Improved synthesis of enantiopure 4-hydroxy[2.2]paracyclophane

Article abstract of DOI:10.1021/jo100468s

(Figure presented) 4-Hydroxy[2.2]paracyclophane is readily prepared via an improved synthetic protocol from unsubstituted [2.2]paracyclophane. The key step is a Dakin oxidation of 4-formyl[2.2]paracyclophane. This allows a rapid access to large quantities of the product and an easy synthesis of the enantiopure form.

Full text of DOI:10.1021/jo100468s

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aldehydes4 and imines,5 oxidations of thioethers,6 trime-
Improved Synthesis of Enantiopure
4-Hydroxy[2.2]paracyclophane
thylsilylcyanation,7 and allylboration.8
Most of such ligands can be described as derivatives of
4-hydroxy[2.2]paracyclophane (3) and are obtainable from
this compound.
€
Christian J. Friedmann, Sefer Ay, and Stefan Brase*
Since its first synthesis by Cram in 1955,9 much effort has
been made to develop more efficient synthesis protocols to
this compound.10,11 Today, 3 is usually prepared on a large
scale by a method developed by Krohn and Hopf (Scheme 1).12
Institute of Organic Chemistry, Karlsruhe
Institute of Technology, Fritz-Haber-Weg 6,
D-76131 Karlsruhe, Germany
braese@kit.edu
SCHEME 1. Conventional Synthesis of 312
Received March 20, 2010
Starting from unsubstituted [2.2]paracyclophane (1), an aro-
matic substitution with bromine under iron catalysis is carried
out. The obtained 4-bromo[2.2]paracyclophane (2) is then
converted into the desired product by a one-pot proce-
dure consisting of a Br-Li exchange, transmetalation onto
boron, and an oxidative workup with a mixture of potassium
hydroxide and hydrogen peroxide. Although this procedure is
accomplishable, the second step is usually critical, and the yield
ranges from 6513 to 82%.14
In order to obtain enantiopure material, the subsequent
resolution requires two additional steps. Auxiliaries known
in the literature15 for this purpose are acid chlorides, derived
from camphanic acid (4)16 and naproxene (5, Figure 1),17
which convert 3 into the corresponding diastereomeric
4-Hydroxy[2.2]paracyclophane is readily prepared via an
improved synthetic protocol from unsubstituted [2.2]para-
cyclophane. The key step is a Dakin oxidation of 4-formyl-
[2.2]paracyclophane. This allows a rapid access to large
quantities of the product and an easy synthesis of the
enantiopure form.
The synthesis of enantiopure ligands has attained consid-
erable attention due to their pivotal role in asymmetric
synthesis, and consequently, the need for efficient routes to
such compounds is of increasing interest. In the field of
asymmetric catalysis, mono- and bidentate ligands based on
a [2.2]paracyclophane scaffold1 (1) have been successfully
employed as potent catalysts2 in different kinds of reactions,
like asymmetric hydrogenations,3 addition reactions to
(9) In his pioneering work on paracyclophane chemistry, Cram identified
3 as a byproduct in his attempts to reduce the corresponding diazonium salt
with hypophosphoric acid: Cram, D. J.; Allinger, N. L. J. Am. Chem. Soc.
1955, 77, 6289–6294.
(10) (a) Cram, D. J.; Day, A. C. J. Org. Chem. 1966, 31, 1227–1232.
(b) Norcross, B. E.; Becker, D; Cukier, R. I.; Schultz, R. M. J. Org. Chem.
1967, 32, 220–222.
(11) Other examples, apart from methods described elsewhere: (a)
Zhuravsky, R.; Starikova, Z.; Vorontsov, E.; Rozenberg, V. Tetrahedron:
Asymmetry 2008, 19, 216–222. (b) Rozenberg, V.; Zhuravsky, R.; Sergeeva,
E. Chirality 2006, 18, 95–102. (c) Hitchcock, P. B.; Rowlands, G. J.; Parmar,
R. Chem. Commun. 2005, 4219–4221. (d) Vorontsova, N.; Rozenberg, V.;
Vorontsov, E.; Antonov, D.; Starikova, Z. Eur. J. Org. Chem. 2003, 761–770.
(e) Braddock, D. C.; MacGilp, I. D.; Perry, B. G. J. Org. Chem. 2002, 67,
8679–8681. (f) Rozenberg, V.; Danilova, T.; Sergeeva, E.; Vorontsov, E.;
Starikova, Z.; Korlyukov, A.; Hopf, H. Eur. J. Org. Chem. 2002, 468–477. (g)
Cipiciani, A.; Fringuelli, F.; Mancini, V.; Piermatti, O.; Scappini, A. M.;
Ruzziconi, R. Tetrahedron 1997, 53, 11853–11858.
