Asymmetric synthesis of ethano-Troeger bases using CuTC-catalyzed diazo decomposition reactions

Article abstract of DOI:10.1039/c1ob06751f

Nonracemic ethano-bridged Troeger bases are prepared using CuTC-catalyzed decompositions of diazo compounds. Excellent levels of diastereo- and enantio-control (dr and ee up to 12:1 and 95% respectively) are now obtained with aryl diazoketone precursors.

Full text of DOI:10.1039/c1ob06751f

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PAPER
¨
Asymmetric synthesis of ethano-Troger bases using CuTC-catalyzed diazo
decomposition reactions†
Ankit Sharma,a Ce´line Besnard,b Laure Gue´ne´eb and Je´roˆme Lacour*a
Received 18th October 2011, Accepted 24th October 2011
DOI: 10.1039/c1ob06751f
Nonracemic ethano-bridged Tro¨ger bases are prepared using CuTC-catalyzed decompositions of diazo
compounds. Excellent levels of diastereo- and enantio-control (dr and ee up to 12 : 1 and 95%
respectively) are now obtained with aryl diazoketone precursors.
Table 1 Screening of copper sourcesa
Ethano-Tro¨ger bases 1,1 which are structural analogues of the
important methano-Tro¨ger derivatives 2,2,3 present the advantage
of being, unlike their congeners, configurationally stable under
acidic conditions.1a,1c,1f ,4,5 Recently, new routes to these compounds
have been made available through the use of nitrogen ylide
chemistry.1f ,6 In particular, Rh2(OAc)4-catalyzed reactions of aryl
diazoesters and methano-Tro¨ger bases 2 afforded functionalized
derivatives of type 3 (Fig. 1). In a single step, the ethano-bridge
was constructed, a new carbon quaternary stereogenic center was
introduced,7 and enantiopure derivatives 2 were transformed into3
with very high levels of chirality transfer (ee up to 99%).6 Herein,
we extend the scope of this chemistry as we report that CuTC
Entry
Catalyst
Yield (%)b,c
drc
eed
1
2
Cu powder
Cu(OTf)2
60
89
85
72
91
82
89
85
90
87
85
1.0 : 1
3.5 : 1
12 : 1
3.0 : 1
5.1 : 1
5.5 : 1
8.7 : 1
5.0 : 1
4.3 : 1
10 : 1
11 : 1
25
10
15
36
81
81
82
82
83
87
90
(copper(I) thiophene-2-carboxylate) is an excellent catalyst.8,9
A
3
4
5
6
7
8
9
10
11
[Cu(CH3CN)4][BF4]
Cu2(OTf)2·Toluene
CuBr
Cu2(OAc)2
CuCl
rather large range of diazocarbonyl reagents can be used. Aryl
diazoketones provide good levels of diastereo- and enantio-control
(dr and ee up to 12 : 1 and 95% respectively) in particular.
Cu(hfacac)2·H2O
Cu(acac)2
CuI
CuTC
a Conditions (+)-(S,S)-2 (0.1 mmol), diazo reagent 4 (0.2 mmol), 5
mol% of catalyst, 0.25 mL of toluene, 90 ◦C, 6 h. b Combined yield
for both diastereoisomers using 1,3,5-trimethoxybenzene as the internal
1
standard. c Determined by H-NMR analysis (400 MHz). d Of the major
diastereoisomer; determined by CSP-HPLC analysis.
Fig. 1 Traditional ethano- and methano-Tro¨ger bases (1 and 2) and
arylester functionalized ethano-derivatives 3.
context,10 we were keen on studying the decomposition of a large
variety of diazocarbonyls under copper catalysis in the presence
of compounds 2.
