Cu-catalyzed hydroxycyclopropanol ring-opening cyclization to tetrahydrofurans and tetrahydropyrans: short total syntheses of hyperiones

Article abstract of DOI:10.1039/d0sc05556e

Tetrahydrofurans (THFs) and tetrahydropyrans (THPs) are important core scaffolds frequently found in many molecules of medicinal importance. Herein, we report a novel copper-catalyzed hydroxycyclopropanol ring-opening cyclization methodology to synthesize di- or tri-substituted THFs and THPs. In this reaction, a strained C-C bond was cleaved and a new Csp³-O bond was formed to produce the aforementioned O-heterocycles. The new THF synthesis features a broad substrate scope, scalability, and good functional-group tolerability. It enabled us to complete the shortest enantioselective syntheses of hyperiones A and B (3 and 4 steps, respectively), which is significantly shorter than the previously reported two total syntheses (≥10 steps).

Full text of DOI:10.1039/d0sc05556e

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Cu-catalyzed hydroxycyclopropanol ring-opening
cyclization to tetrahydrofurans and
tetrahydropyrans: short total syntheses of
hyperiones†
Cite this: DOI: 10.1039/d0sc05556e
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
*
Weida Liang, Xinpei Cai and Mingji Dai
Tetrahydrofurans (THFs) and tetrahydropyrans (THPs) are important core scaffolds frequently found in many
molecules of medicinal importance. Herein, we report a novel copper-catalyzed hydroxycyclopropanol
ring-opening cyclization methodology to synthesize di- or tri-substituted THFs and THPs. In this
reaction, a strained C–C bond was cleaved and a new Csp3–O bond was formed to produce the
aforementioned O-heterocycles. The new THF synthesis features a broad substrate scope, scalability,
and good functional-group tolerability. It enabled us to complete the shortest enantioselective syntheses
of hyperiones A and B (3 and 4 steps, respectively), which is significantly shorter than the previously
reported two total syntheses ($10 steps).
Received 7th October 2020
Accepted 20th November 2020
DOI: 10.1039/d0sc05556e
rsc.li/chemical-science
metalloalkyl species are involved, b-H elimination is always
a potential problem, not to mention the difficulties involved in
Introduction
generating such metalloalkyl species. Furthermore, both the
metalloalkyl species and the tethered alcohols are nucleophilic,
therefore, an external oxidant is needed.
O-Heterocycles, particularly tetrahydrofurans (THFs) and tetra-
hydropyrans (THPs) are privileged scaffolds in bioactive natural
products1 and lifesaving drug molecules.2 For example, THF-
containing macrolide amphidinolide C1 (1, Fig. 1) has
demonstrated potent inhibition activity against cancer cell
proliferation.3 Eribulin features three THFs and three THPs in
its backbone and is an FDA-approved drug for treating various
cancers.4 Sofosbuvir, a nucleoside analog, is a pricy, but effec-
tive medication to treat hepatitis C.5 Due to the importance of
THFs and THPs in natural products, human medicines, and
other elds, many synthetic strategies and methodologies have
been developed for their syntheses.6 Regardless of the recent
advances, achieving high stereoselectivity via a ring forming
process in preparing disubstituted or polysubstituted THFs and
THPs still remains a synthetic challenge.7 One commonly used
ring closing strategy for THF and THP synthesis is via an
intramolecular C–O bond formation. Intramolecular nucleo-
philic substitution via a 5 or 6-exo-tet cyclization process (cf. 5
/ 4) falls into this category.6d–g Related transition metal, elec-
trophilic reagent (such as I2 or PhSeCl) or radical promoted
additions to the activated p-systems including olens, alkynes,
and allenes have been developed as well.6h–o A transition metal-
catalyzed direct Csp3–O ring closure is deemed an attractive
strategy (cf. 6 / 4), but poses signicant challenges. When
Department of Chemistry and Center for Cancer Research, Purdue University, West
Lafayette, IN 47907, USA. E-mail: mjdai@purdue.edu
† Electronic supplementary information (ESI) available. CCDC 1996070. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0sc05556e
Fig. 1 THFs and THPs in natural products and drugs and our synthetic
strategy.
