Synthesis of function-oriented 2-phenyl-2H-chromene derivatives using l-pipecolinic acid and substituted guanidine organocatalysts

Article abstract of DOI:10.1016/j.tetlet.2010.02.143

Organocatalytic domino oxa-Michael/aldol reactions between salicylaldehyde with electron deficient olefins are presented. We screened guanidine, 1,1,3,3-tetramethylguanidine (TMG) and l-pipecolinic acid as organocatalysts for this transformation. 3-Substi

Full text of DOI:10.1016/j.tetlet.2010.02.143

Tetrahedron Letters 51 (2010) 2567–2570
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of function-oriented 2-phenyl-2H-chromene derivatives
using L-pipecolinic acid and substituted guanidine organocatalysts
b
b
b
Bhaskar C. Das a,b, , Seetaram Mohapatra , Philip D. Campbell , Sabita Nayak ,
*
Sakkarapalayam M. Mahalingam b, Todd Evans c,
*
a Department of Nuclear Medicine, Albert Einstein College of Medicine, Bronx, NY 10461, USA
b Department of Developmental and Molecular Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA
c Department of Surgery, Weill Cornell Medical College, Cornell University, New York, NY 10065, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Organocatalytic domino oxa-Michael/aldol reactions between salicylaldehyde with electron deficient ole-
Received 7 December 2009
Revised 22 February 2010
Accepted 22 February 2010
Available online 4 March 2010
fins are presented. We screened guanidine, 1,1,3,3-tetramethylguanidine (TMG) and
organocatalysts for this transformation. 3-Substituted 2-phenyl-2H-chromene derivatives are synthe-
sized with high yields and with poor enantioselectivity (5–17% ee) using -pipecolinic acid while TMG
works well with cinnamaldehyde without using co-catalyst. These 3-substituted-2-phenyl-2H-chromene
derivatives are further derivatized to synthesize triazole and biotin-containing chromene derivatives, to
facilitate purification of protein targets.
L-pipecolinic acid as
L
Ó 2010 Elsevier Ltd. All rights reserved.
For our ongoing chemical biology projects we are interested in
developing new compounds that interact with developmentally
important receptor-mediated pathways such as the TGF-b
pathway, acting as antagonist or agonist.1,2 For this purpose, we
described a chemical genetic approach by testing a library of 2-
substituted-2H-chromene derivatives for their bioactivity on
developing zebrafish embryos. The zebrafish embryo provides an
ideal vertebrate model system for in vivo small molecule screens
because of its optical transparency, accessibility during embryonic
development, and permeability to small molecules. We synthe-
sized a small library of 2-substituted-2H-chromene derivatives
that were screened for bioactivity using the zebrafish model.3
Compound BT7 (1) (Fig. 1) was found to modulate a specific rele-
vant pathway, namely p-SAPK/JNK, which is known to be down-
stream of TGF-b, and can mediate Smad-independent signaling.3
Our next aim is to isolate BT7-interacting protein(s) by attach-
ing a biotin moiety, which would facilitate purification of the pro-
tein target.
