An efficient epimerization of biotin sulfone derivatives to 2-epi-biotin analogs

Article abstract of DOI:10.1016/j.tetlet.2007.03.129

Reduction of biotin sulfone derivatives leads to 2-epi-biotin analogs. The stereochemistry of the side chain at C₂ can be simply deduced from the diagnostic chemical shift pattern of the benzylic protons, rather than through the conventional an

Full text of DOI:10.1016/j.tetlet.2007.03.129

Tetrahedron Letters 48 (2007) 3685–3688
An efficient epimerization of biotin sulfone derivatives
to 2-epi-biotin analogs
Kyungsoo Oh*
Department of Chemistry and Chemical Biology, Indiana University Purdue University Indianapolis, Indianapolis, IN 46202, USA
Received 1 March 2007; revised 20 March 2007; accepted 21 March 2007
Available online 25 March 2007
Abstract—Reduction of biotin sulfone derivatives leads to 2-epi-biotin analogs. The stereochemistry of the side chain at C2 can be
simply deduced from the diagnostic chemical shift pattern of the benzylic protons, rather than through the conventional analysis of
the coupling constants using the empirical Karplus equation.
Ó 2007 Elsevier Ltd. All rights reserved.
Biotin (1), also known as a vitamin H, is a water-soluble
biocatalyst, which participates in the reversible fixation
of carbon dioxide in the biosynthesis of organic mole-
cules.1 Its early discovery from egg yolk followed by iso-
lation from beef liver and milk concentrates initiated a
considerable effort toward the understanding of its
important biological roles in human nutrition and ani-
mal health.2 Moreover, the application of the biotin–
(strept)avidin system has provided a useful tool for iden-
tification and purification of the protein complement of
cells in the genomic and post-genomic era.3 Since there
are only a few naturally occurring proteins that bind
to biotin, the cross-contamination during the protein
purification using biotinylation is quite rare. One of
the difficulties in protein purification using biotinylation
is due to the fact that biotin has the strongest non-cova-
lent interaction known in nature with avidin (Kd =
10ꢀ15 M). This extremely high affinity of biotin for avi-
din raised some concern upon the recovery of protein–
receptor covalent complexes. Several approaches have
been tested to facilitate the release of biotinylated tar-
gets from (strept)avidin complexes, however use of bio-
tin derivatives that possess decreased affinity for
(strept)avidin has not been extensively exploited due to
the lack of a general synthetic approach to biotin ana-
logs.4 To facilitate the recovery of the biotinylated target
from (strept)avidin complexes, we have focused on two
variables in biotin analog structures; (1) the oxidation
state of the sulfur,5 and (2) the stereochemistry of the
valeryl chain. Herein, we report our preliminary findings
regarding an efficient epimerization of the valeryl side
chain of biotin derivatives through reduction of biotin
sulfone derivatives.6
O
HN
NH
OH
S
O
(+)-Biotin (1)
Commercially available (+)-biotin (1) was oxidized to
biotin sulfone (2) in excellent yield using H2O2 in acetic
acid7 or in the presence of a large excess of OxoneÒ in
methanol and water.8 The resulting biotin sulfone (2)
was first globally protected as the benzyl derivative 3.
The benzyl ester moiety in 3 was then reduced using
lithium trimethoxy aluminum hydride6a and the subse-
quent reduction of sulfone alcohol 4 using lithium alu-
minum hydride gave alcohol 5 in modest yield.
Alternatively, sulfone ester 3 was reduced directly to
thiacyclopentane alcohol 5 in excellent yield.6 To the
best of our knowledge, there is no precedence for the
epimerization of substituted thiacyclopentane dioxides
by reduction protocols.9 Thus, we initially assigned the
stereochemistry of the C2 substituent of 5 based on anal-
ogous biotin derivatives.10 The coupling constant
1
between H2 and H3 in the H NMR spectrum of 5 was
5.7 Hz, which was in agreement with similar structures of
biotin derivatives reported by Bates and Rosenblum.11
However, there was some discrepancy in the chemical
*
Tel.: +1 317 278 7531; fax: +1 317 274 4701; e-mail: oh@chem.
iupui.edu
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.03.129