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Our continued interest in cyclopropanol ring-opening cross its application in the shortest total syntheses of natural prod-
coupling reactions made us wonder if metalloalkyl species such ucts hyperiones A (12a, 3 steps) and B (12b, 4 steps).
as 6 could be generated from a cyclopropanol precursor (cf. 7 /
8, Fig. 1). Cyclopropanols are highly strained molecules and can
be readily prepared via the Kulinkovich reaction, the Simmons–
Smith reaction, and other protocols.8 Despite of their intrinsic
Results and discussion
Our investigation started with hydroxycyclopropanol 13a, which
ring strain, cyclopropanols are bench stable and can be stored
was prepared from ethyl acetate and 1-phenylbut-3-en-1-ol using
for months or years. Meanwhile, their strain offers unusual
the method developed by Cha and co-workers.13 To prove the
concept, we rst explored the use of stoichiometric Cu(II) salt and
were delightful to discover that desired cis 3-methylketone-5-
phenyl substituted THF product 14a was obtained in 42% or
reactivity. Synthetic chemists have been harnessing this reac-
tivity and developed a variety of cyclopropanol ring-opening
reactions to install different functional groups such as aryl,
alkynyl, alkenyl, acyl, activated alkyl, halogen, nitrile, azide,
35% yield when 2.0 equiv. of Cu(OTf)2 or Cu(OAc)2 were used,
amine, and others at the b-position of carbonyl compounds.9
respectively (entry 1 and 2, Table 1). Encouraged by these results,
we started to evaluate different oxidants to render the Cu(II) salt
catalytic. Both oxygen (entry 3) and 1,4-benzoquinone (BQ, entry
5) were effective oxidants when 0.1 equiv. of Cu(OTf)2 was used,
We have developed palladium-catalyzed hydroxycyclopropanol
ring-opening carbonylative lactonization to facilitate total
syntheses of complex natural products.10 Meanwhile, we devel-
oped a series of copper-catalyzed cyclopropanol ring-opening
but tBuOOH failed the task (entry 4). Cu(I) salt such as CuBr was
triuoromethylation, triuoromethylthioation, amination, and
less efficient than Cu(OTf)2 (entry 6). Solvent evaluation revealed
alkylation with activated alkyl electrophiles.11 However, cyclo-
that toluene was better than 1,2-dichloroethane (DCE),
propanol ring-opening b-oxygenation via a transition metal-
dichloromethane (DCM), benzene, and others. Molecule sieves
were not necessary. Interestingly, commonly used phenanthro-
line (entry 10) and bathophenanthroline (entry 11) ligands for
catalyzed Csp3–O cross coupling reaction has not been real-
ized. Our experience in this area taught us that copper-
homoenolates derived from cyclopropanol ring opening are
copper-catalyzed reactions completely inhibited the trans-
less prone to b-H elimination in comparison to the corre-
formation. Further reducing the amount of BQ led to signicantly
sponding palladium intermediates. We wondered the possi-
decreased yield. We then explored several commercially available
bility of trapping the copper-homoenolate derived from
BQ derivatives including p-xyloquinone (entry 13), 2,6-dimethyl
BQ (entry 15), methyl BQ (entry 16), and 1,4-naphthoquinone
a cyclopropanol with an alcohol via an oxidative cross coupling
process. If such process happens in an intramolecular fashion,
it would lead to valuable O-heterocycles such as THFs and THPs
(cf. 7 / 8 / 9). It would also represent one of the very few
methods to rearrange strained molecules to O-heterocycles.12
Table 1 Reaction condition optimization
This method would offer several advantages. First, the starting
material can be convergently assembled from readily available
ester 10 and (bis)homoallylic alcohol 11 by using the diaster-
eoselective cyclopropanol synthesis method developed by Cha
and co-workers.13 Second, the stereochemistry of the THF or
THP products would rely on the stereochemistry of 7 not the
ring forming process. Third, the intramolecular process may
enable the Csp3–O bond formation of sterically hindered
tertiary alcohols. Fourth, the product would be equipped with
an aryl or alkyl ketone for further elaboration. Meanwhile,
several challenges lie in the proposed cyclopropanol ring-
opening cyclization process. First, for unsymmetrical cyclo-
propanol such as 7, competitive ring-opening processes exist.
