Attack on fluorinated 2-aryloxazolines by organolithiums: Dearomatisation, lithiation or substitution

Article abstract of DOI:10.1016/j.tetlet.2011.02.091

Treatment of 4-, 3- or 2-aryl-4,5-diphenyloxazolines with isopropyllithium gives the products of dearomatising addition, fluorine-directed lithiation or nucleophilic aromatic substitution of fluoride depending on substitution pattern and conditions. In th

Full text of DOI:10.1016/j.tetlet.2011.02.091

Tetrahedron Letters 52 (2011) 2436–2439
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Attack on fluorinated 2-aryloxazolines by organolithiums: dearomatisation,
lithiation or substitution
⇑
James Clayton, Jonathan Clayden
School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Treatment of 4-, 3- or 2-aryl-4,5-diphenyloxazolines with isopropyllithium gives the products of dearo-
matising addition, fluorine-directed lithiation or nucleophilic aromatic substitution of fluoride depending
on substitution pattern and conditions. In the case of the 4-fluoroaryl substrates, fluorinated 1,4-cyclo-
hexadiene may be obtained in good yield.
Received 24 January 2011
Revised 17 February 2011
Accepted 21 February 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Synthesis
Dearomatisation
Metallation
Oxazoline
Organofluorine chemistry
Carbolithiation
1. Introduction
Introduction of fluorine dramatically changes the physicochem-
ical properties of a molecule.6 The C–F bond is highly polarised,
In pioneering work on the dearomatisation of naphthyl and
pyridyl oxazolines, Meyers1 showed that the regio- and stereo-
chemical outcomes of nucleophilic attack on the aromatic ring
may be controlled by an appropriate chiral oxazoline substituent.
Building on Meyers’ work, we showed recently2 that the dearomat-
isation of phenyl rings, previously reported to occur only with the
assistance of coordinating metal,3 is also possible under certain
conditions. Dearomatisation of a phenyloxazoline was achieved
in good yield only when 2-aryloxazoline (such as 1) carried the
previously unexplored 4,5-anti-diphenyl substitution pattern, and
when a secondary organolithium nucleophile R1Li was added in
the presence of the deaggregating co-solvent DMPU.4 Dearoma-
tised products 2 of these reactions were transformed, after a series
of functionalisations of the newly revealed diene, into carbasugar
analogues such as 3 (Scheme 1).2,5
inducing powerful electrostatic interactions, and substituting OH
with F preserves electronegativity whilst removing a hydrogen bond
donor/acceptor. Fluorinated drugs often have increased biological
activity profiles compared to their non-fluorinated counterparts.7
The term ‘polar hydrophobicity’ has been given to the unique prop-
erties of C–F bonds which enhance binding to biological receptors.8
Widespread interest in the synthesis of fluorinated carbocycles
and sugar analogues, notably in the groups of Linclau, O’Hagan and
Gouverneur,9 led us to consider broadening the scope of our dear-
omatisation methodology to the potential synthesis of fluorinated
carbasugars by employing phenyl rings carrying fluorine substitu-
ents as substrates for dearomatising attack by organolithiums. A
wide range of fluorinated aryl rings are commercially available or
are accessible using newly developed fluorination methods,10 and
we hoped the electronegative fluorine would assist nucleophilic at-
tack on the arene. Subsequent diastereoselective transformations
of the product fluorodienes would lead to functionalised fluori-
nated carbocycles.
Ph
Ph
Ph
Ph
O
N
O
N
HO
HO
1 R1-Li,
A series of fluorinated aryloxazolines were made by our one-pot
method11 (Scheme 2). Aminoalcohol 4 was acylated with a fluor-
obenzoyl chloride and the resulting hydroxyamide 6 was cyclised
stereospecifically in the presence of excess triethylamine with
methanesulfonyl chloride, to give 2-fluoroaryl-4,5-anti-diphenyl-
oxazolines 7–10 with reliable inversion of stereochemistry at the
oxygen-bearing centre.
R2
steps
R1
2 R2X
THF, DMPU
-78 °C
reference 2
HO
OH
OMe 1
OMe
2
OH
3
Scheme 1. Synthesis of carbasugars by nucleophilic dearomatisation of a 2-aryl-
4,5-diphenyloxazoline.
Dearomatisation of the fluorinated oxazolines was attempted
under the conditions previously optimised for 2-phenyl and
⇑
Corresponding author. Fax: +44 161 2754939.
