Aerobic Catalytic Features in Photoredox- and Copper-Catalyzed Iodolactonization Reactions

Article abstract of DOI:10.1021/acs.orglett.8b02771

Mechanistic evaluations and comparison of two important aerobic catalytic oxidation processes, aerobic copper catalysis and photoredox catalysis, are performed. Interesting and distinct catalytic behaviors were observed for a common reaction of iodolacton

Full text of DOI:10.1021/acs.orglett.8b02771

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Aerobic Catalytic Features in Photoredox- and Copper-Catalyzed
Iodolactonization Reactions
Jeewani Poornima Ariyarathna,† Fan Wu,† Sara Katelyn Colombo, Charleese Marisa Hillary,
and Wei Li*
Department of Chemistry and Biochemistry, School of Green Chemistry and Engineering, The University of Toledo, 2801 West
Bancroft Street, Toledo, Ohio 43606, United States
S
* Supporting Information
ABSTRACT: Mechanistic evaluations and comparison of two important aerobic catalytic oxidation processes, aerobic copper
catalysis and photoredox catalysis, are performed. Interesting and distinct catalytic behaviors were observed for a common
reaction of iodolactonization of alkenoic acids. Namely, the aerobic copper catalysis requires the formation of a copper
carboxylate, whereas the aerobic photoredox catalysis requires the addition of proton sources to proceed to completion.
Furthermore, the iodolactone products obtained from these catalytic processes are extensively derivatized to a number of
functionalized lactones, including aryl lactones generated from the nickel-catalyzed reductive coupling with aryl halides.
Scheme 1. Background and Reaction of Interest
he utilization of molecular oxygen as a terminal oxidant in
chemical transformations, often referred to as aerobic
T
oxidation, is an attractive feature that has been widely studied
and applied in many important catalytic processes.1 For
example, aerobic copper catalysis has long been utilized in the
Glaser−Hay couplings,2 the Chan−Lam couplings,3 and C−H
functionalizations.4 On the other hand, photoredox catalysis,
with its ability to engage organic substrates in single electron
transfer (SET) processes, has recently emerged as a powerful
catalytic platform to engineer novel chemical transformations.5
Not surprisingly, aerobic oxidation has made its way into
photoredox catalysis through the efforts of several groups.6 As
part of our interest in the development of catalytic halogenation,
the oxidation of halide salts with a benign terminal oxidant is
crucial for the overall conditions.7 In this regard, copper and
photoredox catalysis are ideal catalytic avenues for the aerobic
oxidation of halides. More importantly, discovery and under-
standing of new mechanistic features in different oxidation
processes is pivotal for the development of practical aerobic
oxidation procedures (Scheme 1a).
The reaction of interest for our study is the iodolactonization
of alkenoic acids. This is a well-known strategy for the
generation of iodolactones, highly versatile synthetic inter-
mediates amenable to access a range of functionalized products.8
This strategy has demonstrated proven applications for the
synthesis of complex natural products and medicinally relevant
molecules.9 A significant drawback for this strategy is the
frequent usage of a stoichiometric amount of highly electrophilic
halogen sources, such as I2, PhI(OAc)2, NIS, NaIO4, ICl, etc.,
often resulting in limited functional group compatibility
Received: August 29, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02771
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(Scheme 1b).10 The adoption of a stable halide salt as a halogen
precursor via in situ aerobic oxidation is a useful and practical
alternative.11 The reason we are interested in the use of iodide
salt is because it is easily oxidized and versatile for functional
group interconversion. Intrigued by the different facets of
mechanistic features that various aerobic oxidation processes
may possess, we decided to utilize the iodolactonization reaction
as a standard template to study and compare aerobic copper and
photoredox catalysis. Herein, we report our findings of distinct
catalytic behaviors from these two aerobic catalyses in the
iodolactonization reaction (Scheme 1c). We have fully explored
the synthetic versatility of the iodolactone product through
extensive derivatizations and, most notably, the nickel-catalyzed
reductive couplings of iodolactones and aryl iodides to furnish
useful aryl lactone structures.
With these goals in mind, we began our study with the
alkenoic acid 1 and potassium iodide 2 as our standard
substrates. For the copper-catalyzed variant (catalytic protocol
A), initial screening of copper catalysts revealed that the
utilization of copper(II) triflate [Cu(OTf)2] afforded only a 3%
yield, suggesting no catalytic turnover (Table 1, entry 1).
