Asymmetric total synthesis of gracilamine and determination of its absolute configuration

Article abstract of DOI:10.1021/acs.orglett.7b02517

(+)-Gracilamine, a biologically attractive and structurally unique pentacyclic Amaryllidaceae alkaloid, was biomimetically synthesized in 11 linear steps in 9.9% overall yield from the known racemic oxocrinine. The synthesis features an asymmetric hydrogenation, a ring-opening/benzylic oxidation/cyclization sequence, and a biomimetic intramolecular cycloaddition. This total synthesis not only allows the assignment of its absolute configuration, but also provides experimental support for the hypothesis that naturally occurring (+)-gracilamine is biogenetically derived from the crinine-type alkaloid (+)-epivittatine.

Full text of DOI:10.1021/acs.orglett.7b02517

Letter
pubs.acs.org/OrgLett
Asymmetric Total Synthesis of Gracilamine and Determination of Its
Absolute Configuration
Xiao-Dong Zuo,† Shu-Min Guo,† Rui Yang,† Jian-Hua Xie,*,† and Qi-Lin Zhou†,‡
†State Key Laboratory and Institute of Elemento-organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China
‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China
S
* Supporting Information
ABSTRACT: (+)-Gracilamine, a biologically attractive and structurally unique pentacyclic Amaryllidaceae alkaloid, was
biomimetically synthesized in 11 linear steps in 9.9% overall yield from the known racemic oxocrinine. The synthesis features an
asymmetric hydrogenation, a ring-opening/benzylic oxidation/cyclization sequence, and a biomimetic intramolecular
cycloaddition. This total synthesis not only allows the assignment of its absolute configuration, but also provides experimental
support for the hypothesis that naturally occurring (+)-gracilamine is biogenetically derived from the crinine-type alkaloid
(+)-epivittatine.
atural products, such as the Amaryllidaceae alkaloids, are
an invaluable source of potential leads for drug
oxidation/hydrolysis of the benzylic C−N bond of a crinine-
type alkaloid such as (+)-epivittatine (2)8 provides aldehyde 3.
Reaction of 3 with leucine followed by intramolecular [3 + 2]
cycloaddition of the resulting imine (4) affords 5, which has a
pentacyclic skeleton. Esterification of the carboxyl group and
selective methylation of the less hindered pyrrolidine ring of 5
yield gracilamine.
N
discovery.1 Amaryllidaceae alkaloids are a large class of
structurally complex molecules isolated from plants of the
Amaryllidaceae family,2 extracts of which have long been used
as traditional remedies for treatment of various diseases.3
Gracilamine (1, Scheme 1) is an unusual pentacyclic
̈
On the basis of this hypothetical biosynthetic pathway, Ma et
al.5a completed the total synthesis of gracilamine in 2012. They
used a key intermediate 6 without a fused pyrrolidine ring and
completed the synthesis of 1 in racemic form through a
biomimetic intramolecular [3 + 2] cycloaddition to assemble its
two fused five-membered BE-rings (Scheme 1b). This highly
elegant work provides the first solid evidence supporting the
proposed biosynthetic pathway. Just recently, another precise
biomimetic synthesis was reported by Banwell et al. which also
supports the hypothesis of an intramolecular [3 + 2]
cycloaddition to install the fused five-membered BE-rings of
1.5c They employed an aldehyde 8 containing C3a-arylhexahy-
droindole unit (ACD-rings), which was synthesized by a
palladium-catalyzed intramolecular Alder-ene reaction, as a key
intermediate (Scheme 1b). However, because the absolute
configuration of gracilamine remains unknown, the biogenetic
precursor of the molecule has not been conclusively identified.