E-mail address: clayden@man.ac.uk (J. Clayden).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.091

J. Clayton, J. Clayden / Tetrahedron Letters 52 (2011) 2436–2439
2437
Ph
(S)
HO
Ph
(R)
NH2
Ph
Ph
Ph
4
Ph
(R)
O
Ph
(R)
N
HO
O
O
N
Ph
1 i-PrLi,
THF, DMPU
O
Cl
1. NEt3,
(4 equiv)
2. MeSO2Cl,
(1.5 equiv)
NH
D
d- 9 (R = OMe) 45%
d-10 (R = H) 44%
Ph
Ph
2 MeOD
-78 °C
F
CH2Cl2,
0-25 °C
0-25 °C
O
N
5
6
7-10
R
R
R
R
Ph
Ph
O
N
F
Ph
Ph
Ph
Ph
Ph
Ph
N
Ph
Ph
1 i-PrLi,
THF, DMPU
R
Me
F
15 14%
O
N
O
N
O
O
N
9 (R = OMe)
10 (R = H)
2 MeI
-98, -78
or -40 °C
F
OMe
F
F
80%
Scheme 4. Competing metallation with a 3-fluoro substituent.
7
8
9
10
F
90%
46%
OMe
88%
Scheme 2. Fluorinated aryloxazolines as starting materials for dearomatising
addition.
Ph
Ph
Ph
Ph
Ph
Ph
O
N
O
N
O
N
1 i-PrLi, -78 °C
Ph
Ph
Ph
Ph
THF, DMPU
F
+ F
O
N
O
N
2 CF3CO2H
MeI
+
8
16
17
24 %
34%
Ph
Ph
Ph
Ph
Li
Scheme 5. Competing dearomatisation and SNAr reaction with
substituent.
a
2-fluoro
8 %
O
N
O
N
13
i-PrLi, -78 °C
THF, DMPU
combined
F
F
F
12
Ph
Ph
O
N
cepted by methylation at ꢀ78 °C to give 15 in 14% yield, this result
remained the same when 9 was treated with isopropyllithium at
ꢀ98 or ꢀ40 °C: no dearomatisation was detected by NMR in the
crude products of these reactions.
Under our standard conditions of isopropyllithium in THF/
DMPU, 2-fluorophenyloxazoline 8 performed capriciously but gave
some intriguing results. Meyers had used 2-fluoro oxazolines to
form new C–C bonds by oxazoline-promoted SNAr displacement
of fluoride with an aryl or alkyl organo-metallic reagent.17 After
treatment of 8 with i-PrLi and quenching with trifluoroacetic acid,
the NMR spectrum of the crude reaction mixture indicated that
both substitution and addition had taken place to return 16 and
17 in roughly a 3:2 ratio (Scheme 5).
Attempts to improve the yield of 17 met with little success: the
reaction was repeated at ꢀ98 °C, with a freshly opened bottle of
isopropyllithium, and with different rates of addition of alkyllith-
ium: in all cases no, or very little, 17 could be detected in the crude
reaction product, but instead only the product 16 of substitution.
Functionalisation of the fluorodienes 14 and 17 was attempted,
but unfortunately without success, in accordance with previous
reports.18
In summary, extension of oxazoline-promoted diastereoselec-
tive nucleophilic dearomatisation1,2 to fluorinated aryl rings gives
results dependent on substitution pattern. A fluoro substituent in
the 4-position promotes dearomatisation but only when the reac-
tion is quenched with a proton source. Moving the fluoro substitu-
ent to the 3-position cooperatively activates the 2-position
towards ortho-lithiation, and 2-alkylated products can be obtained.
The acidifying effect of the two directing groups is evidently great-
er than the dearomatisation-promoting power of the oxazoline.
With a fluoro-substituent in the 2-position, dearomatisation by at-
tack at the 2- or 6-position is finely balanced: in general the 2-fluo-
roaryl oxazoline leads to SNAr substitution of fluoride by
alkyllithium, with the product of a capricious dearomatisation
(by attack at the 6-position of the ring) observed only when the
reaction was performed on a small scale.
NH4Cl
7
F
F
11
SeO2, TBHP
CH2Cl2, rt.
quant
14 74 %
Scheme 3. Dearomatisation of a 4-fluoro oxazoline.
2-(methoxyphenyl)oxazolines. Aryl oxazoline 7 was dissolved in
THF and DMPU and cooled to ꢀ78 °C. Addition of isopropyllithium2
led to a deep green colour commonly associated with a dearoma-
tised aza-enolate. The colour lasted until the reaction was
quenched with methyl iodide (MeI). After workup, the reaction
yielded a mixture of inseparable dearomatised 12 (apparently as
single diastereoisomer) and rearomatised compounds 13
(Scheme 3). Similar results were obtained when THF was replaced
with cumene or toluene.
