Synthesis and characterization of novel bile acids derived H-phosphonates conjugates

Article abstract of DOI:10.1080/10426500701807533

The H-phosphnates of bile acid, conjugated with several alcohols and nucleosides, were synthesized in one pot by a tandem transesterification with diphenyl phosphate. The structures of these novel bile acid conjugates were confirmed by IR, 31P NMR, 1H NMR and HRMS. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500701807533

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Phosphorus, Sulfur, and Silicon
and the Related Elements
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Synthesis and Characterization
of Novel Bile Acids Derived H-
Phosphonates Conjugates
Yan Li a , Yong Ju a & Yufen Zhao a
a Key Laboratory of Bioorganic Phosphorus Chemistry
and Chemical Biology, Ministry of Education,
Department of Chemistry , Tsinghua University ,
Beijing, P. R. China
Published online: 20 Jun 2008.
To cite this article: Yan Li , Yong Ju & Yufen Zhao (2008) Synthesis and
Characterization of Novel Bile Acids Derived H-Phosphonates Conjugates,
Phosphorus, Sulfur, and Silicon and the Related Elements, 183:2-3, 706-711, DOI:
10.1080/10426500701807533
To link to this article: http://dx.doi.org/10.1080/10426500701807533
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Phosphorus, Sulfur, and Silicon, 183:706–711, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701807533
Synthesis and Characterization of Novel Bile Acids
Derived H-Phosphonates Conjugates
Yan Li, Yong Ju, and Yufen Zhao
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical
Biology, Ministry of Education, Department of Chemistry, Tsinghua
University, Beijing, P. R. China
The H-phosphnates of bile acid, conjugated with several alcohols and nucleosides,
were synthesized in one pot by a tandem transesterification with diphenyl phos-
phate. The structures of these novel bile acid conjugates were confirmed by IR, 31
P
NMR, 1H NMR and HRMS.
Keywords Bile acid; H-phosphonates; transesterification
INTRODUCTION
Bile acids are natural origin compounds and play a very important role
in biological systems.1 The medicinal applications of bile acids and their
conjugates were reported in numerous literature,2 such as antiviral
and the antifungal properties,3 absorption enhancersm4 and carriers
of liver-specific drugs.5 Their biological importance, the amphiphilic
properties, the rigid concave of the backbone, and the unique disposition
of hydroxyl groups make these compounds attracting building blocks
for the design of synthetic receptors.5
Cholic acid 1 is one of the most commonly occuring bile acids
(Scheme 1).2 It possesses one carboxylic acid and three hydroxyl groups,
which renders various chemical modifications possible. However, one of
the major problems associated with the synthesis of cholic acid deriva-
tives is that the “natural” hydroxyl groups is relatively inert and dif-
ficult to derivatise, especially the 7α- and 12α-OH.6 Several studies
have reported the conversion of the hydroxyl groups to other functional
components, which result in novel amphiphilic structures with new
properties.6–8
Project supported by the NSF of China (No. 20772071) and SRFDP of Higher Educa-
tion (No. 20050003104).
Address correspondence to Yong Ju, Key Laboratory of Bioorganic Phosphorus Chem-
istry and Chemical Biology, Department of Chemistry, Tsinghua University, Beijing
100084, China. E-mail: juyong@tsinghua.edu.cn
706

Novel Bile Acids Derived H-Phosphonates Conjugates
707
In this respect, we report the preparation of bile acid–derived poly-
H-phosphonates. H-Phosphonate has proven to be a versatile building
block in organic syntheses, which facilitate the conversion of hydroxyl
groups into multifunctional components. It can be further trans-
ferred to phosphates, phosphorothionates, and phosphoroselenoates by
oxidation9 and phosphoramidate conjugates by Atherton-Todd reaction
in high yield.10 Previously, we have reported the synthesis of many
bioconjugates with diphenyl phosphite 2 (DPP) as a phosphorylation
reagent.9–11 DPP can undergo fast transesterification with various al-
cohols in pyridine. In order to obtain mono-phenyl H-phosphonates,
a large access of DPP (7 eqiv.) was required to prevent the forma-
tion of double-exchange H-phosphonates.11 However, if one of the hy-
droxyl components is sterically hindered, the desired mono-phenyl
H-phosphonate could be obtained in high yield with equal amounts
of DPP.12 The hydroxyls in the bile acids are oriented along one face
and are typically steric hindered groups. Taking account of these
facts, bile acids are ideal starting templates that result in a novel
multifunctional scaffold containing phosphorus. It is worth noting
that a few H-phosphonates and phosphoramides derivatives, such as
carbohydrates13 and nucleic acids,12 have been fully studied. To the
best of our knowledge, however, there is little information about the
synthesis and properties of steroid H-phosphonates conjugates.
SCHEME 1 The chemical structure of cholic acid and diphenyl.
RESULTS AND DISCUSSION
The general synthetic strategy for bile acid H-phosphonates conjugates
was shown in Schemes 2 and 3. Our approach starts from methyl cholate
because the methyl esters can be prepared easily in large quantities.
However, many other bile acid esters and amides are also appropriate
starting materials. The reaction was monitored by ESI-MS and 31P
NMR. As is shown in Scheme 2, the first step of transesterification is fast
and the all the hydroxyl groups were almost quantitatively converted to
mono-phenyl H-phosphonates 4 after 5 h under anhydrous condition.
The second step of transesterification is relatively slow and sensitive

