArCH), 130.2 (ArCH), 129.9 (ArCH),
129.1 (ArCH), 128.9 (ArCH), 127.1 (ArCH), 125.8 (ArCH),
123.1 (ArC), 72.0 (CH2O), 69.0 (CHN), 42.1 (PhCH2); m/z
(EIþ) 337.1461 (35%, C24H19NO requires 337.1467), 246
(100, Mþ-C7H7), 218 (21), 203(52), 191 (8), 177 (12), 91(11).
4.1.2. Anthracene-9-carboxylic acid (1-benzyl-2S-
hydroxy-ethyl)-amide 5. 9-Anthranoyl chloride 4 (1.73 g,
7.17 mmol) was dissolved in THF (30 cm3) in the presence
of triethylamine (2.60 cm3, 18.68 mmol). A solution of (S)-
3-phenyl-2-amino-1-propanol (1.09 g, 7.19 mmol) in THF
(35 cm3) was then added dropwise to the reaction mixture at
0 8C. The reaction was stirred at 0 8C for 1 h, warmed to
room temperature followed by filtration. The solvent was
removed to afford a yellow solid, which was dissolved in
CH2Cl2 (20 cm3), washed with water (3£10 cm3), and the
organic phase dried over Na2SO4. The solvent was removed
to obtain a yellow solid of the hydroxyl-amide 5 (1.54 g,
60%) that was used without purification in subsequent steps.
A sample was purified for analytical purposes by two
recrystallizations from EtOAc/petrol) giving the title
compound as yellow needles, mp 190–194 8C (EtOAc/
petrol); [a]D¼þ13.3 (c 1, CHCl3); (Found: C, 80.95; H,
5.97; N, 3.87. C24H21NO2 requires C, 81.10; H, 5.96; N,
3.94%); nmax (film)/cm21 1634, 1519, 1455; dH [300 MHz;
(CD3)2SO] 8.65 (1H, d, J¼9.0 Hz, ArCH), 8.59 (1H, s,
10H), 8.11–8.03 (3H, m, ArCH), 7.57–7.35 (8H, m,
ArCH), 7.22 (1H, dd, J¼8.3, 6.7 Hz, ArCH), 7.02 (1H, d,
J¼8.7 Hz, NH), 5.07 (1H, t, J¼5.6 Hz, OH), 4.65 (1H, m,
CH), 3.56 (1H, ddd, J¼11.6, 10.5, 5.6 Hz, CHHOH), 3.59
(1H, ddd, J¼11.6, 10.5, 5.6 Hz, CHHOH), 3.11 (1H, dd,
J¼13.7, 4.0 Hz, PhCHH), 2.63 (1H, dd, J¼13.7, 11.0 Hz,
PhCHH); dC [75 MHz; (CD3)2SO] 168.0 (CvO), 139.7
(ArC), 134.1 (ArC), 131.0 (ArC), 130.9 (ArCH), 129.7
(ArCH), 128.6 (ArCH), 128.3 (ArCH), 127.5 (ArCH), 127.1
(ArCH), 126.6 (ArCH), 126.4 (2£ArCH), 126.2 (ArCH),
126.0 (ArCH), 125.8 (ArCH), 64.4 (CH2OH), 53.5
(CHNH), 37.0 (PhCH2); m/z (EIþ) 355.1576 (33%,
4.1.4. Anthracene-9-carboxylic acid (1S-benzyl-2-
chloro-ethyl)-amide 7. TiCl4 (0.01 cm3, 0.07 mmol,
2 mol %) was dissolved in THF (5 cm3) and the hydroxyl
amide (1.18 gm, 3.33 mmol) was added as a solution in THF
(15 cm3) via a dropping funnel followed by Et3N (0.71 cm3,
5.00 mmol), THF (5 cm3), diphenylphosphorochloridate
(1.04 cm3, 5.00 mmol) and THF (10 cm3). The resulting
mixture was stirred at room temperature for 1 h before
quenching with water (15 cm3). The organic layer was
extracted with CH2Cl2 and the combined organic layers
dried over Na2SO4 and filtered. Removal solvent afforded
the crude material (1.72 gm, 2.93 mmol, 88% yield) that
was used without purification in subsequent steps. A sample
