Stereoselective total synthesis of all the stereoisomers of (+)- and (?)-febrifugine and halofuginone

Article abstract of DOI:10.1016/j.tetlet.2020.152151

A convenient method for the total synthesis of all the stereoisomers of febrifugine and halofuginone using D-arabinose and L-arabinose as the key starting materials is reported. Apart from the inherent stereocenters in these pentose sugars, the method utilizes the selective hydrogenolysis of the anomeric O-benzyl group, stereoselective Grignard reaction and Wacker oxidation as the key steps to obtain the important precursors for the total synthesis.

Full text of DOI:10.1016/j.tetlet.2020.152151

Journal Pre-proofs
Stereoselective Total Synthesis of All the Stereoisomers of (+)- and (-)-Febri‐
fugine and Halofuginone
Ramu Sridhar Perali, Anjaneyulu Bandi
PII:
S0040-4039(20)30611-0
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152151
TETL 152151
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Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
25 May 2020
11 June 2020
15 June 2020
Please cite this article as: Perali, R.S., Bandi, A., Stereoselective Total Synthesis of All the Stereoisomers of (+)-
and (-)-Febrifugine and Halofuginone, Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.
2020.152151
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Graphical Abstract
Stereoselective Total Synthesis of All the
Stereoisomers of (+)- and (‒)-Febrifugine
and Halofuginone
Leave this area blank for abstract info.
Ramu Sridhar Perali and Anjaneyulu Bandi

1
Tetrahedron Letters
Stereoselective Total Synthesis of All the Stereoisomers of (+)- and (‒)-Febrifugine
and Halofuginone
Ramu Sridhar Perali and Anjaneyulu Bandi
aSchool of Chemistry, University of Hyderabad, Gachi Bowli, Hyderabad – 500046, India.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A convenient method for the total synthesis of all the stereoisomers of febrifugine and
halofuginone using D-arabinose and L-arabinose as the key starting materials is reported. Apart
from the inherent stereocenters in these pentose sugars, the method utilizes the selective
hydrogenolysis of the anomeric O-benzyl group, stereoselective Grignard reaction and Wacker
oxidation as the key steps to obtain the important precursors for the total synthesis.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Carbohydrates
Glycals
Stereoselective
Total synthesis
Natural products
Introduction
The biological properties of (+)-halofuginone 2 in mammals
was initially come across during its usage as veterinary medicine
to treat intestinal protozoal infections.12 In the year 2000, (+)-
halofuginone 2 has received orphan drug designation by the FDA
for the treatment of scleroderma, a chronic connective tissue
disease.13 Currently, 2 is in phase II clinical trials as a therapeutic
to treat cancer, muscular dystrophy and fibrotic diseases.
Recently it was also found that 2 inhibits the Th17 cells, which
play an important role in autoimmune diseases, by activation of
amino acid starvation response (AAR) directly by inhibiting the
prolyl-tRNA synthetase activity of glutamyl-prolyl-tRNA
synthetase (EPRS).14
(+)-Febrifugine
1 and (+)-isofebrifugine 3 are natural
alkaloids isolated from a Chinese medicinal plant, Ch’ang Shan1
in 1940. Later they were also isolated from common garden
plant, hydrangea.2 For over 2000 years, the extracts of Ch’ang
shan have been used as natural remedy for the treatment of
stomach cancer and malaria.1 In solution, (+)-Febrifugine 1
slowly epimerizes into its diastereomer (+)-isofebrifugine 3,
which is comparatively less potent than 1.3 Due to the facile
epimerization of 1 to 3, a couple of structural revisions of these
two natural products were reported in the literature.4 Thus, these
compounds have been the subject of interest concerning their
structural and biological perspective.5 Although Baker and Co-
workers reported the chemical structures and the racemic
synthesis of 1 and 3,6 the absolute stereochemistry of these
compounds was revealed in 1999 by Kobayashi’s total synthesis.7
Since then a couple of methods for the total synthesis of 18 and
3,9 in both racemic and enantioselective fashion, have been
reported. Although 1 was found to be an effective antimalarial
and even more potent than chloroquine,10 due to severe toxicity
inducing side-effects11 like nausea, vomiting and liver damage
associated with it have precluded its transformation into a
medicine. In search of a better analogue of 1 possessing
improved activity and low toxicity profile, the structure-activity
studies led to the discovery of (+)-halofuginone 2 (Figure 1),8 in
which the metabolically vulnerable quinazoline protons in 1 were
replaced by chlorine and bromine atoms.
Figure 1. Molecular structures of (+)-Febrifugine (1), (+)-
halofuginone (2), (+)-isofebrifugine (3) and (+)-isohalofuginone (4).
However, there were very few reports on the stereoselective
synthesis of (+)-halofuginone (2) and (+)-isohalofuginone (4).
Apart from this, the evaluation of biological properties of
enantiomers of 1 to 4 is also very important in therapeutic
perspective of these compounds. Thus, herein we report a
stereoselective method for the total synthesis of all the
stereoisomers of febrifugine and halofuginone.
———
 Corresponding author. Tel.: +91-40-2313-4823; fax: +0-000-000-0000; e-mail: p_ramu_sridhar@uohyd.ac.in