(1) Modern Cyclophane Chemistry; Gleiter, R., Hopf, H., Eds.; Wiley-
VCH: Weinheim, 2004.
(2) For a general review, see: Gibson, S. E.; Knight, J. D Org. Biomol.
Chem. 2003, 1, 1256–1269.
(3) With phosphorous-based ligands: (a) Zanotti-Gerosa, A.; Malan, C.;
Herzberg, D. Org. Lett. 2001, 3, 3687–3690. (b) Pye, P. J.; Rossen, K.;
Reamer, R. A.; Tsou, N. N.; Volante, R. P.; Reider, P. J. J. Am. Chem. Soc.
1997, 119, 6207–6208.
€
(4) 1,4-Additions: Ay, S.; Nieger, M.; Brase, S. Chem.;Eur. J. 2008, 14,
11539–11556. 1,2-Additions: (a) Bolm, C.; Whelligan, D. K. Adv. Synth.
(12) Krohn, K.; Rieger, H.; Hopf, H.; Barrett, P. G. Chem. Ber. 1990, 123,
1729–1732.
(13) Results from our laboratories.
(14) Kane, V. V.; Gerdes, A.; Grahn, W.; Ernst, L.; Dix, I.; Jones, P. G.;
Hopf, H. Tetrahedron Lett. 2001, 42, 373–376.
€
Catal. 2006, 348, 2093–2100. (b) Lauterwasser, F.; Gall, J.; Hofener, S.;
Brase, S. Adv. Synth. Catal. 2006, 348, 2068–2074. (c) Lauterwasser, F.;
€
€
Nieger, M.; Mansikkamaki, H.; Nattinen, K.; Brase, S. Chem.;Eur. J. 2005,
€
€
11, 4509–4525.
(5) (a) Hermanns, N.; Dahmen, S.; Bolm, C.; Brase, S. Angew. Chem., Int.
(15) Only indirect resolution methods using enzymes are known. In all
cases, 4-acetoxy[2.2]paracyclophane is used as a substrate hydrolyzed via
kinetic resolution to enantioenriched 3: (a) Pamperin, D.; Schulz, C.; Hopf,
H.; Syldatk, C.; Pietzsch, M. Eur. J. Org. Chem. 1998, 1441–1445. (b)
Pamperin, D.; Ohse, B.; Hopf, H.; Pietzsch, M. J. Mol. Catal. B: Enzym.
1998, 317–319. (c) Cipiciani, A.; Fringuelli, F.; Mancini, V.; Piermatti, O.;
Scappini, A. M.; Ruzziconi, R. Tetrahedron 1997, 53, 11853–11858.
(16) Rozenberg, V.; Danilova, T.; Sergeeva, E.; Vorontsov, E.; Starikova,
Z.; Korlyukov, A.; Hopf, H. Eur. J. Org. Chem. 2002, 3, 468–477.
(17) Zhang, T.-Z.; Dai, L.-X.; Hou, X.-L. Tetrahedron: Asymmetry 2007,
18, 251–259.
€
Ed. 2002, 41, 3692–3694. (b) Dahmen, S.; Brase, S. J. Am. Chem. Soc. 2002,
124, 5940–5941.
€
(6) Vetter, A. H.; Berkessel, A. Tetrahedron Lett. 1998, 39, 1741–1744.
(7) Belokon, Y.; Moscalenko, M.; Ikonnikov, N.; Yashkina, L.;
Antonov, D.; Vorontsov, E; Rozenberg, V. Tetrahedron: Asymmetry 1997,
8, 3245–3250.
(8) Vorontsova, N. V.; Zhuravsky, R. P.; Sergeeva, E. V.; Vorontsov,
E. V.; Starikova, Z. A.; Rozenberg, V. I. Russ. Chem. Bull. 2007, 546, 2225–
2231.
4612 J. Org. Chem. 2010, 75, 4612–4614
Published on Web 06/04/2010
DOI: 10.1021/jo100468s
r
2010 American Chemical Society

Friedmann et al.
JOCNote
SCHEME 3. Ligands prepared from (Sp)-316,18,24-26
FIGURE 1. Common reagents for the resolution of 3.
SCHEME 2. New Synthesis and Resolution Procedure for 3
n-hexane. Only two crystallization steps are necessary to obtain
analytically pure material of (Sp,R)-8 in >98% de and 22%
yield.22
Since (Sp,R)-8 is unstable under acidic conditions, it can be
easily converted into the enantiopure aldehyde (Sp)-6. By
filtration over a column of silica using dichloromethane as
the eluent, it can be hydrolyzed and (Sp)-6 is obtained in an
almost quantitative yield. This enantiopure aldehyde can be
oxidized under Dakin conditions into phenol (Sp)-3. The
oxidation step occurs without racemization, which was
verified by comparison of the enantiomeric excesses of sub-
strate and product.
esters. The separation is achieved via fractional crystalliza-
tion and delivers material of >98% de in 14-15% yield.
