Biomimetic Total Syntheses of Sanctis A-B with Structure Revision

Article abstract of DOI:10.1021/acs.orglett.9b04486

The first concise total syntheses of sanctis A and B were reported, and it enabled revision of the structure of sanctis B through single-crystal X-ray diffraction. The established synthetic approach mainly mimics a biosynthetic olefin isomerization/hemiac

Full text of DOI:10.1021/acs.orglett.9b04486

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Letter
Biomimetic Total Syntheses of Sanctis A−B with Structure Revision
Luqiong Huo, Chunmao Dong, Miaomiao Wang, Xiuxiang Lu, Wenge Zhang, Bao Yang, Yunfei Yuan,
Shengxiang Qiu, Hongxin Liu,* and Haibo Tan*
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ABSTRACT: The first concise total syntheses of sanctis A and B
were reported, and it enabled revision of the structure of sanctis B
through single-crystal X-ray diffraction. The established synthetic
approach mainly mimics a biosynthetic olefin isomerization/
hemiacetalization/dehydration/[3 + 3]-type cycloaddition cascade
sequence, offering a viable synthetic methodology to efficiently
access sanctis A−B and their analogues.
ith the field of phenolic plant metabolites, A-type
procyanidins and their analogues are important,
because of the prevelance of their represented architectural
W
dioxabicyclo-[3.3.1]nonane scaffold in a wide array of
1
pharmaceuticals and natural products. Especially in “dragon’s
blood”, which is one of the major components of the
traditional Chinese hemostatic prescription “Yun-Nan-Bai-
2
Yao”, A-type procyanidins have also been verified to exist.
Notably, the available data revealed that A-type procyanidins
and their analogues are usually endowed with diverse biological
properties and potential therapeutic applications, including
1
c,3
4
5
antioxidant,
anti-inflammatory, anti-MRSA, tyrosinase
6
7
inhibitory, antimicrobial, and antibiofilm activities. Their
potent bioactivities and fascinating architectural features have
made them appealing targets for synthetic chemists in recent
8
years.
Sanctis A-C are other A-type procyanidin analogues, which
Figure 1. Represent A-type procyanidins and sanctis A−C.
were isolated from the Lichen Parmotrema sancti-angelii in
9
2
018 (Figure 1). Different from the well-known A-type
condensation and dehydroxylation. Subsequent olefin isomer-
ization and hemiacetalization reactions can rationally offer
hemiacetal 9. Biosynthetically, hemiacetal 9 may be occupied
by orcinol i to undergo a remarkable spontaneous hemi-
acetalization/dehydration/formal [3 + 3] cycloaddition
cascade process, similar to our previous strategy utilized in
the biomimetic total synthesis of myrtucommuacetalone and
procyanidin, sanctis A−C (1−3) feature a unique tetracyclic
dioxabicyclo[3.3.1]nonane skeleton with a pendent methyl
group, instead of an aromatic ring, displayed at C9. The
unprecedented characteristics have driven us to become
fascinated with their biological activities, which are significant
to gain insight into the structure−activity relationship of A-
1
0,11
type procyanidins.
Nevertheless, the biological activities
14−16
myrtucommulone J,
finishing the construction of unique
were rare in the original report, probably because of their poor
availability. Attracted by the potential biological activities and
their unique structural features, herein, we report the first
bioinspired total syntheses of sanctis A−B (1−2).
tetracyclic dioxabicyclo[3.3.1]nonane scaffold. Presumably,
both of the target sanctis A−B (1−2) might be generated
simultaneously, depending on the alternative regioselectivity in
the process of the formal [3 + 3]-type cycloaddition.
With the inspiration of their archtectural characteristics, our
hypothetical biogenetic pathway of sanctis A−B (1−2) was
proposed (Scheme 1), which was mainly reflected by their
Received: December 14, 2019
1
2
origination from natural product methyl hematommate (7)
1
3
and oricinol i with a series of functional group elaboration
and dimerization. Briefly, the key biosynthetic precursor 8
might be generated from methyl hematommate 7 via aldol
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.9b04486
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Scheme 1. Proposed Biogenesis of Sanctis A−B
Table 1. NMR Comparison of Sanctis B and iso-Sanctis B
Sanctis B
iso-Sanctis B
No.
2
2′
ΔδH
No.
