New cyclic dimers of cholic acid

Article abstract of DOI:10.1007/s00706-004-0228-0

Two new cyclic dimers of cholic acid were obtained in the reaction of 3-O-acetyl methyl cholate with oxalyl chloride. The oxalates bound the cholate subunits "side-to-side" as a result of acylation of 7α and 12α OH groups in the substrate. The selective deprotection of hydroxy groups at C-3 and C-24 proved to be rather difficult and led to various products depending on the reaction conditions. Springer-Verlag 2005.

Full text of DOI:10.1007/s00706-004-0228-0

Monatshefte f€ur Chemie 136, 153–158 (2005)
DOI 10.1007/s00706-004-0228-0
New Cyclic Dimers of Cholic Acid
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´
Zenon )otowski and Dariusz Guzmanski
Institute of Chemistry, University of Bialystok, PL-15443 Bialystok, Poland
Received April 23, 2004; accepted (revised) May 26, 2004
Published online January 14, 2005 # Springer-Verlag 2005
Summary. Two new cyclic dimers of cholic acid were obtained in the reaction of 3-O-acetyl methyl
cholate with oxalyl chloride. The oxalates bound the cholate subunits ‘‘side-to-side’’ as a result of
acylation of 7ꢀ and 12ꢀ OH groups in the substrate. The selective deprotection of hydroxy groups at
C-3 and C-24 proved to be rather difficult and led to various products depending on the reaction
conditions.
Keywords. Acylation; Bile acids; Macrocycles; Steroids; Supramolecular Chemistry.
Introduction
The combination of unique construction and unusually specific properties of bile
acids makes them nearly perfect building blocks in the design and synthesis of
molecular receptors, enzyme models, and transporters, for example, drugs across
the phospholipid membranes [1]. Two groups of cyclic structures prepared from
bile acids, are cyclocholates [2, 3] and cholaphanes [4, 5].
The syntheses of cyclic supramolecular hosts based on bile acids depended so
far exclusively on the formation of bonds among the steroidal units by using the
24-carboxyl group of one of these units, and the 3ꢀ hydroxy group of the other
(‘‘head to tail’’ method). In this paper we report the synthesis of a new type of
cyclic dimers of cholic acid in which two steroidal molecules were bound with the
less reactive 7ꢀ and 12ꢀ hydroxy groups.
Results and Discussion
The compounds mentioned above (side-bonded dimers 1 (‘‘trans-dimer’’) and 2
(‘‘cis-dimer’’) were obtained by esterification of 3-O-acetyl methyl ester of cholic
acid (5) with oxalyl chloride in pyridine.
In the first experiment, to the solution of 5 an exactly equimolar amount of
oxalyl chloride was added. As a result, three less polar products were formed and a
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Corresponding author. E-mail: zlch@uwb.edu.pl

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154
Z. )otowski and D. Guzmanski
Formulae 1
Formulae 2
small amount of the substrate was recovered. Based on the chemical shifts of the
C-7 and C-12 protons the following results were obtained: (i) in the case of the
least polar product: both 7ꢀ and 12ꢀ hydroxy groups were esterified and thus one
of the expected dimers 1 or 2 was created. This was confirmed by mass spectrom-
etry technique; (ii) in both more polar compounds only one of the hydroxy groups
was esterified (the same in both steroidal subunits). Thus, ‘‘non-closed’’ dimers 3
and 4 were obtained, in which two molecules of cholic acid were acylated with one
molecule of oxalyl chloride.
1
The structures of 3 and 4 were determined by comparison of their H NMR
spectra with those of cholic acid and deoxycholic acid, which does not possess the
7ꢀ hydroxy group (Scheme 1). The spectrum of 3 (Fig. 1C) shows that only 7ꢀ
hydroxy groups where acylated contrary to compound 4 (Fig. 1D), which under-
went acylation exclusively at 12ꢀ-OH groups.
Based on these results it was derived, that three dimeric by-products were
formed: two ‘‘non-closed’’ dimers 3 and 4, and a ‘‘cross-dimer’’, in which the 7ꢀ
hydroxy group of the first steroidal subunit and the 12ꢀ hydroxy group of the second
subunit were acylated. The third product underwent rapid cyclization with the sec-
ond molecule of oxalyl chloride yielding the least polar product – ‘‘cis-dimer’’ 2. In

New Cyclic Dimers of Cholic Acid
155
Scheme 1
Fig. 1. 1H NMR spectra (chemical shifts range 3.6–5.4 ppm) of cholic acid (A), deoxycholic acid
(B), dimer 3 (C), and dimer 4 (D)
the case of exhaustive acylation with oxalyl chloride a ‘‘trans-dimer’’ 1 was
expected to be formed. This was confirmed in a second experiment (Scheme 1)
The dimers 1 and 2 were subjected to reduction under various conditions in
order to obtain compounds with free 3ꢀ and 24 hydroxy groups, but none of these
experiments was successful: lithium tri-t-butoxyaluminum hydride [6] appeared to
be inactive, whereas lithium aluminum hydride [7] reduced only the oxalate esters
leading thereby to decomposition of the dimers (Table 1).
An attempt to reduce the ester groups in 1 with the LiAlH4–BF3=Et2O system
[8] was also undertaken, but none of them was reduced to an ether group as
expected. A very complex mixture was formed instead in which 6 predominated.