Enantiopure 4-hydroxy[2.2]paracyclophane (3) is obtained
after ester cleavage with LiAlH4.
4-Hydroxy[2.2]paracyclophane is a key compound in the
synthesis of [2.2]paracyclophane-based building blocks and
ligands for asymmetric catalysis (Scheme 3).2,23
We discovered that 3 can be synthesized in a more effective
way that also facilitates its synthesis in enantiopure form.
[2.2]Paracyclophanecarboxaldehyde (6), which is a key com-
pound in our route, can be obtained by a Rieche formylation18
of unsubstituted [2.2]paracyclophane (1) in quantitative yield
and used without further purification in the next step.19 It is
known that aromatic phenols can be synthesized through a
Dakin oxidation20 of the corresponding aldehydes. To the best
of our knowledge, this was not applied for the synthesis of
4-hydroxy[2.2]paracyclophane. We were able to show that race-
mic 4-hydroxy[2.2]paracyclophane (3) can be formed through
oxidation of 6 with hydrogen peroxide and concentrated sulfuric
acid on a large scale in an excellent yield of 90% (Scheme 2,
upper row). The enantiomers of aldehyde 6 can be separated
by a procedure developed by Quici.21 Therefore, 6 is condensed
with 7 by simple refluxing in benzene. The obtained diastereo-
meric imines 8 are separated by fractional crystallization in
By an ortho-selective substitution reaction, for instance,
ketones (Sp)-9 and FHPC (R = H)24 can be obtained that
serve as starting materials for the synthesis of N,O-imine
ligands.4 The derivatization into the thiol (Sp)-11 or indoles
(Sp)-1225 via the triflate (Sp)-1017 is also possible. (Sp)-10 can
also be used for the synthesis of phosphine-based ligands like
(Sp)-13.17 Even chiral, nonracemic quinones are accessible.26
In addition, the phenol 3 can itself serve as a chiral auxiliary.27
In conclusion, we have developed a new access to enan-
tiopure 4-hydroxy[2.2]paracyclophane, which is a key com-
pound in [2.2]paracyclophane chemistry. Our route offers an
efficient synthesis since cheaper and safer reagents are used
for the resolution/oxidation sequence.
Experimental Section
rac-4-Formyl[2.2]paracyclophane (6). [2.2]Paracyclophane (1)
(2.26 g, 10.8 mmol) was dissolved in CH2Cl2 (100 mL) and
cooled to 0 °C. Titanium(IV) chloride (2.38 mL, 21.7 mmol) and
€
(18) Rieche, A.; Gross, H.; Hoft, E. Chem. Ber. 1960, 93, 88–94.
(19) (a) Hopf, H.; Raulfs, F.-W. Isr. J. Chem. 1985, 25, 210–216.
(b) Ramm, S. M.; Hopf, H.; Jones, P. G.; Bubenitschek, P.; Ahrens, B.;
Ernst, L. Eur. J. Org. Chem. 2008, 2948–2959.
(20) (a) Dakin, H. D. Am. Chem. J. 1909, 42, 477–498. (b) Renz, M;
Meunier, B. Eur. J. Org. Chem. 1999, 737–750. The use of the Dakin reaction
in the paracyclophanes series: (c) Rozenberg, V. I.; Antonov, D. Y.;
Sergeeva, E. V.; Vorontsov, E. V.; Starikova, Z. A.; Fedyanin, I. V.; Schulz,
C.; Hopf, H. Eur. J. Org. Chem. 2003, 2056–2061. (d) Dominguez, B.;
Zanotti-Gerosa, A.; Hems, W. Org. Lett. 2004, 6, 1927–1930. (e) Dominguez,
B.; Hems, W. P.; Zanotti-Gerosa, A. PCT Int. Appl. WO2004111065 (A1), 2004.
(21) It was also shown that a resolution of the corresponding aldehyde (6)
with enantiopure 1-phenyl-2-(4-methylphenyl)ethylamine proceeds in 29%
yield over two steps: Banfi, S.; Manfredi, A.; Montanari, F.; Pozzi, G.; Quici,
S. J. Mol. Catal. A: Chem. 1996, 113, 77–86.
(22) The absolute configuration of the diastereomers of 8 was verified by
comparison with literature values (see ref 21).
€
€
(23) Brase, S.; Dahmen, S.; Hofener, S.; Lauterwasser, F.; Kreis, M.;
Ziegert, R. E. Synlett 2004, 2647–2669.
(24) Hopf, H.; Barrett, D. G. Liebigs Ann. Chem. 1995, 449–451.
(25) Ortner, B.; Hubner, H.; Gmeiner, P. Tetrahedron: Asymmetry 2001,
12, 3205–3208.
