Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic Ketones to Macrolactones by Cerium and Cyanoanthracene Catalysis

Article abstract of DOI:10.1002/anie.202012720

We describe a cerium-catalyzed aerobic oxidative ring expansion for the expedient construction of synthetically challenging macrolactones under visible-light conditions. Cyanoanthracene has been employed as co-catalyst to accelerate the turnover of the cerium cycle leading to a fast conversion within 20 min of irradiation. Taking advantage of the high efficiency and operationally simple conditions, a collection of over 100 macrolactones equipped with ring systems ranging from 9- to 19-membered macrocycles have been prepared from simple building blocks. Moreover, the enabling potential of this strategy to simplify the generation of molecular complexity has been demonstrated through the concise synthesis of sonnerlactone.

Full text of DOI:10.1002/anie.202012720

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic
Ketones to Macrolactones via Cerium and Cyanoanthracene
Catalysis
Authors: Jianbo Du, Xiaokun Yang, Xin Wang, Qing An, Xu He, Hui
Pan, and Zhiwei Zuo
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10.1002/anie.202012720
Angewandte Chemie International Edition
RESEARCH ARTICLE
Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic
Ketones to Macrolactones by Cerium and Cyanoanthracene
Catalysis**
Jianbo Du†[a], Xiaokun Yang†[a], Xin Wang[a], Qing An[a], Xu He[a], Hui Pan[a], and Zhiwei Zuo*[b]
[a]
J. Du,† X. Yang,† X. Wang, Q. An, X. He, H. Pan
School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
University of Chinese Academy of Science, Beijing 100049, China
[b]
Prof. Dr. Z. Zuo
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
E-mail: zuozhw@sioc.ac.cn
These authors contributed equally to this work.
[†]
[**]
The authors are grateful for financial support provided by the National Natural Science Foundation of China (No.21772121, 21971163).
Supporting information for this article is given via a link at the end of the document.
ꢀꢀCerium catalyzed aerobic oxidative ring expansion of cyclic ketones
Abstract: We describe a cerium catalyzed aerobic oxidative ring
expansion for the expedient construction of synthetically challenging
macrolactones under visible-light conditions. Cyanoanthracene has
been employed as co-catalyst to accelerate the turnover of cerium
cycle leading to a fast conversion within 20 min of irradiation. Taking
advantage of the high efficiency and operationally-simple conditions,
a collection of over 100 macrolactones equipped with ring systems
ranging from 9- to 19-membered macrocycles have been prepared
from simple building blocks. Moreover, the enabling potential of this
strategy to simplify the generation of molecular complexity has been
demonstrated through the concise synthesis of sonnerlactone.
O
R’
O
Ce photocatalyst
organo photocatalyst
OH
O
m
m
R
R
oxygen, blue LEDs
R’
O
n
n
ꢁ 9- to 19-membered macrolactones
ꢁ macrolactone natural products
hydroxyalkylation
O
OH
O
Me
O
+
O
m
R’
n
R
HO
OH
ꢁ ketones, epoxides and
oxetanes building blocks
ꢁ (3R, 5S)-Sonnerlactone
Introduction
ꢀ Environmental benign conditions
operationally-simple protocol
w/o strong oxidants
ꢀ Rapid construction of macrolactones
over 100 structurally diverse lactones
reaction time: 20 min
Over the past decades, the utilization of visible light energy
via photoinduced electron transfer or energy transfer processes
have enabled a wide range of selective transformations under
environmentally benign conditions.[1] Among light-absorbing
molecules, rare earth metal complexes, particularly abundant
lanthanide compounds, possess distinctive excitation and
emissive properties, and have rendered versatile applications in
photoluminescence and related areas.[2] Despite the fact that
their intriguing electronic properties, photoactivities, and
affordability could open new avenues for the development of
activation modes and large-scale applications, lanthanide
photocatalysts currently remain underexploited in synthetic
chemistry, compared to iridium and ruthenium photocatalysts.[3]
Over the last five years, groundbreaking reports have emerged
in lanthanide photocatalysis. Preeminently, the Schelter group
exploited the outstanding reduction capacity of excited
cerium(III) complexes and achieved several photocatalytic
dehalogenations and Miyuara borylation reactions.[3b-f] We
incorporated the LMCT excitation of Ce(IV) complexes for the
direct and facile activation of commonly occurring O-centered
functionalities, which has led to a series of selective C–H bond
and C–C bond cleavage and functionalizations of feedstock
chemicals.[3g-m] Recently, this operationally simple cerium-
catalyzed LMCT paradigm has also been employed in
decarboxylative transformations.[3n-p] Notably, the Glorius group
recently demonstrated an intriguing use of Gd(OTf)3 as effective
Figure 1. Photocatalytic ring expansion of cyclic ketones via cerium catalysis.