Metal-mediated ring opening is required to ensure selective
Entry Reaction conditions
Yielda
1
2
3
4
5
6
7
8
9
Cu(OTf)2 (2.0), DCE, M.S., 24 h
Cu(OAc)2 (2.0), DCE, M.S., 24 h
Cu(OTf)2 (0.1), O2, DCE, M.S., 12 h
Cu(OTf)2 (0.1), tBuOOH (2.0), DCE, M.S., 6 h
Cu(OTf)2 (0.1), BQ (2.0), DCE, M.S., 12 h
CuBr (0.1), BQ (2.0), DCE, M.S., 12 h
Cu(OTf)2 (0.1), BQ (2.0), DCM, 4 h
Cu(OTf)2 (0.1), BQ (2.0), benzene, 4 h
Cu(OTf)2 (0.1), BQ (2.0), tol., 4 h
42%
35%
41%
0%
51%
27%
67%
57%
68%
0%
cleavage of the less substituted C–C bond and radical-mediated 10
ring opening needs to be suppressed to avoid the cleavage of the
more substituted C–C bond and formation of a more stable
Cu(OTf)2 (0.1), BQ (2.0), L1 (0.2 equiv.), tol., 4 h
Cu(OTf)2 (0.1), BQ (2.0), L2 (0.2 equiv.), tol., 4 h
Cu(OTf)2 (0.1), BQ (0.5), O2 (balloon), tol., 4 h
Cu(OTf)2 (0.1), p-xyloquinone (2.0), tol., 4 h
11
12
13
0%
65%
76b,c
%
radical. Second, once intermediate 8 is formed, reductive
14
Cu(OTf)2 (0.1), p-xyloquinone (0.5), O2 (balloon), tol., 4 h 75%
elimination to form the Csp3–O bond should be facilitated
15
Cu(OTf)2 (0.1), 2,6-dimethyl BQ (2.0), tol., 4 h
Cu(OTf)2 (0.1), methyl BQ (2.0), tol., 4 h
Cu(OTf)2 (0.1), 1,4-naphthoquinone (2.0), tol., 4 h
Cu(OTf)2 (0.1), tol., 16 h
70%
74%
62%
6%
before the copper-homoenolate decomposes to an enone or 16
other byproducts. Third, a proper oxidant needs to be identied
which could promote the catalytic cycle without oxidizing the
tethered alcohol or promoting undesired radical cyclopropanol
ring-opening processes. Herein, we report the details of our
efforts in realizing such a copper-catalyzed cyclopropanol ring-
opening cyclization to synthesize substituted THFs or THPs and
17
18
a
b
c
Isolated yield. 73% yield for gram scale. 92% ee when 14a was
prepared from the corresponding enantioenriched homoallylic
alcohol with 92% ee; L1: phenanthroline; L2: bathophenanthroline;
DCE: 1,2-dichloroethane; BQ: 1,4-benzoquinone; M.S.: molecular sieve.
