Synthesis and RNase A inhibition study of C2-symmetric bis-isochromenyl sulfones

Article abstract of DOI:10.1016/j.tetlet.2010.03.075

A new class of C₂-symmetric bis-isochromene derivatives with 3,3′-linkage has been synthesized from bis-propargyl sulfones. The method involves treatment of the sulfones with triethylamine to form the isochromene derivatives presumably via the intramolecular Michael addition to the intermediate bis-allenic sulfones. Interestingly, the product expected from the Garratt-Braverman pathway was not obtained. The bis-isochromene 7d displayed RNase A inhibition activity, much stronger than the isochromene 8 and bis-isocoumarin 9.

Full text of DOI:10.1016/j.tetlet.2010.03.075

Tetrahedron Letters 51 (2010) 2828–2831
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and RNase A inhibition study of C2-symmetric bis-isochromenyl
sulfones
*
Tapobrata Mitra, Sansa Dutta, Amit Basak
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India
a r t i c l e i n f o
a b s t r a c t
A new class of C2-symmetric bis-isochromene derivatives with 3,30-linkage has been synthesized from
bis-propargyl sulfones. The method involves treatment of the sulfones with triethylamine to form the iso-
chromene derivatives presumably via the intramolecular Michael addition to the intermediate bis-allenic
sulfones. Interestingly, the product expected from the Garratt–Braverman pathway was not obtained. The
bis-isochromene 7d displayed RNase A inhibition activity, much stronger than the isochromene 8 and
bis-isocoumarin 9.
Article history:
Received 23 February 2010
Revised 15 March 2010
Accepted 17 March 2010
Available online 28 March 2010
Ó 2010 Elsevier Ltd. All rights reserved.
(1H)-Isochromen-2-ones, better known as isocoumarins, have
drawn considerable interest as a wide variety of natural and unnat-
ural products containing such moieties are endowed with a range
of biological activities.1 Moreover, isocoumarins serve as useful
and versatile synthetic intermediates.2 In recent past, the activity
of isocoumarins as potent anticancer agents has generated keen
interest in these molecules and their analogues.3 Because of the
similarity in chemical and biological activities between the oxygen
species and their nitrogen counterparts, (1H)-isochromen-1-imine
analogues have also drawn interest especially their synthesis4 in
recent years. It occurred to us that bis-isochromen-1-imine deriv-
atives with a C2-symmetry might be even more attractive as many
of the biological targets like homodimeric proteases5 have similar
symmetry elements. ds-DNA also has a pseudo C2-symmetry and
thus interactions with other C2-symmetric molecules can be ex-
pected to be stronger. Literature survey revealed that till now,
there is no report of synthesis of bis-(1H)-isochromen-1-imines.
Herein, we report a near quantitative synthesis of such imines with
C2-symmetry starting from bis-propargyl sulfones. It may be noted
that all the current methods available for the synthesis of iso-
chromenones involve a metal ion-catalyzed intramolecular addi-
tion of amide oxygen to an acetylenic functionality. For example,
Lieu et al.4a have recently reported a AgSbF6-mediated intramolec-
ular enyne–amide cyclization for the synthesis of isochromenone-
imines. The method requires 25 mol % catalyst and refluxing in THF
at 80 °C for 8 h. In this year, Ma et al.4b reported that the same reac-
tion can be carried out using 5 mol % of AgOTf and refluxing in
dichloroethane at 60 °C for 2.5 h. On the contrary, the present
method is mild, quantitative, rapid (over within minutes) and easy
to execute (no requirement of metal ion, requires only 1.0 equiv of
Et3N).
Before embarking upon any synthetic endeavour, we considered
the possible reaction pathways of o-amido bis-allenic sulfones,
derivable from the corresponding bis-propargyl sulfones6 of the
type A (Scheme 1). The allenic sulfone can undergo Garratt–Brav-
erman cyclization7 via conformation B to form the sulfolene D
while intramolecular nucleophilic addition of amide oxygen or
nitrogen involving conformation C will lead to bis-isochromene
imines E or bis-isoquinolines F.