For this purpose, we sought to develop function-oriented BT7
analogues and, finally, to attach biotin using functional group
transformations. Domino or cascade reactions that involve the for-
mation of multiple stereo centers comprise one-pot a rapidly
growing research field with respect to the synthesis of small mol-
ecules with complex architectures.4 However, the development of
catalytic diastero- and enantioselective domino reactions is still a
challenging task.5a–c In this context, the development of organocat-
alytic asymmetric domino reactions has been pursued. Nitro-
chromenes are versatile synthetic intermediates in organic
synthesis owing to the various possible transformations of the ni-
tro group into other useful functional groups. Useful synthetic
methods reported for the preparation of 3-nitro- and 3-formyl-2-
phenyl-2H-chromene are the domino Michael/aldol reactions of
salicylaldehyde with b-nitrostyrene and cinnamaldehyde.5d–g Basi-
cally this involves L-proline and L-proline-based catalysts, along
with a co-catalyst used for the above-mentioned purpose. How-
ever, this gives moderate to good yields and poor enantioselectiv-
ity.6 Based on the development of these reactions and our research
interest of finding catalytic domino reactions that give our func-
tion-oriented BT7 analogues (2, 3, and 4), we envisioned a reaction
between differently substituted salicylaldehyde with b-nitrosty-
rene and cinnamaldehyde using simple catalyst like guanidine,
1,1,3,3-tetramethylguanidine (TMG)7 and pipecolinic acid. We
choose these catalysts to improve selectivity, yield, reaction
time, and to use an alternative protocol in which the absence of
co-catalyst can provide a better yield.8
Biotin
r
e
k
n
i
l
Cl
Cl
EWG
Cl
O
O
O
Cl
Cl
Cl
2: EWG = NO2
3: EWG = CHO
4: EWG = CN
1: BT-7
Linker : triazole/tetrazole/ester
* Corresponding authors. Tel.: +1 718 430 2422; fax: +1 718 430 8853.
E-mail addresses: bdas@aecom.yu.edu (B.C. Das), tre2003@med.cornell.edu
(T. Evans).
Figure 1.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.143

2568
B. C. Das et al. / Tetrahedron Letters 51 (2010) 2567–2570
Table 2
In an initial catalyst screen for the reaction between salicylalde-
Scope of the domino Michael/aldol reaction between various salicylaldehydes with E-
hyde 5a and b-nitrostyrene 6, we found to our delight that amines
7, 8, and 9 were catalysts for the domino Michael/aldol reaction. In
further experiments, other factors that influence the reaction were
thoroughly investigated such as solvent, catalyst loading, and reac-
tion temperature. In the beginning of our study b-nitrostyrene 6
and salicylaldehyde 5a were taken as the model substrates. The re-
sults are listed in Table 1. Catalyst 9 gave 81% yield of 2a at 80 °C in
toluene with poor enantioselectivity (5% ee).
After successfully optimizing the catalyst 9, solvent (toluene),
temperature (80 °C) and reaction time (24 h), we further explored
to generalize this reaction. For this purpose we used substituted
salicylaldehyde as substrates. Table 2 shows a number of examples
of this chemistry. Significant variation in the electronic and steric
b-nitrostyrene9 6a
NO2
R2
NO2
R2
CHO
OH
9
(20 mol%)
+
O
2
toluene, 80oC
60-80%, 24 h
R1
R1
6
5
Entry
1
Aldehyde (5a–e)
Product (2a–e)
NO2
Yieldb (%)
81
eec (%)
CHO
5
2
OH
O
Cl
CHO
OH
Cl
NO
NO
2
features of salicylaldehydes is tolerated for the L-pipecolinic acid
9 catalyzed cascade process. The reaction between 3,5-dichlorosal-
icylaldehyde 5b with b-nitrostyrene 6 in the presence of 20 mol %
2
3
4
5
76
79
80
60
O
O
O
O
Cl
Cl
Cl
CHO
OH
Cl
of L-pipecolinic acid 9 furnished our target BT7 nitro chromene
2
derivative 2b in 76% yield with poor enantioselectivity ee 2% (Table
2, entry 2). We then screened the 5-substituted salicylaldehydes,
5-chloro (5c), 5-bromo (5d) and b-nitrostyrene 6 under optimized
conditions, which gave the corresponding chromene derivatives 9c
and 9d in 79%, and 80% yields with poor enantioselectivity 17% and
18% ee, respectively (Table 2, entries 3 and 4). 5-hydroxy salicylal-
dehyde 5e gave the corresponding nitrochromene derivative in
moderate yield 60% with 13% ee (Table 2, entry 5).
The catalytic activity of 9 was further examined with the 2-ami-
no-benzaldehyde 10 and b-nitrostyrene 6 under the same reaction
conditions and gave the 3-nitro-2-phenyl-1,2-dihydro-quinoline10
11 in 85% yield and 26% ee (Scheme 1).
