Design and synthesis of guanidine-containing novel retinoids

Article abstract of DOI:10.1016/j.tetlet.2009.08.008

Guanidine-containing new retinoids 1-4 were synthesized through the addition of guanidine to the activated carboxylic acids (retinoids) promoted by carbonyldiimidazole (CDI). A set of structurally and functionally diverse guanidine derivatives of retinoids were obtained in high yields (78-82%). We are in the process of studying the biological effect of these molecules on retinoic acid signaling pathways.

Full text of DOI:10.1016/j.tetlet.2009.08.008

Tetrahedron Letters 50 (2009) 5860–5863
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Design and synthesis of guanidine-containing novel retinoids
b
b
Bhaskar C. Das a,b,c, , Sakkarapalayam M. Mahalingam , Pranoy Mohapatra
*
a Department of Nuclear Medicine, Albert Einstein College of Medicine, Bronx, NY 10461, USA
b Department of Developmental & Molecular Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA
c Albert Einstein Cancer Center, Bronx, NY 10461, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Guanidine-containing new retinoids 1–4 were synthesized through the addition of guanidine to the acti-
vated carboxylic acids (retinoids) promoted by carbonyldiimidazole (CDI). A set of structurally and func-
tionally diverse guanidine derivatives of retinoids were obtained in high yields (78–82%). We are in the
process of studying the biological effect of these molecules on retinoic acid signaling pathways.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 14 May 2009
Revised 4 August 2009
Accepted 6 August 2009
Available online 9 August 2009
In the context of our ongoing chemical biology project, studying
the role of retinoic acid signaling pathways during zebra fish
embryogenesis, we synthesized novel retinoid libraries. Retinoids
(retinol [vitamin A] and its biologically active metabolites) are
essential signaling molecules that control various developmental
pathways and influence the proliferation and differentiation of a
variety of cell types in the adult.1,2 A number of synthetic retinoids
have been synthesized that interact selectively with their recep-
tors.3 Understanding the importance of the retinoids we were
interested in synthesizing a small library of new retinoids. We first
tried to synthesize compounds 5 and 6 as new retinoic acid ana-
logues by introducing constrained phenyl ring system in the place
of conjugated alkene backbone (spacers in ATRA) to avoid the ATRA
(all-trans retinoic acid) metabolism into its isomers, 9-cis-RA and
13-cis-RA (Fig. 1). Then we hypothesized that, acid group could
be further derivatized to different bioactive retinoids to increase
efficacy and potency and this may open new avenue of retinoic
acid signaling pathways during zebra fish embryogenesis. For this
purpose we synthesized novel guanidine-containing retinoids.
We transformed the carboxylic acid group to the corresponding
acyl guanidine derivative because, guanidines are strongly basic
and are fully protonated under physiological conditions which is
crucial for specific ligand-receptor interactions and possess a wide
range of interesting and important biochemical and pharmaceuti-
cal properties.4–7 Guanidino-containing drugs such as MIBG and
MGBG were shown several decades ago to have antitumor proper-
ties and have been subjected to intense preclinical and clinical
evaluation.8 So we undertook the projects to synthesize novel gua-
nidine-containing retinoids for studying the retinoic acid signaling
pathways. To our surprise, a search of the literature failed to
NH
NH2
O
O
1
7
11
6
N
2
OH
OH
9
15
13
H
14
10
12
A
4
8
1
3
5
ATRA
O
O
NH
NH2
R
R
N
H
B
5: R = CH3
6: R = H
7a: R = CH3
7b: R = H
Figure 1.
* Corresponding author. Tel.: +1 718 430 2422; fax: +1 718 430 8853.
E-mail address: bdas@aecom.yu.edu (B.C. Das).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.08.008

B. C. Das et al. / Tetrahedron Letters 50 (2009) 5860–5863
5861
uncover any examples of guanidine-containing retinoic acid deriv-
atives (Fig. 1). Herein we first report the synthesis of guanidine-
containing retinoids by simple addition of guanidine to the acti-
vated carboxylic acids (retinoids) promoted by CDI.
The guanidino retinoid 1 was synthesized from the all-trans ret-
inoic acid (ATRA) using CDI as a carboxylic acid activating agent in
DMF. The activated ATRA was treated with guanidine in DMF at
room temperature and generated the guanidino retinoid 1 with
an 82% yield (Scheme 1). A signal at 160.4 ppm in 13C NMR spec-
trum confirms the presence of guanidine group.
