A Divergent Enantioselective Strategy for the Synthesis of Griseusins

Article abstract of DOI:10.1002/anie.201505022

The first enantioselective total synthesis of griseusin A, griseusin C, 4′-deacetyl-griseusin A, and two non-native counterparts in 11–14 steps is reported. This strategy highlights a key hydroxy-directed C H olefination of 1-methylene isochroman with an

Full text of DOI:10.1002/anie.201505022

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201505022
German Edition: DOI: 10.1002/ange.201505022
Total Synthesis
A Divergent Enantioselective Strategy for the Synthesis of Griseusins
Yinan Zhang, Qing Ye, Xiachang Wang, Qing-Bai She, and Jon S. Thorson*
Abstract: The first enantioselective total synthesis of griseu-
sin A, griseusin C, 4’-deacetyl-griseusin A, and two non-native
counterparts in 11–14 steps is reported. This strategy highlights
for their potent antibiotic, antifungal, and anticancer activ-
ities, their fundamental mechanism of action remains unclear.
For example, the recent identification of a representative
naturally occurring griseusin as COMPARE-negative impli-
cates a potentially novel anticancer mechanism.[1g] Griseusin
synthetic development has been inspired by these cumulative
properties, beginning with the first total synthesis of an
enantiomer of griseusin A by Kometani et al. in 1983.[3] Yet,
although notably elegant strategies to construct the griseusin
core scaffold have since been developed,[4] the total syntheses
of naturally occurring griseusins have not been reported,
a major challenge of which stems from C1 epimerization
within the context of spiropyran construction. Toward this
end, herein we describe an efficient divergent enantioselec-
tive strategy for griseusin synthesis and the corresponding
total synthesis of griseusin A, 4’-deacetyl griseusin A, griseu-
sin C, and two unnatural analogues. Highlights of the
fundamental strategy include the rapid assembly of core
À
a key hydroxy-directed C H olefination of 1-methylene
isochroman with an a,b-unsaturated ketone followed by
subsequent stereoselective epoxidation and regioselective cyc-
lization to afford the signature tetrahydro-spiropyran ring.
Colorectal cancer cell cytotoxicities of the final products
highlight the impact of the griseusin tetrahydro-spiropyran
ring on bioactivity. As the first divergent enantioselective
synthesis, the strategy put forth sets the stage for further
griseusin mechanism-of-action and SAR studies.
T
he griseusins produced by Streptomyces griseus and Nocar-
diopsis sp. are pyranonaphthoquinone metabolites that con-
tain a fused spiro-ring C/E system (Figure 1).[1] Ring E of this
signature structural motif is further elaborated through
oxidation, acetylation, and/or glycosylation and, similar to
members of the simpler frenolicin-type pyranonaphthoqui-
nones,[2] some griseusin members also contain an open D-ring.
Additional distinguishing features among members include
stereo-inversion at C3, C4, C3’, C4’, C6’, and/or the ring C/E
spiro-ring junction C1. Although griseusins have been noted
À
chiral fragments, a novel C H activation to facilitate early-
stage fragment coupling assembly, and an enabling diaste-
reoselective epoxidation–cyclization cascade approach to
tetrahydro-spiropyran formation that avoids the C1 epimeri-
zation. The subsequent comparison of the anticancer cyto-
toxicities of this griseusin series for the first time reveals that
E-ring substitution modulates potency. This enabling syn-
thetic method sets the stage for future in-depth SAR and
mechanistic studies of this intriguing natural product family.
An initial intent was to leverage our recently reported
diastereoselective oxa-Pictet–Spengler-based strategy devel-
oped for the construction of frenolicin-type pyranonaphtho-
quinones.[5] However, all attempts to do so proved to be
incompatible with the requisite griseusin tetrahydro-spiro-
pyran ring. Thus, an alternative approach was designed based
on the division of the griseusins into four main subclasses that
diverge synthetically from griseusin C (3, Scheme 1). In this
strategy, the key griseusin a-hydroxy tetrahydro-spiropyran
moiety was obtained from a diastereoselective epoxidation–
cyclization cascade of the key intermediate 1-methylene
À
isochroman 4, a precursor prepared by direct C H activation
and conjugation of 5 and 6. Sharpless dihydroxylation[6] was
thereby conceived to set the key stereocenters en route to 5
from commercially available naphthalene 7. Importantly, the
modular design of this strategy was anticipated to enable
access to a range of divergent griseusin-based analogues.
The synthesis commenced with the asymmetric prepara-
tion of intermediate 5 (Scheme 2) through preparation of
benzoquinone 8 from commercially available 1,5-dihydroxy-
naphthalene 7 (55% overall yield, 20 gram scale).[5] Deace-
tylation under acid reflux gave 5-hydroxy benzoquinone 9,
which was converted to naphthalene bromide 10 through
sequential reduction, acetonide protection, and methylation
(three steps, 65% overall yield). Heck coupling of 10 with
Figure 1. Naturally occurring griseusins.