We found that rearomatisation of the presumed intermediate
azaenolate 11 could be avoided by quenching instead with ammo-
nium chloride. With two equiv i-PrLi, in THF and 10 equiv DMPU, a
single diastereoisomer of the fluorinated 1,4-cyclohexadiene 14
was formed in 74% yield (Scheme 3). Stereochemistry was tenta-
tively assigned to 14 as shown in Scheme 3 on the basis of related
reactions.2,12 14 could be rearomatised quantitatively to 13 by
treatment with selenium dioxide.13
Similar treatment of 3-fluorooxazolines 9 or 10 with isopro-
pyllithium also led to intense, deep colouration—purple in the case
of 9 and green in the case of 10, but on treatment with saturated
aqueous ammonium chloride no dearomatised products were evi-
dent. Given the well-known metallation-directing power14 of oxaz-
oline15 and fluoro16 substituents, we supposed that competing
deprotonation of the 2-position of the ring might be occurring.
Quenching the reactions of 9 or 10 with MeOD showed that this
was indeed the case: deuteration in the 2-position was indicated
in both cases by mass spectroscopy and by disappearance of the
1H doublet at d 7.78 or 7.75 (Scheme 4). 2-Lithio-9 was also inter-
a

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J. Clayton, J. Clayden / Tetrahedron Letters 52 (2011) 2436–2439
2.3. (4R,5R)-4,5-Dihydro-1-(20-isopropylphenyl)-4,5-
2. Experimental
diphenyloxazole 16 and (4R,5R)-1-((R)-20-Fluoro-6-
isopropylcyclohexa-10,40-dienyl)-4,5-dihydro-4,5-
diphenyloxazole (17)
2.1. (4R,5R)-1-((R)-40-Fluoro-60-isopropylcyclohexa-1,4-dienyl)-
4,5-dihydro-4,5-diphenyloxazole (14)
Oxazoline 8 (50 mg) was placed in a flame dried round bot-
tomed flask equipped with a stirrer bar, sealed and flushed with
N2, dry THF (5 mL) was added and the oxazoline dissolved. DMPU
(0.22 mL, 10 equiv) was added and the solution was cooled to
ꢀ78 °C. Isopropyllithium solution (0.66 mL, 0.45 M in pentane
2 equiv) was added to the stirred solution and the reaction mixture
turned deep green. After 2 min, the reaction was quenched with
TFA (0.5 mL). The reaction mixture was allowed to warm to room
temperature before the addition of methanol (1 mL). The reaction
mixture was partitioned between ether and saturated aqueous so-
dium hydrogen carbonate. The organic layer was then washed four
times with water to remove DMPU and dried with MgSO4. The sol-
vent was removed under reduced pressure and purification using
preparative HPLC (2% EtOAc in petroleum ether) gave compounds
16 (18 mg, 34%) and 17 (12 mg, 24%) as colourless clear oils.
Compound 16: Rf 0.5 (20% EtOAc in petroleum ether:); MS m/z
(ES+) 342.2 (100%, M+H+); HRMS: found 342.1860, M+H+ requires
342.1852; mmax (film)/cmꢀ1 3029 (Ar C–H), 1643 (C@N); 1H NMR
(CDCl3, 500 MHz) d 7.81 (d, J 7 Hz, 6H,), 7.30 (m, J 7 Hz, 1H, Ar),
5.29 (d, J 8 Hz, 1H), 5.21 (d, 1H), 4.00 (sept, J 7 Hz, 1H, CH i-Pr),
1.23 (d, J 7.5 Hz, 6H, both i-PrMe); 13C NMR (CDCl3, 125 MHz) d
163.9, 148.4, 141.1, 139.4, 130.0, 129.2, 2 ꢁ 127.9, 127.8, 127.4,
126.7, 2 ꢁ 125.6, 125.4, 125.0, 2 ꢁ 124.8, 124.5, 87.3, 78, 28.5,
21.6, 21.3.
Compound 17: Rf 0.4 (20% EtOAc in petroleum ether); MS m/z
(ES+) 362.2 (100%, M+H+); HRMS: found 362.1926, M+H+ requires
362.1915; mmax (film)/cmꢀ1 2959 (C–H), 1679 (C@N); 1H NMR
(CDCl3, 400 MHz) d 7.35–7.13 (m, 10H, Ar), 5.77–7.64 (m, 2H,
C30 + C40CH), 5.72 (d, J 7.5 Hz, 1H, C5 H), 5.43 (d, J 7.5 Hz, 1H, C4
H), 3.51–3.43 (m, 1H, C20 H), 3.04–2.83 (m, 2H, C50 CH2), 2.14
(dsept, JF 7, J 3.5 Hz, 1H, i-Pr CH), 0.97 (d, J 7 Hz, 3H, i-Pr Me),
0.79 (d, J 7 Hz, 3H, i-Pr Me); 13C NMR (CDCl3, 100 MHz) d 160.6,
140.2 (d, JF 144 Hz), 127.8, 2 ꢁ 127.7 (d, JF 8 Hz), 127.3, 126.7,
2 ꢁ 125.7, 2 ꢁ 124.6, 124.5 (d, JF 2 Hz), 121.3 (d, JF 10 Hz), 105.0,
87.3, 77.4, 43.2, 30.2 (d, JF 14 Hz), 27.7(d, JF 25 Hz), 19.5, 15.6.