O
CDI, DMF, 1 h
then
H N
OH
NH
NH
ATRA
2
2
6 h, r.t, 82%
NH
NH
O
N
H
2
Retrosynthetic analysis suggests that our constrained retinoids
5 and 6 could be synthesized by the Wittig reaction of the ylides
8 and 9 with the aldehyde 10 (Scheme 2), and by subsequent
hydrolysis.
1
Scheme 1.
In the process of synthesizing 5, starting from the allylic alcohol
11, we encountered the unusual Wittig salt 12 which led to the for-
mation of the unusual compound 13 when the ylide 12 was treated
with aldehyde 10 (Scheme 3).9
To explore the mechanism of the unusual Wittig salt 12 forma-
tion, we first tried to understand the substitution effect at a-posi-
O
O
R
R
OH
OCH3
PPh Br
+
3
O
tion of the allylic alcohol 11. For this purpose we synthesized the
allylic alcohol 14 from Grignard reaction of b-cyclocitral with phe-
nyl magnesium bromide in THF. The allylic alcohol 14 was con-
verted into the corresponding ylide using PPh3Br followed by the
5: R = CH3
6: R = H
8: R = CH3
9: R = H
10
Scheme 2. Retrosynthetic analysis.
.
PPh3 HBr,
CH3CN
tBuONa, DMF
OH
H
H3CO2C
CHO
HO
90 oC, 6 h, 92%
PPh3.Br
13
11
10
12
O
.
PPh3 HBr,
1. nBuLi, ether then 10
CH3OH
2. NaOH (aq), EtOH
rt, 72 h, 78%
Scheme 3.
Ph
1. PPh3.Br,
CH3CN, 90 oC, 6 h
O
OH
Ph
PhMgBr
H
2. tBuONa, DMF
THF, r.t, 3 h
H
14
H3CO2C
CHO
HO
15
10
O
Scheme 4.
O
.
OH
1. PPh Br,
3
OH
o
CH CN, 90 C, 6 h
3
+
H
t
2. BuONa, DMF
HO
16
17
5
(1:0.6)
H CO C
CHO
3
2
O
10
Scheme 5.

5862
B. C. Das et al. / Tetrahedron Letters 50 (2009) 5860–5863
Wittig reaction with aldehyde 10 furnishing the unusual product
15 with 83% yield (Scheme 4).
These new derivatives 20 and 21, which also have the hydro-
phobic cyclohexene ring and constrained phenyl ring system in
the place of conjugated alkene backbone in ATRA, and the
carboxylic acid group remain intact. Comparing the structural
relation of our original target-constrained retinoids 5 and 6 with
the new unusual products 13 and 15, these new compounds hav-
ing the extension of the double bond conjugation via the 4th
position of the cyclohexene ring unit may block to avoid the
retinoic acid metabolism caused by Cytochrome P450 enzymes
(CYPs) which will be the extra complement in this new finding
as novel retinoids. It is believed that the physiologically most
prominent pathway starts with the rate-limiting hydroxylation
at C-4 position of the cyclohexenyl ring leading to the formation
of 4-hydroxy-ATRA, then by a reductase enzyme into 4-oxo-
ATRA that is then further transformed by CYP(s) into more polar
metabolites.10 So, this finding clearly demonstrates the utility of
this methodology in synthesizing retinoic acid analogues where
newly synthesized retinoids are more resistant to be metabo-
lized by enzymes.
Encouraged by these remarkable results and having established
a new strategy for synthesizing the new retinoid derivatives and
for increasing the biological activity of this compound, we have de-
signed and synthesized the guanidino retinoid 2. The guanidino
retinoid 2 was synthesized from the acid 15 using CDI as a carbox-
ylic acid activating agent in DMF. The activated carboxylic acid was
treated with guanidine in DMF at room temperature and generated
the guanidino retinoid 2 with 78% yield (Scheme 7). A signal at
163.6 ppm in 13C NMR spectrum confirms the presence of guani-
dine group.
In case of the alpha position of the allylic alcohol was substi-
tuted with hydrogen 16 (1o alcohol) gave both unusual 17 and
usual 5 Wittig products with 1:0.6 ratio (Scheme 5). From these
experiments we found that when alpha position was substituted
with methyl, phenyl gave exclusively the unusual Wittig salt,
whereas the alpha position substituted with hydrogen gives both
usual and unusual products.
