Efficient synthesis of dichlorodenafil, an unapproved sildenafil analogue appearing in non-prescription supplements

Article abstract of DOI:10.1248/cpb.c12-00907

We have developed an efficient synthesis of dichlorodenafil (4), an unapproved sildenafil analogue isolated from dietary supplements. Our sequence employs POCl₃-mediated chlorination of readily available chloroacetyl compound 7 followed by sele

Full text of DOI:10.1248/cpb.c12-00907

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Note
Chem. Pharm. Bull. 61(5) 572–575 (2013)
Vol. 61, No. 5
Efficient Synthesis of Dichlorodenafil, an Unapproved Sildenafil
Analogue Appearing in Non-prescription Supplements
Jong Yup Kim,a In Gyun Hwang,b Jae Ho Oh,b Il Hyun Kang,b Sung Won Kwon,a and
,a
Deukjoon Kim*
a Research Institute of Pharmaceutical Sciences, College of Pharmacy, Seoul National University; Seoul 151–742,
b
Korea: and Korea Food and Drug Administration; Cheongwon-gun 363–951, Korea.
Received October 18, 2012; accepted February 7, 2013
We have developed an efficient synthesis of dichlorodenafil (4), an unapproved sildenafil analogue iso-
lated from dietary supplements. Our sequence employs POCl3-mediated chlorination of readily available
chloroacetyl compound 7 followed by selective hydrolysis of the chloro-heterocycle function. Our synthesis
confirms the structure of the illegal additive, and will provide regulatory agencies with ready access to au-
thentic standard samples of dichlorodenafil (4) to aid in their mission to protect the public from unapproved
and potentially harmful erectile dysfunction (ED) drug analogues that are added to herbal and dietary
supplements without providing users with appropriate toxicological or pharmacological information.
Key words dichlorodenafil; sildenafil analogue; illegal food additive; synthesis
Since sildenafil (1) was introduced onto the U.S. market shown in Fig. 2. This new unapproved ED drug analogue pos-
in 1998 for the treatment of erectile dysfunction (ED), other sesses a (Z)-dichlorovinyl moiety in place of the sulfonyl-N-
ED drugs such as vardenafil (2) and tadalafil (3) have subse- methylpiperazine group in sildenafil (1), and was given the
quently been commercialized1,2) (Fig. 1). These synthetic ED name of dichlorodenafil (4). Subsequently, the KFDA amended
medications function by inhibiting the phosphodiesterase type its Standards and Specification of Food to enable the regula-
5 enzyme (PDE-5). For the past several years, unapproved tion of dichlorodenafil (4).5)
synthetic analogues (mostly based on sildenafil, vardenafil,
In order to corroborate the structure (4) that was proposed
and tadalafil) have been routinely identified in ‘all-natural’ for dichlorodenafil based on the spectroscopic analysis, and
herbal remedies and dietary supplements.3) These compounds to provide a standard sample to aid in its detection in dietary
generally exhibit only minor structural differences compared supplements, we embarked on a synthesis of this substance. To
to their approved or commercial counterparts. Based on their our knowledge, no synthetic approach to dichlorodenafil has
structural similarities, it is reasonable to expect these ana- been described in the literature to date. We describe in this
logues to exhibit similar biological activities. In most cases, note an efficient synthesis and confirm that dichlorodenafil
however, there is no information available to the public with possesses structure 4.
regard to the toxicological or pharmacological effects of these
illegal and unapproved analogues.4) Thus, the presence of
Results and Discussion
analogues in herbal supplements and dietary supplements, es-
pecially in an unapproved dosage, could pose a significant risk that dichlorodenafil (4) could be obtained by selective hy-
to public health. drolysis of chloro-heterocyclic function in 5. Kagan and co-
As outlined in our synthetic plan (Chart 1), we envisioned
In 2010, the Korea Food and Drug Administration (KFDA) workers reported that upon exposure to 1.1 to 1.3eq of PCl5,
and the Korea Customs Service (KCS) jointly announced the acetophenone produced a complex mixture which included the
isolation of a new ED drug analogue discovered as an ad- corresponding dichlorovinyl compound (16%, unspecified E/Z
ditive in dietary supplements imported by international air ratio) and chloroacetylbenzene (4%).6) We further envisaged
mail. Spectroscopic studies established the structure of this that chloroacetyl 7-chloropyrazolopyrimidinone 6, prepared
analogue as 5-(5-((Z)-1,2-dichlorovinyl)-2-ethoxyphenyl)-1- by ring chlorination7,8) of known chloroacetyl pyrazolopyrim-
methyl-3-propyl-1H-pyrazolo[4,3-d]pyrimidin-7(6H)-one (4) as idinone 7, might be elaborated to the desired (Z)-dichlorovinyl
Fig. 1. Representative ED Drugs
The authors declare no conflict of interest.
© 2013 The Pharmaceutical Society of Japan
*To whom correspondence should be addressed. e-mail: deukjoon@snu.ac.kr