2
2′
ΔδC
H-1
H-2
H-3
H-4
H-5
H-6
H-7
H-8
H-9
H-10
6.32, s
−
−
−
−
−
4.52, t
2.08, m 2.13, m
−
−
6.28, s
−
−
−
−
−
4.55, t
−0.04
−
−
−
−
C-1
C-2
C-3
C-4
C-5
C-6
C-7
C-8
C-9
C-
109.9
135.9
113.0
154.4
115.7
155.3
24.2
32.8
98.3
152.2
112.7
141.8
106.2
162.2
113.5
158.1
24.1
32.8
98.8
154.6
−2.8
+5.9
−6.8
+7.8
−2.2
+2.8
−0.1
+0.0
+0.5
+2.4
−
+0.03
+0.05
−
−
−
−
1
0
1
2
3
4
H-11
H-12
−
−
−
−
−
C-
117.1
138.1
111.3
157.2
101.7
19.9
117.5
138.2
111.6
157.5
101.9
20.1
+0.4
+0.1
+0.3
+0.3
+0.2
+0.2
+0.2
+5.0
+1.0
+4.9
1
−
C-
1
C-
1
C-
1
With the scenario in mind, the total syntheses of sanctis A−
B (1−2) mimicking their biogenesis pathway were followed
H-13 6.23, d 6.26, d
H-14
+0.03
(
see Scheme 2). Methyl orsellinate 12 could be obtained
−
Scheme 2. Biomimetic Total Syntheses of Sanctis A (1) and
iso-Sanctis B (2′)
H-15 6.15,
brd
H-16 2.50, s
6.15, d
2.53, s
1.77, s
+0.00
+0.03
+0.05
C-
15
C-
1
6
7
8
9
0
H-17 1.72, s
C-
1
C-
1
C-
1
C-
2
27.3
27.5
H-18
−
168.7
51.7
173.7
52.7
H-19 3.79, s
H-20 2.10, s
3.93, s
2.43, s
+0.14
+0.33
19.4
24.3
4
1
-
8.94, s
8.03, s
12.08, s +4.14
8.04, s +0.01
OH
4-
OH
informative results collectively pointed to the possibility of
incorrect structural establishment for sanctis B (2), via the
misinterpretation of NMR spectra attributable to its architec-
tural complexity. Moreover, the structure of synthetic product
2′ could be further confirmed by the two-dimensional (2D)
NMR interpretation (see Scheme 3), thus strengthening the
commercially or synthesized from the readily available methyl
17,18
acetoacetate 10 and lactone 11 in 33% yield.
The selective
formylation of methyl orsellinate 12 then was performed
19
smoothly in the titanium tetrachloride reaction system,
resulting in the synthesis of methyl hematommate 7 in 87%
yield. 7 could be further converted to biosynthetic precursor 8
using the Wittig reaction with a yield of 83%, as expected. With
biosynthetic precursor 8 in hand, the key cascade reaction was
conducted with PTSA as a catalyst in toluene at 90 °C.
Gratifyingly, the proposed hemiacetalization/dehydration/
formal [3 + 3] cycloaddition cascade reaction proceeded
smoothly, delivering the desired products sanctis A (1) and the
putative structure of sanctis B (2′) in a combined yield of 89%
with a ratio of 1:9, which could be successfully separated via
silica gel column chromatography.
Notably, the nuclear magnetic resonance (NMR) and high-
resolution electrospray ionization mass spectrometry (HRE-
SIMS) spectra of sanctis A (1) fully matched those of the
natural isolate. However, the NMR spectra of the synthetic
compound 2′, possessing the putative structure of sanctis B,
which was unambiguously established by a single crystal X-ray
crystallography, were distinctively inconsistent with those of
the natural isolate sanctis B (2), as shown in Table 1, although
the HRESIMS data showed a very close similarity. The
Scheme 3. HMBC Correlations of 2 and 2′
aforementioned conclusion. Based on the aforementioned
results, we tentatively concluded that the structure of natural
product sanctis B (2) was wrongly elucidated and, accordingly,
gave the synthetic compound 2′ with a trivial name of iso-
sanctis B.