photocatalysts in a cycloaddition/ring expansion reaction that
enables the rapid construction of complex indole manifolds
under mild conditions.[3q] Herein, we described an aerobic
oxidative ring expansion of cyclic ketones through the catalytic
use of readily available cerium(III) triflate for the straightforward
synthesis
of
challenging
macrolactones.
Importantly,
cyanoanthracene has been employed as an effective co-catalyst
to enhance the turnover of the cerium cycle, rendering a
selective ring expansion within a short reaction time of 20 min.
Macrolactones have been the subject of great interest in
organic synthesis owing to the challenges inherent in their
synthesis complexity and their prominent roles in antibiotic
discovery and development.[4] The substantial difficulties in the
ring closure, especially in the construction of medium-sized
macrocycles have rendered effective synthetic methods
currently in high demand.[5] The ring expansion strategy,
bypassing the entropically disfavored cyclization with
fragmentation-initiated rearrangement, has been frequently
demonstrated effective in the constructions of these challenging
scaffolds.[6] Nevertheless, the thermodynamically demanding C–
C bond fragmentation step has typically required the use of
forcing conditions.[7] Preeminently, the recent advancement in
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10.1002/anie.202012720
Angewandte Chemie International Edition
RESEARCH ARTICLE
2 mol% Ce(OTf)3
O
O
O
O
catalytic cleavage and functionalization of C–C bonds, through
transition metal catalysis[8] as well as photoredox catalysis,[9]
have demonstrated vast potential to expand synthetic capacity
of ring expansions.
4 mol% (n-Bu)4PCl
OH
EAꢀꢁO2 balloon
blue LEDs (0.14 W/cm2)
1
irradiation time: 2h
2
from cycloheptanone
and ethylene oxide
The b-scission of alkoxy radicals,[10] one of the well-
established reaction pathways that traditionally relies on the use
of various radical precursors, has recently been incorporated
into catalytic activations for effective C–C bond cleavage under
mild and practical conditions.[9] Among ring expansions, one
peculiar type of b-scission on lactol scaffolds was demonstrated
by Schreiber, Suginome and Suarez et al. using
stoichiometrically generated O–X species such as peroxides,
hypoiodites, through reactions with strong oxidants (hydroxyl
Entry
catalysts
Yield (%)
1
16 (45a)
55
2 mol% Ce(OTf)3
2
2 mol% Ce(OTf)3, 1 mol% 4-CzIPN
2 mol% Ce(OTf)3, 1 mol% Mes-Acr+ ClO4
–
3
54
4
27
2 mol% Ce(OTf)3, 1 mol% Eosin Y
2 mol% Ce(OTf)3, 1 mol% 3
2 mol% Ce(OTf)3, 1 mol% 4
2 mol% Ce(OTf)3, 1 mol% 5
2 mol% Ce(OTf)3, 1 mol% 6
2 mol% Ce(OTf)3, 1 mol% 7
2 mol% Ce(OTf)3, 1 mol% 8
2 mol% Ce(OTf)3, 1 mol% 9
2 mol% Ce(OTf)3, 1 mol% 10
5
55
peroxide, mercury oxide, etc.) to generate olefinic- and iodo-
6
76
[9o]
lactones, respectively.[11] Recent work by Knowles
groups
and our
[3g,j]
7
81 (80b)
74
have demonstrated the efficacy of direct catalytic
8
generation of alkoxy radicals for ring expansions via b-scission
of C–C bonds. We hypothesized that the mild nature of Ce-
LMCT excitation, as well as the Lewis acidity of cerium salts,
could be combined to enable a direct catalytic ring expansion of
alkoxyketones via the lactol tautomer to generate macrolactones
in a straightforward assembly obviating the requirement for
overly strong or dangerous oxidants.