3686
K. Oh / Tetrahedron Letters 48 (2007) 3685–3688
O
O
H2O2, AcOH
O
NaH, DMF
25 C
°
HN
NH
HN
NH
O
BnN
O
NBn
O
O
or
O
O
BnBr
25 °C
50-60 %
Oxone®
MeOH/H2O
OH
S
OH
OBn
S
S
4
4
4
O
25 C
°
1
2
3
> 95%
Et2O/THF THF
0 °C 0 °C
> 90% < 50%
LiAlH4
NBn
LiAlH(OMe)3
O
O
LiAlH4
Et2O
BnN
BnN
O
NBn
4
3
5
2
OH 0 °C
OH
S
S
4
4
55%
O
5
4
Scheme 1. Synthesis of 5 from biotin sulfone 2.
shifts of H2, H5exo, and H5endo of 5 when compared to
the N,N-benzyl protected biotin derivatives 911a and
1111b (see Supplementary data) (Scheme 1).
application of the empirical generalized Karplus equa-
tion.11 In biotin, the dihedral angle, calculated from
the generalized Karplus equation, between the cis pro-
tons H2–H3 is approximately 45°, while the dihedral
angle from the literature X-ray crystallographic data
In order to confirm the structure of 5, an alternative syn-
thetic sequence was pursued. Global protection of (+)-
biotin (1) and the subsequent reduction of the ester moi-
ety in 6 provided alcohol 7. To our surprise, the NMR
spectrum of the resulting biotin alcohol 7 did not match
with that of 5. The coupling constant between H2 and
H3 in 7 was 5.6 Hz, however the chemical shifts of H2,
H5exo, and H5endo in 7 were strikingly similar to those
of the known biotin derivatives 911a and 1111b with the
correct C2 stereochemistry. We hypothesized that an
epimerization was occurring during the reduction of sul-
fone 4. This was confirmed by oxidation of sulfide alco-
hol 7 using OxoneÒ, which delivered the identical
sulfone alcohol 4. Finally reduction of biotin sulfone
alcohol 4 using LiAlH4 in diethyl ether once again gave
alcohol 5 (Scheme 2).
3
3
3
is 54°. Furthermore, the J2,3, J4,5endo, and J4,5exo cou-
pling constants revealed that in all cases cis coupling
constants were at least 1.5 Hz larger than the corre-
sponding trans coupling constants. Thus, the trans cou-
pling constants for 8–11 ranged from 1.66 to 4.56 Hz,
while the cis coupling constants ranged from 4.64 to
3
6.08 Hz with an exception of J3,4 (9.12–9.59 Hz). The
preferred conformation of 5 is predicted as a twist-en-
velop with C2 out of the plane formed by C3, C4,
and C5 to minimize eclipsing interactions between the
3
C2exo substituent, H3, and H4. Therefore, our J2,3 cou-
pling constant (5.6 Hz) of 5 initially led to the misinter-
pretation of the stereochemistry of 5. The coupling
constants based upon the empirical Karplus equation
as a sole determinant of structural analysis of the hexa-
hydrothioimidazolone ring thus should be interpreted
with considerable caution where the Cs symmetrical
envelope conformation of 5 is distorted to accommo-
The relative stereochemistry of substitution in hexa-
hydrothienoimidazolones could be determined by
O
O
O
LiAlH4
HN
NH
NaH, DMF BnN
BnBr
NBn
BnN
NBn
O
O
THF
OH
OBn
OH
S
S
S
25
C
0-25
53%
C
°
°
4
4
4
77%
1
6
7
O
O
LiAlH4
Et2O
Oxone
BnN
O
NBn
O
BnN
NBn
MeOH/H2O
25
90%
C
OH
OH
°
S
S
0
C
°
4
4
50-60%
4
5
Scheme 2. Synthesis of 7 from biotin benzyl ester 6.