[*] Dr. Y. Zhang, Dr. X. Wang, Prof. J. S. Thorson
Center for Pharmaceutical Research and Innovation
College of Pharmacy, University of Kentucky
789 South Limestone Street, Lexington, KY 40536 (USA)
E-mail: jsthorson@uky.edu
Dr. Q. Ye, Prof. Q.-B. She
Markey Cancer Center, University of Kentucky
741 South Limestone Street, Lexington, KY 40536 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505022.
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À
Table 1: Optimization of the C H olefination reaction to set the stage for
Scheme 3.[a]
Entry
Pd catalyst
Oxidant
T [8C]
Yields of 13/4/5 [%][b]
1[c]
2
3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OPiv)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
AgOAc
80
80
80
80
80
80
80
40
60
100
80
80
47/10/40[d]
60/32/6[d]
53/37/<5[d]
64/10/22
55/32/10
40/35/10
22/0/71
Ag2CO3
Ag2CO3/celite
AgOPiv
Ag2CO3
–
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3/celite
Ag2CO3
4
5[e]
6[f]
7
8[g]
9[h]
10[i]
11[j]
12[k]
25/19/50
48/30/20
60/32/3
48/40/<5[d]
85/0/9d
Scheme 1. Retrosynthetic analysis of griseusins. TBS=tert-butyldi-
methyl silyl.
[a] Reactant 5 (0.05 mmol), Pd catalyst (0.01 mmol), oxidant (0.2 mmol),
Li2CO3 (0.05 mmol), DCE (0.3 mL), and reactant 6 (0.06 mmol) was
stirred at 808C for 16–24 h. [b] Yield was determined by 1H NMR analysis
of the crude reaction mixture using 1-bromo-3,5-dichlorobenzene as an
internal standard. [c] Previously described method.[10] [d] Yield of isolated
product. [e] 8 Equiv Ag2CO3 was added. [f] 200% Pd(OAc)2 was used.
[g] 80 h reaction time. [h] 40 h reaction time. [l] 8 h reaction time.
[j] 10 mmol scale. [k] 20 mol% (BnO)2P(O)OH as ligand and no base.
be due to steric inhibition of palladium coordination in the
secondary oxidative cyclization and competition by the
corresponding hetero-Michael addition,[10] consistent with
the observed major product 13 (Figure S1). Extensive screen-
ing of alternative solvents, ligands, and auxiliary oxidants led
to a ca. threefold improvement in yield of the desired 4
(entry 2, Table 1 and Tables S1–S4). While palladium piva-
late, silver pivalate,[12] or excess oxidant/catalyst failed to
provide further improvements, silver carbonate on celite gave
4 in 37% yield (entries 3–7, Table 1). Lower temperature
reduced the yield even with extended reaction time whereas
elevated temperature increased reaction rates but with no
improvement in desired product (entries 8–10, Table 1). The
yield of 13 and 4 on a larger scale were comparable (entry 11,
Table 1) and they were easily isolated by standard silica gel
chromatography. It is also important to note that the use of
(BnO)2P(O)OH as the ligand[13] dramatically improved the
yield of 13 (entry 12, Table 1) as an alternative route to 4
through a,b-dehydrogenation.[14] Within this context, enol
silylation and subsequent Saegusa–Ito oxidation[15] were
found to be the best conditions to achieve the desired product
(4, two steps, 45% yield under un-optimized conditions, 62%
yield in total from 5 to 4).
Scheme 2. Preparation of intermediates 5 and 6. NBS=N-
bromosuccinimide.
methyl but-3-enoate afforded the desired ester 11 in 74%
yield. Whereas the ester 11 was unstable under standard
Sharpless dihydroxylation conditions (strong base), alterna-
tive use of NaHCO3 led to intermediate 5 in excellent yield
(84%, 97% ee).[7] The corresponding synthesis of building
block 6 was achieved in three steps from ethyl (R)-3-
hydroxybutyrate (12)[8] to set the stage for the key hydroxy-
À
directed C H olefination reaction.
With the key intermediate 4 in hand, our attention shifted
to deprotection and tetrahydro-spiropyran formation
(Scheme 3). The TBS group could be easily removed by
hydrogen fluoride in CH3CN to give cyclized 3’-dehydroxy
griseusin precursor (15, 91%) as the predominant product.
Subsequent optimization of potential neutralizing bases in
this reaction revealed the addition of two equivalents of
À
Although palladium catalyzed C H activation has been
extensively studied, its use in total synthesis remains limited.[9]
Following the precedent set by Yu and co-workers for
[10]
À
hydroxy-directed C H olefination in the context of iso-
chromanone synthesis,[11] the desired methylene isochroman 4
was obtained in poor yield (10%, entry 1, Table 1). This may
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Scheme 3. Tetrahydro-spiropyran formation. Key NOESY correlations in
final products 17 and 18 are highlighted in gray. DCE=1,2-dichloro-
ethane, DBU=1,8-diazabicyclo[5.4.0]undec-7-ene, TEA=triethylamine,
Tf =trifluoromethanesulfonyl.