Further we explored to figure out whether this unusual Wittig
salt formation occurs when alcohol group is outside or it is com-
mon in case alcohol group is inside the ring system. For this we
have chosen the allylic alcohols 18 and 19 which were treated with
triphenylphosphonium bromide in acetonitrile at 90 °C. Without
further purification the ylides were subjected to the Wittig reac-
tion with aldehyde 10. To our surprise reactions utilizing allylic
alcohols inside the ring system 18 and 19 generated the more
normal Wittig products 20 and 21 with 86% and 80% yields, respec-
tively, via the usual Wittig ylides 22 and 23 (Scheme 6). This
clearly proves that the unusual reaction occurs when the allylic
alcohol is outside the ring system.
OH
PPh Br
3
.
PPh Br,
3
CH CN
3
R
R
1
1
o
R
R
2
2
90 C, 6 h
18: R1 = R2 = H
22: R1 = R2 = H
19: R1 = R2 = CH3
23: R1 = R2 = CH3
Spurred by this result, we next tried to expand this guanidation
to the retinoic acid analogues 20 and 21.11 As expected, we ob-
tained the guanidino retinoids 3 (80%) and 4 (78%) with good yields
(Scheme 8).
In all cases, the product was isolated as an air stable solid with-
out column chromatography. After completion of the reaction, the
reaction mixture was poured into water and the final products 1–4
were filtered and washed with water and were readily adaptable
to synthesis for the construction of combinatorial libraries also
to scale-up.12 Biological evaluation of these novel retinoids is
currently underway in our laboratory. It is worth mentioning that,
O
OH
t
BuONa, DMF
H
H CO C
CHO
3
2
20: R1 = R2 = H; 86%
21: R1 = R2 = CH3; 80%
R
1
10
R
2
Scheme 6.
Ph
Ph
CDI, DMF, 1 h
then
H N
NH
NH
H
HO
2
2
H N
N
2
6 h, r.t, 78%
2
O
NH
O
15
Scheme 7.
O
O
NH
NH
OH
N
H
2
H
CDI, DMF, 1 h
H
then
H N
NH
NH
R
R
2
2
2
2
R
R
3
3
6 h, r.t
3: R1 = R2 = H; 80%
20: R1 = R2 = H
4: R1 = R2 = CH3; 78%
21: R1 = R2 = CH3
Scheme 8.

B. C. Das et al. / Tetrahedron Letters 50 (2009) 5860–5863
10. Napoli, J. L. FASEB J. 1996, 10, 993–1001.
5863
this is the first report where guanidine-containing novel retinoids
are synthesized and could potentially be useful to study the reti-
noic acid biology and may be used as therapeutic candidates for
diseases induced by RA signaling.
11. General procedure for preparation of retinoids 20 and 21: A flask equipped with a
magnetic stirring bar, a septum inlet was charged with ylide 22/23 (1 mmol),
dry DMF (5 mL), and tBuONa (3 mmol) under nitrogen. The mixture was stirred
at room temperature for 5–10 min. To this solution was added aldehyde 10
(1 mmol) and the resulting mixture was then stirred at room temperature for
12 h. the reaction mixture was treated with water (20 mL) and neutralized
with 1 M HCl and the product was extracted with ethyl acetate (3 ꢀ 10 mL)
washed with brine, and dried over Na2SO4. The product was isolated by
chromatography over silica gel.
In conclusion, we have synthesized novel retinoids (13, 15, 20,
and 21) and found that when alcohols are outside the ring they
produce unusual Wittig salts, whereas when alcohols are inside
the ring they produce usual Wittig salts. These acids are further
transformed to synthesize guanidine derivatives of new retinoids
through 1,10-carbonyldiimidazole (CDI), which is based on the
addition of guanidine to the activated carboxylic acids promoted
by CDI. Structurally and functionally diverse guanidine-containing
new retinoids were obtained in high yields with high purity. Bio-
logical evaluation of these novel retinoids is currently underway
in our laboratory.
Data for compounds 20 and 21: Compound 20: Mixture of two isomers
(E:Z = 1:0.38) 4-(3-methyl-cyclohex-2-enylidenemethyl)-benzoic acid mp =
176–180 °C; 1H NMR (300 MHz, CDCl3):
d 1.70–1.80 (m), 1.80–1.91 (m),
2.10–2.22 (m), 2.38–2.48 (m), 2.58–2.70 (m), 6.04 (s), 6.15 (s), 6.23 (s), 6.45 (s),
7.36–7.39 (m), 7.46–7.61 (m), 7.65–7.78 (m), 8.04–8.07 (m); 13C NMR (75 MHz,
CDCl3): d 23.2, 24.6, 26.8, 30.9, 31.7, 32.8, 121.1, 121.8, 124.3, 125.8, 127.8,
129.0, 129.2, 129.4, 130.4, 132.5, 132.6, 141.1, 141.8, 143.7, 144.2, 171.8. HR-
MS: (C15H16O2) calcd ([M–H]+) 227.1078; found 227.1086.