With this background, we proceeded to synthesize the starting
materials 1a–d. Thus EDC-mediated coupling of 2-iodo benzoic
acid with amino acid benzyl esters produced the aminoacyl deriv-
atives. Sonogashira coupling8 with propargyl alcohol followed by
mesylation (mesyl chloride, Et3N) and bromination (LiBr, THF)9
gave the bromide. The latter upon treatment with Na2S/TBAB in
THF–water afforded the sulfide 2a–d which on oxidation with
mCPBA produced the sulfone (Scheme 2). The simple aryl sulfone
3 was similarly prepared from the intermediate bromide 6d.
With the starting bis-propargyl sulfones in hand, the stage was
set to check their reactivity under basic conditions (Scheme 3). As
an initial experiment, the sulfone 1d was dissolved in CDCl3 in an
NMR tube and 1.0 equiv amount of triethylamine was added and
the 1H NMR was recorded again at different time points (Fig. 1).
We are amazed to see that within 2 min (the time taken to record
the spectrum after the addition of Et3N), the AB quartet for the
methylenes adjacent to the sulfonyl in the substrate at ꢀd 4.04
was completely replaced by a similar AB quartet at ꢀd 3.6 pair of
doublets (J = 14.2 Hz). At the same time, a new singlet appeared
at ꢀd 6.1 which was assigned to the 4 and 40 hydrogens of isochro-
men-1-imine.10 After that there was no change in the spectrum
pointing out that the reaction was complete within the first few
minutes. It was repeated on a larger scale in CHCl3 and Et3N and
the product was isolated pure in >95% yield by column chromatog-
raphy over silica gel using hexane–EtOAc (1:1) as an eluent. That
the product was not a result of Garratt–Braverman cyclization
* Corresponding author.
E-mail address: absk@chem.iitkgp.ernet.in (A. Basak).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.075

T. Mitra et al. / Tetrahedron Letters 51 (2010) 2828–2831
2829
O
O
S
S
C
C
O
O
H
H
H
H
O
O
N
N
N
N
R1* R1*
R1*
R1*
NH
C
HN
C
R1*
O
O
O
R1*
O
A
C
S
O
O
N as nucleophile
B
S
O
R1*
O
N
N
R1*
GB Products
O
O
N
C
F
S
O
CONHR1*
O
O
O
O
O as nucleophile
H
S
N
R1*
R1*
O
E
CONHR1*
R1*
=
R
R = Me, CHMe2, CH2CHMe2, CH2Ph
D
CO2Bn
Scheme 1. Possible pathways for the bis-allenic sulfone.
OH
Br
50%
I
i
O
O
85%
CO2H
4
R
HN
6a-6d
COOBn
90%
SPh
HN
R
5a-5d
COOBn
iii
O
ii
95%
HN
R
COOBn
3d (R= CH2Ph)
iv 85%
O
O
S
S
SO2Ph
O
O
O
O
O
iv
R
R
HN
85%
R
R
HN
HN
R
COOBn COOBn
COOBn
3 (R = CH2Ph)
COOBn COOBn
2a-2d
1a-1d
a, R = Me, b, R = CHMe2, c, R = CH2CHMe2, d, R = CH2Ph
i = MsCl, NEt3, 0 0C, DCM; LiBr, THF; ii = Na2S, TBAB, THF-H2O; iii = PhSH, NEt3, DCM; iv = mCPBA, DCM
Scheme 2. Synthesis of the target bis-propargyl sulfones.