In order to further reassign the structure of sanctis B (2), a
1
13
concerted effort to compare the H and C NMR data
between sanctis B (2) and iso-sanctis B (2′) were conducted,
and it was readily found that sanctis B (2) should exist as a
similar isomer also possessing a dioxabicyclo[3.3.1]nonane
B
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a
Table 2. Optimization of Reaction Conditions
b
c
entry
solvent
toluene
toluene
toluene
toluene
toluene + dioxane
toluene + THF
THF
DCM
catalyst
catalyst loading
temperature, T (°C)
time
yield (%)
ratio of 1:2′:2
1
2
3
4
5
6
7
8
9
PTSA
PTSA
PPTS
TFA
PTSA
PTSA
PTSA
PTSA
1.0 equiv
1.0 equiv
1.0 equiv
50 μL
1.0 equiv
1.0 equiv
1.0 equiv
1.0 equiv
rt
90
120
90
90
90
rt
4 d
1 h
3 h
3 h
3 h
3 h
3 h
3 h
1 h
70
89
30
30
<10
<10
nr
50
70
1:9:0
1:9:0
0:1:0
0:1:0
−
−
−
0:1:0
0:1:0
d
e
f
rt
90
g
2N HCl + ACN
a
b
c
1
d
Reaction conditions: i (1.2 mmol), 8 (1.0 mmol), solvent (5 mL), rt, 1−3 h. Isolated yield. The ratio was accorded by H NMR. Toluene (4
e
f
g
mL), dioxane (1 mL). Toluene (4 mL), THF (1 mL). No reaction. ACN (2.5 mL), 2N HCl (2.5 mL).
scaffold, because of their closely similar NMR profiles.
However, the absence of the proton signal at δH 12.08
suggested that the phenolic OH in sanctis B (2) was not
involved in the 1,3-hydrogen bonding effect. Further
elucidation of the HMBC spectrum of 2 disclosed that it
necessitated to be a regioisomer of iso-sanctis B (2′). This
conclusion could be unambiguously confirmed by the critical
and achieve the biomimetic total synthesis of sanctis B (2)
with its further structural confirmation, the concise synthetic
process with protection for the para-phenolic free hydroxyl
group of 8 was performed as shown in Scheme 4.
Scheme 4. Biomimetic Total Syntheses of Sanctis B (2)
HMBC correlations of H -20 to C-1, C-2, and C-3, H-3 to C-5
3
and C-6, in conjunction with 4-OH to C-3, C-4, and C-5.
Thus, the reported structure of sanctis B was corrected and
depicted as compound 2 in Scheme 3.
The major difference between the structures of sanctis B (2)
and iso-sanctis B (2′) was attributed to the alternative
selectivity for the two phenolic groups of intermediate 8
participating in the cyclization process (see Table 2).
Therefore, the endeavors with the objective to diversely access
sanctis A−B (1−2) as well as iso-sanctis B (2′) in one
operation were performed, and several contributing variables,
such as solvents, substrate loadings, and potential acid
catalysts, were screened. As a result, the reproducibility of
this reaction necessitated strong acids and nonpolar organic
solvents, but were accompanied by the reduction of yields (see
Table 2, entries 1−8). Moreover, all of them just afforded the
products sanctis A (1) and/or iso-sanctis B (2′) with a ratio of
The selective O-allylation of the para-phenolic free hydroxyl
group of phenol 12 with allylic bromide 13 could successfully
afford the desired ether intermediate 14 in 69% yield, and
∼1:9 (see Table 2, entries 1 and 2). Especially, the usage of
PTSA as the acidic catalyst and toluene as the solvent gives rise
to the best yield (89%). Notably, when an aqueous condition
was used to mimic the biosynthetic environment in a natural
organism, the hemiacetalization/dehydration/formal [3+ 3]
cycloaddition cascade sequence proceeded smoothly to
generate the target products sanctis A (1) and iso-sanctis B
further TiCl -mediated formylation would transform it to the
4
aldehyde 15 in 67% yield. The critical α,β-unsaturated ketone
moiety was readily constructed using the classical Wittig
reaction, offering the key precursor 16 in 85% yield, which
then underwent the key hemiacetalization/dehydration/formal
[3 + 3] cycloaddition cascade reaction with orcinol i to
(
2′) with 70% yield, strongly suggesting the possibility for the
efficiently finish the establishment of tetracyclic dioxabicy-
clo[3.3.1]-nonane scaffold, generating the expected product 17
through the active chromenylium intermediate iii in a highly
efficient manner. Finally, the deprotection of the allylic group
would successfully afford the target product sanctis B (2) in
73% yield. To our delight, the NMR and HRESIMS spectra of
synthetic product perfectly matched with those of the natural
isolate sanctis B (2). Moreover, the X-ray diffraction (XRD)
analysis experiment was also conducted for the readily
crystallized synthetic sanctis B (2), thus unambiguously
confirming its revised structure. Therefore, we successfully
process to be a biomimetic transformation mimicking the
natural generation of sanctis A (1) and its derivatives
downstream, although the use of stronger acid and the absence
of sanctis B (2) should be further investigated.
Considering the absence of sanctis B (2) in the cascade
reaction process, we speculated that the priority to generate
iso-sanctis B (2′) instead of sanctis B (2) was most probably
attributed to the superior reactivity for the para-phenolic free
hydroxyl functionality than that of the ortho-phenolic one in 8
under the nonenzymatic conditions. To probe the mechanism
C
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Letter
complete the first biomimetic total synthesis of sanctis B (2)
and distinctively revised its structure via XRD analysis.