was purified for analytical purposes by column chromato-
graphy (10% EtOAc/petrol) followed by recrystallization
from toluene to give white crystals, mp 198–203 8C
(toluene); [a]D¼214 (c 0.5, CHCl3); nmax (ATR)/cm21
1673, 1488, 1424; dH (250 MHz; CDCl3) 8.51 (1H, s,
ArCH), 8.32 (1H, d, J¼8.4 Hz, ArCH), 8.08–7.95 (3H, m,
ArCH), 7.61–7.30 (9H, m, ArCH), 6.27 (1H, d, J¼7.9 Hz,
NH), 5.14 (1H, m, CH), 4.09 (1H, dd, J¼11.5, 4.2 Hz,
CHHCl), 3.82 (1H, dd, J¼11.5, 3.5 Hz, CHHCl), 3.21–3.10
(2H, m, PhCH2); dC (100 MHz; CDCl3) 172.3 (CO), 131.4
(ArC), 130.6 (ArC), 129.7 (ArCH), 129.4 (ArCH), 129.1
(ArCH), 128.9 (ArCH), 128.4 (ArCH), 127.7 (ArCH), 127.5
(ArCH), 127.2 (2£ArCH), 125.9 (ArCH), 125.6 (ArCH),
125.3 (ArCH), 51.8 (CH), 47.7 (CH2Cl), 38.3 (CH2Ph); m/z
(ES) 374.1314 (42% C24H21NOCl requires 374.1312), 205
(100); (EIþ) 374 (32), 373 (22), 307 (21), 289 (12), 222 (87),
205 (44), 177 (13), 154 (100), 136 (74).

R. A. Bawa, S. Jones / Tetrahedron 60 (2004) 2765–2770
2769
4.1.5. 9-(4S-Benzyloxazolin-2-yl)anthracene 2 (from 7).
Potassium t-butoxide (1.2 gm, 10.66 mmol) was added as a
solid to a stirred solution of chloride 7 (3.12 gm, 5.31 mmol)
in THF (20 cm3). The reaction mixture was stirred for 1 h at
room temperature, filtered through short pad of silica,
eluting with EtOAc (5 cm3). Removal of solvent gave the
title compound 2 (1.28 g, 3.80 mmol, 72%).
11,12-dicarbonyl N-methylamide 16. N-Methylmaleimide
(0.611 g, 5.51 mmol) was added in one portion as a solid at
90–95 8C to a stirred solution of 9-bromoanthracene 13
(0.500 g, 1.95 mmol) in toluene (12 cm3). The resulting
mixture was left stirring for further 7 h at 90–95 8C, cooled
to room temperature and the solvent removed under
reduced pressure to give the target compound, which was
recrystallized from toluene to afford a white solid of the title
compound 16 (0.554 g, 77%), mp 228–232 8C; (Found: C,
62.0; H, 3.8; N, 3.8; Br, 21.9. C19H14BrNO2 requires C,
62.0; H, 3.8; N, 3.80; Br, 21.70%); nmax (ATR)/cm21 1776,
1690, 1456, 1426; dH (300 MHz; CDCl3) 7.82 (1H, m,
ArCH), 7.63 (1H, m, ArCH), 7.32 (1H, m, ArCH), 7.25–
7.12 (5H, m, ArCH), 4.74 (1H, d, J¼3.3 Hz, CH), 3.37 (1H,
d, J¼8.6 Hz, COCHCHCO), 3.25 (1H, dd, J¼8.6, 3.3 Hz,
COCHCH), 2.48 (3H, s, NCH3); dC (100 MHz; CDCl3)
175.3 (CvO), 173.4 (CvO), 141.0 (ArC), 140.0 (ArC),
138.1 (ArC), 136.6 (ArC), 128.0 (ArCH), 127.9 (ArCH),
127.5 (ArCH), 127.1 (ArCH), 125.8 (ArCH), 125.3 (ArCH),
124.5 (ArCH), 123.5 (ArCH), 64.6 (CBr), 53.4 (CH), 49.0
(CH), 45.1 (CH), 24.5 (NCH3); m/z (ES) 390.0117 (100%,
MþþNa; C19H14NO2Na79Br requires 390.0106); m/z (EIþ)
370 (6%), 369 [14, Mþ (81Br)], 367 [21, Mþ (79Br)], 259
(15), 258 (97, C14H881Brþ), 256 (100, C14H879Brþ), 202
(11), 177 (13), 176 (16), 101 (8).