(26) Miyahara, Y.; Inazu, T.; Yoshino, T. Tetrahedron Lett. 1982, 23,
2189–2190.
(27) Fringuelli, F.; Piermatti, O.; Pizzo, F.; Ruzziconi, R. Chem. Lett.
2000, 29, 38–39.
J. Org. Chem. Vol. 75, No. 13, 2010 4613

Friedmann et al.
JOCNote
dichloromethoxymethane (1.00 mL, 11.3 mmol) were added
subsequently. The mixture was stirred at room temperature for
6 h, poured into water (100 mL), and stirred for another 2 h. The
two phases were separated, and the aqueous phase was extracted
with CH2Cl2 (100 mL). The combined organic phases were dried
over MgSO4 and concentrated under reduced pressure. The
crude product (2.56 g, >99%) was obtained as a colorless solid
and used without further purification. 1H NMR (400 MHz,
CDCl3): δ = 9.95 (s, 1H), 7.02 (d, J = 1.9 Hz, 1H), 6.73 (dd, J =
7.8 Hz, 1.9 Hz, 1H), 6.59 (d, J = 7.8 Hz, 1H), 6.56 (dd, J = 7.9
Hz, 1.8 Hz, 1H), 6.50 (dd, J = 7.9 Hz, 1.8 Hz, 1H), 6.43 (dd, J =
7.9 Hz, 1.8 Hz, 1H), 6.37 (dd, J = 7.9 Hz, 1.8 Hz, 1H), 4.10 (ddd,
J = 11.8 Hz, 9.6 Hz, 1.5 Hz, 1H), 3.27 (ddd, J = 12.5 Hz, 10.4
Hz, 2.0 Hz, 1H), 3.21 (dd, J = 11.7 Hz, 4.5 Hz, 1H), 3.18 (dd,
J = 11.7 Hz, 4.5 Hz, 1H), 3.14-2.97 (m, 3H), 2.95 (ddd, J =
13.1 Hz, 10.1 Hz, 6.7 Hz, 1H). 13C NMR (100 MHz, CDCl3):
δ = 191.1, 143.2, 140.6, 139.5, 139.4, 138.1, 136.6, 136.3, 136.1,
133.2, 132.9, 132.3, 132.1, 35.2, 35.1, 35.0, 33.6. Enantiopure
material was prepared according to ref 21. HPLC conditions:
OD column, n-heptane/2-propanol 98:2, 1 mL/min, 15 °C, 15.32
min (S); 22.27 min (R).
solution was stirred for 16 h. The solvent was evaporated under
reduced pressure, and the residue was taken up in CH2Cl2 (100mL)
and water (100 mL). The two phases were separated, and the
aqueous phase was extracted with CH2Cl2 (100 mL). The com-
bined organic phases were dried over MgSO4 and concentrated
under reduced pressure. The crude product was purified by column
chromatography on silica gel (eluent: cyclohexane/EtOAc, 1:1) to
afford rac-4-hydroxy[2.2]paracyclophane (4.26 g, 90%) as a pale
yellow solid. 1H NMR (400 MHz,CDCl3): δ = 7.01 (dd, J = 7.8
Hz, 1.8 Hz, 1H), 6.54 (dd, J = 8.0 Hz, 1.9 Hz, 1H), 6.43 (dd, J =
8.0 Hz, 1.9 Hz, 1H), 6.38 (dd, J = 8.0 Hz, 1.9 Hz, 1H), 6.37 (d, J =
7.7 Hz, 1H), 6.24 (dd, J = 8.0 Hz, 1.9 Hz, 1H), 5.50 (d, J = 1.8 Hz,
1H), 4.54 (bs, 1H), 3.35-2.65 (m, 8H). 13C NMR (100 MHz,
CDCl3): δ = 153.4, 141.8, 139.4, 138.6, 135.2, 133.4, 132.6, 131.6,
127.7, 125.3, 124.8, 122.4, 35.2, 34.7, 33.8, 31.0. HPLC conditions:
AS column, n-heptane/2-propanol 90:10, 1 mL/min, 10 °C, 7.44
min (R); 8.74 min (S).
Acknowledgment. Financial support from the Landesgra-
€
duiertenforderung Baden-Wurttemberg (fellowship for C.J.F.)
is gratefully acknowledged.
€
rac-4-Hydroxy[2.2]paracyclophane (3). rac-4-Formyl[2.2]para-
cyclophane (5.00 g, 21.2 mmol) was dissolved in CH2Cl2 (100 mL)
and MeOH (100 mL). Then concd H2SO4 (0.16 mL) and H2O2
(2.40 mL, 35% in water) were subsequently added, and the
Supporting Information Available: General information
and 1H NMR spectra for compounds 3, 6, and 8. This material
is available free of charge via the Internet at http://pubs.acs.org.
4614 J. Org. Chem. Vol. 75, No. 13, 2010