Having succeeded in synthesizing the BT7 nitrochromene deriv-
ative 2b, our attention was next focused on the attachment of bio-
tin. Yao and co-workers reported the [3+2] cycloaddition of 3-
nitro-2-phenyl-2H-chromene with sodium azide under catalyst
free conditions at 80 °C in DMSO.11 Following this procedure we
synthesized the BT7 triazole 12 by using 6,8-dichloro-3-nitro-2-
phenyl-2H-chromene 2b and sodium azide at 80 °C in DMSO in
82% yield. We next examined the feasibility of attaching biotin to
the BT7 triazole analogue 12 via triazole moiety using DCC under
various reaction conditions, however, we did not observe the ex-
pected product formation (Scheme 2).
17
16
13
Br
HO
CHO
OH
Br
HO
NO
2
CHO
OH
NO2
a
b
c
Reaction with 5 (1 mmol), 6 (1.2 mmol) and catalyst (20 mol %) in toluene.
Isolated yield of pure copmpound.
Determined by chiral-HPLC analyses using chiralpak AD.
NO2
NO2
CHO
NH2
9
(20 mol%)
+
N
H
toluene, 80oC
87 %
11
6
10
(26%, ee)
Scheme 1.
As we failed to make amide linkage of d-biotin with 12, we then
decided instead of acid–amine coupling, alkyne-azide click chemis-
N
NH
N
Cl
NO
2
D-Biotin, DCC
DMF, Et N
Cl
3
NaN
Table 1
3
O
Catalyst screen for the amine-catalyzed enantioselective domino reactions between
5a and 6
O
o
DMSO, 80 C
Cl
Cl
2b
12
NO
2
20 mol%
catalyst
NO
2
CHO
OH
Scheme 2.
+
O
solvent
2a
5a
6
try could be implemented. Keeping this idea in mind we planned to
synthesize 14. According to Scheme 3 and 14 could be synthesized
from Michael adduct 3.
NH
NH
NH
OH
N
H
N
N
H N
2
2
O
To synthesize compound 3, we screened the catalyst, 7, 8, 9,
7
8
9
and
hyde 13 as substrates. To our surprise, the reaction proceeded only
with 8 and gave 3 in 78% yield.12 After 72 h with
-proline only 10%
conversion was observed. Reaction of 5b with 13 in the presence of
-pipecolinic acid and 7 was carried out but this failed to proceed at
L-proline with 3,5-dichlorosalicylaldehyde 5b and cinnamalde-
Entry
Catalyst
Reaction condition
Yielda,b (%)
L
1
2
3
4
5
7
7
8
8
9
Toluene, rt, 5 d
Toluene, 80 °C, 48 h
Toluene, rt, 5 d
Toluene, 80 °C, 48 h
Toluene, 80°C, 24 h
11
68
30
75
L
all even after 72 h. It is very interesting to note that in the case of b-
nitrostyrene, all four catalysts gave 3-nitro-substituted chromene
derivatives in moderate to good yields, but in the case of cinnamal-
dehyde, only catalyst 8 (TMG) gave 3-formyl substituted chromene
derivatives. The detailed mechanism of this transformation is cur-
81 (5%, ee)b
a
Reaction with 5 (1 mmol), 6 (1.2 mmol) in 2 ml toluene, isolated yield after
column purified.
b
Determined by chiral-HPLC analyses using chiralpak AD.

B. C. Das et al. / Tetrahedron Letters 51 (2010) 2567–2570
2569
O
Cl
C
NH2
D-Biotin, EDC
DMF, cat. DMAP
8
1. Zn, CBr
Ph P, DCM
3
4
BH3:THF
Cl
Cl
CHO
OH
Cl
(20 mol%)
2b
H
NaBH4, rt- 70 0
O
rt, 12 h
Ph
2. nBuLi, THF,
O
3
Cl
CHO
18
0
Cl
Cl
13
toluene, 80 C
-78 C, 2 h
Cl
o
O
5b
HN
NH
N
N
Cl
H
N
CuI, DIPEA
N
Cl
S
O
14
Acetonitrile, rt
O
O
O
Cl
15
Cl
19
Scheme 3.