O
O
R
S
S
S
C
O
C
O
O
R
O
R
O
O
O
O
i
O
O
N
N
R
HN
NH
HN
R
COOBn
7a-7d
R
COOBn
NH
COOBn
COOBn
COOBn
COOBn
1a-1d
ii
S
SO2Ph
SO2Ph
SO2Ph
O
O
O
O
O
ii
O
O
i
R
N
O
O
NH
R
COOBn
O
8
10
9
COOBn
3
a, R = Me, b, R = CHMe2, c, R = CH2CHMe2, d = CH2Ph
i = Et3N, CHCl3, 10 min ; ii = HCl, MeOH, 24h
Scheme 3. Synthesis of bis-isochromen-1-imine and bis-isochromenone.

2830
T. Mitra et al. / Tetrahedron Letters 51 (2010) 2828–2831
Figure 1. 1H NMR at different time points upon base treatment of sulfone 1d.
was apparent from the symmetrical nature of its structure (like
appearance of only one signal for the 4,40-positions and absence
of the amide NH signal). The other alternative isoquinolone struc-
ture, however, could not be ruled out on the basis of NMR or mass
spectra. The structures were finally confirmed to be the (1H)-iso-
chromen-1-imine derivatives by facile acid-mediated hydrolysis
to the bis-isocoumarin, which could be characterized by single
crystal X-ray (the ORTEP diagram shown in Fig. 2).
The generality of this methodology for the synthesis of other
bis-(1H)-isochromen-1-imine derivatives was demonstrated by
carrying out the reaction with various other sulfones, derived from
different amino acids. In each case the imine derivatives were ob-
tained in excellent yields. The reaction also works with monoprop-
argyl sulfones 3. The results are shown in Table 1.
Table 1
Results of triethylamine treatment
Substrate Product Time (min) Yield (%) Yield of deprotected
isocoumarins (%)
1a
1b
1c
1d
3
7a
7b
7c
7d
8
2
5
10
2
97
95
95
98
98
>95 (for 9) >95 (for 10)
2
Because of the reported antiangiogenic activity of isocouma-
rins,11 we became interested to check such activity of the synthe-
sized bis-isochromene 7d, bis-isocoumarin
9 and also the
Figure 3a. RNase A inhibition study of 8, 7d 9.
monoisochromene 8 by looking at the inhibitory activity against
RNase A. Incidentally, RNase A and angiogenin have high degree
of homology12 and hence inhibition of RNase A is regarded as a
model for the development of angiogenin inhibitors.13 Thus, a solu-
tion containing RNA was incubated14 with RNase A in the presence
of various compounds and then analyzed by agarose gel electro-
phoresis. The results are shown in Figure 3a and b and the intensi-
Figure 3b. RNase A inhibition study of 8 and 7d at different concentrations.
ties of various bands are shown in Table 2a and b. From the tables,
it is clear that although all the three compounds showed RNase
inhibition, the activity shown by the bis-isochromene 7d is signif-
icantly higher than the other compounds (Fig. 3a) and also the per-
centage of inhibition increased as the concentrations of 8 and 7d
were increased (Fig. 3b). It is also interesting to note that simple
monoisochromene has only weak inhibition thus emphasizing
the importance of a bis-system and the importance of a C2-symme-
try is being explored.
In conclusion, we have developed a simple method for the prep-
aration of 3,30-bis-isochromen-1-imine derivatives connected via a
functional linker involving double Michael type addition to bis-
allenic sulfone, generated in situ from bis-propargyl sulfone. We
have also demonstrated that intramolecular nucleophilic addition
Figure 2. ORTEP diagram of bis-isocoumarin 9.

T. Mitra et al. / Tetrahedron Letters 51 (2010) 2828–2831
2831
Table 2a
4. (a) Liu, G.; Zhou, Y.; Ye, D.; Zhang, D.; Ding, X.; Jiang, H.; Liu, H. Adv. Synth. Catal.
2009, 351, 2605; (b) Bian, M.; Yao, W.; Ding, H.; Ma, C. J. Org. Chem. 2010, 75,
269; (c) Varela-Ferna´ndez, A.; Gonza´lez-Rodr´ıguez, C.; Varela, J. A.; Castedo, L.;
Saa´, C. Org. Lett. 2009, 11, 5350.