As mentioned previously, nitrogen ylide chemistry can be
considered for the transformation of methano-Tro¨ger bases into
ethano-derivatives. Given the fact that copper sources, metal or
salts, are used more frequently than Rh(II) complexes in this
The reaction of (+)-(S,S)-2 with diazo arylketone 4a (Table 1)
was used to select the best possible copper catalyst.11 More than
ten sources of copper were used (Table 1). Satisfactorily, all of
them afforded the desired product 5a in moderate to excellent
yields (60–91%). Interestingly, strong differences were noticed in
terms of diastereoselectivity and enantiospecificity. CuI and CuTC
salts were the most effective (Table 1, entries 10 and 11), with a
slight preference for the latter in terms of selectivity. As CuTC, to
our knowledge, had never been reported in diazo decomposition
chemistry,12 a broader comparison was performed with 10 diazo
aDepartment of Organic Chemistry, Quai Ernest Ansermet 30, Ch-1211,
Geneva 4. E-mail: jerome.lacour@unige.ch; Fax: +41 22 379 3215
bLaboratoire de Cristallographie, University of Geneva, Quai Ernest Anser-
met 24, CH-1211, Geneva 4, Switzerland
† Electronic supplementary information (ESI) available: Synthetic pro-
cedures and spectral characterization of the major diastereoisomers of
products 5. Crystal structure determinations of 5a, 5c and 5i. CCDC
reference numbers [840414–840416]. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1ob06751f
966 | Org. Biomol. Chem., 2012, 10, 966–969
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Table 2 Copper vs. rhodium catalysisa
CuTC
Yieldb
Rh2(OAc)4
Yieldb
Entry
Diazo
R1
R2
Product
drc
drc
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4f
4g
4h
4i
Me
Me
Me
Me
Et
nPr
iPr
Ph
Ph
5a
5b
5c
5d
5e
5f
5g
5h
5i
80
70
80
82
91
80
78
80
85
87
40
11 : 1
—
70
np
78
80
19
25
np
49
20
25
85
5 : 1
—
COMe
CO2Me
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2iPr
CO2tBu
CO2Et
4.3 : 1
5.5 : 1
5.0 : 1
5.0 : 1
6.0 : 1
4.3 : 1
3.9 : 1
4.0 : 1
—
2.2 : 1
2.0 : 1
2.4 : 1
2.7 : 1
—
3.3 : 1
1.2 : 1
1 : 1
9
10
11
Me
Me
OEt
4j
4k
5j
5k
—
a Conditions: rac-2 (0.4 mmol), diazo reagent 4 (0.8 mmol), CuTC (5 mol%) or Rh2(OAc)4) (5 mol%), 1.0 mL of toluene, 100 ◦C, 16 h; reported results
are the average of at least two experiments. The relative configuration displayed is that of the major diastereoisomers of compounds 5. np stands for no
product of type 5. b Isolated yield (%, both diastereoisomers). c dr ratios determined by 1H-NMR analysis (400 MHz) of the crude reaction mixtures.
reagents (Table S1, ESI†). In all cases, better yields and selectivity
ratios were obtained with CuTC over CuI. CuTC was thus selected
for the remainder of the study.
minor isomers or directly from crude reaction mixtures. The
relative configuration of these adducts was established by X-
ray diffraction analysis (Fig. 2, 5a and 5c only). In all cases,
it was found that the ketone functional groups occupy the
more hindered position on the ethane bridge.13 All data indi-
cate that this relative configuration is conserved for the major
diastereoisomers within the series of products 5c to 5r (Tables 2
and 3).
A series of a-diazo reagents (4b to 4k) were then tested and
the results were directly compared to reactions performed with
Rh2(OAc)4. 5 mol% amounts of CuTC and Rh2(OAc)4 were used
throughout the study. The results are presented in Table 2. First, a-
diazo-b-diketone 4b (R1 = Me, R2 = COMe) was reacted with rac-2
at 100 ◦C in toluene.18 To our satisfaction, product 5b was isolated
with a 70% yield (Table 2, entry 2); the analogous reaction with
Rh2(OAc)4 being to the contrary totally unproductive. Encouraged
by this result, non-symmetrically substituted a-diazo-b-ketoester
derivatives were used. Compounds 4c and 4d reacted equally well
(R1 = Me, R2 = CO2Me and CO2Et, 80 and 82% respectively).