Scheme 5.
rently being investigated in our laboratory. Aldehyde 3 was further
derivatized to 14. Compound 3 was treated with triphenylphos-
phine, CBr4 in Zn, resulting in dibromo derivative, which was fur-
ther treated with n-BuLi in À78 °C to furnish alkyne 14.13 Before
doing click chemistry with d-biotin azide, we initially attempted
a model reaction, using simple benzyl azide in the presence of
CuI and DIPEA, as base and successfully achieved the triazole 15
(Scheme 3).14 After obtaining the triazole derivative we also
planned to synthesize ester and amide linkage of d-biotin. Thus,
the aldehyde 3 was reduced by sodium borohydride in methanol,
which produced the corresponding alcohol 16 in 92% yield.15
We then esterified this alcoholic derivative of BT7 16 with d-
biotin using DCC, DMAP in dichloromethane and produced the d-
biotin-attached BT7 analogue 17 in 74% yield (Scheme 4).15 After
synthesizing 17, we focused to synthesize d-biotin-attached amide
(Scheme 5).16 It is interesting to note that we tried with many
reducing agents (but not by enzymatic process) to reduce only ni-
tro group to amine keeping the vinylic double bond intact in the
compound 2b but failed. However, the nitro compound 2b was re-
duced with BH3/THF to form the amine 18 which was isolated by
crystallization using ether–hexane system in 82% yield. We ob-
served both double bond and nitro group were reduced.8e The
amine 18 was coupled with biotin using EDCI and DMAP in DMF
to give BT7 biotin amide 19, which was purified by crystallization
using ether and hexanes in 78% yield.16
able 2H-chromene and 1,2-dihydro-quinoline derivatives. We
successfully attached the biotin moiety to our lead compound
BT7. For the first time, we observed the catalytic activity of L-pip-
ecolinic acid to synthesize 3-substituted 2-phenyl chromene deriv-
atives with poor enantioselectivities (5–117% ee). We are in the
process of developing chiral L-pipecolinic acid-based and TMG-
based catalysts to improve activity, improve stereoselectivity in
various organic transformations and other enantioselective dom-
ino reactions. The detailed biological study of these compounds
as possible TGF-b pathway modulators is ongoing in our
laboratory.
Acknowledgments
The author B.C.D. is thankful to AECOM for start up funding. The
instrumentation in the AECOM Structural NMR Resource is sup-
ported by the Albert Einstein College of Medicine and in part by
Grants from the NSF (DBI9601607 and DBI0331934), the NIH
(RR017998) and the HHMI Research Resources for Biomedical Sci-
ences. We gratefully acknowledge the CRO laboratories inc. Dallas,
TX, USA for providing HPLC data. Authors are thankful to reviewer
for valuable suggestion.
After synthesizing of BT7 nitro 2b and aldehyde 3, we next
turned our attention to functionalize BT7-carbonitrile 4. For this
purpose the reaction between 5b and phenyl-acrylonitrile was
Supplementary data
Supplementary data (copies of 1H, 13C NMR and mass spectra)
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2010.02.143.
executed with catalysts 7, 8, 9, and L-proline, to our surprise in
all cases we failed to achieve the synthesis of compound 4. From
this observation, it might be possible that, Michael acceptors play-
ing crucial role in this domino oxa-Michael/aldol reaction, because
in the case of b-nitrostyrene, all four catalysts worked well, for cin-
References and notes
namaldehyde only catalyst 8 worked, for acrylonitrile, L-proline
worked (data not shown), but in the case of cinnamonitrile none
of the four catalysts worked.