Scheme 4. Completion of the 1–3, 19, and 20 total syntheses. Key
NOESY correlations in final products 1–3, 19, and 20 are highlighted
in gray.
triethylamine to afford desired 16 in 85% yield (Table S5).
While few, if any, examples exist of sequential unsaturated
ketone epoxidation and cyclization, the seminal work of Tan
and co-workers on the stereoselectivity of allyl silyl ether-
based spiroketal formation served as a basis for this strat-
egy.[16] Subsequent screening of a range of oxidants, solvents,
and Lewis or Brønsted acids revealed that the addition of
dimethyldioxirane (DMDO)[17] to a 1:1 mixture of 16 and
TfOH in DCM afforded the desired griseusin C-type pre-
cursor 17 (80% yield, > 98:2 d.r., Tables S6 and S7). In
contrast, the same reaction in the presence of InCl3 in THF
led to the corresponding 1,3’-epi-griseusin C precursor 18
(59% yield, 73:27 d.r.). Distinct from previous examples,[16c,d]
diastereoselectivity may derive from facial selectivity of 16
epoxidation (TfOH predominantly si-face; InCl3 re-face bias),
in which the sequential cyclization follows a C1-inversion
mechanism in both cases. For all products, the relative
configuration at newly generated stereocenters was assigned
by cross peaks in NOESY and other 2D NMR spectra
(Scheme 3).
From 17, the remaining steps proceeded as planned
(Scheme 4). Specifically, lithium borohydride reduction gave
the natural 4’-axial diastereomer 21 exclusively. Selective 4’-
acetylation was accomplished using a sterically hindered base
(dicyclohexylmethyl amine) to yield 22. Final global depro-
tection of the pyranonaphthoquinone core afforded the
desired natural products griseusin A (1), 4’-deacetyl griseusin
(2), griseusin C[1f] (3, also known as 4’-dehydro-deacetyl
griseusin A[1h]), and two griseusin analogues, 3’-dehydroxy
griseusin C (19), and 1,3’-epi-griseusin C (20). Optical rota-
tions of the synthetically derived griseusins agreed well with
Table 2: Cytotoxicity of 1–3, 19, and 20.[a]
Compounds
IC50 [nm]
HCT116
DLD-1
SW620
1
2
3
19
20
201Æ3
305Æ5
127Æ2
133Æ2
345Æ5
170Æ9
258Æ9
94Æ3
138Æ8
210Æ8
67Æ2
57Æ2
63Æ1
290Æ6
242Æ6
[a] Data are presented as IC50 ÆSD values. Experiments were performed
in triplicate.
values previously reported for the corresponding natural
23
20
D
products (1[1b] ½aŠ ¼À148, synthetic ½aŠ ¼À153; 2[1e]
D
24
D
20
20
D
15
D
½aŠ ¼À198, synthetic ½aŠ ¼À202; 3[1h] ½aŠ ¼À114, synthetic
½aŠ ¼À100).
D
The cancer cell line cytotoxicity of this set was subse-
quently evaluated against three colon cancer cell lines
(HC116, DLD-1, and SW620; Table 2). All griseusins were
found to be potent cancer cell line cytotoxins to which the
stereochemistry and substituent pattern of ring E contributed
to potency modulation. Specifically, although all members
were found to display notable potency (IC50 < 350 nm),
griseusin C (3) and its corresponding 3’-dehydroxy analogue
19 were found to be most active. C4’-reduction and/or
modification led to reductions in potency, whereas analogues
with variation at C3’ (hydroxy versus deoxy) were roughly
equipotent.
In conclusion, this work highlights the first concise,
asymmetric divergent strategy for the synthesis of naturally
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commercially available materials. Key highlights include the
À
first applications of hydroxy-directed C H olefination in
a total synthesis and the key regioselective and diastereose-
lective cyclization to efficiently access the signature tetrahy-
dro-spiropyran ring E while avoiding C1 epimerization, an
issue that has plagued griseusin total synthesis efforts to date.
Subsequent bioactivity assays reveal that the stereochemistry
and functionalization of ring E modulates anticancer potency
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Acknowledgements
This work was supported, in part, by the University of
Kentucky College of Pharmacy, the University of Kentucky
Markey Cancer Center, National Institutes of Health grant
CA175105 (Q.-B. She), and the National Center for Advanc-
ing Translational Sciences (UL1TR000117).
À
Keywords: aromatic polyketides · C H activation ·
natural products · pyranonaphthoquinones · total synthesis
How to cite: Angew. Chem. Int. Ed. 2015, 54, 11219–11222
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Received: June 2, 2015
Revised: June 24, 2015
Published online: July 31, 2015
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