K. Oh / Tetrahedron Letters 48 (2007) 3685–3688
3687
O
O
O
BnN
m-CPBA
NBn
BnN
NBn
BnN
NBn
CH2Cl2
-78
OH
OH
OH
S
S
S
C
°
4
4
4
O
O
7
β−12 58 %
α−12 16 %
LiAlH4
THF
0-25
70%
C
°
O
O
O
LiAlH4
BnN
NBn
Oxone BnN
MeOH/H2O
NBn
BnN
NBn
O
O
THF
OBn
OBn
S
OH
S
25 C
S
0-25
50-60%
C
°
°
4
4
4
100 %
7
3
O
13
Scheme 3. Synthesis of 7 by reduction of sulfoxides.
date the eclipsing interactions between the substituent,
H4, and the side chain.10
biotin sulfone derivatives upon reduction is unique to
the sulfone moiety, as the related sulfoxides 12 and 13,
prepared either using m-CPBA or OxoneÒ, do not lead
to the epimerization at C2 (Scheme 3). A selective oxida-
tion of alcohol moiety in 5 and 7 to carboxylic acid
would furnish N,N-benzyl biotin derivatives,14 which
upon the known debenzylation conditions lead to bio-
tin15 and 2-epi-biotin.
In contrast to coupling constant analysis, the NMR
interpretation based upon the chemical shift patterns is
more straightforward in the determination of the stereo-
chemistry of the side chain of the tetrahydrothiophene
ring. As the previous NMR studies of biotin and related
hexahydrothienoimidazolone derivatives showed, the
chemical shifts of H2, H3, H4, H5endo, and H5exo in 8–
11 are relatively insensitive to the substitution at C2.
Although the chemical shift of H2 in 5 comes between
the H5endo and H5exo, this chemical shift pattern has been
ruled out as a signature of the stereochemistry at C2.
The diagnostics of the stereochemistry of the side chain
at C2 comes from, as observed, the analysis of benzylic
In conclusion, we demonstrated that the reduction of
biotin sulfone derivatives leads to 2-epi-biotin analogs
and the stereochemistry of the side chain at C2 can be
deduced from the diagnostic chemical shift patterns of
the benzylic protons, rather than by analysis of the cou-
pling constants using the empirical Karplus equation.
The binding affinity assay of the prepared sulfone, sulf-
oxides, and 2-epi-biotin derivatives to strept(avidin) is
currently underway in our laboratory and our results
will be reported in due course.
0
0
protons Ha and Hb (or Ha and Hb ) due to the inherent
asymmetry of the hexahydrothienoimidazolone and pre-
ferred conformation of the benzyl groups. In com-
pounds 5, 8,11a and 10,11b which have a C2exo
substituent, the difference in chemical shift between Ha
0
0
and Ha (or Hb and Hb ) is small (see Supplementary
Acknowledgements
data). However, in 7, 9,11a and 1111b the chemical shift
0
difference between Hb and Hb is greater than that
The School of Science at IUPUI is acknowledged for
start-up. This investigation was partially supported
through the Research Support Funds Grant (RSFG-
IUPUI).
0
between Ha and Ha because the magnetic dissimilarity
between Hb and Hb is enhanced by the proximity of
0
Hb to the C2endo substituent (see also NMR spectra of
6 in Supplementary data). Therefore, the stereochemis-
try at C2 of the hexahydrothienoimidazolone can be
determined through the diagnostic chemical shift pat-
tern of the benzylic protons,12 not through the analysis
of the coupling constant using the empirical Karplus
equation.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.03.129.
A facile epimerization at C2 of the biotin sulfone deriv-
atives upon reduction using LiAlH4 implies a possible
reduction mode of the sulfone moiety. Although the de-
tailed mechanism of the epimerization needs further
investigation,13 a plausible intermediate such as an a-
monoanion or an a,a0-dianion of the sulfone could be
envisioned.9 The epimerization observed at C2 of the
References and notes
1. Uskokovic, M. R. In Encyclopedia of Chemical Techno-
logy, 3rd ed.; Kirk, R. E., Othmer, D. E., Eds.; Wiley:
New York, 1984; Vol. 24, p 41.