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Z. )otowski and D. Guzmanski
Table 1. Reduction of dimers 1 and 2 with hydrides under various conditions
Substrates
Reducing agent
Reaction conditions
Result
1
LiAlH(O-t-Bu)3 (excess)
LiAlH4 (excess)
THF, reflux., 12 h
THF, ꢁ70ꢂC, 4 h
THF, 0ꢂC, 45 min
no reaction
no reaction
5
1, 2
1, 2
LiAlH4 (excess)
Scheme 2
It turned out that both oxalate esters remained unchanged and only one of the ester
groups in the side chain was reduced to CH2OH (Scheme 2).
Conclusions
Both cyclic dimers 1 and 2 were readily formed by reaction of 5 with oxalyl
chloride. It was proved that selective reduction of ester groups at C-3 and in the
side chain in 1 or 2 is rather difficult. The oxalyl esters proved to be the most
reactive groups towards LiAlH4 in THF, whereas the same reducing agent in the
BF3=Et2O complex did not reduce the oxalate system, but the COOMe in the side
chain to CH2OH, with rather poor yield.
Experimental
Melting points were determined on a Kofler apparatus of the Bo€etius type. NMR spectra were taken
with a Bruker AC 200F spectrometer with TMS as internal standard. Infrared spectra were record-
ed on a Nicolet series II Magna-IR 550 FT-IR spectrometer. Mass spectra were obtained with an
AMD-604 spectrometer. The reaction products were isolated by column chromatography performed
on 70–230 mesh silica gel (J.T. Baker). Thin-layer chromatograms were developed on aluminum
TLC sheets precoated with silica gel F254 (Merck) and visualized with 50% H2SO4 after heating.
All solvents were dried and freshly distilled prior to use. Methyl cholate 5 was purchased from
Steraloids Inc., and it was used without further purification.
Reaction of 5 with Oxalyl Chloride – Dimers 2, 3, and 4
To a solution of 408 mg of 5 (0.88 mmol) in 5 cm3 of anh. pyridine 77mm3 (0.9mmol) of oxalyl
chloride (room temp.) were added dropwise with vigrous stirring. After the whole amount of the