Copper(II) acetate monohydrate [Cu(OAc)2·H2O], on the
other hand, afforded a 64% yield. Interestingly, the addition of
diisopropylamine (DIPA) as a base with Cu(OTf)2 afforded the
desired product in 55% yield (Table 1, entries 2−4). With
Cu(OAc)2 as the catalyst, we further examined the addition of a
mild inorganic base and discovered that KH2PO4 was the
optimal base to give the iodolactone product in 91% yield (Table
1, entries 5−8). Satisfied with this condition, we then focused
our attention on the development of the photoredox-catalyzed
protocol (catalytic protocol B). Screening a number of
photocatalysts revealed Ir[dF(CF3)ppy]2(bpy)PF6 A as the
best catalyst, providing a 40% yield (Table 1, entries 9−12). In
addition, solvent screening identified methanol as the optimal
solvent. However, regardless of how we changed our conditions,
we were never able to increase the yield beyond 50%. In
particular, addition of base additives often resulted in lower
yields. By contrast, the addition of an acid additive improved the
reaction yield to 64% (Table 1, entries 13 and 14). Furthermore,
a slight increase of the reaction time to 24 h eventually led to
reaction completion with a 93% yield (Table 1, entry 15).
With the optimal conditions in hand, we conducted and
evaluated a series of control reactions for each catalytic protocol
(Figure 1). For the copper-catalyzed reaction, the exclusion of
a
Table 1. Reaction Optimization
Figure 1. Control experiments and catalyst turnover. a Ring opening
product with methanol.
the catalyst completely inhibited the reaction. Under a nitrogen
environment, the reaction produced a 7% yield. To compare
directly with the photocatalytic condition, running the reaction
at 40 °C afforded a 41% yield. For the photoredox process,
removing the photocatalyst still afforded the iodolactone in 12%
yield, suggesting that oxygen under blue LED light can function
for iodide oxidation to a certain extent. Similar to the copper
catalysis, a nitrogen environment led to catalyst inhibition with a
less than 5% yield. The addition of base was detrimental to the
reaction yield. Furthermore, running the reaction in the absence
of light completely inhibited the reaction. Finally, we also
evaluated the catalytic efficiency of the photoredox procedure by
measuring the turnover numbers.12 In this case, the catalyst
loading can be lowered to 0.001 mol % to afford a 36% yield.
Excluding the background reaction, the turnover number was
calculated to be 48 000, suggesting that the photocatalytic
process is incredibly efficient.
We also performed time studies for both reactions (Figure 2).
A number of features were notable here. First, the photoredox
conditions had an induction time of approximately 30 min prior
to the product formation. Careful UV−vis studies indicated that
the immediate formation of trioiodide I3− species, followed by a
slow equilibrium to iodine was most likely to be responsible for
the initial induction time.13 For the copper-catalyzed process, an
initial burst of product formation was followed by a steady
increase in product formation. UV−vis studies of mixing
stoichiometric Cu(OAc)2 and potassium iodide also revealed
a
Reaction conditions for photoredox catalysis: alkenoic acid 1 (0.25
mmol), photocatalyst (0.5 mol %), KI (0.275 mmol), solvent (1 mL),
40 °C. Reaction conditions for copper catalysis: alkenoic acid 1 (0.5
mmol), copper catalyst (15 mol %), KI (0.5 mmol), KH2PO4 (0.5
b
mmol), solvent (1 mL), 60 °C. Yields were determined by crude 1H
NMR using 1,3-benzodioxole as the internal standard. Yield shown in
parentheses was isolated yield.
−
the formation of I3 . As the reaction progressed, consumption of
B
DOI: 10.1021/acs.orglett.8b02771
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
turnover. The formation of product 3 can occur with iodination
of 1, D, or E.16 In addition, the presence of copper carboxylate D
or E may facilitate the equilibrium shift of I3− to iodine with no
induction time observed initially.17 On the other hand, the
photoredox protocol requires no such ligand exchange process.
Instead, accumulation of the oxygen radical anion F may inhibit
the reaction. This is primarily the reason why, without an
appropriate acid additive, the reaction never afforded yields
greater than 50% and, with the acid additive, the yield was
increased to 93%. These mechanistic insights suggest that, for
the reaction of interest, aerobic copper catalysis proceeds
smoothly under basic conditions, whereas photoredox catalysis
can be facilitated under acidic conditions. The mechanistically
distinct behaviors observed here will be further explored in other
aerobic catalytic conditions.
