Improved synthesis and preparative scale resolution of racemic monastrol

Article abstract of DOI:10.1016/S0040-4039(02)01269-8

The Yb(OTf)₃ catalyzed Biginelli cyclocondensation reaction of 3-hydroxybenzaldehyde, ethyl acetoacetate and thiourea afforded the corresponding dihydropyrimidine-2-thione, called monastrol, in 95% isolated yield. The chiral resolution of racem

Full text of DOI:10.1016/S0040-4039(02)01269-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5913–5916
Improved synthesis and preparative scale resolution of
racemic monastrol
Alessandro Dondoni,* Alessandro Massi and Simona Sabbatini
Dipartimento di Chimica, Laboratorio di Chimica Organica, Universita` di Ferrara, Via L. Borsari 46, I-44100 Ferrara, Italy
Received 13 June 2002; accepted 28 June 2002
Abstract—The Yb(OTf)3 catalyzed Biginelli cyclocondensation reaction of 3-hydroxybenzaldehyde, ethyl acetoacetate and
thiourea afforded the corresponding dihydropyrimidine-2-thione, called monastrol, in 95% isolated yield. The chiral resolution of
racemic monastrol, a mitosis blocker by kinesin Eg5 inhibition, was carried out on a preparative scale (ca. 100 mg) through
diastereomeric N-3 ribofuranosyl amides. © 2002 Elsevier Science Ltd. All rights reserved.
The importance of the dihydropyrimidine (DHPM) ring
as a pharmacophore is well established through the
pharmacological activities of various derivatives as cal-
cium channel antagonists (drugs candidates against car-
diovascular diseases) and a1a adrenergic receptor
antagonists (drugs for the treatment of the benign
prostatic hyperplasia).1 DHPM derivatives are readily
accessible products via the ketoester–aldehyde–urea (or
thiourea) cyclocondensation reaction known as the
Biginelli reaction.2 The scope of this pharmacophore
has been further increased by the identification of the
4-(3-hydroxyphenyl)-2-thione derivative ( )-1 called
monastrol (Fig. 1),3 as a cell permeable lead compound
for the development of new anticancer drugs. In fact,
out of a 16 320-member collection of small molecules,
monastrol ( )-1 has been identified as a compound that
specifically affects cell-division (mitosis) by a new mech-
anism which does not involve tobulin targeting. It has
been established that the activity of ( )-1 consists of the
specific and reversible inhibition of the motility of the
mitotic kinesin Eg5, a motor protein known to be
required for spindle bipolarity. Given this biological
activity, usable quantities of monastrol ( )-1 in pure
enantiomeric forms constitute a target of great impor-
tance. The only synthesis so far reported of ( )-1 is that
of Kappe and co-workers4 by microwave-promoted
condensation of ethyl acetoacetate, 3-hydroxybenzalde-
hyde, and thiourea in polyphosphate ester as reaction
medium. The registered yield of isolated racemic ( )-1
was ca. 60%. The separation of individual enantiomers
was then carried out on a 0.02 mg scale by enantioselec-
tive HPLC and the absolute configuration established
by CD spectroscopy. These results prompted us to
report here both an improved synthesis of ( )-1 in ca.
95% yield as well the chiral resolution of the racemate
through diastereomeric N-3 glycosyl amides to give the
individual R- and S-enantiomer on a preparative scale
(typically 100 mg).
While in our recent work on the synthesis of glycosy-
lated dihydropyrimidin-2-ones the three component sys-
tem CuCl/AcOH/BF3Et2O proved to be a quite efficient
promoter of the Biginelli reaction,5 the same catalyst
turned out to be not compatible with the sulfurated
version employing thiourea since the reaction between
ethyl acetoacetate, 3-hydroxybenzaldehyde, and
thiourea (3 equiv.) did not produce the desired product.
Instead the use of Yb(OTf)3 (0.1 equiv.) was quite
beneficial since with this Lewis acid as a promoter the
reaction afforded monastrol ( )-1 (mp 184–186°C, lit.4
184–186) in 95% isolated yield by aqueous work-up6
followed by flash chromatography (silica, cyclohexane/
EtOAc 2:1) (Scheme 1). This result demonstrates the
Keywords: multicomponent reaction; Biginelli reaction; 3,4-dihy-
dropyrimidine-2-thione; monastrol.
* Corresponding author. E-mail: adn@dns.unife.it
Figure 1. The new mitosis blocker 4-(3-hydroxyphenyl)-3,4-
dihydropyrimidine-2-thione, monastrol ( )-1.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01269-8