2
Tetrahedron
Results and Discussion
these two diastereomeric piperidines using the silica gel
column chromatography and assigned the configuration at the
Our protocol for the total synthesis began with the use of D-
two stereocenters successfully17 (Scheme 2).
arabinal as chiral pool starting material. Thus, D-arabinose was
initially transformed to the 2-acetoxy-3,4-di-O-acetylglycal 5
involving peracetylation and glycosyl bromide formation using
HBr in acetic acid (33 wt%) followed by 1,8-di-azabicyclo
[5.4.0]undec-7-ene (DBU) mediated dehydrohalogenation
reaction.15 Glycal 5 in presence of BF3·Et2O as a promotor
To obtain the minor diastereomer 13a in good quantity, we
planned to invert the stereocentre at mesylate carbon in the azido
mesylate mixture 12a and 12b. Towards this, substitution of the
secondary mesylate in the azido mesylates 12a and 12b with
cesium acetate provided the diastereomeric mixture of azido
acetates 14a and 14b which upon deacetylation gave the azido
alcohols 15a and 15b in excellent yield. Mesylation of this
alcohol gave the azido mesylates 12a and 12b followed by a one-
pot reductive cyclization provided the piperidine derivatives 13a
and 13b in 7:3 ratio, respectively (Scheme 3). The structure 13b
was further confirmed by crystallization of the corresponding
hydrochloride salt and analyzed by single crystal XRD (Figure
2).18
underwent
a
highly facial-diastereoselective glycosylation
reaction with benzyl alcohol providing dihydropyranone 6.
Diastereoselective reduction of ketone in 6 under Luche
conditions15 provided the allyl alcohol derivative 7 as a single
diastereomer in good yield. Benzylation of this alcohol, to give 8,
followed by regioselective hydrogenolysis with concomitant
hydrogenation of the alkene provided the cyclic hemiacetal 916 in
77 % yield. Treatment of 9 with allyl magnesium bromide
furnished the diols 10a and 10b, which were further mesylated to
give the diastereomeric mixture of dimesylates 11a and 11b, in
3:7 ratio,17 respectively (Scheme 1).
Scheme 3. Stereoselective synthesis of the piperidine
framework.
Figure 2. The ORTEP diagram of compound 13b.HCl.
Scheme 1. Synthesis of dimesylate precursor for the
piperidine ring formation.
In identical to the synthesis of 13a and 13b, L-arabinose was
converted to glycal ent-5 and was further transformed to the
dibenzylpyranose derivative ent-8 as a single enantiomer.
Proceeding reactions on using ent-8 afforded the diastereomeric
piperidine derivatives ent-13a and ent-13b (Scheme 4).
Regioselective nucleophilic substitution of the primary
mesylate in 11a and 11b with sodium azide provided the azido
mesylates 12a and 12b. Conversion of the azide to amine under
Staudinger reduction conditions followed Et3N assisted
cyclization provided the piperidine derivatives 13a and 13b in
3:7 ratio, respectively. Fortunately, we were able to separate
Scheme 4. Synthesis of ent-13a and ent-13b.
Our next objective is conversion of enantiomerically pure
piperidine derivatives to all the stereoisomers of febrifugine and
Scheme 2: Stereoselective synthesis of the piperidine
framework of (+)-febrifugine and (+)-halofuginone.