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compound 5 upon exposure to PCl5 through optimization of POCl3 for 72h with concomitant chlorination of the pyrazo-
the chlorination conditions of Kagan et al. The known chlo- lopyrimidinone nucleus to yield a mixture of 5 and 5′. Prema-
roacetyl compound 7 could be prepared via Friedel–Crafts ture interruption of the reaction [POCl3, reflux, 3h] resulted in
acylation of the commercially available pyrazolopyrimidinone the isolation of chloropyrazolopyrimidine 6. The reaction mix-
intermediate 8.9)
ture was poured into ice and allowed to stand at room temper-
Initially, we pursued a stepwise synthesis of dichlorodenafil ature overnight to destroy excess POCl3. It is worth mention-
(4) as summarized in Chart 2. To this end, known chloroace- ing that when the rate of quenching was hastened by stirring
tyl compound 7 was converted to the corresponding 7-chlo- instead of standing overnight, the process then gave varying
ropyrazolopyrimidinone 6 by treatment with POCl3 in 88% amounts of starting material 7 due to hydrolysis, which we
yield.7,8) We were pleased to find that exposure of the resultant attributed to a temperature rise during the presumably faster
chloroacetyl 7-chloropyrazolopyrimidinone 6 to excess PCl5 (4 quench. Addition of t-BuOH and stirring the resulting solution
equiv.) in refluxing toluene for 12h furnished a mixture of 5 at room temperature for 12h effected selective hydrolysis of
(66% isolated yield) and 5′ in favor of the desired (Z)-isomer the chloroheterocycle to furnish the desired dichlorodenafil (4)
5. Finally, selective hydrolysis of the chloroheterocycle 5 by in 66% isolated yield along with isomeric 4′ (8%).10)
treatment with conc.-HCl in t-BuOH at room temperature for
12h delivered the desired dichlorodenafil (4) in 84% isolated
Conclusion
1
yield.10) In the H-NMR spectrum, the appearance of the vinyl
In summary, we have developed an efficient synthesis of
proton in (Z)-isomer 4 downfield relative to that in (E)-isomer dichlorodenafil (4), an unapproved sildenafil analogue isolated
4′ as indicated in the Chart 2, and the nOe interaction be- from dietary supplements. Our sequence employs POCl3-me-
tween the vinyl proton and the aromatic protons [between the diated chlorination of readily available chloroacetyl compound
H-23 and H-15 and H-23 and H-17] in (Z)-isomer 4 firmly 7 followed by selective hydrolysis of the chloroheterocycle
established the configuration of the dichlorovinyl group as (Z) function. Our synthesis confirms the structure of this illegal
for the major isomer. The spectral data of our synthetic mate- additive, and will provide regulatory agencies with ready ac-
rial were identical to those of dichlorodenafil isolated from cess to authentic standard samples of dichlorodenafil (4) to
dietary supplements.
aid in their mission to protect the public from unapproved
Having accomplished an efficient synthesis and structure and potentially harmful ED drug analogues that are added to
confirmation of dichlorodenafil (4), we proceeded to develop a herbal and dietary supplements without providing users with
streamlined, two-step preparation by using POCl3 for dichlo- appropriate toxicological and pharmacological information.
rovinylation as well as the ring chlorination (Chart 3). To our
satisfaction, the chloroacetyl group in 7 was efficiently elabo- Dichlorodenafil (4) Chloroacetyl compound
Experimental Procedure for Two-Step Synthesis of
(1.42g,
6
rated to the desired (Z)-dichlorovinyl substituent in refluxing 3.65mmol) was suspended in POCl3 (15mL) and heated to
reflux for 72h. After cooling to room temperature, the mix-
ture was poured into crushed ice (150g), and allowed to stand
at room temperature overnight. To the resulting mixture was
added t-BuOH (20mL), and this solution was stirred at room
temperature for 12h. The solution was extracted with CH2Cl2
(30 mL×3), and then, the combined organic layers were suc-
cessively washed with sat. NaHCO3 and brine, dried over
MgSO4, and concentrated in vacuo. The residue was purified
by silica gel column chromatography (hexane/ethyl acetate=
2/1) to give 4 (0.977g, 2.40mmol, 66%) and 4′ (123mg,
0.302mmol, 8%) as white solids.
Fig. 2. Structure of the Illegal ED Drug Analogue Dichlorodenafil (4)
(Z)-7-Chloro-5-(5-(1,2-dichlorovinyl)-2-ethoxyphen-
Chart 1. Synthetic Plan