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A. Dondoni et al. / Tetrahedron Letters 43 (2002) 5913–5916
Scheme 1. Conditions: thiourea (3 equiv.), Lewis acid (0.1
equiv.), THF, reflux, 12 h.
Scheme 3. Conditions: DMF, rt, 12 h.
compatibility of the lanthanide Lewis acid promoter
Yb(OTf)3 with the sulfurated version of the Biginelli
reaction.7
94% overall yield by flash chromatography (silica,
cyclohexane/EtOAc 4:1) and the excess of the resolving
agent 5 was almost quantitatively recovered. The selec-
tive N-acylation at the N-3 of ( )-4 as well as the
permanence of the silyl protective group on the oxygen
atom of the phenyl ring were demonstrated by the
absence of the coupling constant J3,4 and the strong
NOE between a methyl of the TBDMS group and an
aromatic proton.
Aiming at a chiral resolution of ( )-1 on a preparative
scale we first envisioned the separation of enantiomers
via diastereomeric esters using the a-linked C-ribosyl
carboxylic acid 2.8 The use of a sugar derivative as the
chiral resolving agent followed our ongoing work on
the synthesis of glycosylated monastrol analogues via
Biginelli cyclocondensation reaction.9 Under unopti-
mized conditions, esterification of the free hydroxy
group of racemic monastrol ( )-1 with 2 promoted by
suitable condensation agents (Scheme 2) afforded the
1:1 mixture of the diastereomeric (4R)- and (4S)-3,4-
dihydropyrimidine-2-thione derivatives 3 in 45% overall
yield. Unfortunately, the separation of these
diastereoisomers by column chromatography using a
variety of solvents failed in our hands.
The two diastereomeric amides 7 were separated by
flash chromatography (silica, toluene/diisopropyl ether
9:1 containing 0.3% of AcOH) starting from a typical
scale of 400 mg mixture.12 The first eluted was pure
compound (4R)-710 ([h]D −27, c 1.2, CHCl3) in 40%
yield13 followed by the diastereoisomer (4S)-7 (38%
yield)13 which however was impure by 15% of O-silyl-
ated monastrol ( )-4.10 Chromatography (silica, cyclo-
hexane/EtOAc 4:1) of the latter material afforded pure
(4S)-710 ([h]D 69, c 1.3, CHCl3) in 36% yield.13 The
chiral sugar moiety and the O-TBDMS group were
then removed in one step from each individual
diastereoisomer (4R)-7 and (4S)-7 by treatment with
EtONa in EtOH at room temperature overnight to give
the two monastrol enantiomers (4R)(−)-1 ([h]D −71, c
1.6, MeOH; [h]436 −204, c 1.6, MeOH) and (4S)(+)-1
([h]D 71, c 1.8, MeOH; [h]436 204, c 1.8, MeOH, lit.4
[h]436 1.1, c 0.007, MeOH) in almost quantitative yield
(95%) (Scheme 5). The assignment of the absolute
configuration at C-4 of these compounds was confirmed
by comparison of their CD spectra (Fig. 2) with those
reported by Kappe.4
Another possible route for the resolution of ( )-1 was
via diastereomeric amides exploiting the selective N-
acylation2 of the NH group at the position 3 with
respect to the position 1. However the protection of the
phenolic hydroxy group was first required in order to
avoid a competitive esterification. The selective O-silyl-
ation of ( )-1 with tert-butyldimethylsilyl chloride was
readily carried out by the standard method (Scheme 3)
and the structure of the resulting O-TBDMS derivative
( )-410 (mp 146–148°C from cyclohexane/EtOAc)
demonstrated by suitable NMR analysis. Accordingly a
strong NOE was observed between H-1 and the adja-
cent Me at C-6 and a coupling constant J3,4=3.0 Hz
was measured.
In order to prove that the basic removal of the chiral
auxiliary acid occurred without loss of stereochemical
integrity at C-4 of the DHPM ring, the enantiomers
(4R)(−)-1 and (4S)(+)-1 were transformed into the cor-
responding diastereoisomeric N-glycosyl amides (4R)-
Selective amide formation at N-3 of ( )-4 with the
b-linked C-glycosyl carboxylic acid 511 was achieved via
condensation with the acyl chloride 6 (3 equiv.) in
toluene as a solvent (Scheme 4). The 1:1 mixture of
diastereomeric (4S) and (4R) amides 7 was obtained in
Scheme 2. Conditions: DMF, 120°C, 4 h.