9
77
10
11
12
73
77
78
3
5
4
R1
R1=H, R2=H
E = +1.16 V
E* = -2.05 V
!= 5.1 ns, "=0.26
R1=CN, R2=Ph
E = +1.61 V
R1=Ph, R2=Ph
E = +1.20 V
E* = -1.79 V
E* = -1.21 V
Results and Discussion
!= 8.1 ns, "=0.84
!= 5.2 ns, "=0.55
R2
1.00
6
V
Given the prevalence of hydroxyl and carbonyl groups in the
backbone of bioactive macrolides, we first aimed to establish an
oxidative ring-expansion protocol for keto-macrolactones.
Oxygen was chosen as the benign oxidant for practical
1.50
7
8
9
10
R1=PMP, R2=PMP R1=Ph, R2=H
R1=Ac, R2=H
E = +1.37 V
E* = -1.65 V
R1=Cl, R2=Cl
E = +1.38 V
E* = -1.60 V
R1=CN, R2=CN
E* = + 1.99 V
E = -0.91 V
considerations, despite that the occurrence of oxygen
E = +1.06 V
E* = -1.88 V
E = +1.20 V
E* = -1.88 V
[12]
photosensitizing
might lead to undesired oxidation pathways.
!= 5.3 ns, "=0.60 != 6.1 ns, "=0.42
!= 3.9 ns
!= 8.2 ns, "=0.51 != 13.4 ns, "=0.90
We initiated our investigation using a-alkoxyketone 1 as the
template substrate, which can be readily accessed through a
simple enolate alkylation of cycloheptanone with ethylene oxide
and exists as a 1:1 equilibrium mixture of ketone and lactol.
Under the irradiation of blue LEDs (lc, 400 nm, light intensity,
0.14 W/cm2), the corresponding keto-lactone 2 can be generated
with 16% yield in 2 hours using a low loading of Ce(OTf)3
catalyst under atmospheric pressure of oxygen (Figure 2). Only
trace hydroxy-lactone can be observed, indicating a high level of
selectivity for the keto-lactone formation. Critically, the oxidation
of the primary hydroxyl group was not detected. This poor
conversion rate can be slightly improved with prolonged
irradiation time, although at the cost of forming a mixture of
overoxidation by-products.
Yields were determined by GC-FID analysis. a yields at 12 h; b yields at 20 min.
Figure 2. Photocatalytic ring expansion of cyclic ketones via cerium catalysis.
OTf = trifluoromethanesulfonate.
anthronitrile 5 proved to be the most effective co-catalyst after
careful assessment, leading to an optimal yield of 81%.
Intriguingly, the reaction could be promoted to completion within
20 min using this co-catalyst. Seemingly, there is no direct
correlation between the acceleration effect with oxidation
potential
of
different
photocatalysts,
or
with
their
photoluminescence properties (lifetime and quantum efficiency).
The oxidized form of the anthracenes could effectively engage
single electron transfer events with in situ generated Ce(III)
alkoxide complexes (Ep= 0.40 V vs. SCE). As previously
reported fluorescence quenching experiments on anthracenes
revealed decreased quenching rates for CN substituted
anthracenes with molecular oxygen,[13] we speculate that
relatively suppressed oxygen photosensitization with catalyst 5
and 10 would in turn facilitate more rapid turnover of the Ce
catalytic cycle, despite that the efficiency of catalyst 10 was
somewhat compromised by its low solubility in ethyl acetate.
Considering that previous mechanistic studies on cerium
alkoxide complexes have manifested a relatively high efficiency
of alkoxy radical generation in the cerium-LMCT paradigm,[3l] we
reasoned that the acceleration of Ce(IV) regeneration would be
rather critical for the efficient production of the desired
macrolactone. Gratifyingly, the introduction of another
photocatalyst, including commonly used 2,4,5,6-tetra(carbazol-
9-yl)benzene-1,3-dicarbonitrile (4CzIPN), Mes-Acr+, Eosin Y and
anthracene catalysts, led to significantly improved catalytic
efficiencies. Among anthracene catalysts, 9-phenyl-10-
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10.1002/anie.202012720
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RESEARCH ARTICLE
Scheme 1. Scope of cerium catalysed oxidative ring expansion. General reaction conditions: alkoxyketone substrate (0.1 mmol), Ce(OTf)3 (0.02 equiv.),
10-phenyl-9-anthracenecarbonitrile (0.01equiv.), (n-Bu)4PCl (0.04 equiv.), ethyl acetate (0.05 M), Blue LED (400 nm, 0.14 W/cm2 ). All yields are yields of
isolated products. Diastereoselectivity was determined by 1H NMR analysis. aTFA (0.6 equiv.) was used. Boc = tert-butyloxycarbonyl; TBS = tert-
butyldimethylsilyl; Bz = benzoyl.