Under copper catalysis, better diastereoselectivity ratios were
obtained (dr 4.3 : 1 and 5.5 : 1) in comparison with that established
with Rh2(OAc)4 (2.2 : 1 and 2.0 : 1 respectively, Table 2, entries 3
and 4). The diastereoisomers that predominate with CuTC are
also the major components of the crude reaction mixtures with
Rh2(OAc)4.
This better selectivity with CuTC was confirmed with more
sterically demanding reactants 4e to 4j. Excellent yields and better
dr values were obtained (5e to 5j: 78 to 91%, dr up to 6.0 : 1,
Table 2, entries 5 to 10) and this was irrespective of the size
increase of R1 or R2. These results contrast sharply with that
obtained in the presence of Rh2(OAc)4 for which products, 5, are
isolated on average with a maximum yield of 49%. In the case
of 4g, no product was formed. Only in the reaction of a-diazo
b-diester 4k, Rh2(OAc)4 outperformed CuTC (85 vs. 40%, Table 2,
entry 11).
Fig. 2 ORTEP views of the major diastereoisomers of 5a and 5c crystal
structures; enantiomers (5RN,11SN,14SC) are shown. Hydrogen atoms are
omitted for clarity and thermal ellipsoids are drawn at 50% probability.
Finally, the scope of the chirality transfer was investigated
under CuTC catalysis.14 For this series of reactions, enantiopure
(+)-(S,S)-2 was used as the substrate (Table 3, entries 1 to 9).
Whereas moderate enantiomeric purity levels were noticed for
products 5b and 5d made using diazo compounds derived from b-
diketones or ketoesters (ee 64 and 50% respectively), results were
better with diazo reactants made from arylketones (4a, and 4l
to 4q).15 In all cases, the major diastereoisomers of products 5
were obtained with good diastereoselectivity (11 : 1 > dr > 8 : 1)
For 5a, 5c and 5i, monocrystals were afforded for the ma-
jor (racemic) diastereoisomers either after separation from the
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Table 3 Efficient chirality transfera
Entry
diazo
R1
R2
X
Product
Yieldb
drc
eed
1
2
3
4
5
6
7
8
9
4a
4b
4d
4l
4m
4n
4o
4p
4q
4a
Me
Me
Me
Et
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
OMee
5a
5b
5d
5l
5m
5n
5o
5p
5q
5r
73
70
67
83
11 : 1
—
5.5 : 1
11 : 1
8 : 1
90
64
50
90
COMe
CO2Et
Ph
Ph
Ph
85 (80)f
80
90 (95)f
82
Me
Me
Me
Me
Me
p-MePh
m-MePh
p-ClPh
p-NO2Ph
Ph
9 : 1
83
10 : 1
10 : 1
11 : 1
12 : 1
84
83 (75)f
85
93 (99)f
95
10
80
92
a Conditions: (+)-(S,S)-methano-Tro¨ger bases (0.1 mmol), 5 mol% CuTC, 0.25 mL of dry toluene, 90 ◦C, 30 min; major diastereoisomer of 5 shown.
Reported results are the average of at least two experiments. b Isolated yield (%) of the major diastereoisomer. c dr ratios determined by 1H-NMR
spectroscopy (400 MHz) on the crude reactions mixtures. d ee values determined by CSP-HPLC. e ee 98%. f After crystallization.
and isolated yields (80–85%). In terms of enantioselectivity, the
substitution of methyl by ethyl or phenyl substituents a to the
Experimental
Representative preparation of racemic ethano-Tro¨ger base: 5a
carbonyl group had little effect in comparison with 5a; products
5l and 5m (Table 3, entries 4 and 5) being afforded with the same
90% ee value than 5a. In the case of 5m, a simple recrystallization
in acetone by slow evaporation of the solvent afforded the adduct
in 95% ee (Table 3, entry 5). Increased steric hindrance on the
reactants does not seem to be a factor under CuTC cataly-
sis.