A. Dondoni et al. / Tetrahedron Letters 43 (2002) 5913–5916
5915
Scheme 4. Reagents and conditions: (a) SOCl2, DMF, toluene, 100°C, 2 h; (b) ( )-4 (0.33 equiv.), toluene, 100°C, 4 h.
Scheme 5. Conditions: EtOH, rt, 12 h.
thus providing for the first time usable quantities of
these isomers for biological studies.
Acknowledgements
We gratefully acknowledge MURST and University of
Ferrara for financial support.
References
1. Kappe, C. O. Eur. J. Med. Chem. 2000, 35, 1043.
2. (a) Kappe, C. O. Tetrahedron 1993, 49, 6937; (b) Kappe,
C. O. Acc. Chem. Res. 2000, 33, 879.
3. Mayer, T. U.; Kapoor, T. M.; Haggarty, S. J.; King, R.
W.; Schreiber, S. L.; Mitchison, T. J. Science 1999, 286,
971.
Figure 2. Experimental CD spectra of (4S)(+)-1 and (4R)(−)-
1.
4. Kappe, C. O.; Shishkin, O. V.; Uray, G.; Verdino, P.
Tetrahedron 2000, 56, 1859.
5. Dondoni, A.; Massi, A.; Sabbatini, S. Tetrahedron Lett.
710 and (4S)-7,10 respectively, by protection of the
hydroxy group and reaction with the acyl chloride 6.
Quite gratifyingly, each of these compounds appeared
not to be contaminated by the other diastereoisomer by
NMR analysis. Hence an enantiomeric purity ]97–
98% can be attributed to each monastrol isomer (4R)-
(−)-1 and (4S)(+)-1.
2001, 42, 4495.
6. Treatment of the crude reaction mixture with EtOAc and
repeated extractions with water allowed the separation of
most of the unreacted thiourea.
7. Ma, Y.; Qian, C.; Wang, L.; Yang, M. J. Org. Chem.
2000, 65, 3864.
8. Togo, H.; Ishigami, S.; Fujiii, M.; Ikuma, T.; Yokoyama,
M. J. Chem. Soc., Perkin Trans. 1 1994, 2931.
In conclusion an improved synthesis and a preparative
scale resolution of monastrol ( )-1 have been achieved,

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A. Dondoni et al. / Tetrahedron Letters 43 (2002) 5913–5916
9. Dondoni, A.; Massi, A.; Sabbatini, S.; Bertolasi, V. J. Org.
(s, 1H, H-1), 6.26 (s, 1H, H-4), 6.14 (dd, 1H, J1%,2%=1.2,
J2%,3%=5.8 Hz, H-2%), 6.01 (d, 1H, H-1%), 5.87 (dd, 1H,
J3%,4%=7.5 Hz, H-3%), 4.76 (ddd, 1H, J4%,5%a=4.5, J4%,5%b=6.0
Hz, H-4%), 4.65 (dd, 1H, J5%a,5%b=10.8 Hz, H-5%a), 4.55 (dd,
1H, H-5%b), 2.32 (s, 3H, CH3); MALDI-TOF MS: 879.1
(M++H), 902.6 (M++Na), 918.6 (M++K).
Chem. 2002, in press.
10. ( )-4: white solid; 1H NMR (DMSO-d6): l=10.18 (s, 1H,
H-1), 9.66 (bd, 1H, J3,4=3.0 Hz, H-3), 7.26–7.19 (m, 1H,
Ph), 6.85–6.69 (m, 3H, Ph), 5.13 (bs, 1H, H-4), 4.02 (q, 2H,
J=7.0 Hz, OCH2CH3), 2.26 (s, 3H, CH3), 1.11 (t, 3H,
OCH2CH3), 0.95 (s, 9H, C(CH3)3), 0.19 (s, 6H, Si(CH3)2).
11. Dudfield, P. J.; Le, V.-D.; Lindell, S. D.; Rees, C. W. J.
1
Chem. Soc., Perkin Trans. 1 1999, 2937.
(4R)-7: white foam; H NMR (DMSO-d6) selected data:
12. A careful purification of diisopropyl ether was required for
this operation. See: Armarego, W. L. F.; Perrin, D. D.
Purification of Laboratory Chemicals; Butterworth-Heine-
mann: Oxford, 1998; pp. 250–251.
l=11.78 (s, 1H, H-1), 6.20 (d, 1H, J1%,2%=5.5 Hz, H-1%),
6.11 (s, 1H, H-4), 5.89 (dd, 1H, J2%,3%=5.0, J3%,4%=5.2 Hz,
H-3%), 5.78 (dd, 1H, H-2%), 4.79 (ddd, 1H, J4%,5%a=4.2,
J4%,5%b=4.8 Hz, H-4%), 4.64 (dd, 1H, J5%a,5%b=10.8 Hz, H-5%a),
13. Yield calculated with respect to the initial amount of
O-TBDMS derivative ( )-4 subjected to the chiral resolu-
tion.
4.57 (dd, 1H, H-5%b), 2.00 (s, 3H, CH3); MALDI-TOF MS:
879.0 (M++H), 902.4 (M++Na), 918.7 (M++K). (4S)-7:
white foam; 1H NMR (DMSO-d6) selected data: l=11.92