Total Synthesis of Asperphenins A and B

Article abstract of DOI:10.1021/acs.orglett.8b02652

The first total synthesis of asperphenins A and B has been accomplished in a concise, highly stereoselective fashion from commercially available materials (15 steps, 9.7% and 14.2% overall yields, respectively). The convergent route featured the judicious

Full text of DOI:10.1021/acs.orglett.8b02652

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Total Synthesis of Asperphenins A and B
Jia-Lei Yan,† Yingying Cheng,† Jing Chen,† Ranjala Ratnayake,‡,§ Long H. Dang,‡,§,∥
Hendrik Luesch,‡,§ Yian Guo,*,† and Tao Ye*,†
†State Key Laboratory of Chemical Oncogenomics, Key Laboratory of Chemical Genomics, Peking University Shenzhen Graduate
School, Xili, Nanshan District, Shenzhen 518055, China
‡Department of Medicinal Chemistry, §Center for Natural Products, Drug Discovery and Development (CNPD3), and ∥Department
of Medicine, University of Florida, Gainesville, Florida 32610, United States
S
* Supporting Information
ABSTRACT: The first total synthesis of asperphenins A and B has been accomplished in a
concise, highly stereoselective fashion from commercially available materials (15 steps, 9.7%
and 14.2% overall yields, respectively). The convergent route featured the judicious choice of
protecting groups, fragment assembly strategy and a late-stage iron-catalyzed Wacker-type
selective oxidation of an internal alkene to the corresponding ketone.
ments and ECD calculations. Structurally, these two natural
etabolites isolated from marine fungus often possess
Munique structural features and incorporate new or products are composed of a β-hydroxy fatty acid, a tripeptide,
and a trihydroxybenzophenone. Asperphenins A and B exhibit
significant cytotoxicity against several human cancer cell lines,
e.g., with IC50 values of 0.8 and 1.1 μM, respectively, against
RKO colorectal carcinoma cells.
unusual assemblages of functional groups. Many of them
provide novel lead molecules for probing fundamental
biological processes and the development of novel chemo-
therapeutic agents that target cancers. Our laboratory is
engaged in a program devoted to the total synthesis and
evaluation of marine natural products.1 Herein, we disclose the
first total synthesis of asperphenins A and B by utilizing a
highly efficient and convergent approach.
Asperphenins A and B (Scheme 1) were isolated from a
culture broth of marine-derived Aspergillus sp. collected from
the shore of Jeju Island, Korea.2 The relative and absolute
stereochemical configuration of asperphenins had been
established by a combination of spectroscopic analyses,
chemical degradation, Mosher ester analysis, CD measure-
Scheme 1 outlines our retrosynthetic analysis plan. We
anticipated that the condensation of acid 3 with either
fragment 4 or 5 followed by removal of protecting groups
would give rise to the natural products. We envisioned the
amine moiety of 4 or 5 could arise from either asymmetric
Mannich reaction of ketone 6a or a stereoselective Wittig
olefination of aldehyde 6b with a leucinol-derived triphenyl-
phosphonium salt. Both 6a and 6b, in turn, could be accessed
from a reaction of aryl iodide 8 with lactols 7a and 7b,
respectively.
The synthesis of fragment 3 commenced with the known
compound 9,3 which underwent titanium tetrachloride
mediated enantioselective aldol reaction with caprinaldehyde
to provide 104 as a single diastereomer in 81% yield (Scheme
2). Protection of the resulting alcohol as its TBS ether followed
by hydrolytic cleavage of the chiral auxiliary furnished the acid
11 in 76% yield. In parallel, coupling of N-Cbz-L-asparagine
and L-glutamine methyl ester under the influence of HATU in
the presence of HOAt and DIPEA furnished the corresponding
dipeptide, which was subjected to hydrogenolysis of the Cbz
protecting group to produce amine 14 in 76% yield over two
steps. A second HATU/HOAt-mediated condensation of acid
11 and amine 14 provided the corresponding amide 15 in 85%
yield. Saponification of the methyl ester with LiOH in aqueous
methanol followed by acidification afforded the acid 3 in 81%
yield.