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10.1002/anie.202012720
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RESEARCH ARTICLE
Furthermore, control experiments revealed the essential roles of
cerium catalyst and blue light for this photocatalytic ring
expansion (see SI). Notably, no desired product was formed in
the presence of cyanoanthracene 5 without Ce(OTf)3, excluding
photosensitizing singlet oxygen as a possible intermediate for
the oxidative ring expansion.
employed to construct benzo-fused macrolactones with high
efficiencies. Moreover, this transformation is not sensitive to the
steric hindrance around the pendant hydroxyl group, as tert-butyl
groups and gem-dimethyl groups can be efficiently introduced
through commercial tert-butyloxirane (17, 21, 24 and 31) and
isobutylene oxide (34). Nevertheless,
a substoichiometric
amount of TFA was found beneficial for the ring expansion of
cyclohexanones and a few other types of ketones (22, 27-30, 46,
48 and 53), possibly through a more favorable equilibrium
towards lactol. The direct activation of cyclic ketones with
pendant hydroxyl group under these mild reaction conditions
allows for good compatibility with various commonly occurring
functionalities, including sulfones (12), alkene (13, 23, 38, 45),
carbamates (14 and 53), ethers (16, 25, 30, 33, 40, 44, 47, 48),
alkynes (16 and 20), esters (17 and 54), halogen atoms (22 and
37), and amide (39). Under the mild reaction conditions, b-
functionalities of the newly formed ketone carbonyl that are
prone to migration or elimination to generate a a,b-unsaturated
carbonyl moiety remain intact (38, 40, 45, 47 and 48).
Importantly, the stereochemistry of the secondary hydroxyl
group in the cyclic ketone material is transferred into the
macrolactone without any erosion (35). Despite the intermediacy
of high energy open-shell species and constant irradiation,
strained dibromocyclopropane moiety (37) was carried through
the reaction unscathed.
Given that small and medium-ring cyclic ketones are
abundant and readily available, the use of inexpensive
photocatalysts and operationally simple conditions make the
rapid access of diverse macrocycle structures easily achievable.
This has enabled the rapid preparation of a large collection of
keto-lactones (over 100 structurally diversified macrolactones,
see supporting information) equipped with ring systems ranging
from 9- to 19-membered macrocycles and bearing varied
substitutes, (representative examples are demonstrated in
Scheme 1).[14] Notably, the building blocks employed for these
syntheses, cyclic ketones, epoxides and oxetanes are easily
accessible from commercial sources. In practice, the
combination of cyclohexanones and epoxides provides
a
general and efficient route for the syntheses of 9-membered
macrolactones (11-25, 54-55 and S1-S19). Regarding the
construction of 10-membered-ring system,
a
challenging
macrocyclic scaffold which are the omnipresent in naturally
occurring medium-sized-ring lactones, flexible and adaptive
synthetic plans can now be devised using this photocatalytic
strategy. Two different synthetic approaches, the modular
assembly from cyclohexanones and oxetanes (26-30) or from
cycloheptanones and epoxides (31-44, and S20-S27), can
expediently and reliably deliver the required 10-membered
macrocycle with good-to-excellent efficiencies, enabling the
To probe the plausible mechanism for the cooperative
catalytic system, fluorescent quenching experiments of
cyanoanthracene 5 by cerium (III) complexes were firstly
performed, yet the lack of an obvious quenching phenomenon
indicates that photoexcited 5 is not capable of regenerating
selection of the more accessible set of building blocks in practice. Ce(IV) species. Probing the oxidized radical cation of 5 (E1/2
=
As demonstrated in products 2 and 26, 27 and 32, 29 and 34,
different substitution patterns can also be established with
different sets of building blocks, providing opportunities to further
improve structural diversity. Moreover, 11-membered keto-
lactone can be facilely assembled from the cycloheptanones and
oxetanes (46, 48, and S37-S40), or cyclooctanones and
epoxides (45, 47, and S28-S36). This photocatalytic ring
expansion can also be employed for the syntheses of
macrolactones with large ring systems (from 12- to 19-
membered ring), with no significant decrease in the efficiency
(49-52, S41-S55). Furthermore, cyclic ketones embedded in
complex skeletons including bridged azacycles (nortropinone,
53) and steroids (androstanolone and cholestanone, 54 and 55)
can also be modularly expanded into macrocycles via this
protocol, demonstrating an effective approach to construct
macrolactone with polycyclic scaffolds.