3
halofuginone. Towards this, the free NH in 13a and 13b was
protected by using Cbz-Cl to give 16a and 16b in excellent yield
(94%).9f Although, various approaches have been reported to
convert the alkene group present in 16a and 16b into the
corresponding methyl ketone, we chose to use the Wacker
oxidation for the first time to obtain the keto-derivatives 17a and
17b in excellent yield. Conversion of this ketone to the silyl enol
ether using TMSOTf and DIPEA followed by bromination with
NBS produced the α-bromo ketones 18a and 18b. Attempts to
purify the 18a and 18b using column chromatography were not
successful. Thus, the obtained crude bromide was subjected to
substitution reaction with quinazolinone that resulted in the
formation of protected (+)-isofebrifugine 19a and (+)-febrifugine
19b8r in good yield. Finally, deprotection of the Cbz-group, as
well as the benzyl, was commenced by refluxing the compounds
19a and 19b in aqueous 6N hydrochloric acid to obtain the (+)-
isofebrifugine 319 and (+)-febrifugine 1 as their di-hydrochloric
acid salt, respectively. On the other hand, the substitution of the
α- bromo ketones 18a and 18b with 7-bromo-6-chloro-4-
hydroxyquinazoline provided the protected (+)-halofuginone
derivatives 20a and 20b. Removal of the protecting groups by
refluxing in 6N hydrobromic acid in water afforded (+)-
isohalofuginone 4 and (+)-halofuginone 2 as their di-HBr salts
(Scheme 5).
·2HBr (ent-2 ·2HBr) (Scheme 6). The structure of the ent-20a
was confirmed by the crystallization of this compound before
subjecting it to the final deprotection reaction (Figure 3).20
Scheme 6. Total synthesis of (‒)-febrifugine and (‒)-
halofuginone and their diastereomers.
Figure 3. ORTEP diagram of compound ent-20a.
Conclusion
In conclusion, an efficient protocol for the total synthesis of
all the possible stereoisomers of febrifugine and halofuginone
have been achieved using naturally abundant pentose sugars, D-
arabinose and L-arabinose as chiral pool starting materials. The
protocol is amicable to scale up. To date, most of the synthetic
analogs of febrifugine and halofuginone were mostly comprise
modification on the quinazolinone ring, whereas the alterations
on the piperidine ring are not much explored. The present
protocol paves the way to acquire molecules possessing the
structural variation on the piperidine framework of this highly
important molecular scaffold. Synthesis of piperidine ring
modified halofuginone analogs is in progress.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgments
The authors thank the DST-SERB grant (File No.
EMR/2016/007816).
References and notes
Scheme 5. Completion of the total synthesis of (+)-
febrifugine and (+)-halofuginone and their diastereomers.
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ent-13a provided the (‒)-isofebrifugine ·2HCl (ent-3 ·2HCl), (‒)-
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provided (‒)-febrifugine ·2HCl (ent-1 ·2HCl), (‒)-halofuginone

4
Tetrahedron
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267.78, monoclinic, space group P 21, a = 8.2091 (11) Å, b =
7.0970 (8) Å, c = 12.9662 (15) Å, V = 753.63(16) Å3, Z = 2, Mo-
Kα radiation (λ= 0.71073), T= 299 K; R1=0.0486, wR2 = 0.1175 [I
≥ 2σ (I)].
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= 638.93, monoclinic, space group P 21, a = 6.1629 (10) Å, b =
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Supplementary Material
Experimental details, characterization data for all compounds,
copies of NMR spectra (1H, 13C and DEPT), and X-ray crystal
structure of compounds (PDF).
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5
Highlights:
 An expeditious total synthesis of all the
stereoisomers of Febrifugine is achieved
 An expeditious total synthesis of all the
stereoisomers of Halofuginone is achieved

Commercially inexpensive carbohydrates were used
as chiral starting materials.
 Wacker oxidation was used as the key step in
generating the key intermediate