1. MacRae, C. A.; Peterson, R. T. Chem. Biol. 2003, 10, 901–908.
2. Peterson, R. T.; Link, B. A.; Dowling, J. E.; Schreiber, S. L. Proc. Natl. Acad. Sci.
U.S.A. 2000, 97, 12965–12969.
3. Torregroza, I.; Evans, T.; Das, B. C. Chem. Biol. Drug. Des. 2009, 73, 339–345.
4. (a) Tietze, L. F.; Brasche, G.; Gericke, K. Domino Reactions in Organic Synthesis;
Wiley-VCH: Weinheim, 2006. p. 672; (b) Tietze, L. F. Chem. Rev. 1996, 96, 115;
(c) Nicolaou, K. C.; Edmonds, D. J.; Bulger, P. G. Angew. Chem., Int. Ed. 2006, 45,
7134; (d) Guo, H.; Ma, J. Angew. Chem., Int. Ed. 2006, 45, 354; (e) Pellieser, H.
Tetrahedron 2006, 62, 2143; (f) Ramon, D. J.; Yus, M. Angew. Chem., Int. Ed. 2005,
44, 1602; (g) Enders, D.; Grondal, C.; Hüttl, M. R. M. Angew. Chem., Int. Ed. 2007,
46, 1570–1581.
5. (a) Rios, R.; Sundén, H.; Vesely, J.; Zhao, G.-L.; Dziedzic, P.; Córdova, A. Adv.
Synth. Catal. 2007, 349, 1028; (b) Penon, O.; Carlone, A.; Mazzanati, A.; Locatelli,
M.; Sambri, L.; Bartoli, G.; Melchiorre, P. Chem. Eur. J. 2008, 14, 4788; (c) Enders,
D.; Wang, C.; Bats, D. W. Angew. Chem., Int. Ed. 2008, 47, 3579; (d) Luo, S.-P.; Li,
Z.-B.; Wang, L.-P.; Guo, Y.; Xia, A.-B.; Xu, D.-Q. Org. Biomol. Chem. 2009, 7, 4539–
4546; (e) Wang, W.; Li, H.; Wang, J.; Zu, L. J. Am. Chem. Soc. 2006, 128, 10354–
10355; (f) Li, H.; Wang, J.; E-Nunu, T.; Zu, L.; Jiang, W.; Wei, S.; Wang, W. Chem.
Commun. 2007, 507–509; (g) Sunden, H.; Rios, R.; Ibrahem, I.; Zhao, G-L.;
Eriksson, L.; Córdova, A. Adv. Synth. Catal. 2007, 349, 827–832; (h) Sundén, H.;
Ibrahem, I.; Zhao, G.-L.; Eriksson, L.; Córdova, A. Chem Eur. J 2007, 13, 574; (i)
Ibrahem, I.; Sundén, H.; Rios, R.; Zhao, G.-L.; Córdova, A. Chimia 2007, 61, 219.
6. (a) Xu, D.-Q.; Wang, Y.-F.; Luo, S.-P.; Zhang, S.; Zhong, A.-G.; Chen, H.; Xu, Z.-Y.
Adv. Synth. Catal. 2008, 350, 2610–2616; (b) Karthikeyan, T.; Sankararaman, S.
Tetrahedron: Asymmetry 2008, 19, 2741–2745.
In summary, we report an organocatalytic domino oxa-Michael/
aldol reaction that gives chromenes in high yields without using
co-catalyst and in less reaction time. Hence, the reaction consti-
tutes a simple catalytic high yield entry to pharmaceutically valu-
D-Biotin, DCC
Cl
NaBH4,
CH3OH, r.t., 2 h
OH
DCM, cat. DMAP
r.t., 12 h
3
O
Cl
16
O
O
HN
Cl
NH
O
S
O
Cl
17
Scheme 4.