708

SCHEME 3 The synthesis and the chemical structure of deoxycholic acid
derived.
to steric hindrance, thereby preventing the formation of disteroid H-
phosphonate.
The deoxycholic acid–derived H-phosphonate conjugates were syn-
thesized in the similar procedure. It is worth mentioning that bile
acids are interesting drug carriers that target a specific organ and im-
prove the lipophilicity of drug molecules.2 The bioactive nucleosides
such as 3ꢀ-Azido-3ꢀ-deoxythymidine (AZT) and 2ꢀ, 3ꢀ-didehydro-2ꢀ, 3ꢀ-
dideoxythymidine (d4T), can be attached to deoxycholic methyl ester us-
ing this method. These molecules contain both steroid and nucleosides
and have the potential to act as prodrugs or drugs by themselves.14
However, the three bulky nucleoside moieties cannot be attached to
the steroid nucleus simultaneously due to the large steric hindrance of
cholic acid. When the same reaction was applied to cholic acid methyl
ester, it only results in a complex mixture that is unlikely to be purified
on a column chromatography of silica gel to obtain the pure product.
The structures of all these new H-Phosphonates and phosphoramide
conjugates of bile acids were confirmed by IR, 31P NMR, 1H NMR,
and HRMS. The NMR spectra shown in Figure 1 clearly present the
shifts of protons among these cholic acid derivatives. In the downfield
of 1H NMR spectra, the three protons that were directly connected with
1
phosphorus are easily confirmed by the large JP-H coupling constant
(700 Hz approximately).
In conclusion, bile acids, which possess up to three sterically hinder
hydroxyl groups, have provided a unique structure for the synthesis
of poly-H-phosphonates derivatives. The extension of this methodology
may find applications in pharmacology and supramolecular chemistry.
EXPERIMENTAL
Cholic acid, deoxycholic acid, and diphenyl phosphite (DPP) were pur-
chased from Sigma Company and used without further purification.
709

710
Y. Li et al.
1
FIGURE 1 Comparison of the 600 MHz H NMR spectra of compounds 3, 5,
and 6 in CDCl3.
Cholic acid methyl ester (CAME, 3) and deoxycholic acid methyl es-
ter (DCAME, 7) were synthesized by refluxing appropriate bile acid
in methanol catalyzed by hydrochloric acid and followed by a recrys-
tallization, which was adapted to a reported procedure.15 Pridine was
purchased from Beijing Chemicals Co. and redistilled prior to use. Typi-
cal procedure for the synthesis and the spectrum data of bile acid dimers
are represented by conjugate 5.
Preparation of Methyl 3α, 7α,
12α-tri(propargy-H-phosphonate)-5β-cholan-24-ate (5)
CAME (254 mg, 0.6 mmol) was dissolved in pyridine 5 ml. Under
an argon atmosphere, the solution was added slowly to DPP (0.534 g,
2.0 mmol) in 4 ml dry pyridine and cooled in an ice bath. The mixture
was then stirred at room temperature for 5 h and the mono-phenyl bile
acid triphosphite was obtained. Then the propargyl alcohol (0.129 ml,
2.2 mmol) was added and was stirred for additional 7 h. The obtained
residue was purified on a column (silica gel; CH2Cl2: AcOEt = 8:3) to
provide pure tri-H-phosphonates (244 mg, 0.34 mmol, light-yellow oil)
in 56% yield. R f : 0.5 (CH2Cl2:CH3OH=20:1); IR (cm–1): 3448.5, 3293.2,
3227.9, 2946.7, 2946.4, 2872.6, 2434.4, 2125.1, 1735.2, 1251.2, 977.3;
1
31P NMR (81 MHz, CDCl3): δ = 6.92, 7.25, 7.99, 8.08; H NMR (600
MHz, CDCl3): δ = 0.67 (s, 3H, 18–CH3), 0.85 (s, 3H, 19–CH3), 0.91 (d,
3H, 21–CH3), 0.98–1.97 (m, 20H, aliphatic H), 2.14–2.32 (m, 4H, 2,4–
CH2), 2.56–2.64 (m, 3H, 3 × ≡ C–H), 3.60 (s, 3H, OCH3), 4.26 (m, 1H,

Novel Bile Acids Derived H-Phosphonates Conjugates
711
3α–CH), 4.59–4.68 (m, 7H, 3×–CH2C ≡ CH, 7α–CH), 4.82 (m, 1H, 12α–
CH), 6.86 (d, 1H, J = 710 Hz, 3ꢀ–PH), 6.91, 6.93*(d, 1H, J = 710Hz,
7ꢀ–PH), 6.96, 6.99* (d, 1H, J = 710 Hz, 12ꢀ–PH); HRMS (ESI) found:
946.2994, [C34H51O11P3+NH4]+ calcd: 946.2982.
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