New Cyclic Dimers of Cholic Acid
157
chloride was added, the mixture was stirred for additional 0.5 h, poured into acidified H2O, and
products were extracted with CHCl3. The organic layer was dried (MgSO4) and the solvent was
removed. The mixture of the three products was subjected to column chromatography. Pure dimers
were eluted consecutively: 2 (benzene=ethyl acetate 83=17, 38 mg, 8%), 3 (benzene=ethyl acetate 8=2,
64mg, 15%), and 4 (benzene=ethyl acetate 8=2, 72 mg, 17%).
Di[3ꢀ-acetoxy-7ꢀ,12ꢀ-dihydroxy-5ꢁ-cholanoic acid methyl ester]7,120;12,70-
dioxalate (2, C58H84O16)
1
Colorless crystals, mp 266–269ꢂC (n-hexane=CH2Cl2); H NMR (200 MHz, CDCl3): ꢂ ¼ 5.30 (m, 2
12ꢁ-H), 5.01 (m, 2 7ꢁ-H), 4.51 (m, 2 3ꢁ-H), 3.66 (s, 2OCH3), 1.96 (s, 2CH3CO), 1.01 (s, 2 19-CH3),
0.79 (m: s, 2 18-CH3 and d, 2 21-CH3) ppm; 13C NMR (50 MHz; CDCl3): ꢂ ¼ 174.0 (2C), 170.9 (2C),
158.2 (2C), 158.0 (2C), 78.9 (2CH), 75.4 (2CH), 74.4 (2CH), 51.5 (2CH3), 48.9 (2CH), 45.4 (2C), 42.7
(2CH), 41.4 (2CH), 38.3 (2CH), 37.1 (2CH2), 35.8 (2CH2), 34.7 (2CH2), 34.2 (2CH), 31.9 (2CH2),
31.2 (2CH2), 30.7 (2CH2), 30.4 (2CH2), 30.0 (2CH2), 29.3 (2C), 28.8 (2CH), 22.7 (2CH2), 21.6
(2CH3), 17.6 (2CH3), 14.1 (2CH3), 12.0 (2CH3) ppm; IR (CHCl3): ꢃꢁ¼ 1758, 1730, 1315, 1251,
1186 cmꢁ1; MS (70 eV): m=z ¼ 1037 (Mþ þ H), 949, 609, 535, 519, 430, 370, 253.
Di[3ꢀ-acetoxy-7ꢀ,12ꢀ-dihydroxy-5ꢁ-cholanoic acid methyl ester]7,70-oxalate
(3, C56H86O14)
1H NMR (200 MHz, CDCl3): ꢂ ¼ 5.09 (m, 2 7ꢁ-H), 4.58 (m, 2 3ꢁ-H), 4.01 (m, 2 12ꢁ-H), 3.65 (s,
2OCH3), 1.96 (s, 2CH3CO), 0.98 (d, J ¼ 6.2 Hz, 2 21-CH3), 0.94 (s, 2 19-CH3), 0.71 (s, 2 18-CH3)
ppm.
Di[3ꢀ-acetoxy-7ꢀ,12ꢀ-dihydroxy-5ꢁ-cholanoic acid methyl ester]12,120-oxalate
(4, C56H86O14)
1H NMR (200 MHz, CDCl3): ꢂ ¼ 5.33 (m, 2 12ꢁ-H), 4.53 (m, 2 3ꢁ-H), 3.86 (m, 2 7ꢁ-H), 3.64 (s,
2OCH3), 2.01 (s, 2CH3CO), 0.92 (s, 2 19-CH3), 0.87 (d, J ¼ 6.1 Hz, 2 21-CH3), 0.78 (s, 2 18-CH3)
ppm.
Reactions of 3 and 4 with Oxalyl Chloride – Dimer 1
Dimers 2 and 3 were subjected to the reaction with oxalyl chloride according to the procedure
described above with two equivalents of the acylating agent. In both cases the dimer 1 was obtained
(elution with benzene=ethyl acetate 85=15) in 55–62% yields.
Di[3ꢀ-acetoxy-7ꢀ,12ꢀ-dihydroxy-5ꢁ-cholanoic acid methyl ester]7,70;12,120-
dioxalate (1, C58H84O16)
1
Colorless crystals, mp 240–243ꢂC (n-hexane=CH2Cl2); H NMR (200 MHz, CDCl3): ꢂ ¼ 5.41 (m, 2
12ꢁ-H), 5.17 (m, 2 7ꢁ-H), 4.56 (m, 2 3ꢁ-H), 3.64 (s, 2OCH3), 1.99 (s, 2CH3CO), 1.01 (s, 2 19-CH3),
0.81 (m: s, 2 18-CH3 and d, J ¼ 5.7 Hz, 2 21-CH3) ppm; 13C NMR (50 MHz, CDCl3): ꢂ ¼ 174.0 (2C),
170.2 (2C), 158.3 (2C), 156.9 (2C), 77.5 (2CH), 74.4 (2CH), 73.7 (2CH), 51.4 (2CH3), 48.3 (2CH),
45.3 (2C), 42.6 (2CH), 41.1 (2CH), 37.9 (2CH), 35.4 (2CH2), 34.7 (2CH2), 34.3 (2CH), 30.9 (2CH2),
30.8 (2CH2), 30.6 (2CH2), 29.6 (2C), 28.8 (2CH), 27.6 (2CH2), 26.3 (2CH2), 26.0 (2CH2), 22.8
(2CH3), 22.5 (2CH2), 21.1 (2CH3), 17.8 (2CH3), 11.9 (2CH3) ppm; IR (CHCl3): ꢃꢁ¼ 1758, 1728,
1314, 1258, 1186cmꢁ1; MS (70 eV): m=z ¼ 1037 (Mþ þ H), 1019, 964, 946, 610, 536, 430, 370, 253.

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Z. )otowski and D. Guzmanski: New Cyclic Dimers of Cholic Acid
Reduction of 1 with LiAlH4–BF3=Et2O – Dimer 6
Compound 1 (65 mg, 0.063mmol) was dissolved in 0.23cm3 of a solution of BF3=Et2O complex
(1.89 mmol) in 7 cm3 of anh. diethyl ether, and added dropwise to a suspension of 29mg of LiAlH4
(0.765 mmol) in 8 cm3 of anh. diethyl ether at 0ꢂC. After the addition was completed, the reaction was
stirred for 1 h at 0ꢂC and then refluxed for 1 h. Then the excess of reducing agent was carefully
quenched with a few drops of H2O, the precipitate was filtered off, and the reaction mixture was
subjected to column chromatography to afford 11mg of compound 6 (17%) (elution with benzene=
ethyl acetate 75=25).
(3ꢀ-Acetoxy-7ꢀ,12ꢀ-dihydroxy-5ꢁ-cholanoic acid methyl ester)(5ꢁ-cholane-
30ꢀ,70ꢀ,120ꢀ,240-tetraol 30-acetate)7,70;12,120-dioxalate (6, C57H84O15)
1H NMR (200 MHz, CDCl3): ꢂ ¼ 5.41 (m, 2 12ꢁ-H), 5.18 (m, 2 7ꢁ-H), 4.56 (m, 2 3ꢁ-H), 3.64 (s,
OCH3), 3.56 (t, J ¼ 6.1 Hz, CH2-OH), 2.00 (s, CH3CO), 1.99 (s, CH3CO), 1.02 (s, 2 19-CH3), 0.81 (m:
s, 2 18-CH3 and d, 2 21-CH3) ppm; IR (CHCl3): ꢃꢁ¼ 3523, 1757, 1729, 1315, 1251, 1186 cmꢁ1
.
Acknowledgments
We are grateful to Prof. J. W. Morzycki for discussions.
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