Scheme 1. Retrosynthetic Analysis of Asperphenin A
Received: August 19, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02652
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
corresponding lactol 7a as a latent hydroxyaldehyde in 95%
yield. Spurred by Knochel’s seminal findings on halogen−
magnesium exchange,7 we opted to convert the readily
available aryl iodide 8 to the corresponding arylmetal species
with i-PrMgCl·LiCl, followed by the addition of lactol 7a to
give rise to biphenyl diol 21 as a mixture of diastereomers in
85% yield. Ley’s TPAP oxidation8 of 21 delivered the
corresponding diketone 6a in 78% yield and set the stage for
the exploration of the key asymmetric Mannich reaction.9
However, this reaction proved to be challenging due to the
high propensity of trapping of the transient enolate intra-
molecularly with an electrophile. Accordingly, treatment of
methyl ketone 6a and tert-butanesulfinyl imine 22 with various
bases, solvents, and concentrations at low temperature resulted
in no observation of the desired product. For example,
exposure of both 6a and 22 in the presence of 1 equiv of
potassium hexamethyldisilazane (KHMDS) provided two aldol
products (23 and 24). Given that the reactions afforded β-
amino ketone 23 as the major product, we reasoned that the
asymmetric Mannich reaction occurred with simultaneous
cyclization of the regenerated enolate onto the ketone to afford
the thermodynamically more stable product.
Scheme 2. Synthesis of Fragment 3
Our planned first approach to asperphenin A required the
synthesis of fragment 4 from ketone 6a via an asymmetric
Mannich reaction (Scheme 3). Thus, 2-methoxy-4-methyl-
To circumvent the problems encountered in the above
strategy toward the construction of chiral amine 4 via an
asymmetric Mannich reaction, we turned our attention to the
strategy where the ketone moiety would have to be installed at
a later stage of the synthesis. Thus, aldehyde 18 was
transformed into lactone 25 by a two-step sequence involving
(1) reduction with sodium borohydride and (2) acid-
promoted lactonization. Lactone 25 was then elaborated to
the lactol 7b in 81% yield by an identical strategy as described
for 7a, including methyl ether deprotection and reprotection of
the resulting free phenol with benzyl bromide and potassium
carbonate, followed by partial reduction of the lactone with
DIBAL-H at −78 °C. Again, the Grignard reagent derived
from aryl iodide 8 added to lactol 7b provided biphenyl diol 27
in 72% yield. To our surprise, the use of the previously
employed Ley’s TPAP oxidation of diol 27 led to the
dicarbonyl product 6b in 27% yield along with the over-
oxidation product 28 (58%) (Scheme 4). Similar results were
obtained with manganese dioxide11 in dichloromethane. 2,3-
Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)12 oxidation of
the benzylic alcohols in 27 provided intractable mixtures with
Scheme 3. Attempts on the Synthesis of Intermediate 4
Scheme 4. Synthesis of Aldehyde 6b
benzoic acid 16 was converted into the corresponding acid
chloride and then coupled with N,N-diethylamine to afford 17
in 95% yield. Regioselective formylation of 17 to aldehyde 18
was achieved through an amide-directed ortho-lithiation (t-
BuLi, TMEDA) and quenching with DMF (86% yield).5
Addition of methylmagnesium bromide to aldehyde 18
afforded the corresponding secondary alcohol, which was
immediately subjected to acid-promoted lactonization to give
rise to 19 in 92% yield over two steps.5b,6 Lactone 19 was
treated with boron tribromide in dichloromethane to produce
the crude deprotected phenol, which was then reprotected by
benzylation using benzyl bromide and potassium carbonate to
afford the benzyl derivative 20 in 81% yield. Partial reduction
of the lactone 19 using DIBAL-H at −78 °C furnished the
B
DOI: 10.1021/acs.orglett.8b02652
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
no desired products. Gratifyingly, treatment of diol 27 with
PCC on alumina13 in DCM smoothly afforded the desired
aldehyde 6b in 76% yield as the sole product.