4.1.6. 9-(4S-Benzyloxazolin-2-yl)-10-hydro-9,10-ethano-
anthracene-11R,12R-dicarbonyl N-methylamide 9 and
9-(4S-benzyloxazolin-2-yl)-10-hydro-9,10-ethanoanthra-
cene-11S,12S-dicarbonyl N-methylamide 10. 9-(4-
Benzyloxazolin-2-yl)anthracene 2 (0.500 g, 1.48 mmol)
was dissolved in dry toluene (10 cm3), the resulting solution
heated to 90–95 8C and N-methylmaleimide (0.442 g,
3.98 mmol) added. This was left at this temperature for 4
and 1/2 h, then cooled to room temperature, and the solvent
was removed under reduced pressure to afford a pale yellow
solid of the two diastereisomers 9 and 10 (50/50 by 1H NMR
spectroscopy). This mixture was purified using column
chromatography (EtOAc/petrol, 30:70) to give a 60/40
mixture of two diastereoisomers (0.279 g, 42%) as a yellow
solid; nmax (ATR)/cm21 1776, 1697, 1456, 1433, mp 101–
104 8C; [a]D¼28.0 (c 0.5, CHCl3); dH (300 MHz; CDCl3)
Diastereoisomer A, 8.28 (1H, d, J¼7.3 Hz, ArCH), 7.32–
7.26 (4H, m, ArCH), 7.23–7.02 (7H, m, ArCH), 6.84 (1H,
d, J¼7.3 Hz, ArCH), 4.81 (1H, m, CH), 4.68 (1H, m, CH),
4.47 (1H, m, CHHO), 4.26 (1H, m, CHHO), 3.76 (1H, d,
J¼8.6 Hz, CH), 3.32 (1H, dd, J¼13.8, 5.0 Hz, CH), 3.21–
3.15 (1H, m, PhCHH), 2.99–2.91 (1H, m, PhCHH), 2.40
(3H, s, CH3); Diastereoisomer B, 8.22 (1H, d, J¼7.1 Hz,
ArCH), 7.32–7.26 (4H, m, ArCH), 7.23–7.02 (7H, m,
ArCH), 6.95 (1H, d, J¼7.2 Hz, ArCH), 4.81 (1H, m, CH),
4.68 (1H, m, CH), 4.47 (1H, m, CHHO), 4.26 (1H, m,
CHHO), 3.82 (1H, d, J¼8.6 Hz, CH), 3.41 (1H, dd, J¼13.7,
5.5 Hz, CH), 3.21–3.15 (1H, m, PhCHH), 2.99–2.91 (1H,
m, PhCHH), 2.42 (3H, s, CH3); dC (75 MHz; CDCl3) 176.9
(CvO), 175.9 (CvO), 141.2 (CvN), 138.6, 138.0, 137.2,
130.1 (ArCH), 129.8 (ArCH), 129.1 (ArCH), 127.6 (ArCH),
127.5 (ArCH), 127.1 (ArCH), 127.0 (ArCH), 126.1 (ArCH),
125.1 (ArCH), 124.3 (ArCH), 123.7 (ArCH), 71.7 (Diaster-
eomer A, CH2O), 71.3 (Diastereomer B, CH2O), 69.1, 68.7,
51.3, 49.4, 49.1, 47.7, 46.4, 42.4, 42.2, 24.7 (NCH3); m/z
(ES) 449.1881 (100%, C29H25N2O3 requires 449.1865).