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
In order to further address the problematic issue on the
absence of sanctis B (2) during the synthetic cascade process,
subsequently, the intertransformation between sanctis B (2)
and iso-sanctis B (2′) through the enol ether intermediate 18
was also investigated (see Scheme 5). Treatment of sanctis B
AUTHOR INFORMATION
Corresponding Authors
■
Hongxin Liu − Guangdong Institute of Microbiology,
Guangdong Academy of Sciences, Guangzhou, China;
Email: liuhx@gdim.cn
Haibo Tan − South China Botanical Garden, Chinese
Academy of Sciences, Guangzhou, China; orcid.org/
Scheme 5. Intertransformation of 2 and 2′
0
000-0002-1652-5544; Email: tanhaibo@scbg.ac.cn
Other Authors
Luqiong Huo − South China Botanical Garden, Chinese
Academy of Sciences, Guangzhou, China, Guangdong
Institute of Microbiology, Guangdong Academy of Sciences,
Guangzhou, China, and University of Chinese Academy of
Sciences, Beijing, People’s Republic of China
Chunmao Dong − South China Botanical Garden,
Chinese Academy of Sciences, Guangzhou, China
Miaomiao Wang − South China Botanical Garden,
Chinese Academy of Sciences, Guangzhou, China, and
University of Chinese Academy of Sciences, Beijing,
People’s Republic of China
Xiuxiang Lu − South China Botanical Garden, Chinese
Academy of Sciences, Guangzhou, China, and University
of Chinese Academy of Sciences, Beijing, People’s Republic
of China
Wenge Zhang − South China Botanical Garden, Chinese
Academy of Sciences, Guangzhou, China, and University
of Chinese Academy of Sciences, Beijing, People’s Republic
of China
(
2) with the standard reaction conditions performed in the
biomimetic cascade process resulted in complete conversion of
to target product 2′ within 0.5 h and at ∼100% yield.
2
However, when 2′ was subjected to the same reaction
conditions, none of any desired product 2 was detected,
even after 3 h. The aforementioned results collectively
highlighted that the 2′ should be a thermodynamically
favorable product in an interchange equilibrium, mainly
because of its less steric hindrance and the formation of a
1
,3-hydrogen bonding effect, logically indicating that 2′ might
be generated in part from 2, because of the presence of a
strong acid in our established reaction conditions. Moreover,
these results also tentatively answered the issue of why sanctis
B (2) was discovered in nature but iso-sanctis B (2′) was only
accessed in our biomimetic total synthesis process.
In conclusion, the chemological evolution of the structural
entities encoded by nature’s molecular archive stimulated us to
complete the biomimetic total syntheses of sanctis A−B (1−2)
and iso-sanctis (2′), in conjunction with unequivocal structure
revision of sanctis B (2) through XRD. Our synthetic
methodology was mainly ascribed to mimic the biosynthetic
olefin isomerization/hemiacetalization/dehydration/[3 + 3]
cycloaddition sequence, which held the promise of illustrating
the possibility of biogenetic pathways for these natural
products. Taking the advantage of simplicity and efficiency,
we believe that the described synthetic strategy and
technologies would provide a general process for the rapid
assembly of other such dimer products. Moreover, this study
would open new pathways to provide sufficient quantities of
these molecules toward the drug discovery in a practical
fashion. Further investigations directed toward this goal are
currently underway and will be reported in due course.
Bao Yang − South China Botanical Garden, Chinese
Academy of Sciences, Guangzhou, China; orcid.org/
0000-0002-5483-7960
Yunfei Yuan − South China Botanical Garden, Chinese
Academy of Sciences, Guangzhou, China
Shengxiang Qiu − South China Botanical Garden,
Chinese Academy of Sciences, Guangzhou, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.9b04486
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
No. 81773602), Natural Science Foundation of Guangdong
■
(
ASSOCIATED CONTENT
sı Supporting Information
■
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04486.
Province (No. 2019A1515011694), Pearl River Science and
Technology New Star Fund of Guangzhou (No.
2
01605120849569), Guangdong Special Support Program
Experimental section, detailed experimental procedures
and full spectroscopic data for all related compounds
(
No. 2017TQ04R599), and the Foundation of Key Laboratory
of Plant Resources Conservation and Sustainable Utilization,
South China Botanical Garden, Chinese Academy of Sciences.
(PDF)
Accession Codes
CCDC 1970760 and 1970762 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
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