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(entry 17) and identied p-xyloquinone as the superior one. indicates a Thorpe–Ingold effect to facilitate the Csp3–O ring
Notably, a combination of BQ (0.5 equiv., entry 12) or p-xyloqui- closing process despite the steric hindrance of the tertiary
none (0.5 equiv., entry 14) with oxygen (balloon) gave similar alcohols. Additionally, hydroxycyclopropanols derived from
results to the use of 2.0 equiv. of BQ or p-xyloquinone. For the different esters can be used to deliver products with different
convenience of operations, we decided to use a single oxidant, p- substituents on the resulting ketones (14q–v). Again, both aryl
xyloquinone, for the substrate scope investigations. Additionally, and alkyl groups are tolerated.
when the reaction was conducted at gram-scale, 73% yield of 14a
We then tried to expand the established reaction conditions
was obtained. Compound 14a could also be produced in enan- for the syntheses of THFs to THPs, but were less successful in
tioenriched form (92% ee) from the corresponding enantioen- this direction (Table 3). For the cases of cyclopropanols of type
riched homoallylic alcohol (92% ee).
15, which are derived from the Kulinkovich reaction between
We then prepared a collection of hydroxycyclopropanols and ethyl acetate and the corresponding bishomoallylic alcohols,
evaluated them under the optimized reaction conditions while we were able to obtain the desired THP products 16a–16d,
(Tables 2 and 3). In general, the reaction has a broad substrate the yields were signicantly lower than the corresponding THF
scope for the synthesis of substituted THFs. A good functional cases. In addition to the desired THP products, for each case, we
group tolerability was observed as well. For example, 3- observed the formation of cyclopropanol ring-opening proton-
methylketone-5-aryl substituted THFs can be prepared (14a–i). ation product (cf. 17a–d). The isolation of 17a–d as the major
Both electron rich and electron decient aryl groups worked products suggests a radical cyclopropane C–C bond cleavage
effectively (14a–g). Heteroaryl groups such as pyrrole (14f), mode to break the more substituted C–C bond. The resulting
thiophene (14h), and indole (14i) were tolerated, but not furan. secondary radical intermediate was then quenched or under-
The aryl groups can also be switched to alkyl groups such as went hydrogen atom transfer (HAT) process to give side prod-
bulky tert-butyl group (14j) and TBS-ether containing alkyl ucts 17a–d in the end. Notably, when an inseparable epimeric
groups (14k and 14l). All of these cases led to the formation of 3- mixture of 15a (R1, R2 ¼ H, Ph or Ph, H) was used, the formation
methylketone-5-aryl/alkyl substituted THFs with cis stereo- of 16a with a 3,6-trans stereochemistry was observed, but not the
chemistry, which are otherwise difficult to synthesize. Notably, one with a 3,6-cis stereochemistry, which indicates that the ring-
tertiary alcohols are suitable substrates as well (14m–p). Higher opening cyclization process is sensitive to the stereochemistry
reaction yields were obtained for these tertiary alcohols, which of the tether secondary alcohol.
To provide insights about the reaction mechanism of the
copper-catalyzed cyclopropanol ring-opening intramolecular
Csp3–O cross coupling reaction, a series of experiments were
Table 2 Substrate scope for the tetrahydrofuran synthesis
conducted. First, Cu(OTf)2 catalyst is essential for the cyclo-
propanol ring opening as well as the THF or THP ring closure.
Without Cu(OTf)2, 95% of the starting material 13a was recov-
ered without any 14a formation. Interestingly, when 13a0, the
epimer of 13a, was used, desired THF product 14a0 with trans
stereochemistry was produced in 30% yield together with 9% of
18 and 44% of enone 19a (Fig. 2). The formation of enone 19a as
the major product from 13a0 indicates again that the stereo-
chemistry of the secondary alcohol of 13a or 13a0 plays an
important role in determining the cyclopropanol ring-opening
pathways, which will be discussed in the proposed catalytic
Table 3 Substrate scope for tetrahydropyran synthesis
a
A 1 : 1 mixture of diastereomers was used as starting material.
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Fig. 3 Proposed reaction mechanism.
on the appended side chain would coordinate with the copper
center to form the same 7-membered metallocycle A to continue
the rest of the cycle (A / B / 14a) and regenerate the Cu(I)
species. When compound 13a was used, intermediate A with the
large phenyl group in the pseudo equatorial position would be
formed. With compound 13a0, intermediate D would be formed.