2570
B. C. Das et al. / Tetrahedron Letters 51 (2010) 2567–2570
7. (a) Yanqing, P.; Gonghua, S.; Feifei, H. Monatsh. Chem. 2005, 136, 727–731; (b)
Nirmal, K. P.; Hitoshi, U.; Masahiro, T. Tetrahedron Lett. 2007, 48, 8700–8703;
(c) Junjie, H.; Yanfen, X.; Yingpeng, S.; Xuegong, S.; Xinfu, P. Catal. Commun.
2008, 9, 2077–2079.
13. Alami, M.; Peyrat, J.-F.; Belachmi, L.; Brion, J.-D. Eur. J. Org. Chem. 2001, 4207–
4212.
14. Hotha, S.; Kashyap, S. J. Org. Chem. 2006, 71, 364–367.
15. Synthesis of biotin attached BT7 analogue 17: To a solution of 6, 8-dichloro-2-
8. (a) Carrie, E. A.; Melissa, M. V.; Scott, J. M. Org. Lett. 2005, 7, 3849–3851; (b)
Zhouyu, W.; Xiaoxia, Y.; Siyu, W.; Pengcheng, W.; Anjiang, Z.; Jian, S. Org. Lett.
2006, 8, 999–1001; (c) Fatome, M.; Andrieu, L.; Laval, J. D.; Royer, R. L. Eur. J.
Med. Chem. 1976, 11, 81–82; (d) Rene, L.; Royer, R. Eur. J. Med. Chem. 1975, 10,
72–78; (e) Varma, R. S.; Kadkhodayan, M.; Kabalka, G. W. Heterocycles 1986, 24,
1647–1652.
phenyl-2H-chromene-3-carbaldehyde 3 (1 mmol) in methanol was added
sodium borohydride (2 mmol). The mixture was then stirred at room
temperature for 2 h and the reaction completion was indicated by TLC. After
evaporation of the solvent, water was added and the product was extracted
using ethyl acetate (3 Â 10 mL). The extracts were washed with H2O
(2 Â 10 mL) and brine, then dried over Na2SO4, and evaporated. The residue
was purified by flash column chromatography using hexanes/ethyl acetate
(8:2) as the eluent to give the alcohol 16 as an oil. Compound 16 was directly
used for next step. To a stirred solution of d-biotin in DCM was added DCC
under nitrogen atmosphere and stirred for 2 h at room temperature. To this
stirred solution was added alcohol 16 in DCM and stirring continued for over
night at room temperature under nitrogen atmosphere. After evaporation of
the solvent, the product 17 was purified by silica gel chromatography
(hexanes/ethyl acetate). 74% yield, 1H NMR (300 MHz, CDCl3): d 7.95 (s, 1H),
7.35–7.38 (m, 5H), 7.31–7.34 (m, 1H), 6.82 (s, 1H), 6.43 (s, 1H), 6.37 (s, 1H),
6.10 (s, 1H), 4.59 (s, 2H), 4.25–4.32 (m, 1H), 4.12–4.18 (m, 1H), 3.04–3.13 (m,
1H), 2.78–2.87 (m, 1H), 2.56–2.60 (d, 1H, J = 12 Hz), 2.18–2.25 (t, 2H, J = 6 Hz),
1.43–1.48 (m, 4H), 1.27–1.29 (m, 2H); 13C NMR (75 MHz, CDCl3): d 173.2,
163.5, 163.2, 147.0, 138.4, 134.1, 130.0, 129.6, 128.3, 126.0, 124.8, 121.6, 120.9,
78.2, 63.8, 56.2, 48.3, 36.6, 33.8, 31.6, 28.8, 25.3, 25.0. ESI MS: [M+H] 533.1055,
calcd 533.0990 for C26H26Cl2N2O4S.