RNase A inhibition studies by gel electrophoresis; compounds were taken in MeOH at
1.15 mM concentration
Lane
Content
Relative intensity
5. (a) Mellons, J. W. Nat. Med. 1996, 2, 274; (b) Petit, S. C.; Simsic, J.; Michael, S. F.;
Swanstrom, R. Perspect. Drug Discovery Des. 1993, 1, 69; (c) Darke, P. L.; Huff, J.
R. Adv. Pharmacol. 1994, 25, 399.
6. (a) Back, T. G. Tetrahedron 2001, 57, 5263; (b) Basak, A.; Khamrai, U. K.
Tetrahedron Lett. 1995, 36, 7913; (c) Basak, A.; Mandal, S.; Das, A. K.; Bertolasi,
V. Bioorg. Med. Chem. Lett. 2002, 12, 873; (d) Basak, A.; Khamrai, U. K.; Mallik, U.
Chem. Commun. 1996, 749.
1
2
3
4
5
RNA
1
RNA + RNase A
RNA + RNase A + 8
RNA + RNase A + 7d
RNA + RNase A + 9
0.0319
0.2115
0.8985
0.1144
7. (a) Garratt, P. J.; Neoh, S. B. J. Org. Chem. 1979, 44, 2667; (b) Cheng, Y. S. P.;
Garratt, P. J.; Neoh, S. B.; Rumjanek, V. H. Isr. J. Chem. 1985, 26, 101; (c)
Braverman, S.; Duar, Y.; Segev, D. Tetrahedron Lett. 1979, 17, 3181; (d) Zafrani,
Y.; Gottlieb, H. E.; Sprecher, M.; Braverman, S. J. Org. Chem. 2005, 70, 10166.
8. (a) Sonogashira, K.; Tohoda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467;
(b) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980,
627.
Table 2b
RNase A inhibition studies of compounds 8 and 7d at different concentration
9. (a) Jones, G. B.; Weight, J. M.; Hynd, G.; Wyatt, J. K.; Rarner, P. M.; Huber, R. S.;
Li, A.; Kilgore, M. W.; Sticca, R. P.; Pollenz, R. S. J. Org. Chem. 2002, 67, 5727; (b)
Basak, A.; Bag, S. S.; Das, A. K. Eur. J. Org. Chem. 2005, 7, 1239.
Lane
Content
Relative intensity
1
2
3
4
5
6
RNA
RNA + RNase A
RNA + RNase A + 8 (1 mg/mL)
RNA + RNase A + 8 (2 mg/mL)
RNA + RNase A + 7d (1 mg/mL)
RNA + RNase A + 7d (2 mg/mL)
1
0.3689
0.5800
0.6173
0.9720
0.9915
10. The absence of any NOE between the H-8 and the H
a or the methyl in the
alanine based isochromene 7a suggested a Z-configuration of the imine (for
NOESY spectrum, see Supplementary data).
11. (a) Lee, J. H.; Park, Y. J.; Kim, H. S.; Hong, Y. S.; Kim, K.-W.; Lee, J. J. J. Antibiot.
2001, 54, 463; (b) Nakashima, T.; Hirano, S.; Agata, N.; Kumagai, H.; Isshiki, K.;
Yoshioka, T.; Ishizuka, M.; Maeda, K.; Takeuchi, T. J. Antibiot. (Tokyo) 1999, 52,
426; (c) Agata, N.; Nogi, H.; Milhollen, M.; Kharbanda, S.; Kufe, D. Cancer Res.
2004, 64, 8512; (d) Agata, N.; Hirano, S.; Abe, C.; Nakashima, T.; Tsuchiya, A.;
Kumagai, H.; Isshiki, K.; Yoshioka, T.; Ishizuka, M.; Takeuchi, T. Res. Commun.