entries 1−4). The literature precedents suggested that Wacker
oxidation of cinnamyl azides17a and 2-styryltetrahydro-2H-
pyrans17b occurred predominantly or exclusively at the
benzylic rather than the homobenzylic carbon. Attempted
Wacker oxidation of both 31 and 32, however, delivered only
trace quantities of the desired ketone. As we searched for
alternative methods of transforming alkenes 31 and 32 into
ketones 33a and 33b, we were drawn to a report by Han and
co-workers of a mild variant of the Wacker-type oxidation.18 In
the Han’s modification of the Wacker oxidation, iron(II)
chloride, polymethylhydrosiloxane, and air were employed as a
highly efficient and selective catalytic system. Because the mild
oxidation conditions enable exceptional functional-group
tolerance, we tested this protocol with alkene 31. We were
pleased to discover that the iron-catalyzed Wacker-type
oxidation of alkene 31 provided ketone 33a in 30% yield. To
our delight, the same iron-catalyzed aerobic oxidation of 32
delivered 33b in 89% yield (Table 1, entries 7 and 8).
In parallel, D-leucinol (29) was transformed into trifluor-
oacetamide 30 by a three-step sequence involving (1)
trifluoroacetylation of the primary amine, (2) conversion of
the alcohol into the bromide with CBr4 and Ph3P, and (3) β-
amino phosphonium salt formation upon treating the resulting
bromide with triphenylphosphine in refluxing toluene. Wittig
reaction of aldehyde 6b with the phosphorane derived from
phosphonium bromide 30 was next examined. Thus, treatment
of the phosphonium salt 30 with 2.1 equiv of n-butyllithium
followed by addition of the aldehyde 6b delivered the alkene
31 enriched in the desired E-isomer (E/Z = 5:1).14 The
desired product 31 could be isolated free of the Z isomer in
77% yield by chromatography on silica gel. The presence of the
trifluoroacetamide in 30 ensured the generation of the dianion
intermediate, which presumably played a critical role to the
stereochemical outcome of this transformation.14b,15 Hydrol-
ysis of the trifluoroacetamide 31 gave allylic amine 5 in nearly
quantitative yield. Acetylation of amine 5 was achieved under
basic conditions with acetyl chloride to afford 32 in 82% yield
(Scheme 5).
Encouraged by the successful transformation of alkene 32 to
the corresponding β-ketone amide 33b, we proceeded with the
total synthesis of asperphenin A and B as outlined in Schemes
6 and 7. Thus, coupling of acid 3 and allylic amine 5 under the
Scheme 6. Synthesis of Asperphenin A
Scheme 5. Synthesis of Amine 5 and Amides 31 and 32
With alkenes 31 and 32 in hand, the stage was set to explore
the crucial Wacker-type oxidation. Attempts to selectively
oxidize the E-alkenes 31 and 32 using molecular iodine or N-
iodosuccinimide16 gave little or no desired products (Table 1,
Scheme 7. Synthesis of Asperphenin B
Table 1. Optimization of the Wacker-Type Oxidation
influence of EDCI in the presence of HOAt and DIPEA
afforded 34 in 71% yield. After removal of the TBS group to
give rise to the corresponding alcohol 35, the alcohol was then
subjected to an iron-catalyzed Wacker oxidation to furnish 36
in 85% yield. The final global hydrogenative debenzylation
went on without incident to provide asperphenin A in 72%
yield (Scheme 6). To this end, we synthesized ent-5 following
the same synthesis as for 5, but starting with L-leucinol. This
was readily achieved, and ent-5 was incorporated into the
synthesis as previously performed to afford asperphenin B with
no adverse consequences (Scheme 7). The spectral data for
synthetic 1 and 2 (1H and 13C NMR and HMRS) were
identical with those published for the natural products, and the
entry alkene
condition
I2, dioxane/H2O
I2, dioxane/H2O
NIS, dioxane/H2O
NIS, dioxane/H2O
PdCl2, CuCl, O2, H2O, DMF
PdCl2, CuCl, O2, H2O, DMF
FeCl2, PMHS, EtOH, air
FeCl2, PMHS, EtOH, air
temp (°C) yield (%)
1
2
3
4
5
6
7
8
31
32
31
32
31
32
31
32
90
90
30
30
50
50
80
80
ND
ND
trace
trace
trace
trace
30
25
89
optical rotation of our products ([α]D −22.0, c 0.1, MeOH,
C
DOI: 10.1021/acs.orglett.8b02652
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
25
for asperphenin A; [α]D −16.2, c 0.1, MeOH, for
ACKNOWLEDGMENTS
■
asperphenin B) corresponded well with the literature value
We acknowledge financial support from the Shenzhen Peacock
Plan (KQTD2015071714043444), the NSFC (21772009), the
S Z S T I C ( J C Y J 2 0 1 6 0 5 2 7 1 0 0 4 2 4 9 0 9 ,
JCYJ20170818090017617, JCYJ20170818090238288), the
GDNSF (2014B030301003), the National Institutes of Health
(NCI Grant No. R01CA172310), the Debbie and Sylvia
DeSantis Chair professorship (H.L.), and the NCI Research
Specialist Award (R50CA211487) (R.R.).
