Expedient synthesis of epigoitrin from L-ascorbic acid

Article abstract of DOI:10.1080/00397911.2013.850092

Epigoitrin is the main bioactive constituent of an important traditional Chinese herbal medicine, Radix isatidis. Reported pharmacological effects of epigoitrin include antiviral, anticancer, and antithyroid activities. Extensive biological exploration of

Full text of DOI:10.1080/00397911.2013.850092

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Expedient Synthesis of Epigoitrin From
L-Ascorbic Acid
Jing-Jing Yang a , Jian-Zhong Wu a & Chunhua Qiao a
a College of Pharamaceutical Science , Soochow University ,
Suzhou , China
Accepted author version posted online: 02 Dec 2013.Published
online: 28 Mar 2014.
To cite this article: Jing-Jing Yang , Jian-Zhong Wu & Chunhua Qiao (2014) Expedient Synthesis
of Epigoitrin From L-Ascorbic Acid, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 44:9, 1240-1244, DOI: 10.1080/00397911.2013.850092
To link to this article: http://dx.doi.org/10.1080/00397911.2013.850092
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Synthetic Communications®, 44: 1240–1244, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397911.2013.850092
EXPEDIENT SYNTHESIS OF EPIGOITRIN FROM
L-ASCORBIC ACID
Jing-Jing Yang, Jian-Zhong Wu, and Chunhua Qiao
College of Pharamaceutical Science, Soochow University, Suzhou, China
GRAPHICAL ABSTRACT
Abstract Epigoitrin is the main bioactive constituent of an important traditional Chinese
herbal medicine, Radix isatidis. Reported pharmacological effects of epigoitrin include anti-
viral, anticancer, and antithyroid activities. Extensive biological exploration of epigoitrin
was constrained by the limited natural source. This article describes our continued effort
toward chemical preparation of epigoitrin from the readily available L-ascorbic acid. Our
new developed synthetic route is composed of nine steps, with an overall yield of 17%.
Keywords Anticancer activity; antiviral activity; chiral pool; epigoitrin; synthesis
INTRODUCTION
Radix isatidis is an important traditional Chinese medicine and was listed in the
2010 edition of Chinese Pharmacopoeia.[1] Reported pharmacological effects of this
plant include antiviral, antibacterial, anticancer, anti endotoxin, and immune system
enhancement activities.[2,3] Among these, the antiviral property is most noteworthy.
About 0.1% of R. isatidis is an alkaloid epigoitrin (Fig. 1), which is used as the bio-
marker for the plant antiviral efficacy.[1,4] Epigoitrin is also found in other crucifer-
ous vegetables such as cabbage, brussels sprouts, and oilseed rape.[5] Pharmacokinetic
study indicated that epigoitrin exhibited greater efficacy and oral bioavailability than
the total alkaloid extraction from R. isatidis.[6]
The general method to obtain epigoitrin has been through column chromatogra-
phy separation of the crude extracts from Isatis indigotica Fort.[7,8] Biotransformation
Received November 22, 2012.
Address correspondence to Chunhua Qiao, College of Pharmaceutical Science, Soochow University,
199 Ren-Ai Road, Suzhou, 215123, China. E-mail: qiaochunhua@suda.edu.cn
1240

TOTAL EXPEDIENT SYNTHESIS OF EPIGOITRIN
1241
Figure 1. Structure of epigoitrin.
of the plant secondary metabolite glucosinolate via the myrosinase-catalyzed hydroly-
sis pathway also provided epigoitrin (Scheme 1).[9,10] In both cases, the natural source
of Isatis indigotica Fort limited the production of suitable amount of this single com-
ponent. Because of the profound antiviral activity and limited natural supply source,
there is a great need to develop a chemical synthesis route for epigoitrin.
Being a small-sized optically pure molecule (MW: 129.18), chemical preparation
of epigoitrin is complicated. Chemical preparation of epigoitrin is rarely reported
in the literature. In 1950, Martin et al. reported the first chemical total synthesis of
epigoitrin.[11] Butadiene monoxide and aqueous ammonia were used as the starting
materials, and this reaction led to the key intermediate 1-amino-3-buten-2-ol as two
racemates. Each pure enantiomer was then obtained by fractional crystallization of
their corresponding oxalates. Later, another research group converted aminoalco-
hol to diastereomeric camphor sulfonic acid ammonium salt to achieve the resolu-
tion of the racemic mixture.[12] Next, 1-amino-3-buten-2-ol was treated with aqueous
alkali and carbon disulfide to furnish epigoitrin.[11] However, butadiene monoxide
Scheme 1. General pathway of myrosinase-catalyzed hydrolysis of epiprogoitrin.[8]