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Vol. 61, No. 5
yl)-1-methyl-3-propyl-1H-pyrazolo[4,3-d]pyrimidine
(8) 107.6°C; electrospray ionization (ESI)-MS m/z 425 (M+H).
(E)-7-Chloro-5-(5-(1,2-dichlorovinyl)-2-ethoxyphen-
(8′)
1H-NMR (400MHz, CDCl3) δ 7.90 (d, J=2.8Hz, 1H), 7.57
(dd, J=8.8, 2.8Hz, 1H), 7.01 (d, J=8.8Hz, 1H), 6.66 (s, 1H), yl)-1-methyl-3-propyl-1H-pyrazolo[4,3-d]pyrimidine
4.37 (s, 3H), 4.14 (q, J=6.8Hz, 2H), 3.05 (t, J=7.6Hz, 2H), 1H-NMR (400MHz, CDCl3) δ 8.04 (d, J=2.0Hz, 1H), 7.67
1.91 (sextet, J=7.6Hz, 2H), 1.37 (t, J=6.8Hz, 3H), 1.03 (t, (dd, J=8.8, 2.0Hz, 1H), 7.03 (d, J=8.8Hz, 1H), 6.46 (s, 1H),
J=7.6Hz, 3H); 13C-NMR (100MHz, CDCl3) δ 158.4, 156.6, 4.37 (s, 3H), 4.15 (q, J=6.8Hz, 2H), 3.05 (t, J=7.6Hz, 2H),
146.8, 145.7, 143.2, 135.3, 130.3, 129.2, 128.4, 128.1, 127.8, 1.91 (sextet, J=7.6Hz, 2H), 1.38 (t, J=6.8Hz, 3H), 1.03 (t,
114.9, 113.5, 64.9, 38.7, 28.0, 22.3, 14.8, 14.2; IR (neat) 1587, J=7.6Hz, 3H); 13C-NMR (100MHz, CDCl3) δ 158.2, 156.7,
1522, 1498, 1443, 1255, 1224, 1044, 900, 802, 671; mp 146.9, 145.8, 143.2, 132.9, 132.3, 131.7, 127.8, 127.7, 126.7,
Reaction conditions: (a) POCl3, reflux, 4h, 88%; (b) PCl5 (4eq.), toluene, reflux, 12h, 81%; and (c) conc.-HCl, t-BuOH, rt, 12h, 84% for 4 and 81% for 4′.
Chart 2. Stepwise Synthesis of Dichlorodenafil (4)
Reaction conditions: (a) POCl3, reflux, 72h; (b) t-BuOH/H2O, rt, 12h.
Chart 3. Two-Step Synthesis of Dichlorodenafil (4)