The unique fused pentacyclic structure bearing seven
contiguous stereocenters, two of which are quaternary centers,
and the potential biological activities of gracilamine have
dinitrogenous Amaryllidaceae alkaloid isolated by Unver and
Kaya in 2005 from Galanthus gracilis, which grows in Turkey.4
The structure and relative stereochemistry of gracilamine
(except for the configuration of the hydroxyl-substituted
carbon atom on the cyclohexane ring) were determined by
NMR spectroscopy and recently confirmed by total synthesis
and X-ray crystallography,5 but the absolute configuration of
the molecule remains to be determined. In addition, because 1
was isolated from the source of plant by an ethanol extraction
process, the ethyl ester unit associated with it is probably an
artifact of the isolation process, the actual natural product being
either the corresponding free acid or another ester.5c More
importantly, its biological activities have not been evaluated
because only a limited amount of the optically pure sample has
been obtained from the natural source (3.8 mg of 1 was isolated
from 5.25 kg of dried, powered G. gracilis).4
̈
Unver and Kaya proposed that gracilamine is biosynthesized
from a crinine- or tazettine-type alkaloid via an intramolecular
[3 + 2] cycloaddition reaction with leucine to form the
pentacyclic skeleton.4 On the basis of this proposal and the fact
that tazettine-type alkaloids are derived from crinine-type
alkaloids,6 Jin suggested a biosynthetic pathway involving a
crinine-type alkaloid (Scheme 1a).7 Specifically, biocatalytic
Received: August 14, 2017
© XXXX American Chemical Society
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Scheme 1. Proposed Biosynthetic Pathway and Biomimetic
Total Synthesis of Gracilamine
Scheme 2. Possible Biogenetic Precursors and Our
Retrosynthetic Analysis of Gracilamine
recently also attracted the attention of other synthetic research
groups. For example, Gao et al.5b reported a total synthetic
strategy featuring a photo-Nazarov reaction, intramolecular
hetero-Michael, and intramolecular Mannich reaction to
construct the core structure of 1. Following the work of Gao
et al., Snyder et al.9 described a formal synthesis of 1 using an
intramolecular Diels−Alder reaction as the key step to install
the octahydroindole-fused tetracyclic ABCD-skeleton. Yu et
al.10 developed an asymmetric route to synthesize the cyclic
diketone bearing a tricyclic ABD-core framework, the key
intermediate of Gao’s synthesis of 1, using a rhodium-catalyzed
[3 + 2 + 1] cycloaddition of an optically active 1-yne−
vinylcyclopropane and CO.11 In addition, Adrio, Carretero, and
co-workers reported catalytic asymmetric construction of the
tricyclic ABD-core of 1 through a copper-catalyzed 1,3-dipolar
cycloaddition of an azomethine ylide and an activated 1,3-
diene.12 However, the asymmetric total synthesis of 1 has not
been accomplished to date. In this paper, we report the first
asymmetric total synthesis of 1 based on the hypothetical
biosynthetic pathway and the determination of the absolute
configuration, as well as the biogenetic precursor of naturally
occurring (+)-1.
On the basis of evidence that the hydroxyl and piperonyl
groups on the cyclohexene ring of 1 are trans to each other, we
deduced that naturally occurring (+)-epivittatine (2)8 is the
most likely biogenetic precursor of (+)-1, although its antipodal
isomer, (−)-epicrinine (10),13 which has not yet been
discovered in nature, is also a possible precursor (Scheme
2a). Thus, we envisaged a new biomimetic synthetic route to
(+)-1 via N-Troc-protected aldehyde 11 (Scheme 2b) instead
of the aldehyde 3, the intermediate in the biosynthetic pathway
proposed by Jin.7 Aldehyde 11, the key compound for the
intramolecular [3 + 2] cycloaddition to form the fused
pentacyclic ring of gracilamine, would be prepared by reduction
of the amide group of tetracyclic compound 12 to the
corresponding hemiaminoacetal group and subsequent ring
opening with TrocCl. Compound 12 would be synthesized by
removal of the Troc group from the amide of 13 and formation
of the five membered aza ring via a cyclization reaction.
Compound 13 would be obtained by ring opening of (+)-O-
TBDPS-epivittatine ((+)-trans-14),5a followed by benzylic
oxidation. The compound (+)-trans-14 would be obtained by
asymmetric hydrogenation of racemic rac-oxocrinine (15)
catalyzed by chiral iridium catalyst Ir−(R)-SpiroPAP ((R)-
16),8d which was highly efficient for the asymmetric hydro-
genation of ketones, ketoesters, enones, and esters.14 Thus,
with a catalytic asymmetric hydrogenation of racemic rac-
oxocrinine (15), a sequence of ring-opening/benzylic oxida-
tion/cyclization, and a biomimetic intramolecular [3 + 2]
cycloaddition the gracilamine 1 could be synthesized in
optically active form. Note that Ma et al. have tried to
synthesize 1 by means of a similar hypothetical biosynthetic
pathway, but they eventually abandoned this route because they
were unable to obtain the key aldehyde intermediate, an
analogue of 11, from rac-oxocrinine (15) with their designed
one-pot ring opening/debenzylation strategy;5a in addition, if
compound 12 could be obtained from direct benzylic oxidation
of compound 14, a more concise route may be possible.