In a 10 mL 2 neck round bottom flask equipped with a magnetic
stirring bar, 100.0 mg of Tro¨ger base 2 (0.399 mmol) was
introduced along with 1 mL of dry toluene and 3.8 mg CuTC
(0.02 mmol, 5 mol%), all under a dinitrogen atmosphere. To
this solution, diazo compound 4a (64 mg, 0.4 mmol) was added
in one portion. The reaction mixture was placed in an already
heated oil bath (100 ◦C) and was stirred for 30 min at this
temperature. Then, 0.4 mmol of the same diazo compound 4a
was added in one portion to the reaction mixture at 100 ◦C. The
reaction was monitored by ESI-MS. The solution was then allowed
to cool to 25 ◦C and the solvent was removed under reduced
pressure. Purification by column chromatography (SiO2, 3 ¥ 30 cm,
acetone/pentane 5 : 95) afforded the major diastereoisomer
{(5S,11R,14R) and (5R,11S,14S)} of the desired product 5a (111.5
mg, 73%) as a white solid. M.p.: 215–217 ◦C; IR: (neat): n-1 = 2923,
1706, 1495, 1260, 1220 cm-1. 1H NMR (400 MHz, CDCl3) d = 7.61
(b, 2H), 7.38 (t, J = 7.8 Hz, 2H), 7.34–7.26 (m, 2H), 7.00 (d, J =
8.0 Hz, 1H), 6.96 (d, J = 8.0 Hz, 1H), 6.85 (d, J = 8.0 Hz, 1H),
6.64 (s, 2H), 5.28 (d, J = 14.6 Hz, 1H), 4.73 (d, J = 17.6 Hz, 1H),
4.60 (d, J = 17.4 Hz, 1H), 4.41 (d, J = 17.6 Hz, 1H), 4.31 (d, J =
17.4 Hz, 1H), 3.52 (d, J = 14.6 Hz, 1H), 2.18 (s, 3H), 2.14 (s, 3H),
1.66 (s, 3H). 13C NMR (100 MHz, CDCl3) d = 205.5, 147.5, 144.7,
141.5, 136.2, 135.9, 135.8, 134.4, 130.5, 129.8, 129.0, 128.8, 128.7,
128.3, 128.1, 127.8, 127.4, 82.3, 59.6, 59.4, 56.4, 25.3, 21.2, 21.1.
HRMS (ESI) for (C26H26N2O + H)+: 383.2117, found: 383.2129.
A series of diazoketones carrying different substituents on the
aryl moiety was also utilized, namely reagents 4n to 4q (R2 =
p-MePh, m-MePh, p-ClPh, p-NO2Ph). Very similar yields and
diastereoselectivity ratios were obtained (Table 3, entries 6 to
9), which are in line with those previously discussed. Reasonably
better enantiomeric purity values (ee ≥93%) were however, ob-
tained with reagents carrying electron-withdrawing groups (p-Cl
and p-NO2: 4p and 4q respectively) in place of electron-donating
ones (ee £84%, p- and m-Me: 4n and 4o).16 Clearly, within this
series of diazo reagents derived from arylketones, the highest
levels of enantioselectivity are observed with the most reactive
electrophilic carbenoids.17 More generally, the more reactive diazo
compounds, i.e. those derived from aryl ketones, give better
results than stable derivatives made from b-diketones and b-
ketoesters (ee 50–64%). Non-racemic 2,8-dimethoxy methano-
Tro¨ger base (ee 98%) was also treated with diazo 4a under the new
conditions, good yield (80%), dr (12 : 1) and ee (92%) values were
obtained.
In conclusion, in the context of nitrogen ylide chemistry, we have
shown that CuTC is a particularly effective source of copper for the
decomposition of diazo reagents. A rather large scope of reactivity
was obtained for the preparation of functionalized ethano-bridged
Tro¨ger bases; aryl diazoketones providing good levels of diastereo-
and enantio-control (dr and ee up to 12 : 1 and 95% respectively)
in particular.
