Asymmetric synthesis of cyclopropanes and dihydrofurans based on phosphine oxide chemistry

Article abstract of DOI:10.1039/b606874j

The asymmetric synthesis of γ-azido trans-cyclopropyl ketones is accomplished via a short, simple and efficient sequence. The cyclopropanation step is achieved by an intramolecular nucleophilic ring closure, with a diphenylphosphinate leaving group, to gi

Full text of DOI:10.1039/b606874j

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Asymmetric synthesis of cyclopropanes and dihydrofurans based on phosphine
oxide chemistry†
David J. Fox,* Sean Parris, Daniel Sejer Pedersen, Charles R. Tyzack and Stuart Warren
Received 15th May 2006, Accepted 21st June 2006
First published as an Advance Article on the web 6th July 2006
DOI: 10.1039/b606874j
The asymmetric synthesis of c-azido trans-cyclopropyl ketones is accomplished via a short, simple and
efficient sequence. The cyclopropanation step is achieved by an intramolecular nucleophilic ring
closure, with a diphenylphosphinate leaving group, to give trans-cyclopropane exclusively.
b-Keto-diphenylphosphine oxides cyclise to form optically active dihydrofurans. All possible
diastereoisomers of dihydrofurans can be prepared selectively starting from the same olefin.
Although the cyclopropane ring is a highly strained structure, it
is found in a wide variety of naturally occurring compounds in-
cluding terpenes, pheromones, fatty acid metabolites and unusual
amino acids.1,2 The rigidity of the three-membered ring makes
it an appealing structural unit for the preparation of molecules
with defined orientation of pendant functional groups. The
stereocontrolled synthesis of cyclopropanes has therefore been the
centre of much research effort.3 The area of homogeneous metal-
catalysed asymmetric addition of carbenes to olefins has been
explored vigorously since the first examples were reported by Noy-
ori and co-workers.4,5 Methods involving palladium,6 copper,7–11
cobalt,12–14 ruthenium,14 and rhodium11,15–17 have been reported.
Scheme 1 Reagents and conditions: i) LDA, THF, −78 to 0 ◦C, 95%
Asymmetric versions of the Simmons–Smith reaction18–21 have
(>95% ee).
also been used extensively in synthesis.22 Michael-induced ring
closure reactions to produce enantio-enriched cyclopropanes have
also been studied using chloro-allyl amides,23,24 sulfur ylides,25–27
sulfoxonium and sulfonium ylides,28–30 sulfoximines,31 nitrogen
ylides,32,33 phosphorus ylides,34,35 and phosphonates.36 Cascade
ring closing reactions involving phosphorus transfer, to generate
both nucleophile and leaving group, have yielded cyclopropanes
generally with high stereospecificity and selectivity. Phosphine
oxides,37–40 phosphonates,10,41,42 and phosphonium salts43 have
been employed in this manner. We have previously reported that
lithiated c-benzoyloxy phosphine oxide 1 (Scheme 1) undergoes
a three-step cascade reaction to produce trans-cyclopropane 4 in
high yield without loss of stereochemical information.38
Subjecting independently synthesised intermediates 2 and 3 to
Scheme 2
substituent, which could be transformed into the desired amine
equivalent at a later stage by a catalytic hydrogenation or a
Staudinger reaction.44
We set out to synthesise three cyclopropane precursors 5, 6
and 7 relying on chemistry developed by Chang and Sharpless.45
They have demonstrated that styrenes 9 (Scheme 3) can be
dihydroxylated and the product diols 10 converted to cyclic
carbonates 11. Regioselective benzylic ring opening with azide
then produces anti-azido alcohols 12.
The synthesis of cyclopropane precursor 5 (Scheme 4) was ac-
complished from the optically active diol46 13, which was converted
to cyclic carbonate 14 by treatment with 1,1ꢀ-carbonyldiimidazole
in dichloromethane. Ring opening of the cyclic carbonate
produced the desired regioisomer 15 exclusively and benzoylation
of the alcohol produced cyclopropane precursor 5.
the same reaction conditions also produced cyclopropyl ketone 4.
Hence, three different precursors of the same cyclopropyl ketone
product are possible. To determine if this approach to cyclopropyl
ketones is a general synthetic concept we decided to explore
the reaction in more detail. In particular, we were interested
in introducing an amino group at the benzylic position to give
us protected c-amino trans-cyclopropane ketones 8 (Scheme 2).