(lit.2 [α]D −24.7, c 0.1, MeOH, for asperphenin A; [α]D
25
25
−18.4, c 0.1, MeOH, for asperphenin B), leading us to
conclude that synthetic 1 and 2 were of the same absolute
stereochemistry as natural asperphenins A and B.
With the synthetic asperphenins A (1) and B (2) in hand,
the screening of cytotoxic activities toward a number of cancer
cell lines was investigated. The initial cytotoxicity evaluation of
1 and 2 was performed across a panel of the HIF-dependent
HCT116 colorectal cancer cell lines using isogenic
(HCT116HIF‑1α−/−HIF‑2α−/− and HCT116WT KRAS) knock-out
cells to identify if these compounds preferentially target HIF/
KRAS pathways (Figure 1). Both compounds were only
REFERENCES
■
(1) (a) Al-Awadhi, F. H.; Gao, B.; Rezaei, M. A.; Kwan, J. C.; Li, C.;
Ye, T.; Paul, V. J.; Luesch, H. J. Med. Chem. 2018, 61, 6364. (b) Guo,
Y.; Zhao, M.; Xu, Z.; Ye, T. Chem. - Eur. J. 2017, 23, 3572. (c) Zhou,
J.; Gao, B.; Xu, Z.; Ye, T. J. Am. Chem. Soc. 2016, 138, 6948.
(d) Gunasekera, S. P.; Li, Y.; Ratnayake, R.; Luo, D.; Lo, J.;
Reibenspies, J. H.; Xu, Z.; Clare-Salzler, M. J.; Ye, T.; Paul, V. J.;
Luesch, H. Chem. - Eur. J. 2016, 22, 8158. (e) Qu, S.; Chen, Y.; Wang,
X.; Chen, S.; Xu, Z.; Ye, T. Chem. Commun. 2015, 51, 2510. (f) Lei,
H.; Yan, J.; Yu, J.; Liu, Y.; Wang, Z.; Xu, Z.; Ye, T. Angew. Chem., Int.
Ed. 2014, 53, 6533.
(2) Liao, L.; Bae, S. Y.; Won, T. H.; You, M.; Kim, S.-H.; Oh, D.-C.;
Lee, S. K.; Oh, K.-B.; Shin, J. Org. Lett. 2017, 19, 2066.
(3) Kitir, B.; Baldry, M.; Ingmer, H.; Olsen, C. A. Tetrahedron 2014,
70, 7721.
Figure 1. Effect of asperphenins A (1) and B (2) on isogenic
HCT116 colon cancer cells and normal colon cells.
́
(4) (a) Yadav, J. S.; Dachavaram, S. S.; Gree, R.; Das, S. Tetrahedron
Lett. 2015, 56, 3999. (b) Yadav, J. S.; Rajendar, G.; Ganganna, B.;
Srihari, P. Tetrahedron Lett. 2010, 51, 2154.
(5) (a) Snieckus, V. Chem. Rev. 1990, 90, 879. (b) Li, Z.; Gao, Y.;
Tang, Y.; Dai, M.; Wang, G.; Wang, Z.; Yang, Z. Org. Lett. 2008, 10,
3017.
(6) Brimble, M. A.; Flowers, C. L.; Hutchinson, J. K.; Robinson, J.
E.; Sidford, M. Tetrahedron 2005, 61, 10036.
(7) (a) Knochel, P.; Dohle, W.; Gommermann, N.; Kneisel, F. F.;
Kopp, F.; Korn, T.; Sapountzis, I.; Vu, V. A. Angew. Chem., Int. Ed.
2003, 42, 4302. (b) Krasovskiy, A.; Knochel, P. Angew. Chem., Int. Ed.