Representative preparation of enantio-enriched ethano-Tro¨ger
base: 5a
The method is the same as above, using enantiopure (+)-(S,S)-2
(25 mg, 0.1 mmol) as the substrate in place of rac-2 and 4a (32 mg,
968 | Org. Biomol. Chem., 2012, 10, 966–969
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0.2 mmol) and 0.95 mg of CuTC (5 mol%) as the catalyst. The
reaction was kept at 90 ◦C instead of 100 ◦C (Table 3). The major
diastereoisomer (5S,11R,14R)-5a was isolated in 73% yield (28 mg,
5 Configurationally stable methano-Tro¨ger bases can be obtained upon
double protonation or introduction of substituents on the aromatic
framework. See: A. Greenberg, N. Molinaro and M. Lang, J. Org.
Chem., 1984, 49, 1127; A. Tatiboue¨t, M. Demeunynck, C. Andraud,
A. Collet and J. Lhomme, Chem. Commun., 1999, 161; D. A. Lenev,
K. A. Lyssenko, D. G. Golovanov, V. Buss and R. G. Kostyanovsky,
Chem.–Eur. J., 2006, 12, 6412.
20
ee 90%), [a]D = +198 (c 0.02, CH2Cl2). CSP-HPLC separation:
(S,S)-Whelk-O 1 column, n-hexane/i-PrOH 99 : 1, 0.7 mL min-1,
23 ◦C, l = 254 nm; tR (5S,11R,14R) = 9.0 min, tR (5R,11S,14S) =
10.2 min.
6 A. Sharma, L. Guene´e, J. V. Naubron and J. Lacour, Angew. Chem.,
Int. Ed., 2011, 50, 3677.
7 The presence of two different substituents around the diazo moiety
leads to the formation of a carbon stereocenter on the ethano-bridge,
and hence the formation of diastereomers.
Acknowledgements
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11 For information, this reaction had afforded 5a (70%) with a moderate
diastereoselectivity (dr 5 : 1) using 1 mol% of Rh2(OAc)4. The transfer
of chirality had been low in this case (ee 10%). No change was observed
with 5 mol% of catalyst (Table 2).
12 CuTC was mentioned once in the context of carbene self-dimerization.
Fischer complexes were used as the substrates and a transmetalation
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13 At the new carbon stereogenic centers, results indicate that large
and small substituents replace preferentially the pro-(S) and pro-(R)
hydrogen atoms of the parent (S,S)-1, respectively (and vice versa for
(R,R)-1).
14 In the presence of either 1 or 5 mol% of Rh2(OAc)4, the reaction
of enantiopure 2 provides no product with 4b or essentially racemic
material with diazo 4a and 4d (eemax 10%).
We thank the University of Geneva and the Swiss NSF for financial
support. We also acknowledge the contributions of the Sciences
Mass Spectrometry (SMS) platform at the Faculty of Sciences,
University of Geneva.
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15 In all likelihood, the chirality transfer occurs with retention of
configuration as in the case of Rh2(OAc)4.
16 A single recrystallization of 5p in acetone afforded the adduct in 99%
ee.
17 Interestingly, this trend is opposite to that observed with arylester
diazo derivatives under Rh2(OAc)4 catalysis. See reference 6 for further
information.
4 (a) O. Trapp, G. Trapp, J. W. Kong, U. Hahn, F. Vo¨gtle and V. Schurig,
Chem.–Eur. J., 2002, 8, 3629; (b) O. Trapp and V. Schurig, J. Am. Chem.
Soc., 2000, 122, 1424.
18 For the Rh(II)-catalyzed reactions, 1 mol% of Rh2(OAc)4 is actually
sufficient to reach full conversion. 5 mol% of catalyst was used to allow
a direct comparison with copper-mediated reactions.
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The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 966–969 | 969
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