Oxazoline 7 (0.18 g) was placed in a flame dried round bot-
tomed flask equipped with a stirrer bar, sealed and flushed with
2, dry THF (10 mL) was added and the oxazoline dissolved. DMPU
N
(0.7 mL, 10 equiv) was added by syringe through the septum and
the solution was cooled to ꢀ78 °C. Isopropyl lithium solution
(2.28 mL, 0.5 M in pentane, 2 equiv) was added to the stirred solu-
tion and the reaction turned deep green. After 2 min, saturated
aqueous NH4Cl solution (1 mL) was added to the reaction mixture
and the green colour disappeared. The flask was then taken out of
the dry ice bath and allowed to reach room temperature before
the addition of methanol (1 mL). The reaction mixture was parti-
tioned between ether and saturated aqueous NH4Cl. The organic
layer was then washed four times with water to remove DMPU
and dried with MgSO4. The solvent was removed under reduced
pressure before purification by flash chromatography (3% EtOAc
in petroleum ether) to give compound 14 (0.153 g, 74%) as an opa-
que oil.
Rf: 0.62 (20% EtOAc in petroleum ether); MS m/z (ES+) 362.2
(100%, M+H+); HRMS: found 362.1910, M+H+ requires 362.1915;
mmax (film)/cmꢀ1 2959 (C–H), 1717 (C@C), 1619 (C@N); 1H NMR
(CDCl3, 500 MHz) d 7.43–7.10 (m, 10H, Ar), 6.80 (ddd, JF 7.5, J 4.5,
3 Hz, 1H, C60 CH), 5.24 (ddd, JF 17.5, J 5, 2 Hz, 1H, C6’ CH), 5.14
(d, J 7 Hz, 1H, C5 CH), 5.03 (d, J 7 Hz, 1H, C4 CH), 3.54–3.49 (m,
a
1H, C20 CH), 3.01 (ddd, JF 23, J 6, 3 Hz, 1H, C50 CH) 2.89 (ddd, JF
b
23, J 9, 5 Hz, 1H, C50 CH), 2.43–2.33 (m, 1H, i-Pr H), 0.93 (d, J
7 Hz, 3H, i-Pr Me), 0.71 (d, J 7 Hz, 3H, i-Pr Me); 13C NMR (CDCl3,
125 MHz) d 163.4 (d, JF 2 Hz), 157.19 (d, JF 254 Hz), 141.9, 140.6,
131.3 (d, JF 11 Hz), 2 ꢁ 128.9 (d, JF 11 Hz), 128.5, 128.4, 127.8,
2 ꢁ 126.7, 2 ꢁ 125.8, 125.7, 100.3 (d, JF 14 Hz), 88.3, 79.1, 42.3 (d,
JF 6.5 Hz), 31.0, 27.9 (d, JF 29 Hz), 20.6, 16.3.
2.2. (4R,5R)-1-(30-Fluoro-40-methoxy-2-methylphenyl)-4,5-
diphenyl-4,5-dihydrooxazole (15)
Oxazoline 9 (100 mg, 1 equiv) was placed in a flame dried round
bottomed flask equipped with a stirrer bar, sealed and flushed with
N2, dry THF (3 mL) was added and the oxazoline dissolved. DMPU
(0.34 mL, 10 equiv) was added by syringe through the septum
and the solution was cooled to ꢀ78 °C. Isopropyllithium solution
(0.8 mL, 0.7 M in pentane, 2 equiv) was added to the stirred solu-
tion and the reaction turned deep green. After 2 minutes, methyl
iodide (0.2 ml, 11.6 equiv) was added to the reaction mixture.
The solution became orange/brown. The flask was taken out of
the dry ice bath and allowed to reach room temperature. The reac-
tion mixture was partitioned between ether and saturated aqueous
NH4Cl solution. The organic layer was washed four times with
water to remove DMPU and dried with MgSO4. The solvent was re-
moved under reduced pressure and purified by flash chromatogra-
phy (10% EtOAc in petroleum ether) to give compound 15 as an
opaque oil (15.3 mg, 14%).
Acknowledgements
We are grateful to the EPSRC and to GlaxoSmithKline for sup-
port, and to Dr Diane Coe for many helpful discussions. We
acknowledge the EPSRC. National Mass Spectrometry Service Cen-
tre, Swansea, for providing mass spectrometric data.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.02.091.
References and notes
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(100%, M+H+), 384 (90%, M+Na+); HRMS: found 362.1548, M+H+ re-
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