Compound 21: Mixture of two isomers (E:Z = 1:0.17) 4-(3,5,5-trimethyl-
cyclohex-2-enylidenemethyl)-benzoic acid mp = 150–155 °C; 1H NMR
(300 MHz, CDCl3 and few drops of CD3OD): d 0.89 (s, 6H), 0.94 (s, 1H), 1.78
(s, 4H), 1.91 (3, 3H), 2.10 (s, 0.4H), 2.33 (s, 2H), 5.97 (s, 1H), 6.13 (s, .17H), 6.26
(s, 1H), 6.38 (s, 0.17H), 7.28–7.31 (m, 3.21 H), 7.94–7.97 (m, 2.50H); 13C NMR
(75 MHz, CDCl3 and few drops of CD3OD): d 24.5, 28.7, 31.3, 40.2, 45.2, 124.9,
126.4, 129.1, 130.0, 139.1, 140.5, 143.8, 169.7. HR-MS: (C17H20O2) calcd
([MꢁH]+) 255.1391; found 255.1399].
Acknowledgments
The author B.C.D. is thankful to AECOM for start-up funding.
The instrumentation in the AECOM Structural NMR Resource is
supported 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
Sciences.
12. General procedure for preparation of guanidino retinoids 1–4: A solution of
guanidine hydrochloride (2 mmol) in DMF:dioxane (1:1; 5 mL) was added
sodium tert-butoxide (2 mmol) and the reaction mixture was heated under
nitrogen at 50–55 °C for 30 min. The mixture was cooled to room temperature,
the solid sodium chloride was filtered and the filtrate was added to the 1-h
stirred solution of retinoid and CDI in DMF at room temperature. The progress
of the reaction was monitored by TLC. After completion of the reaction, water
(10 mL) was added, the solid product was collected by filtration and the solid
was washed with cold water to remove excess guanidine.
Supplementary data
Data for compounds 1–4: Compound 1: N-[3,7-dimethyl-9-(2,6,6-trimethyl-
cyclohex-1-enyl)-nona-2,4,6,8-tetraenoyl]-guanidine. 1H NMR (300 MHz,
CDCl3): d 1.02 (s, 6H), 1.40–1.50 (m, 2H), 1.52–1.60 (m, 2H), 1.69 (s, 3H),
1.95–2.08 (m, 5H), 2.28 (s, 3H), 2.50–2.51 (m, 3H), 5.73–5.85 (m, 1H), 6.17–
6.28 (m, 3H), 6.30–6.40 (m, 1H), 6.83–6.95 (m, 1H); 13C NMR (75 MHz, CDCl3):
d 13.1, 13.8, 13.9, 19.2, 22.0, 29.3, 33.1, 34.3, 127.8, 127.9, 129.8, 130.2, 130.4,
130.5, 130.6, 136.5, 136.9, 137.5, 137.7, 138.6, 138.7, 149.4, 160.4, 170.0; HR-
MS: (C21H32N3O) calcd ([M+H]+) 342.2545; found 342.2556.
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.2009.08.008.
References and notes
Compound 2: N-[4-(3-benzyl-2,4,4-trimethyl-cyclohex-2-enylidenemethyl)-
benzoyl]-guanidine. 1H NMR (300 MHz, CDCl3): d 0.99 (s, 6H), 1.51–1.55 (m,
2H), 1.79 (s, 3H), 2.50–2.51 (m, 2H), 2.66–2.68 (m, 2H), 2.73 (s, 1H), 3.66 (s,
2H), 6.56 (m, 1H), 7.14–7.17 (m, 3H), 7.26–7.36 (m, 3H), 7.43–7.46 (d, J = 9 Hz,
1H), 7.90–7.92 (d, J = 6 Hz, 1H), 8.03–8.05 (d, J = 6 Hz, 1H); 13C NMR (75 MHz,
CDCl3): d 16.8, 24.8, 28.3, 35.7, 36.4, 122.8, 126.3, 128.6, 129.1, 129.4, 129.9,
137.3, 140.5, 141.5, 143.9, 144.7, 163.6, 176.3. HR-MS: (C25H29N3O) calcd
([M+H]+) 388.2391; found 388.2390.