3688
K. Oh / Tetrahedron Letters 48 (2007) 3685–3688
2. For an excellent review, see: De Clercq, P. J. Chem. Rev.
(personal communication), see: Han, Q.; Lafontaine, J.;
Bacheler, L. T.; Rayner, M. M.; Klabe, R. M.; Erickson-
Viitanen, S.; Lam, P. Y. S. Bioorg. Med. Chem. Lett. 1996,
12, 1371.
1997, 97, 1755.
3. For examples, see: (a) van Werven, F. J.; Timmers, H. T.
Nucleic Acids Res. 2006, 34, e33; (b) Nguyen, G. H.;
Milea, J. S.; Rai, A.; Smith, C. L. Biomol. Eng. 2005, 22,
147; (c) Chen, I.; Ting, A. Y. Curr. Opin. Biotechnol. 2005,
16, 35; (d) de Boer, E.; Rodriguez, P.; Bonte, E.;
Krijgsveld, J.; Katsantoni, E.; Heck, A.; Grosveld, F.;
Strouboulis, J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100,
7480, and references cited therein.
11. (a) Bates, H. A.; Rosenblum, S. B. J. Org. Chem. 1986, 51,
3441; (b) Bates, H. A.; Rosenblum, S. B. Tetrahedron
1985, 41, 2331, and references cited therein.
12. For additional NMR data of 2-epi-biotin derivatives, see:
(a) Chavan, S. P.; Chittiboyina, A. G.; Ramakrishna, G.;
Tejwani, R. B.; Ravindranathan, T.; Kamat, S. K.; Rai,
B.; Sivadasan, L.; Balakrishnan, K.; Ramalingam, S.;
Deshpande, V. H. Tetrahedron 2005, 61, 9273; (b) Chen,
F.-E.; Chen, X.-X.; Dai, H.-F.; Kuang, Y.-Y.; Xie, B.;
Zhao, J.-F. Adv. Synth. Catal. 2005, 347, 549; (c)
Moolenaar, M. J.; Speckamp, W. N.; Hiemstra, H.;
Poetsch, E.; Casutt, M. Angew. Chem., Int. Ed. Engl.
1995, 34, 2391; (d) Turos, E.; Parvez, M.; Garigipati, R.
S.; Weinreb, S. M. J. Org. Chem. 1988, 53, 1116.
13. Although we observed a partial epimerization of biotin
sulfone using 1 N NaOH, decomposition was a major
pathway upon a prolong exposure of other biotin sulfone
derivatives under basic conditions, see: Sachon, E.; Tas-
seau, O.; Lavielle, S.; Sagan, S.; Bolbach, G. Anal. Chem.
2003, 75, 6536.
4. Desarnaud, F.; Marie, J.; Larguier, R.; Lombard, C.;
Jard, S.; Bonnafous, J.-C. J. Chromatogr. 1992, 603, 95,
and references cited therein.
5. For the relative avidin binding affinity of biotin and
metabolites, see: (a) Zempleni, J.; Mock, D. M. J. Nutr.
1999, 129, 494S; (b) Finn, F. M.; Yamanouchi, K.; Titus,
G.; Hofmann, K. Bioorg. Chem. 1995, 23, 152.
6. For reduction of biotin sulfone analogs and related
examples, see: (a) Bates, H. A.; Smilowitz, L.; Lin, J. J.
Org. Chem. 1985, 50, 899; (b) Kotake, H.; Inomata, K.;
Murata, Y.; Kinoshita, H.; Katsuragawa, M. Chem. Lett.
1976, 1073; (c) Martin, S. F.; Anderson, B. G.; Daniel, D.;
Gaucher, A. Tetrahedron 1997, 53, 8997.
7. Sachon, E.; Tasseau, O.; Lavielle, S.; Sagan, S.; Bolbach,
G. Anal. Chem. 2003, 75, 6536.
8. Trost, B. M.; Curran, D. P. Tetrahedron Lett. 1981, 22,
1287.
14. For a leading reference of oxidation of alcohol to
carboxylic acid in the presence of sulfide moiety, see:
Goud, P. M.; Sheri, A.; Desai, P. V.; Watkins, E. B.;
Tekwani, B.; Sabnis, Y.; Gut, J.; Rosenthal, P. J.; Avery,
M. A. Med. Chem. Res. 2005, 14, 74.
9. Webber, W. P.; Stromquist, P.; Ito, T. I. Tetrahedron Lett.
1974, 15, 2595.
10. Similar N,N0-m-bromobenzyl derivatives are known, but
No NMR data was available for direct comparison
15. Chavan, S. P.; Tejwani, R.; Ravindranathan, T. J. Org.
Chem. 2001, 66, 6197.