We found that asymmetric hydrogenation of rac-1515
catalyzed by (R)-16 and subsequent protection with TBDPSCl
afforded O-TBDPS-protected (+)-epivittatine ((+)-trans-14,
43%, 93% ee) and O-TBDPS-protected (−)-crinine16
((−)-cis-14, 38%, 97% ee) on a gram scale (Scheme 3).8d We
initially converted (+)-trans-14 to tetracyclic compound (+)-12
via direct benzylic oxidation, but all attempts were unsuccess-
ful.17 The reason for this is likely owing to the effect of steric
B
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Scheme 3. Asymmetric Synthesis of N-Troc-Protected
Scheme 4. Asymmetric Synthesis of (+)-Gracilamine
Aldehyde 11
asymmetric total synthesis of (+)-gracilamine. The absolute
configuration of (+)-1 was established to be (3aR,4S,5S,6-
R,7aS,8R,9aS) as depicted in Scheme 4 on the basis of the
known structure of (+)-epivittatine (2).8c,d This result suggests
that 2 is the biosynthetic precursor of naturally occurring
(+)-gracilamine.
For assessment of biological activity, we also synthesized
(−)-gracilamine ((−)-1) from O-TBDPS-protected (−)-epi-
crinine ((−)-trans-14), which was obtained in the step of
asymmetric hydrogenation of rac-15 with (S)-16 (Scheme 5).
By using the procedure described for (+)-gracilamine, we
synthesized (−)-gracilamine in 9.3% overall yield.
hindrance of the 5,10b-enthano bridge on benzylic hydrogens.
Thus, we then tried selective opening of the aza-five membered
ring of (+)-trans-14, followed by benzylic oxidation and
cyclization. The conversion of (+)-trans-14 to (+)-12 was
accomplished on a multiple-gram scale in high yield (Scheme
3). The (+)-trans-14 was converted to (+)-17 in 87% yield by
reaction with TrocCl (2,2,2-trichloroethyl chloroformate).
Fortunately, we found that the NaClO2 is a suitable oxidant
for the benzylic oxidation of (+)-17, yielding (+)-13 in 70%
yield according to Zhang’s protocol.18 After removal of the
Troc group of (+)-13 with Zn/HOAc and subsequent
cyclization in the presence of NaH, (+)-12 was obtained in
95% yield (two steps). Reduction of (+)-12 with LiAlH4
afforded 19 as a 4:1 mixture of diastereoisomers in 95%
yield. Reaction of (+)-19 with TrocCl afforded N-Troc-
protected aldehyde (+)-11 in 87% yield.
Condensation of aldehyde (+)-11 with leucine ethyl ester
hydrochloride, followed by intramolecular [3 + 2] cycloaddition
of the resulting imine intermediate according to Ma’s
procedure,5a produced (+)-20 as a single isomer in 59% yield
(Scheme 4). Treatment of (+)-20 with Zn/HOAc to remove
the Troc group and selective methylation of the resulting
unprotected pyrrolidine ring with MeI afforded (+)-22 in 83%
yield (two steps). Deprotection of the hydroxyl group of
(+)-22 with NH4·HF2 afforded (+)-gracilamine ((+)-1). The
NMR spectroscopic data and the optical rotation ([α]2D5 +20.0
(c 0.133, MeOH); natural: [α]D +21.8 (c 0.133, MeOH)) of
our synthetic sample were identical to those reported for
naturally occurring (+)-gracilamine.4 Thus, we completed the
synthesis of (+)-1 in an overall yield of 9.9% in 11 steps from
known rac-oxocrinine (rac-15).19 This represents the first
Scheme 5. Asymmetric Synthesis of (−)-Gracilamine
In conclusion, the first asymmetric total synthesis of
(+)-gracilamine has been achieved. Using an iridium-catalyzed
asymmetric hydrogenation of a cyclohexenone as the key step,
and the following ring-opening/benzylic oxidation/cyclization
sequence, in addition to a biomimetic intramolecular [3 + 2]
cycloaddition, we synthesized (+)-gracilamine in 11 linear steps
in 9.9% overall yield from known racemic oxocrinine (15 linear
steps in 6.3% overall yield from commercially available
materials; lit.5 1.4−4.5% overall yield/17−18 linear steps).