4.1.9. 9,10-Dihydro-9-methyl-9,10-ethanoanthracene
11,12-dicarbonyl N-methylamide 14.18 9-Methyl-
anthracene 11 (0.192 g, 1.00 mmol) was dissolved in
toluene (10 cm3) then the resulting solution was heated to
90–95 8C and N-methylmaleimide (0.111 g, 1.00 mmol)
was added in one portion as a solid. The reaction mixture
was stirred at 90–95 8C for 2 h, cooled to room temperature
and finally the solvent was removed under reduced pressure
to afford the title product 14 as a white solid in 98%
conversion as calculated from the 1H NMR spectrum, which
was recrystallized from toluene (0.247 g, 82%), mp 146–
150 8C (lit.18 267–168 8C);† dH (300 MHz; CDCl3) 7.44–
7.39 (2H, m, ArCH), 7.30–7.11 (6H, m, ArCH), 4.78 (1H,
d, J¼3.3 Hz, COCHCH), 3.28 (1H, dd, J¼3.3, 8.4 Hz,
COCHCH), 2.86 (1H, d, J¼8.4 Hz, COCHCHCO), 2.53
(3H, s, NCH3), 2.31 (3H, s, CH3); dC (75 MHz; CDCl3)
177.2 (CvO), 176.6 (CvO), 144.9 (ArC), 142.2 (ArC),
141.4 (ArC), 139.0 (ArC), 127.3 (ArCH), 127.1 (ArCH),
126.9 (2£ArCH), 125.2 (ArCH), 124.2 (ArCH), 122.5
(ArCH), 122.4 (ArCH), 51.0 (CH), 48.9 (CH), 45.9 (CH),
45.4 (C), 24.7 (NCH3), 15.7 (CH3).
4.1.7. 9,10-Dihydro-9,10-ethanoanthracene-11,12-di-
carbonyl N-methylamide 15.17 Anthracene 12 (0.178 g,
1.00 mmol) was dissolved in toluene (10 cm3), the resulting
solution heated to 90–95 8C, and N-methylmaleimide
(0.111 g, 1.00 mmol) was added in one portion. The
reaction mixture was stirred at 90–95 8C for 2 h, cooled
to room temperature and the solvent was removed under
reduced pressure to afford the title product 15 as a grey
solid, which was recrystallized from toluene (0.169 g, 59%),
mp 278–279 8C (lit.17 262–264 8C); dH (300 MHz; CDCl3)
7.40 (2H, m, ArCH), 7.27 (2H, m, ArCH), 7.20–7.12 (4H,
m, ArCH), 4.80 (2H, s, ArCH), 3.22 (2H, s, COCH), 2.52
(3H, s, CH3); dC (75 MHz; CDCl3) 177.3 (CvO), 141.8
(ArC), 138.9 (ArC), 127.4 (ArCH), 127.1 (ArCH), 125.3
(ArCH), 125.7 (ArCH), 47.4 (CH), 45.9 (CH), 24.7 (CH3).
Spectroscopic data was in agreement to that in the literature.
4.1.10. 9-(3-Benzyloxazolinoyl)anthracene dimer 8. The
title compound was obtained as a yellow solid on leaving a
toluene solution of oxazoline 2 to slowly evaporate over the
period of several weeks, mp 219–221 8C; [a]D¼26.0 (c 1,
CHCl3); nmax (film)/cm21 1651, 1476, 1454; dH (250 MHz;
CDCl3) 7.46–7.29 (11H, m, ArCH), 7.06 (4H, m, ArCH),
6.75 (5H, m, ArCH), 6.61 (4H, m, ArCH), 5.98 (2H, t,
J¼3.5 Hz, PhCH), 4.87 (2H, m, CH), 4.46 (2H, t, J¼9.0 Hz,
CHHO), 4.26 (2H, dd, J¼9.0, 7.6 Hz, 2£CHHOH), 3.25
(4H, m, PhCH2); dC (100 MHz; CDCl3) 169.3 (CvN),
142.6 (ArC), 142.1 (ArC), 142.0 (ArC), 141.9 (ArCH),
†
The reported melting point for this compound is as stated but probably
refers to 167–168 8C. No other spectroscopic data is reported for this
compound.
