Total synthesis of two isoflavone C-glycosides (6-tert-butyl puerarin and 6-tert-butyl-4′-methoxypuerarin) through the deoxybenzoin pathway

Article abstract of DOI:10.1016/j.tetlet.2017.06.020

The total synthesis of two isoflavone C-glycosides (6-tert-butylpuerarin and 6-tert-butyl-4′-methoxypuerarin) was achieved through the deoxybenzoin pathway with overall yields of 14.6% and 14.2%. The key intermediate 12 was obtained by de-tert-butylation

Full text of DOI:10.1016/j.tetlet.2017.06.020

Accepted Manuscript
Total Synthesis of two isoflavone C-glycosides (6-tert-butyl puerarin and 6-
tert-butyl-4'-methoxypuerarin) through the deoxybenzoin pathway
Yunpeng Zou, Shouguo Zhang, Xiaoxue Wen, Gang wang, Yunbo sun, Shuchen
Liu, Tao Peng, Yue Gao, Lin Wang
PII:
S0040-4039(17)30740-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.06.020
TETL 49012
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
11 May 2017
3 June 2017
6 June 2017
Please cite this article as: Zou, Y., Zhang, S., Wen, X., wang, G., sun, Y., Liu, S., Peng, T., Gao, Y., Wang, L., Total
Synthesis of two isoflavone C-glycosides (6-tert-butyl puerarin and 6-tert-butyl-4'-methoxypuerarin) through the
deoxybenzoin pathway, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.06.020
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Total Synthesis of two isoflavone C-glycosides (6-tert-butyl puerarin and
6-tert-butyl-4'-methoxypuerarin) through the deoxybenzoin pathway
Yunpeng Zou a, b, #, Shouguo Zhang b, #, Xiaoxue Wen b, Gang wang b, Yunbo sun b, Shuchen
Liu b, Tao Peng b, *, Yue Gao a, b, * and Lin Wang a, b,
*
a College of Life Science and Bio-engineering, Beijing University of Technology, Beijing 100124, P. R. China
b Beijing Institute of Radiation Medicine, Beijing 100850, P. R. China
ABSTRACT
The total synthesis of two isoflavone C-glycosides (6-tert-butylpuerarin and
6-tert-butyl-4'-methoxypuerarin) was achieved through the deoxybenzoin pathway with overall
yields of 14.6% and 14.2%. The key intermediate 12 was obtained by de-tert-butylation of 10 with
trifluoroacetic acid and Friedel-Crafts acetylation of 2-C-β-D-glucopyranoside 11. The ring
closure of 12 with the POCl3/DMF reagent resulted glucosyl isoflavone formation 13, which was
debenzylated and demethylated by BBr3 to obtain 14 and 15. This pathway represents a novel
synthetic pathway based on Friedel–Crafts acetylation and Vilsmeier-Haack cyclization to achieve
isoflavone C-glycosides in high yields.
KEYWORDS: C-glycoside; Isoflavone; 6-tert-butylpuerarin; deoxybenzoin pathway;
6-tert-butyl-4'-methoxypuerarin
Isoflavone C-glycosides, in which the sugar moiety is attached by a C–C bond
directly to the isoflavone ring, are not easily hydrolyzed in acidic gastric juices
compared with O-glycosides and aglycone. These glycosides exhibit various
biological activities such as radioprotective,1 anti-myocardial ischemic,2 mitogenic,
and colony-stimulating,3 and antidiabetic4 activities. Among these compounds,
puerarin, which is found mainly in Pueraria radix, show strong anti-myocardial
ischemic effects;5 it expands the coronary artery and cerebrovascular system,6
significantly reduces myocardial oxygen consumption,7 and improves cardiac systolic
function. To improve the efficacy of puerarin and enhance its concentration in the
blood, 6-tert-butylpuerarin and 6-tert-butyl-4'-methoxypuerarin were totally
synthesized to prevent the 7-O-β-D-glucuronide and 4'-O-sulfate formation of the
compound in the liver.8
Numerous active C-glucosylflavonoids, such as vicenin-1,9 flavocommelin,10
saponarin,11 and orientin,12 were synthesized starting from C-glucosyl acetophenones.
The condensation of C-glucosyl acetophenone 1 with
3,4-bis(benzyloxy)benzaldehyde led to the production of C-glucosylchalcone 2, which
yielded orientin (Scheme 1)12 after I2–dimethyl sulfoxide promoted intramolecular
cyclodehydrogenation and a final debenzylation by hydrogenolysis. The synthetic
pathway of C-glucosylisoflavonoids was highly similar to that of
C-glucosylflavonoids. 8-β-D-glucopyranosylgenistein was synthesized via
C-glucosylchalcone 3 formation by aldol condensation of C-glucosyl acetophenone 1
with 4-(benzyloxy)benzaldehyde. The oxidative rearrangement of chalcone 3 with
thallium(III) nitrate yielded a dimethyl acetal 5, which was then cyclizated by
refluxing in 10% HCl and de-O-benzylated by hydrogenolysis to give the desired 8-β-