The absolute configuration of naturally occurring (+)-gracil-
amine was assigned. This work provides experimental support
for the hypothesis that naturally occurring (+)-gracilamine is
biogenetically derived from the crinine-type alkaloid (+)-epi-
vittatine. The reported reaction sequence lends itself to the
production of analogues that might be useful for probing the
structure−activity relationship profile of this class of com-
pound. These works are now in progress in our laboratory.
C
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Letter
326. For selected papers on applications of them for asymmetric
hydrogenations, see: (d) Xie, J.-H.; Liu, X.-Y.; Yang, X.-H.; Xie, J.-B.;
Wang, L.-X.; Zhou, Q.-L. Angew. Chem., Int. Ed. 2012, 51, 201.
(e) Zhang, Q.-Q.; Xie, J.-H.; Yang, X.-H.; Xie, J.-B.; Zhou, Q.-L. Org.
Lett. 2012, 14, 6158. (f) Yang, X.-H.; Xie, J.-H.; Liu, W.-P.; Zhou, Q.-L.
Angew. Chem., Int. Ed. 2013, 52, 7833. (g) Yang, X.-H.; Wang, K.; Zhu,
S.-F.; Xie, J.-H.; Zhou, Q.-L. J. Am. Chem. Soc. 2014, 136, 17426.
(h) Yang, X.-H.; Yue, H.-T.; Yu, N.; Xie, J.-H.; Zhou, Q.-L. Chem. Sci.
2017, 8, 1181. (i) Liu, Y.-T.; Chen, J.-Q.; Li, L.-P.; Shao, X.-Y.; Xie, J.-
H.; Zhou, Q.-L. Org. Lett. 2017, 19, 3231.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02517.
■
S
Experimental procedures and characterization data for
the intermediates and products (PDF)
(15) rac-15 was prepared in four steps with 64% yield from
commercially available materials by using Node’s biomimetic
procedure. See: Kodama, S.; Takita, H.; Kajimoto, T.; Nishide, K.;
Node, M. Tetrahedron 2004, 60, 4901.
(16) For isolation of (−)-crinine, see: (a) Mason, L. H.; Puschett, E.
R.; Wildman, W. C. J. Am. Chem. Soc. 1955, 77, 1253. (b) Berkov, S.;
Sidjimova, B.; Evstatieva, L.; Popov, S. Phytochemistry 2004, 65, 579.
For the enantioselective synthesis of (+)-epivittatine, see: (c) Overman,
L. E.; Sugai, S. Helv. Chim. Acta 1985, 68, 745.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jhxie@nankai.edu.cn.
ORCID
Jian-Hua Xie: 0000-0003-3907-2530
Qi-Lin Zhou: 0000-0002-4700-3765
Notes
(17) A wide range of oxidants including DIAB, PhIO, NaClO2,
TBHP, and NBS and metal catalysts such as Ru(bpy)3Cl2, Rh2(cap)4,
FeCl3, and Co(OAc)2 were evaluated.
The authors declare no competing financial interest.
(18) Song, A.-R.; Yu, J.; Zhang, C. Synthesis 2012, 44, 2903.
(19) It is noteworthy that a small amount of crystals, which were
suitable for X-ray analysis, were obtained as a racemic mixture,
although our synthetic (+)-gracilamine was obtained with high
enantioselectivity (98% ee); see the Supporting Information.
ACKNOWLEDGMENTS
■
This project was supported by the National Natural Science
Foundation of China (Nos. 21325207, 21421062, and
21532003), the National Basic Research Program of China
(2012CB821600), the “111” project (B06005) of the Ministry
of Education of China, and the National Program for Support
of Top-notch Young Professionals.
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