4.1.8. 9,10-Dihydro-9-bromo-9,10-ethanoanthracene-

2770
R. A. Bawa, S. Jones / Tetrahedron 60 (2004) 2765–2770
2. Jones, S.; Atherton, J. C. C. Tetrahedron: Asymmetry 2001, 12,
1117–1119.
137.1 (ArCH), 130.3 (ArCH), 128.6 (ArCH), 128.1 (ArCH),
127.9 (ArCH), 126.8 (ArCH), 126.2 (ArCH), 125.9 (ArCH),
125.6 (ArCH), 125.5 (ArCH), 125.4 (ArCH), 71.6 (CH2O),
66.7 (NCH), 59.8 (PhC), 55.8 (PhCH), 41.3 (PhCH2); m/z
(ESþ) 675.2988 (7% C48H39N2O2 requires 675.3012);
(FABþ) 675 (12%, MHþ), 613 (19), 461 (20), 460 (71),
443 (13), 392 (27), 391 (100), 354 (14), 339 (27), 338 (96),
337 (41).
3. Atherton, J. C. C.; Jones, S. Tetrahedron Lett. 2001, 42,
8239–8241.
4. Atherton, J. C. C.; Jones, S. J. Chem. Soc., Perkin Trans. 1
2002, 2166–2173.
5. Atherton, J. C. C.; Jones, S. Tetrahedron Lett. 2002, 43,
9097–9100.
6. Gant, T. G.; Meyers, A. I. Tetrahedron 1994, 50, 2297–2360.
7. Kostyanovsky, R.; Giella, I.; Markov, V.; Samojlova, Z.
Tetrahedron 1974, 30, 39–45.
4.2. General procedure for metal triflate-catalysed
Diels–Alder reaction
8. Frump, J. A. Chem. Rev. 1971, 71, 483–505.
9. Phillips, A. J.; Uto, Y.; Wipf, P.; Reno, M. J.; Williams, D. R.
Org. Lett. 2000, 2, 1165–1168.
9-(3-Benzyloxazolinoyl)anthracene 2 (0.100 g, 0.297
mmol) was dissolved in CH2Cl2 and N-methylmaleimide
(0.033 g, 0.297 mmol) was added as a solid followed by the
addition of metal triflate (0.0003 mmol, 10 mol %) in
CH2Cl2. The reaction mixture was stirred overnight at room
temperature, brine (5 cm3) added and the organic layer
separated and dried over Na2SO4. The solution was filtered
and the solvent removed to give crude material that was
10. Jones, S.; Selitsianos, D. Org. Lett. 2002, 4, 3671–3673.
11. Performed using Quantum CAChe 5.04, Fujitsu Ltd. Calcu-
lations carried out by generating an optimised map about the
dihedral angle at 158 intervals, minimising using PM3
methods.
12. Sanyal, A.; Snyder, J. K. Org. Lett. 2000, 2, 2527–2530.
13. Corbett, M. S.; Liu, X.; Sanyal, A.; Snyder, J. K. Tetrahedron
Lett. 2003, 44, 931–935.
1
analyzed by H NMR spectroscopy.
14. Performed using Quantum CAChe 5.04, Fujitsu Ltd. Heats of
formation calculated using PM3 parameters.
15. Fukuzumi, S.; Ohkubo, K.; Okamoto, T. J. Am. Chem. Soc.
2002, 124, 14147–14155.
Acknowledgements
We would like to thank the Committee of Higher Education
in Libya and The Libyan Arab People’s Bureau for a
scholarship (R.A.B.).
16. Benniston, A. C.; Harriman, A.; Lynch, V. M. Tetrahedron
Lett. 1994, 35, 1473–1476.
17. Rigaudy, J.; Baranne-Lafont, J.; Defoin, A.; Cuong, N. K.
Tetrahedron 1978, 34, 73–82.
References and notes
18. Schummann, E. L.; Roberts, E. M.; Claxton, G. P. US
3123618, 1964..
1. Jones, S. J. Chem. Soc., Perkin Trans. 1 2002, 1–21.