BnO
OBn
BnO
OBn
BnO
OBn
HO
OH
OBn
OBn
OBn
OH
BnO
BnO
BnO
HO
OBn
OBn
OH
OH
O
O
O
O
O
O
BnO
OH
OBn
R
BnO
OH
BnO
HO
OBn O
2: R = OBn
3: R = H
OBn O
OBn O
4
OH
O
1
orientin
BnO
OBn
HO
BnO
OBn
OH
OBn
OH
OBn
BnO
HO
BnO
O
O
O
O
OMe
OMe
BnO
OH
HO
BnO
O
OBn O
5
OH
O
OBn O
6
OH
OH
OBn
8-β-D-Glucopyranosylgenistein
Scheme 1: Total synthesis of a flavone C-glycoside (orientin) and an isoflavone C-glycoside
(8-β-D-glucopyranosylgenistein)
D-glucopyranosylgenistein (Scheme 1).13
However, previous literature reviews 13-17 showed that the synthesis of isoflavone
C-glycosides only involves the chalcone pathway starting from
C-glucosylacetophenone. Moreover, numerous aryl C-glycosides without acetyl
groups were not used in synthesizing isoflavone C-glycosides, thereby limiting the
synthesis of various isoflavone C-glycosides. Furthermore, highly toxic thallium (III)
nitrate is used in the conventional total synthesis of isoflavone C-glycosides by
oxidative rearrangement of chalcones.13, 15-17 Therefore, a facile green synthesis
method should be developed for synthesizing of C-glucosylisoflavonoids. In this
paper, two isoflavone C-glycosides (6-tert-butylpuerarin and
6-tert-butyl-4'-methoxypuerarin) was totally synthesized through a simple
deoxybenzoin route in five steps for overall yields of 14.6% and 14.2%.
Results and Discussion
Results are shown in Scheme 2. 4,6-di-tert-butylbenzene-1,3-diol (8) was reacted
with 2,3,4,6-tetra-O-benzylglucopyranosyl trifluoroacetimidate (9) to obtain the
desired 2-C-β- D-glucopyranoside 10 using the O→C glycoside rearrangement
methods with TMSOTf as an activator at 0°C;18, 19 moreover, the reaction mixture
temperature was elevated gradually to ambient temperature. We tried to react
2,3,4,6-tetra-O-benzyl-glucopyranosyl trifluoroacetimidate with
1-(5-tert-butyl-2,4-dihydroxyphenyl)-2-(4-methoxyphenyl)ethanone, but the desired
C-glycoside was not detected. This failure was due to the electron-withdrawing acyl
group in the phenol acceptor that decreased the electron cloud density of the oxygen
atoms in the phenolic hydroxyl group, thereby resulting in decreased reactivity with
glycosyl donor. 1-(5-tert-butyl-2,4-dihydroxyphenyl)ethenone was used as the phenol
acceptor, only a minimal amount of the corresponding β-C-glycoside [δ:13.20 (s, 1H;
OH-6), 9.31 (s, 1H; OH-4), 7.59 (s, 1H; H-8), 2.59 (s, 3H; H-1), 1.38 (s, 9H; (CH3)3C)]
was obtained.