9. General procedure for synthesis 3-nitro-2-phenyl-2H-chromenes: A mixture of
salicylaldehyde (1 mmol) and b-nitrostyrene (1.2 mmol) in dry toluene under
nitrogen atmosphere was added 20 mol %
L-pipecolinic acid at room
temperature. The reaction mixture was stirred at 80 °C under nitrogen
atmosphere for 24 h. The reaction was quenched with saturated NH4Cl and
extracted with ethyl acetate (3 Â 10 mL). The extracts were washed with H2O
and brine, then dried over Na2SO4, and evaporated. The residue was purified by
flash column chromatography using hexanes/ethyl acetate (8:1) as the eluent
to give 3-nitro-2-phenyl-2H-chromenes. The enantiomeric excess was
determined by HPLC with chiralpak AD column.
10. (a) Ballini, R.; Bosica, G.; Fiorini, D.; Palmieri, A. Green Chem. 2005, 7, 825–827;
(b) Yan, M.-C.; Tu, Z.; Lin, C.; Ko, S.; Hsu, J.; Yao, C.-F. J. Org. Chem. 2004, 69,
1565–1570.
11. Pateliya, M. H.; Rama Raju, B.; Kavala, V.; Kuo, C.-W.; Yao, C.-F. Tetrahedron
2009, 65, 5799–5804.
12. Synthesis of 6,8-dichloro-2-phenyl-2H-chromene-3-carbaldehyde (3): A mixture
of 3,5-dichloro salicylaldehyde (1 mmol) and cinnamaldehyde (1.2 mmol) in
dry toluene under nitrogen atmosphere was added 20 mol % 1,1,3,3-
tetramethylguanidine at room temperature. The reaction mixture was stirred
at 80 °C for 48 h under nitrogen atmosphere till the complete disappearance of
starting material monitored by TLC. The reaction was quenched with saturated
NH4Cl and extracted with ethyl acetate (3 Â 10 mL). The extracts were washed
with H2O (2 Â 10 mL) and brine, then dried over Na2SO4, and evaporated. The
residue was purified by flash column chromatography using hexanes/ethyl
acetate (8:1) as the eluent to give 6,8-dichloro-2-phenyl-2H-chromene-3-
carbaldehyde 3. 78% yield, 1H NMR (300 MHz, CDCl3): d 9.75 (s, 1H), 7.35–7.39
(m, 2H), 7.41–7.28 (m, 5H), 7.17–7.18 (d, 1H, J = 3 Hz), 6.49 (s, 1H); 13C NMR
(75 MHz, CDCl3): d 189.9, 150.0, 139.4, 138.2, 136.1, 133.3, 129.4, 129.1, 127.4,
126.8, 124.0, 122.3, 75.1.
16. Synthesis of biotin attached BT7 analogue 19: To a stirred solution of d-biotin
(1.1 mmol) in DMF was added EDCI (3 mmol), DMAP (3 mmol) and compound
18 (1 mmol) under nitrogen atmosphere and stirred for overnight at room
temperature. Reaction mixture was diluted with water and extracted with
ethyl acetate (3 Â 10 mL) and dried over Na2SO4. After evaporation of the
solvent, the product was crystallized over ether–hexanes system resulted in
compound 19 in 78% yield.1H NMR (300 MHz, CDCl3): d 7.85 (m, 1H), 7.71–7.21
(m, 7H), 6.61 (br s, 1H), 6.35 (s, 2H), 5.41 (s, 1H), 4.71 (m, 1H), 4.35 (m, 1H),
4.21 (m, 1H), 3.32 (m, 2H), 2.92–2.51 (m, 2H), 2.22–1.91 (m, 2H), 0.9–1.71 (m,
6H); 13CNMR (75 MHz, CDCl3): 173.0, 163.6, 149.8, 138.8, 129.4, 128.5, 127.9,
127.0, 125.0, 124.7, 122.4, 79.1, 61.9, 60.0, 56.3, 45.0, 39.5, 35.5, 32.5, 28.8, 26.1
ESI MS:[M+H]+: 520.1151, calcd 519.1150 for C25H27Cl2N3O3S.