Moreover, we decided to invert the stereochemistry at the benzylic
position to see what consequences this would have for the reaction
outcome. Rather than a free amine we chose to use an azide
University Chemical Laboratory, Lensfield Road, Cambridge, UK CB2
1EW. E-mail: djf34@cam.ac.uk
† Electronic supplementary information (ESI) available: experimental and
analytical details for all compounds. See DOI: 10.1039/b606874j.
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(5S,1ꢀS)-18, which was confirmed by comparison of NMR data
of dihydrofurans 21 and 23.
Ring opening of cyclic carbonates according to the procedure
by Chang and Sharpless45 requires forcing conditions (110 ◦C
for 48 h) and we hoped that we could suppress dihydrofuran
formation by using milder conditions. Hence, the cyclic carbonate
was replaced with a more reactive cyclic sulfite. Treatment of diol
(4R,5R)-17 (Scheme 6) with thionyl chloride in dichloromethane
produced the cyclic sulfite 24. Treatment of the cyclic sulfite with
Scheme 3 i) Asymmetric dihydroxylation; reagents and conditions: ii)
(EtO)2CO, NaOH; iii) NaN3, H2O, DMF.
Scheme 4 Reagents and conditions: i) 1,1ꢀ-carbonyldiimidazole, CH2Cl2,
97%; ii) NaN3, H2O, DMF, 110 ◦C, 87%; iii) PhCOCl, DMAP, Et3N,
CH2Cl2, 73%.
The synthesis of cyclopropane precursor 6 was attempted by
the same method starting from phosphine oxide47 16 (Scheme 5).
Sharpless asymmetric dihydroxylation of olefin 16 produced a
mixture of the open-chain diol (4R,5R)-17 and the cyclic hemi-
ketal (5S,1ꢀR)-18, which was treated with 1,1ꢀ-carbonyldiimidazole
in dichloromethane to give cyclic carbonate 19. However, when
ring opening was attempted none of the desired azide 6 was
obtained. Instead, dihydrofuran 20 was formed as the only product
in high yield. We suspected that the dihydrofuran was formed
via intramolecular ring opening of the cyclic carbonate by the
carbonyl oxygen with inversion of configuration at C4. An X-
ray crystal structure of dihydrofuran 20 confirmed that this was
indeed the case48 (Scheme 5). Interestingly, when bis-benzoylation
of diol38 (4S,5S)-17 was attempted dihydrofuran 23 was isolated.
However, in this instance the dihydrofuran was formed with
retention of configuration at C4 by dehydration of cyclic ketal
Scheme 6 Reagents and conditions: i) SOCl2, pyridine, CH2Cl2, 47%; ii)
NaN3, DMF, 60 ◦C. Inset: X-ray crystal structure of azide 25 with ellipsoids
at 50% probability.
Scheme 5 Reagents and conditions: i) AD-mix-b, 74%; ii) 1,1ꢀ-carbonyldiimidazole, CH2Cl2, 98% (>95% ee); iii) NaN3, H2O, DMF, 110 ◦C, 89%; iv)
PhCOCl, pyridine, 95%; v) PhCOCl, DMAP, Et3N, CH2Cl2, 67%. Inset: X-ray crystal structure of alcohol 20 with ellipsoids at 50% probability.
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sodium azide in DMF at 60 ◦C did indeed produce the desired
anti-azido alcohol 6. However, under the reaction conditions
the reaction did not stop but proceeded to give a mixture of
ketone 7 and azido dihydrofuran 25 as well as a small amount
(6%) of the previously observed dihydrofuran 20. Ketone 7 and
dihydrofuran 25 can be formed after ring opening of the cyclic
sulfite by either phosphinoyl transfer or dehydration of the cyclic
ketal (Scheme 6). An X-ray crystal structure of dihydrofuran 25
confirmed the proposed stereochemistry.48 Ketone 7 is one of
the desired cyclopropane precursors and was spectroscopically
identical to the same compound synthesised by a different method
(vide infra).
In the light of these experiments we concluded that it was
unlikely that cyclopropane precursor 6 would provide an easy
entry to cyclopropyl ketones and the approach was abandoned
in favour of the remaining two cyclopropane precursors 5 and
7. However, the discovery that dihydrofurans such as 20 and 23
(Scheme 5) can be obtained as single diastereoisomers is of some
interest and will be discussed in more detail below.