O
O
O
O
+
SOCl2
OBn
OH
Cl
7
OBn
OBn
HO
HO
OH
O
O
O
BnO
BnO
BnO
BnO
OH
O
r.t.
BnO
a
+
O
CF3
NPh
BnO
OBn
OBn
HO
10
OBn
9
8
BnO
BnO
BnO
OBn
OBn
HO
OH
OBn
OBn
OBn
OBn
OH
BnO
BnO
BnO
O
O
HO
O
O
O
O
e
c
d
b
HO
HO
OH
HO
OH
HO
7
O
O
O
O
O
OR
14: R = CH3
15: R = H
11
13
12
Scheme 2. Total synthesis of two isoflavone C-glycosides (6-tert-butyl puerarin and
6-tert-butyl-4'-methoxypuerarin). Reagents and conditions: a) TMSOTf, CH2Cl2, 0°C to room
temperature (r.t.), 51.6%; b) CF3COOH, Na2S2O4, r.t., 66.2%; c) AlCl3, Et2O, r.t., 63.6%; d)
POCl3, DMF, 70°C, 68.3%; e) BBr3, CH2Cl2, -78°C, 14 (95.5%), 15 (98.6%).
This low C-glycoside yield from this reaction was likely a result of the poor matching
of the reactivities of the glycosyl donor and phenol acceptor.
Subsequent de-tert-butylation encounters many difficulties. Several conditions of
de-tert-butylation of β-C-glycoside 10 have been attempted and were unsuccessful,
such as treatment of 10 with HBr,20 TfOH,21 AlCl3 in toluene,22 AlCl3 in
dichloromethane,23 sulfuric acid in toluene,24 and AlCl3 in nitromethane and toluene.25
We noticed that 6-tert-butyl group can be removed from β-C-glycoside 10 by stirring
at room temperature for 90 min in the presence of trifluoroacetic acid to allow it to
synthesize 11.26 Subsequently, treating 4-methoxyphenylacetyl acid with thionyl
chloride at room temperature for 4 h resulted in the corresponding acid chloride 7,27
which was reacted with 2-C-β-D-glucopyranoside 11 in the presence of anhydrous
aluminium chloride to result in deoxybenzoin 12 with a 63.6% yield.18, 28
However, the desired deoxybenzoin was not detected when a de-tert-butyl product,
which was obtained through the deprotection of the 4,6-di-tert-butyl groups in 10, was
reacted with 7 and anhydrous AlCl3. This finding illustrated that the large steric
hindrance of the tert-butyl substituent allows the acyl group to be selectively attached
to C-6 in the C-glycoside 11.
Upon obtaining the glucosylisoflavone 13 from 12, we first tried to take advantage
of the reaction of deoxybenzoin 12 in DMF with morpholine and triethyl orthoformate
at 140 °C; however, a poor yield (12%) of 13 was obtained owing to the destruction
of the sugar ring caused by high temperature.29 An increased glucosylisoflavone yield
(68.3%) 13 was prepared by treatment of 12 in DMF with the POCl3/DMF reagent
(prepared at 10°C) at 70 °C for 6 h.30 The reaction mechanism could be as follows:
The reaction of DMF with POCl3 leads to the production of carbocation a, which

attacks the methylene carbon of deoxybenzoin 12 consecutively with a ring closure to
produce glucosylisoflavone 13, as outlined in Scheme 3. However, this method was
[Me2N-CH-O-POCl2]+Cl-
a
HCONMe2
POCl3
+
R
R
R
OPOCl2
NMe2
HO
OH
HO
OH
OH
HO
OH
O
-
-OPOCl2
a
O
O
O
O
R
R
R
HO
O
HO
OH
HO
O
-H+
NMe2
NMe2
HOPOCl2
-
+
OPOCl2
+
Me2NH2
+
O
O
O
O
O
O
R:2,3,4,6-tetra-O-benzyl-β-D-glucopyranoside
Scheme 3. Proposed reactions and mechanisms for converting deoxybenzoin 12 to
glucosylisoflavone 13
not suitable for any of the deoxybenzoins containing a phloroglucinol structure,
because a carbonyl group in the phloroglucinol nucleus that is attacked by the DMF-
POCl3 reagent leads to nuclear formylation and polymerization.30
The subsequent debenzylation and demethylation of 13 with BBr3 proceeded
rapidly to yield 6-tert-butyl-4'-methoxypuerarin (14) and 6-tert-butyl puerarin (15)
with 95.5% and 98.6% yields,31-33 respectively. When isoflavone 13 and BBr3 were
stirred at -78 ° C for 1 h, the de-benzyl product 14 was first produced. The reaction
system was then stirred at room temperature for 10 min to yield the demethylated
product 15. Under hydrogenolysis using 10% Pd–C, the debenzylation of 13 did not
proceed because of the olefin reduction.34-35
Conclusion
Two isoflavone C-glycosides (6-tert-butyl puerarin and
6-tert-butyl-4'-methoxypuerarin) was totally synthesized through a novel synthetic
pathway based on Friedel-Crafts acetylation and Vilsmeier-Haack cyclization in five
steps with overall yields of 14.6% and 14.2%. The facile green deoxybenzoin route
was developed to analyze C-glucosylisoflavonoids and applicable to the large-scale
synthesis of various isoflavone C-glycosides.
Acknowledgment
We are thankful to the National Science Foundation of China for providing financial
supports (Grant No. 81273431; 21102176 and 21272273)
References
1. Kinfe HH, Long HS, Stander MA, et al. S Afr J Bot. 2015;100: 75-79.
2. Dong Z, Guo J, Xing X, et al. Biomed Pharmacother. 2017;89: 297-304.
3. Kawaguchi K, de Mello Alves S, Watanabe T, et al. Planta Med. 1998;64:653-655.
4. Tao Y, Jiang Y, Li W, et al. Anal Methods-UK. 2016;8:4211-4219.
5. Liu SM, Zhang CP, Shi QM, et al. Brain Res Bull. 2014;101:57-63.