1242
J.-J. YANG, J.-Z. WU, AND C. QIAO
is expensive and difficult to handle, and the high water solubility of aminobutenol
complicated the recovery process from the aqueous reaction system.
Epigoitrin is the breakdown product of epiprogoitrin from R. isatidis. As
a by-product, epigoitrin is produced by enzymatic cleavage of thioglucoside.[12,13]
Thioglucoside was isolated from Crambe abyssinica seeds. In the absence of ferrous
salts, hydrolysis of thioglucoside by myrosinase leads to an intermediate β-hydroxy
isothiocyanate, which spontaneously undergoes cyclization to give epigoitrin.[12,13]
However, the source of the starting material thioglucoside and utilization of enzyme
myrosinase limited the practical application of this method to prepare large-scale
epigoitrin.
Our group had previously developed an effective synthetic route for epi-
goitrin. The overall synthesis was seven steps, with an overall yield of 23%.[14] In
this method, azide hydrogenation to amine is the key reaction step. The reaction
temperature and time have to be strictly controlled to avoid facile intramolecular
cyclization. Meanwhile, the cost of the optical (R)-(+)-4-hydroxy-γ-butyrolactone
as starting material discouraged the application of this synthetic method for large-
scale preparation. Herein, we describe our continued effort to develop a new syn-
thetic method for the preparation of epigoitrin. We employed the readily available
L-ascorbic acid as chirl source. L-ascorbic acid is a natural monosaccharide, has a
rich supply, and is widely used in natural product synthesis.
RESULTS AND DISCUSSION
Our developed synthetic route is shown in Scheme 2. Aldehyde 4 was readily
prepared according to the literature method.[15,16] The chiral center in 4 is derived
from the C5 in L-ascorbic acid, and the configuration is consistent with that in
epigoitrin. By the use of potassium t-butyloxide as a base, the Wittig reaction of
4 with methyltriphenylphosphosphonium bromide led to the formation of alkene
5 in 50% yield after the distillation of the crude mixture under reduced pressure.
Subsequent cleavage of acetonide by acid afforded diol 6. This reaction was initially
carried out in 50% aqueous trifluoroacetic acid (TFA); however, the high polarity
of diol 6 made its extraction from aqueous reaction solution formidable. Because of
Scheme 2. Synthesis of epigoitrin from L-ascorbic acid.

TOTAL EXPEDIENT SYNTHESIS OF EPIGOITRIN
1243
this, TsOH in methanol was employed for the protection to give 6 in good conversion.
Replacement of tosylate by sodium azide provided azido 10. Again, compound
10 was readily lost during reaction workup step because of the low-boiling-point
property. We then converted the tosylate to a phthalimide 8. Amine 9 can be readily
prepared by treating 8 with methylamine at room temperature. After excess
methylamine is removed, addition of thiocarbonyldiimidazole in the same reaction pot
completed the total synthesis of epigoitrin from L-ascorbic acid.
CONCLUSIONS
In summary, the biological important component epigoitrin was successfully
prepared from readily available L-ascorbic acid. This developed synthetic route has
nine steps, with an overall yield of 17%.
EXPERIMENTAL
The 1H NMR and 13 C NMR spectra were recorded using a Varian Unity Inova
1
instrument (operating at 400MHz or 300MHz for proton). H NMR spectrum was
recorded using tetramethylsilane (TMS) as the internal reference; 13 C NMR spec-
trum was recorded using the residual solvent as the internal reference. Chemical
shifts (δ) are given in parts per million (ppm). Coupling constants (J) are reported
as hertz. NMR abbreviations are used as follows: s=singlet, d=doublet, t=triplet,
q=quartet, and m=multiplet. The solutions are taken from a concentrated sample
dissolved in CDCl3, CD3OD, and dimethylsulfoxide (DMSO-d6). High-resolution
mass spectra (HRMS) spectra were acquired on Bruker Esquire liquid chromatog-
raphy-ion trap mass spectrometer. Optical rotations were recorded on Rudolf pola-
rimeter Autopol III. Dry tetrahydrofuran (THF) was distilled from Na/Ph2CO under
argon prior to use. Dry CH2Cl2 was distilled over CaH2 under N2 prior to use. PE
(chromatography eluent) stands for petroleum ether (bp 60–90 °C). DMF stands for
N,N-dimethylformamide. Molecular sieves (4Å) were activated by heating 3×1min
(with the moisture briefly ventilated between the heating sessions by opening the oven
door) in a 500W household microwave oven.
Compound 4 was prepared from L-ascorbic acid according to the literature
method.[15,16] The overall three step yield is 51%. Experimental detail for preparing all
other compounds is provided in the supporting information.
FUNDING
This work was supported by Chinese National Science Foundation (81072514)
to Chunhua Qiao at Soochow University. The Foundation from Jiangsu Province
(SR13200112) and the Priority Academic Program Development (PADD) of Jiangsu
Higher Education Institutions are also acknowledged.

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J.-J. YANG, J.-Z. WU, AND C. QIAO
SUPPLEMENTAL MATERIAL
Supplemental data for this article can be accessed on the publisher’s website.
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