1.61 V vs. SCE) should be feasible via in situ single-electron
oxidation of 5; however, due to substantial spectral overlap of
the anthracene catalyst and Ce(IV) LMCT excitation,
modification to the system for spectroscopic study was required.
Eventually, commercial 1-chloro-bis-phenyl ethynylanthracene
(1CPBEA) was chosen for mechanistic investigations, because
of its red-shifted absorption band that could give out a narrow
window around 370 nm for the observation of Ce(IV) (1CPBEA
could also act as a viable co-catalyst to generated 2 in 76%
yield). 2-Ethyl 1,1-dimethylethene-1,1,2-tricarboxylate (E1/2 = –
1.17 V vs. SCE) was utilized as the sacrificial oxidant to
generate 5+., with irradiation at 470 nm employed to minimize
the LMCT excitation of any potentially generated Ce(IV) species.
Upon irradiation of a mixture of Ce(III), ICPBEA, and the
sacrificial oxidant, the appearance of a broad peak centered
around 370 nm, corresponding to the LMCT band of Ce(IV),
emerged, supporting the single electron oxidation of Ce(III) by
the oxidized radical cation of the anthracene. A plausible dual
catalytic cycle was proposed in Scheme 2. We posited that the
Lewis acidic nature of Ce(III) would enhance the formation of
lactol in the ketone/lactol equilibrium. The in situ formed Ce(III)
lactol complex would be easily oxidized by 5+. into its Ce(IV)
state,[15] which was subsequently photoexcited to produce the O-
centered radical for ring expansion. The resultant alkyl radical
was rapidly trapped by oxygen (rate constant in the order of 109
M-1S-1 for alky radicals) [16] to produce a peroxyl radical, which
could be further quenched by photoexcited cyanoanthracene to
It is worth noting that the C–C bonds between the carbonyl
and the alkylated carbon were selectively cleaved in all these
cases, allowing for the formal insertion of the alkoxyl moiety in a
highly predicable manner. Ring strain has little impact on this
photocatalytic transformation, as
a
variety of commonly
occurring cyclic ketones, from less-strained cyclohexanones and
cycloheptanones to strained cyclic ketones with 10- or higher
membered ring systems can all be effectively cleaved and
transferred into the desired products. Importantly, the
nucleophilicity of the ketone carbonyl exerts negligible influence
on the efficiency of the ring expansion, as tetralones (25,30,
S15-S19) and benzosuberones (43, 44 and 48) can be
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10.1002/anie.202012720
Angewandte Chemie International Edition
RESEARCH ARTICLE
a
generate 5+. and the desired keto -macrolactone. Based on fast
[3m]
kinetics of LMCT-homolysis,
we currently believe that the
resting state of the cerium cycle would be Ce(III) complexes,
and the introduction of a photoinduced electron transfer catalyst
would have accelerated its oxidation.