6. Chen YY, Chen W, Zhang Q, et al. ExpTher Med. 2016;11:1051-1058.
7. Zhang S, Chen S, Shen Y, et al. Biol Pharm Bull. 2006; 29: 945-950.
8. Yasuda T, Kano Y, Saito K, et al. Biol Pharm Bull. 1995;18:300-303.
9. Sato S, Koide T. Carbohydr Res. 2010;345:1825-1830.
10. Misawa K, Gunji Y, Sato S. Carbohydr Res. 2013;374:8-13.
11. Misawa K, Takahashi Y, Sato S. Chem Pharm Bull. 2013;61:776-780.
12. Kumazawa T, Kimura T, Matsuba S, et al. Carbohydr Res. 2001;334:183-193.
13. Sato S, Hiroe K, Kumazawa T, et al. Carbohydr Res. 2006;341:1091-1095.
14. Sato S, Ishikawa H. Synthesis. 2010;2010:3126-3130.
15. Lee DYW, Zhang WY, Karnati VVR. Tetrahedron Lett. 2003;44:6857-6859.
16. Kato E, Kawabata J. Bioorg Med Chem Let. 2010;20:4333-4336.
17. Jesus AR, Dias C, Matos AM, et al. J Med Chem. 2014;57:9463-9472.
18. Zou YP, Zhang SG, Wang G, et al. J. Carbohyd. Chem. 2016;35:387-395.
19. Li Y, Wei G, Yu B. Carbohydr Res. 2006;341:2717-2722.
20. Hewgill FR, Stewart JM. J Chem Soc Perk T 1. 1988;6:1305-1311.
21. Kaib PS, Schreyer L, Lee S, et al. Angew Chem Int Ed Engl. 2016;
55:13200-13203.
22. Molleti N, Kang JY. Org Lett. 2017;19:958-961.
23. Lewis N, Morgan I. Synthetic Commun. 1988;18:1783-1793.
24. Egami H, Ide T, Kawato Y, Hamashima Y. Chem Commun (Camb).
2015;51:16675-1667.
25. Rosevear J, Wilshire JFK. Aust J Chem. 1985;38:1163-1176.
26. McOmie JFW, Saleh SA. Tetrahedron. 1973;29:4003–4005.
27. Jetter MC, Youngman MA, McNally JJ, et al. Bioorg Med Chem Lett.
2007;17:6160-6163.
28. McOmie JFW, Saleh SA. Tetrahedron. 1973;29:4003-4005.
29. Ferrari M. U.S. Patent 5 973 169, 1999.
30. Kagal SA, Nair PM, Venkataraman K. Tetrahedron Lett. 1962;3:593-597.
31. Nakatsuka T, Tomimori Y, Fukuda Y, et al. Bioorg Med Chem Lett.
2004;35:3201-3203.
32. Öztaşkın N, Çetinkaya Y, et al. Bioorg Chem. 2015;60:49-57.
33. Shi D, Li J, Jiang B, et al. Bioorg Med Chem Lett. 2012;22:2827.
34. Tückmantel W, Kozikowski AP, Romanczyk LJ. J. Am. Chem. Soc.
1999;121:12073-12081.
35. Takumi F, Tomoyuki K, Sachiko K, et al. Tetrahedron. 2004;60:9375-9379.

Research highlights
This was a novel synthetic pathway to achieve isoflavone C-glycosides.
The total synthesis of 6-tert-butyl puerarin was achieved.
The first total synthesis of 6-tert-butyl-4'- methoxypuerarin was achieved.
Graphical Abstract
BnO
OBn
HO
OH
OBn
OH
BnO
HO
O
O
O
HO
OH
HO
OH
3 steps
HO
2 steps
O
O
O
OR
6-tert-butyl-4'-methoxypuerarin: R = CH3, Y: 14.2%
6-tert-butyl puerarin : R = H, Y: 14.6%