The phenyl group of D would take the pseudo axial position and
experience strong steric repulsion with the methylene group of
the cyclopropane ring. Therefore, intermediate D would not be
Fig. 2 Mechanistic studies.
cycle. Additionally, when compound 18 was subjected to the
optimized reaction conditions, we didn't observe any 14a or
14a0, which rules out the formation of the THF ring via a Bald-
win's rule disfavored 5-endo-trig oxa-Michael addition. Radical
inhibitors such as TEMPO and BHT were explored as well. The favored. When 15a was used, the coordination of the secondary
formation of 14a or 16c was dramatically inhibited by the
addition of 1.2 equiv. of TEMPO to the reaction mixture.
Cyclopropanol 13a or 15c were recovered in 84% or 50% yield,
respectively. For the case of 15c, a new byproduct 19b was iso-
lated in 20% yield, which presumably came from a 5-exo-trig
cyclization of the corresponding enone byproduct or direct
trapping of a b alkyl carbocation with the tertiary alcohol.
However, when BHT was used, desired THF product 14a was
still produced in 67% yield from 13a. For the case of 15c, the
yield of THP product 16c dropped from 40% to 12% and the
yield of 17c increased slight from 51% to 60%. The latter is
likely due to the fact that BHT is a superb hydrogen atom donor,
which could quickly quench the b alkyl radical derived from
a radical cyclopropane ring opening process.
alcohol on the copper center would require an 8-membered ring
formation, which is energetically undesirable. Overall, the
stereochemistry of the secondary alcohol as well as its distance
from the cyclopropanol are signicant for which reaction
pathway will occur. It enforces a strong directing effect on the
reaction.
Based on the above experimental results, a plausible catalytic
cycle was proposed in Fig. 3. Cu(OTf)2 would enter the catalytic
process by reacting with 13a to form a 7-membered metallocycle
intermediate (A). Once intermediate A is formed, selective
cyclopropane ring opening would lead to copper-homoenolate
B, which would be further oxidized by p-xyloquinone to the
corresponding Cu(III) intermediate. Reductive elimination
similar to the Chan–Lam process14 would give product 14a and
generate a Cu(I) species, which would be the starting point for
the rest of the catalytic cycles. This Cu(I) species would coordi-
nate with substrate 13a via a ligand exchange process to form
alkoxy–Cu(I) intermediate C. In our previous studies,11 we
showed that Cu(I) salts were ineffective to promote cyclo-
propanol ring opening. Thus, intermediate C would be oxidized
to a Cu(II) intermediate by p-xyloquinone. The secondary alcohol
Scheme 1 Total syntheses of hyperiones.
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We then began to apply the copper-catalyzed cyclopropanol observation was reported by Barker et al. (ꢀ18.5; c 0.016,
ring-opening cyclization to prepare two THF-containing natural CHCl3)16 and Deska et al. (+19.2; c 0.045, CHCl3).17 The optical
products, hyperiones A and B. Both of them were isolated from rotation of synthetic (ꢀ)-hyperione B is ꢀ35.0 (c 2.4 mg mLꢀ1
,
the leaves of Hypericum chinense,15 an important herb medicine acetone), opposite in sign and similar in magnitude to the re-
to treat fever, hepatitis, and sepsis. Hyperiones A and B are two ported value for natural hyperione B.