Olefin47 26 was the starting point for the synthesis of the
remaining cyclopropane precursor 7 (Scheme 7). Asymmetric
dihydroxylation of olefin 26 gave diol 27 in excellent enantiomeric
excess. Conversion of diol 27 to cyclic carbonate 28 using 1,1ꢀ-
carbonyldiimidazole in dichloromethane followed by regioselec-
tive ring opening of the cyclic carbonate with sodium azide
gave exclusively the desired regioisomer 29. Phosphinoylation
of alcohol 29 with diphenylphosphinoyl chloride in pyridine
produced cyclopropane precursor 7.
Scheme 8 Reagents and conditions: i) LDA, THF, −78 ◦C to room
temperature.
From the results above it is evident that cyclopropane precursor
7 is ideally suited for our purpose. The synthetic sequence is
short with an excellent overall yield (five steps, 62%, >92% ee)
from easily prepared starting materials. Moreover, all experimental
procedures are simple and inexpensive and the products are easily
purified using conventional techniques. The method was applied
to the synthesis of cyclopropane 35 starting from olefin49 30
(Scheme 9). Asymmetric dihydroxylation of olefin 30 gave diol
31 in excellent enantiomeric excess. Owing to the sensitive furan
we anticipated that the Sharpless procedure for ring opening cyclic
carbonates with sodium azide might prove too harsh. Hence, we
decided to convert diol 31 to a cyclic sulfite, which could be ring
opened using milder conditions. Reaction of diol 31 with thionyl
chloride and pyridine in dichloromethane gave cyclic sulfite 32.
Ring opening of the cyclic sulfite using sodium azide in DMF
at 60 ◦C produced the desired anti-azido alcohol 33 as a single
regioisomer. Treatment of alcohol 33 with diphenylphosphinoyl
chloride, triethylamine and DMAP in dichloromethane produced
cyclopropane precursor 34. Cyclopropanation of phosphinate 34
using LDA in THF gave the target cyclopropane 35 albeit in
a low yield. However, no optimisation was undertaken and we
expect that the final cyclopropanation step could be improved.
The enantiomeric excess of cyclopropane 35 was determined to be
>96% by NMR analysis of Mosher’s amide derivatives made from
the free amine obtained by a Staudinger reaction.44
Scheme 7 Reagents and conditions: i) AD-mix-b, 94% (>95% ee); ii)
1,1ꢀ-carbonyldiimidazole, CH2Cl2, 92%; iii) NaN3, DMF, H2O, 110 ◦C,
92%; iv) Ph2POCl, pyridine, 83%.
Cyclopropane precursors 5 and 7 were treated with LDA in
THF at −78 ◦C and slowly allowed to warm to room temperature
(Scheme 8). Phosphine oxide 5 underwent the three-step cascade
reaction to give the desired cyclopropane 8 and ketone 7 produced
the desired cyclopropane 8 by anionic ring closure to give the
target cyclopropane. No cis-cyclopropane product was observed,
presumably because the two substituents on the forming ring
prefer to be anti in the transition state. In both cases the
enantiomeric excess of cyclopropane 8 was determined to be >92%
by NMR of Mosher’s amide derivatives. The free amine necessary
for this purpose was obtained by converting azide 8 to the amine
by a Staudinger reaction (PPh3, THF, H2O).44
Scheme 9 Reagents and conditions: i) AD-mix-b, 65% (>93% ee); ii)
SOCl2, pyridine, CH2Cl2, 99%; iii) NaN3, DMF, 60 ◦C, 84%; iv) Ph2POCl,
Et3N, DMAP, CH2Cl2, 88%; v) LDA, THF, −78 ◦C to room temperature,
30% (>96% ee).