Mol. Pathol. Pharmacol. 2000, 108, 297.
of amides is more facile than the Garratt–Braverman cyclization
pathway which takes place at room temperature in aryl-substi-
tuted allenes. Our observation is similar to our recently reported
synthesis of bis-indoles involving nucleophilic addition of amide
nitrogen to allenic sulfones15 as well as cleavage of DNA via alkyl-
ation pathway.16 The newly synthesized bis-isochromenes and the
biscoumarin possess strong RNase A inhibition activity thus dem-
onstrating the potential importance of these compounds.
12. (a) Raines, R. T. Chem. Rev. 1998, 98, 1045; (b) Vallee, B. L.; Riordan, J. F. Cell.
Mol. Life Sci. 1997, 53, 803.
13. (a) Folkman, J.; Klagsbrun, M. Science 1987, 235, 442; (b) Riordan, J. F. In
Ribonucleases: Structure and Function; D’Alessio, G., Riordan, J. F., Eds.;
Academic Press: New York, 1997; p 445; (c) Fett, J. W.; Strydom, D. J.; Lobb,
R. R.; Alderman, E. M.; Bethune, J. L.; Riordan, J. F.; Vallee, B. L. Biochemistry
1985, 24, 5480; (d) Maity, T. K.; De, S.; Dasgupta, S.; Pathak, T. Bioorg. Med.
Chem. 2006, 14, 1221.
14. Incubation condition: The incubation was done at rt in MeOH for 2 h. For Figure
3a, a 1.15 mM stock solution of each of 7d, 8 and 9 was used. For Figure 3b, the
concentrations of the compounds 7d and 8 are the concentrations of the stock
(effective concentrations are three times less). The stock concentrations of the
Acknowledgements
RNA and RNase A are 10 mg/mL and 0.8
lM, respectively.
DST is thanked for providing a research grant and also for
400 MHz NMR facility under the IRPHA programme. T.M. is grate-
ful to CSIR, Government of India for a fellowship. Authors also
thank Mr. Sanjay Pratihar for his help in solving the crystal
structure.
15. Mitra, T.; Das, S.; Basak, A. Tetrahedron Lett. 2009, 50, 5846.
16. Nicolaou, K. C.; Dai, W.-M. Angew. Chem., Int. Ed. Engl. 1991, 30, 1387.
Spectral data of selected compounds: All the 1H and 13C NMR spectra were
recorded at 400 and 100 MHz, respectively, in CDCl3. Specific rotation was
measured in CHCl3.
For 7a: dH 8.26 (d, J = 7.2 Hz, 2H), 7.51–7.14 (m, 16H), 6.28 (s, 2H), 5.23–5.09
(m, 4H), 4.69 (app. q, J = 7.0 Hz, 2H), 3.98 (d, J = 14.4 Hz, 2H), 3.79 (d,
J = 14.4 Hz, 2H), 1.56 (d, J = 7.0 Hz, 6H); dC 175.1, 155.3, 135.9, 128.8, 127.1,
125.6, 123.4, 121.5, 115.6, 114.1, 80.2, 53.1, 51.7, 28.3, 18.6; ½a D25
ꢂ44.5 (c
ꢁ
Supplementary data
0.25); MS: m/z = 727.34 [MNa+], 705.33 [MH+]; HRMS: calcd for
C40H36N2O8S + H+ 705.2272 found 705.2275.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.03.075.