norlignan natural products derived from the well-known dietary
As mentioned earlier, one advantage of the new THF/THP
lignan sesamin or its epimer asarinin. In 2013, the groups of synthesis is that the product will be equipped with an aryl/
Barker16 and Deska17 independently reported their total alkyl ketone at the C3 position, which could be further func-
syntheses of these two natural products. Barker's syntheses of tionalized to produce a variety of products. For example, the
hyperiones A and B involve 10 steps started from a known carbonyl group of 14a could be easily converted to a gem
oxazolidinone, which can be prepared in one step from diuoro group (cf. 25, Scheme 2) upon the treatment with
commercially available starting material. Their key THF ring DAST.19 Both 14a and 16a could undergo spirocyclization with
formation is a low yielding acid-catalyzed cyclization (23%). The aryl hydrazine 26 to afford 27 and 28 in 82% and 62% yield,
Deska syntheses require 10 steps and involves two enzymatic respectively.20 Additionally, methyl ketone 14a could be con-
reactions and one Ag-catalyzed cycloisomerization to form verted to hydrazone 29, whose structure was conrmed by X-ray
a dihydrofuran ring which was reduced later.
analysis.21 Hydrazone 29 could undergo a Pd-catalyzed cross
The newly developed copper-catalyzed cyclopropanol ring- coupling reaction with aryl bromide 30 to afford 31 in good
opening cyclization enabled us to complete the total synthesis yield.22
of hyperione A in 3 steps and hyperione B in 4 steps (Scheme 1),
which are signicantly shorter than the reported ones. Our
synthesis started with commercially available aldehyde 20,
Conclusions
which underwent Grignard 1,2-addition with allylMgBr to give In summary, we developed a novel copper-catalyzed cyclo-
known allylic alcohol 21 in 96% yield. Kulinkovich reaction propanol ring-opening cyclization to synthesize O-heterocycles
between 21 and ester 22 gave a 2.5 : 1 separable mixture of including THFs and THPs. The reaction has a broad substrate
cyclopropanols 23 and 24 in 66% total yield. Cyclopropanol 23 scope for synthesizing THFs. Its synthetic potential was
underwent the copper-catalyzed ring-opening cyclization demonstrated by a 3 or 4-step total synthesis of hyperione A or
smoothly to complete the total synthesis of hyperione A (12a) in B, respectively, as well as the diversications of the ketone
only 3 steps, but cyclopropanol 24 failed to deliver hyperione B containing THFs/THPs to more sophisticated products. Mech-
(12b) directly. Alternatively, tBuOK-promoted epimerization anistically, a selective C–C bond cleavage followed by an
converted hyperione A to B in 45% yield with 45% of hyperione oxidative Csp3–O bond formation occurred in the reaction
A recovered. Notably, for the conversion of 23 to 12a, BQ is process. The alcohol on the appended side chain functions not
a better oxidant than p-xyloquinone and triuorotoluene is only as a nucleophile for the Csp3–O bond formation, but also
superior to toluene as the solvent. Additionally, homoallylic a directing group to guide the cyclopropane ring opening. This
alcohol 21 could be prepared in enantioenriched form (91% ee) new method is conceptually different from all the other THF/
by using an asymmetric 1,2-addition,18 which eventually led to THP synthesis and is expected to have broad application in
asymmetric total syntheses of (+)-hyperione A (12a, 92% ee) and facilitating the synthesis of natural products and other bioactive
(ꢀ)-hyperione B (ent-12b, 91% ee). The optical rotation of molecules.
synthetic (+)-hyperione A is +17.3 (c 3.0 mg mLꢀ1, CHCl3), same
in sign and about an order magnitude smaller than the reported
Conflicts of interest
value for natural hyperione A (+195.6; c 0.045, CHCl3).15 Similar
There are no conicts to declare.
Acknowledgements
This research was supported by NIH R35 GM128570 and NSF
CAREER 1553820. We thank the NIH P30CA023168 for sup-
porting shared NMR resources to Purdue Center for Cancer
Research. The XRD data was collected on a new single crystal X-
ray diffractometer supported by the NSF through the Major
Research Instrumentation Program under Grant No. CHE
1625543.
Notes and references
Scheme 2 Synthetic applications. (a). DAST, CH2Cl2, reflux, 24 h, 80%;
(b) 26, AcOH, reflux, 6 h, 82% (27), 62% (28); (c) TsNHNH2, MeOH, rt,
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equiv.), dioxane, reflux, 12 h, 73%.
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