As described above (Scheme 5) the treatment of cyclic carbonate
19 with sodium azide resulted in the formation of dihydrofuran 20
with inversion of stereochemistry at the C4 position. Moreover,
we found that it was possible to prepare the diastereomeric
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Table 1 Synthesis of dihydrofuran 20 from cyclic carbonate 19 (Scheme 5)
Entry
Base
Solvent
Time
Temp/◦C
Conversion (%)
Isolated yield (%)
1
2
NaN3
NaH
—
DMF
DMF
48 h
19 h
15 d
48 h
19 d
15 h
5 h
110
110
70
70
70
rt
100a
100a
62c
89
80
—
—
—
—
85
3b
4b
5b
6b
7
DMF-d7
CD3CN
CD3CN
CD3CN
CH2Cl2
—
0c
Et3N
DBU
DBU
66c
100c
100a
rt
a Monitored by TLC. b Experiments performed in NMR tubes containing 5 mg of 19 in 0.5 cm3 solvent, heated oil bath. c Monitored by 1H NMR.
dihydrofuran 23 with retention of configuration at C4 starting
from diol (4S,5S)-17. Because the stereochemistry of the diol
is controlled in the asymmetric dihydroxylation this potentially
gives a method for the selective synthesis of all four possible
dihydrofuran diastereoisomers starting from the same olefin.
In addition, the synthetic route makes it possible to vary the
C4- and C1ꢀ-substituents considerably. Hence, a wide range of
dihydrofurans should be readily accessible by this approach. The
cyclisation of cyclic carbonate 19 to give dihydrofuran 20 was
studied using a range of conditions (Table 1).
As it can be seen from Table 1 the cyclisation will occur even in
neat DMF (entry 3). However, dimethylamine, a common impurity
in DMF, may be acting as a base to accelerate the cyclisation.
For comparison the same reaction conditions were examined with
acetonitrile as the reaction medium (entry 4), which resulted in
no detectable dihydrofuran formation by NMR. NaH (entry 2),
triethylamine (entry 5) and DBU (entries 6 and 7) produced the
desired dihydrofuran 20 in moderate to high yields. However
conditions using DBU in dichloromethane (entry 7) proved to be
superior due to ease of handling, short reaction time and simple
purification to give dihydrofuran 20 in high yield. Optimised
conditions for the synthesis of dihydrofuran diastereoisomer 36
and 20 starting from diol (4R,5R)-17 (Scheme 10) were identified.
Treatment of the diol with a small amount of thionyl chloride
in methanol resulted in the rapid formation of dihydrofuran
Scheme 10 Reagents and conditions: i) SOCl2, MeOH, 63%; ii)
1,1ꢀ-carbonyldiimidazole, CH2Cl2; iii) DBU, 63% (2 steps); iv) MsCl, Et3N,
CH2Cl2; v) PhSH, NaH, THF, 37: 45% (2 steps), 38: 40% (2 steps); vi)
NaN3, DMF, 60 ◦C, 62% (2 steps); vii) Pd(OH)2/C, H2, MeOH, AcOH,
58%. Inset: X-ray crystal structure of THF 39 with ellipsoids at 50%
probability.
36. Alternatively, conversion to the cyclic carbonate followed by
addition of DBU in situ gives dihydrofuran 20.
The stereochemistry of dihydrofuran 36 was confirmed by
conversion to the previously described azide 25 (Scheme 6) by
mesylation of the C1ꢀ-alcohol and displacement of the mesylate
with sodium azide. This proved to be a simple method for
derivatisation at the C1ꢀ-position, which was demonstrated by
the synthesis of sulfides 37 and 38 via displacement of the
mesylates with benzene thiolate. The possibility of reducing the
dihydrofurans to give tetrahydrofurans was also investigated. We
anticipated that good facial selectivity could be achieved due
to the substituent at the C2-position. Catalytic hydrogenation
of dihydrofuran 20 with Pearlman’s catalyst in a mixture of
methanol and acetic acid at atmospheric pressure as expected gave
exclusive reduction from the face opposite the C2-substituent. The
stereochemistry of tetrahydrofuran 39 was confirmed by X-ray
crystallography48 (Scheme 10).
making optically active dihydrofurans. We are currently seeking
to extend this methodology to the synthesis of other substituted
heterocycles.
Acknowledgements
We thank Dr John Davies for crystallography and the EPSRC
for financial assistance towards the purchase of the Nonius CCD
diffractometer. D. S. P. thanks the Alfred Benzon Foundation,
Cambridge Philosophical Society and Nordea Danmark Fonden
for financial support.
Similar methods for the synthesis of dihydrofurans have been re-
ported for the cyclisation of the oxygens of stabilised enolates unto
iodonium ions,50–55 seleniranium ions,56–58 Michael acceptors.59–61
and epoxides.62 However, unlike the previously reported strategies
the present method has the advantage that it is ideally suited for
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