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575
113.5, 112.9, 64.8, 38.2, 28.1, 22.3, 14.9, 14.2; IR (neat) 1501, 682; mp 170.5°C; ESI-MS m/z 407 (M+H).
1467, 1397, 1284, 1262, 1176, 1040, 960, 866, 805; mp 93.7°C;
ESI-MS m/z 425 (M+H).
Acknowledgment This work was supported by the NRF
(Z)-5-(5-(1,2-Dichlorovinyl)-2-ethoxylphenyl)-1- grant funded by the Korea Food and Drug Administration
methyl-3-propyl-1H-pyrazolo[4,3-d]pyrimidin-7(6H)-one (Grant No. 12162KFDA101).
(4) 1H-NMR (400MHz, CDCl3) δ 10.9 (br s, 1H), 8.60 (d,
J=2.4Hz, 1H), 7.62 (dd, J=8.8, 2.8Hz, 1H), 7.05 (d, J=8.8Hz,
1H), 6.75 (s, 1H), 4.32 (q, J=6.8Hz, 2H), 4.28 (s, 3H), 2.95 (t,
J=7.6Hz, 2H), 1.88 (sextet, J=7.6Hz, 2H), 1.62 (t, J=6.8Hz,
3H), 1.05 (t, J=7.6Hz, 3H); 13C-NMR (100MHz, CDCl3) δ
157.2, 153.9, 147.6, 147.0, 138.6, 134.7, 130.6, 129.7, 129.4,
124.6, 120.6, 116.0, 113.2, 65.8, 38.4, 27.9, 22.5, 14.8, 14.2; IR
(neat) 1697, 1494, 1391, 1249, 1158, 1126, 1032, 890, 779, 669;
mp 138.5°C; ESI-MS m/z 407 (M+H).
(E)-5-(5-(1,2-Dichlorovinyl)-2-ethoxylphenyl)-1-
methyl-3-propyl-1H-pyrazolo[4,3-d]pyrimidin-7(6H)-one
(4′) 1H-NMR (400MHz, CDCl3) δ 10.9 (br s, 1H), 8.78 (d,
J=2.8Hz, 1H), 7.72 (dd, J=8.8, 2.4Hz, 1H), 7.07 (d, J=8.8Hz,
1H), 6.54 (s, 1H), 4.34 (q, J=7.2Hz, 2H), 4.27 (s, 3H), 2.94 (t,
J=7.6Hz, 2H), 1.88 (sextet, J=7.6Hz, 2H), 1.62 (t, J=7.2Hz,
3H), 1.03 (t, J=7.6Hz, 3H); 13C-NMR (100MHz, CDCl3) δ
157.0, 154.0, 147.7, 147.0, 138.8, 133.0, 132.3, 131.6, 128.2,
124.7, 120.3, 114.6, 112.8, 65.8, 38.4, 28.0, 22.5, 14.9, 14.3; IR
(neat) 1687, 1561, 1492, 1319, 1244, 1157, 1035, 920, 812, 752,
References
1) Giovannoni M. P., Vergelli C., Graziano A., Dal Piaz V., Curr. Med.
Chem., 17, 2564–2587 (2010).
2) Wang R., Burnett A. L., Heller W. H., Omori K., Kotera J., Kik-
kawa K., Yee S., Day W. W., DiDonato K., Peterson C. A., J. Sex.
Med., 9, 2122–2129 (2012).
3) Park H. J., Jeong H. K., Chang M. I., Im M. H., Jeong J. Y., Choi D.
M., Park K., Hong M. K., Youm J., Han S. B., Kim D. J., Park J. H.,
Kwon S. W., Food Addit. Contam., 24, 122–129 (2007).
4) Gratz S. R., Gamble B. M., Flurer R. A., Rapid Commun. Mass
Spectrom., 20, 2317–2327 (2006).
5) KFDA Notification No. 2011-76 (Amended on June 11, 2011).
6) Kagan J., Arora S. K., Bryzgis M., Dhawan S. N., Reid K., Singh S.
P., Tow L., J. Org. Chem., 48, 703–706 (1983).
7) Kumar R., Joshi Y. C., J. Indian Chem. Soc., 84, 1261–1265 (2007).
8) Kumar R., Joshi Y. C., Indian J. Chem., 46B, 2021–2025 (2007).
9) Kumar R., Joshi Y. C., J. Chem. Sci., 121, 497–502 (2009).
10) Guanghui T., Yi Z., Zheng L., Zhen W., Jingsham S., Sergeja B.,
Tadej S., WO2008/074512 (2008).