For 7d: dH 8.21 (d, J = 7.6 Hz, 2H), 7.42–7.13 (m, 24H), 7.00 (d, J = 7.6 Hz, 2H),
6.14 (s, 2H), 5.11 (s, 4H), 4.79 (dd, J = 8.0, 6.0 Hz, 2H), 3.69 (dd, J = 29.8, 14.8 Hz,
4H), 3.28 (dd, J = 13.2, 5.6 Hz, 2H), 3.17 (dd, J = 13.2, 8.0 Hz, 2H); dC 172.0,
142.3, 138.3, 135.8, 132.2, 131.4, 129.6, 129.1, 128.4, 128.3, 128.1, 127.3, 126.4,
References and notes
125.4, 108.8, 66.5, 60.8, 57.0, 39.9; ½a D25
ꢂ49.9 (c 0.25); MS: m/z = 879.22
ꢁ
[MNa+], 857.25 [MH+]; HRMS: calcd for C52H44N2O8S + H+ 857.2899 found
857.2888.
1. (a) Larsen, T. O.; Breinholt, J. J. Nat. Prod. 1999, 62, 1182; (b) Lee, J. J.; Kim, H.-S.;
Lee, J.-H.; Hong, Y.-S.; Park, Y. J. PCT Int. Appl. WO 2000075124.
For 8: dH 8.25 (d, J = 5.6 Hz, 1H), 7.72 (d, J = 7.2 Hz, 2H), 7.55–7.13 (m, 16H),
6.08 (s, 1H), 5.11 (dd, J = 22.8, 12.4 Hz, 2H), 3.96–3.93 (m, 1H), 3.71 (dd,
J = 24.0, 14.4 Hz, 2H), 3.13–3.08 (m, 1H), 3.01–2.96 (m, 1H); dC 172.8, 143.1,
138.3, 138.1, 135.9, 134.1, 132.3, 131.7, 129.5, 129.4, 129.2, 129.0, 128.5, 128.4,
2. (a) Wang, B.; Li, M.; Xu, S.; Song, H.; Wang, B. Synthesis 2007, 1643; (b) Alper, P.
B.; Nguyen, K. T. J. Org. Chem. 2003, 68, 2051; (c) Hudson, A. R.; Roach, S. L.;
Higuchi, R. I.; Phillips, D. P.; Bissonnette, R. P.; Lamph, J.; Yen, W. W.; Li, Y.;
Adams, M. E.; Valdez, L. J.; Vassar, A.; Cuervo, C.; Kallel, E. A.; Gharbaoui, C. J.;
Mais, D. E.; Miner, J. N.; Marschke, K. B.; Rungta, D.; Negro-Vilar, A.; Zhi, L. J.
Med. Chem. 2007, 50, 4699; (d) Faggi, C.; Garcia-Valverde, M.; Marcaccini, S.;
Menchi, G. Org. Lett. 2010, 12, 788.
3. (a) Soulie, P.; Gamelin, E.; Eder, J. Proc. Am. Soc. Clin. Oncol. 2003, Abstract No.
777.; (b) Bonate, P.; Eder, J.; Soulie, P. Proc. Am. Soc. Clin. Oncol. 2003, Abstract
No. 537.; (c) Kawano, T.; Agata, N.; Kharbanda, S.; Avigan, D.; Kufe, D. Cancer
Chemother. Pharmacol. 2007, 59, 329.
128.3, 128.0, 127.4, 126.4, 125.4, 116.1, 1.8.6, 66.4, 60.2, 60.1, 39.6; ½a D28
ꢂ31.4
ꢁ
(c 0.25); MS: m/z = 560.06 [MNa+], 538.08 [MH+]; HRMS: calcd for
C32H27NO5S + H+ 538.1689 found 538.1685.
For 9: dH 8.29 (d, J = 8.0 Hz, 2H), 7.76 (t, J = 7.6 Hz, 2H), 7.58 (t, J = 7.6 Hz, 2H),
7.48 (d, J = 8.0 Hz, 2H), 6.77 (s, 2H), 4.37 (s, 4H); dC 161.2, 144.2, 135.9, 135.2,
129.8, 128.5, 126.2, 120.8, 110.2, 57.8; MS: m/z = 405.28 [MNa+], 383.28 [MH+];
HRMS: calcd for C20H14O6S + H+ 383.0590 found 383.0596.