Ph
Cl
In preliminary investigations, this reaction manifold is
amenable to other non-oxygenation radical quenching pathways,
including C–C bond formations, and amination using appropriate
radical trapping reagents under operationally simple conditions
(Scheme S1). As a demonstration of the enabling potential of
this cerium catalyzed annulation/expansion platform to simplify
the generation of molecular complexity in organic synthesis, the
natural product sonnerlactone was targeted. Sonnerlactones are
cytotoxic metabolites recently isolated from marine-derived
fungi.[17] The benzo-fused 10-membered lactone ring embedded
in sonnerlactones represents a novel scaffold of naturally
occurring nonanolides, but poses substantial difficulties for
traditional cyclization strategies. Previous syntheses employing
Mitsunobu or RCM reactions have required elaborate
retrosynthetic designs and lengthy preparation of cyclization
precursors.[18] By incorporating our cerium catalyzed ring
expansion into our retrosynthetic planning, a significantly more
concise synthetic plan can be easily envisioned. The modular
assembly of dimethoxy benzosuberones 58 with (R)-propylene
oxide through the annulation/expansion sequence afforded the
benzofused 10-membered macrolactone framework in an
expedient fashion, with the stereochemistry at C3 position
properly established from the inexpensive chiral epoxide
precursor. Notably, the photocatalytic ring expansion was
carried out in a continuous-flow setup using glass made micro-
reactors, and the desired keto-lactone 59 was furnished in 78%
yield at the gram-scale level. A subsequent deprotection and
Noyori reduction [19] with well-established protocols delivered the
desired natural product, accomplishing the synthesis of (3R,
5S)-sonnerlactone in 4 steps (35% overall yield) from the
commercially available ketone.
Ph
1CPBEA
b
CN
e–
[CeIIIORCln]
– H+
E = 0.40 V vs. SCE
5+.
Ph
1
E = 1.61V vs. SCE
CN
*
e–
[CeIIICln]
Ph
CN
[CeIVORCln]
Ph
5
O
O
O
O
!-scission
O2
O
O
OO
+ H+, e–
– H2O
O
O
CeCln
O
OH
Ce(OTf)3
O
O
(n-Bu)4PCl
– H+
O
[CeIIIORCln]
1
2
Scheme 2. Mechanistic investigation and proposed catalytic cycle.
O
OMe
O
Me
O
OMe
Me
1. LHMDS, BF3.OEt2,
Conclusion
O
2. 2 mol% Ce(OTf)3, 1 mol% 5
4 mol% (n-Bu)4PCl, O2
blue LEDsꢀꢁꢂꢃꢄꢅꢁꢆꢇꢈꢉꢊꢄꢆꢋ
63% yield for 2 steps
MeO
O
In summary, we have developed a rapid and straightforward
ring expansion protocol of cyclic ketones under visible light
irradiation and aerobic conditions, utilizing readily available
cerium triflate with the assistance of cyanoanthracene co-
catalyst. The straightforward and operationally simple protocol
MeO
58
59
commercially
available
OH
O
Me
Flow conditions:
3. AlCl3, DCM
Glass micro-reactors,
4.5 mL internal volume
O
4. Ru catalyst,
HCO2H, Et3N
Ce(OTf)3 (60 mg), 5 (14 mg),
(n-Bu4)PCl (60 mg), ketone (1.4 g),
EA (100 mL)
has enabled facile assembly of
a
diverse range of
HO
OH
56% yield
for 2 steps
macrolactones from simple and readily available building blocks.
Furthermore, this synthetic protocol has been applied in the
(3R, 5S)-Sonnerlactone
synthesis of sonnerlactone, resulting in
simplification of synthetic efforts.
a
considerable
Oxygen
input
Cooling fans
Keywords: Lanthanide photocatalysis • visible light • C–C bond
cleavage • ring expansion • cyanoanthracene catalyst
Pump and
control
Micro flow reactors
Blue LEDs
unit
Liquid
solution
Product
collection
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Romero, D. A. Nicewicz, Chem. Rev. 2016, 116, 10075.
Scheme 3. Cerium catalyzed ring expansion in the concise synthesis of
Sonnerlactone.
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RESEARCH ARTICLE
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10.1002/anie.202012720
Angewandte Chemie International Edition
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Jianbo Du†, Xiaokun Yang†, Xin Wang,
Qing An, Xu He, Hui Pan, and Zhiwei
Zuo★
Page No. – Page No.
Photocatalytic Aerobic Oxidative
Ring Expansion of Cyclic Ketones to
Macrolactones by Cerium and
Cyanoanthracene Catalysis
Challenging macrolactones now can be modularly assembled through
a
photocatalytic aerobic oxidative ring expansion of cyclic ketones. With the
assistance of a organo photocatalyst, a-alkoxyketones can be transformed into
macrolactones with 20 min of blue LED irradiation, which ultimately enables a rapid
assembling of a large collection of structurally diverse lactones.
This article is protected by copyright. All rights reserved.