2004, 43, 3333. (c) Krasovskiy, A.; Straub, B. F.; Knochel, P. Angew.
Chem., Int. Ed. 2006, 45, 159.
moderately cytotoxic against parental HCT116 with reduced
cytotoxicity against the human normal colon cells, CCD-
18Co.10 Asperphenin A (1) shows a 2-fold decrease in potency
against HCT116HIF‑1α−/−HIF2α−/− (IC50 shifts from 2.2 uM to
5.1 μM) with a simultaneous decrease in efficacy. For cells
lacking oncogenic KRAS (HCT116WT KRAS), only a slight
decrease in total efficacy was observed compared to the
parental HCT116 cell line.10 Asperphenins may have a slight
selectivity for colon cancer cells over normal cells that could be
explored further with SAR studies.
In summary, stereocontrolled total synthesis of asperphenins
A and B has been accomplished through combination of a
judicious choice of protecting groups, a fragment assembly
strategy, and a late-stage iron-catalyzed Wacker-type selective
oxidation of an internal alkene to the corresponding ketone.
Both of the synthetic samples exhibit interesting results in the
preliminary biological assays.
(8) Saha, T.; Maitra, R.; Chattopadhyay, S. K. Beilstein J. Org. Chem.
2013, 9, 2910.
(9) (a) Babij, N. R.; Wolfe, J. P. Angew. Chem., Int. Ed. 2012, 51,
4128. (b) Davis, F. A.; Yang, B. Org. Lett. 2003, 5, 5011. (c) Tang, T.
P.; Ellman, J. A. J. Org. Chem. 2002, 67, 7819.
(10) For details, see the Supporting Information.
(11) (a) Hollinshead, S. P.; Nichols, J. B.; Wilson, J. W. J. Org. Chem.
1994, 59, 6703. (b) Srivastava, A. K.; Panda, G. Chem. - Eur. J. 2008,
14, 4675.
ASSOCIATED CONTENT
* Supporting Information
■
(12) (a) Torricelli, F.; Bosson, J.; Besnard, C.; Chekini, M.; Burgi,
̈
S
T.; Lacour, J. Angew. Chem., Int. Ed. 2013, 52, 1796. (b) Bosson, J.;
Labrador, G. M.; Pascal, S.; Miannay, F.-A.; Yushchenko, O.; Li, H.;
Bouffier, L.; Sojic, N.; Tovar, R. C.; Muller, G.; Jacquemin, D.;
Laurent, A. D.; Le Guennic, B.; Vauthey, E.; Lacour, J. Chem. - Eur. J.
2016, 22, 18394.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02652.
Experimental details and data (PDF)
(13) Flanagan, S. R.; Harrowven, D. C.; Bradley, M. Tetrahedron
2002, 58, 5989.
(14) (a) Andersson, I. E.; Dzhambazov, B.; Holmdahl, R.; Linusson,
A.; Kihlberg, J. J. Med. Chem. 2007, 50, 5627. (b) Wiktelius, D.;
Luthman, K. Org. Biomol. Chem. 2007, 5, 603.
(15) Maryanoff, B. E.; Reitz, A. B.; Duhl-Emswiler, B. A. J. Am.
Chem. Soc. 1985, 107, 217.
(16) Harikrishna, K.; Mukkamala, R.; Hinkelmann, B.; Sasse, F.;
Aidhen, I. S. Eur. J. Org. Chem. 2014, 2014, 1066.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: bryan_pku@qq.com.
*E-mail: yet@pkusz.edu.cn.
ORCID
Hendrik Luesch: 0000-0002-4091-7492
Yian Guo: 0000-0002-0341-6816
Tao Ye: 0000-0002-2780-9761
Notes
(17) (a) Carlson, A. S.; Calcanas, C.; Brunner, R. M.; Topczewski, J.
Org. Lett. 2018, 20, 1604. (b) Keinan, E.; Seth, K. K.; Lamed, R. J.
Am. Chem. Soc. 1986, 108, 3474.
(18) (a) Liu, B.; Jin, F.; Wang, T.; Yuan, X.; Han, W. Angew. Chem.,
Int. Ed. 2017, 56, 12712. (b) Liu, B.; Han, W. Synlett 2018, 29, 383.
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.8b02652
Org. Lett. XXXX, XXX, XXX−XXX