Compound 3: N-[4-(3-methyl-cyclohex-2-enylidenemethyl)-benzoyl]-guani-
dine. 1H NMR (300 MHz, CDCl3): d 1.57–1.1.70 (m, 2H), 1.80 (s, 3H), 2.03–2.13
(m, 2H), 2.48–2.60 (m, 6H), 6.01 (s, 1H), 6.20 (s, 1H), 7.23–7.27 (d, J = 12 Hz,
2H), 7.98–7.02 (d, J = 12 Hz, 2H); 13C NMR (75 MHz, CDCl3): d 23.2, 24.7, 26.8,
30.7, 124.5, 127.9, 128.8, 129.3, 137.4, 139.3, 140.2, 140.7, 142.7, 163.8, 176.4;
HR-MS: (C16H20N3O) calcd ([M+H]+) 270.1606; found 270.1614.
Compound 4: N-[4-(3,5,5-trimethyl-cyclohex-2-enylidenemethyl)-benzoyl]-
guanidine. 1H NMR (300 MHz, CDCl3): d 0.88 (s, 6H), 1.78 (s, 3H), 1.91 (s,
2H), 2.33 (s, 2H), 2.48–2.53 (m, 4H), 2.74 (s, 1H), 2.90 (s, 1H), 3.52 (s, 1H), 6.01
(s, 1H), 6.32 (s, 1H), 7.24–7.28 (d, J = 12 Hz, 2H), 8.01–8.05 (d, J = 12 Hz, 2H);
13C NMR (75 MHz, CDCl3): d 25.0, 29.1, 31.2, 45.1, 126.0, 127.3, 128.8, 129.5,
137.6, 137.9, 138.0, 140.6, 163.2, 176.5; HR-MS: (C18H24N3O) calcd ([M+H]+)
298.1921; found 298.1928.
1. Tabin, C. J. Development 1992, 116, 289.
2. Degos, L.; Chomienne, C.; Daniel, M. T.; Berger, R.; Dombert, H.; Fenaux, P.;
Castaigne, S. Lancet 1990, 336, 1440.
3. Warrell, R. P.; Frankel, S. R.; Miller, W. H.; Scheinberg, D. A.; Itri, L. M.;
Hittelman, W. N.; Vyas, R.; Andreeff, M.; Tafuri, A.; Jakubowski, A.; Gabrilove, J.;
Gordon, M. S.; Dmitrovsky, E. N. Eng. J. Med. 1991, 324, 1385.
4. (a) Katritzky, A. R.; Rogovoy, V. ArkiVoc 2005, 49; (b) Carmignani, M.; Volpe, A.
R.; Delle Monache, F.; Botta, B.; Espinal, R.; De Bonnevaux, S. C.; De Luca, C.;
Botta, M.; Corelli, F.; Tafi, A.; Ripanti, G.; Monache, G. D. J. Med. Chem. 1999, 42,
3116–3125.
5. Carmignani, M.; Volpe, A. R.; Botta, B.; Espinal, R.; De Bonnevaux, S. C.; De Luca,
C.; Botta, M.; Corelli, F.; Tafi, A.; Sacco, R.; Delle Monache, G. J. Med. Chem. 2001,
44, 2950–2958.
6. Delle Monache, G.; Botta, B.; Delle Monache, F.; Espinal, R.; De Bonnevaux, S. C.;
De Luca, C.; Botta, M.; Corelli, F.; Carmignani, M. J. Med. Chem. 1993, 36, 2956–
2963.
7. Delle Monache, G.; Volpe, A. R.; Delle Monache, F.; Vitali, A.; Botta, B.; Espinal,
R.; De Bonnevaux, S. C.; De Luca, C.; Botta, M.; Corelli, F.; Carmignani, M. Bioorg.
Med. Chem. Lett. 1999, 9, 3249–3254.
8. Ekelund, S.; Nygren, P.; Larsson, R. Biochem. Pharm. 2001, 61, 1183.
9. (a) Das, B. C.; Kabalka, G. W. Tetrahedron Lett. 2008, 49, 4695; (b) Das, B. C.;
Mahalingam, S. M.; Evans, T.; Kabalka, G. W.; Anguiano, J.; Hema, K. Chem.
Commun. 2009, 2133.