With the mechanistic picture being elucidated, we decided to
turn our attention to the scope of the alkenoic acid substrates.18
In this case, both catalytic processes worked efficiently for a
range of α-substituted and 4,4-disubstituted pentenoic acids
(Scheme 2, products 4−10). An exception in this case is
a
Scheme 2. Iodolactonization Substrate Scope
Figure 2. Time studies of both aerobic processes.
the starting material was commensurate with the product
formation.
Based on these studies, we have extracted useful information
about both aerobic oxidation processes. In the copper-catalyzed
process, the formation of the copper carboxylates D is critical for
the catalytic turnover (Figure 3).15 This is supported by the fact
Figure 3. Proposed mechanisms and features.
that when Cu(OTf)2 is used as the catalyst, only 3% of the
desired product is obtained. With the addition of diisopropyl-
amine as an exogenous base, the iodolactone formation can be
increased to 55%. When Cu(OAc)2 is used as the catalyst, the
ligand exchange is much more thermodynamically favorable;
hence, base additives are not required to achieve catalytic
turnovers. However, the addition of certain base additives such
as KH2PO4 can facilitate the ligand exchange and catalyst
a
Standard reaction conditions. Yields for catalytic protocol B were
isolated yields. Yields for catalytic protocol A were determined by
crude 1H NMR using 1,3-benzodioxole as the internal standard.
b
c
Reaction time was 36 h. An additional 25% of ring opening product
by methanol was also observed.
C
DOI: 10.1021/acs.orglett.8b02771
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
compound 6, with which the copper-catalyzed protocol afforded
a much lower yield due to the acidic α-carbonyl proton being
problematic under more basic conditions. Spirocyclic iodolac-
tones could also be obtained with good yields (Scheme 2,
products 11−14). Moreover, benzene backbones could be
tolerated under both catalytic conditions (Scheme 2, products
15 and 16). Finally, fused and bridge iodolactone structures also
proceeded smoothly in good yields (Scheme 2, products 17−
19). Notably, these bicyclic structures were also produced with
exceptional diastereoselectivity.
Scheme 4. Iodolactone Derivatizations
An exceptional feature of the iodolactone product from these
reactions is its versatility in accessing a range of derivatized
products. In particular, we wondered about the capacity of
incorporating these molecules in the nickel-catalyzed reductive
cross electrophile coupling reactions. With that in mind, we
followed a modified coupling protocol based on the elegant
work of Weix,19 Gong,20 and Reisman.21 Gratifyingly, a range of
aryl iodides, with varying electronic properties, all participated
readily to afford the desired aryl lactones in reasonable yields
(Scheme 3, products 20−29). Compared to the electron-rich
carboxylate,22 secondary amine, and azide nucleophiles all
afforded the respective functionalized lactone products 35−38
in good yields.23 In the case of a primary amine as the
nucleophile, a lactam structure was obtained. In this case, the
primary amine initially produced the amino lactone product,
followed by a subsequent intramolecular lactam formation to
produce 39 in great yield. These nucleophilic derivatizations
demonstrated the synthetic versatility of the iodolactone.
In summary, we have conducted mechanistic investigations
on two widely used aerobic oxidation processes. We have
observed very interesting and distinct oxidative features of the
aerobic copper and photoredox catalysis. Specifically, the copper
catalysis requires the generation of a copper carboxylate for
catalytic turnover. In contrast, the aerobic photoredox protocol
requires an infusion of a proton source to drive the reaction to
completion. Additionally, we have demonstrated the synthetic
utility of the iodolactone product in the nickel-catalyzed
reductive coupling reactions with aryl iodides. Finally, we have
also extended the utility of these iodolactones to access a range
of functionalized lactone structures.
Scheme 3. Dielectrophile Coupling Reactions
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02771.
Experimental procedure and characterization data of the
products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Wei.Li@utoledo.edu.
ORCID
a
Reaction conditions: iodolactone (0.375 mmol), aryl iodide (0.25
mmol), NiBr2·diglyme (10 mol %), 2,2′-bipyridine (12 mol %), Mn
(300 mol %), DMA (0.25 M), rt, 16 h.
Wei Li: 0000-0001-8524-217X
Author Contributions
†J.P.A. and F.W. contributed equally.
Notes
aryl iodides, the electron-deficient substrates generally per-
formed better. In addition, sterically demanding substrates also
reacted effectively to form the desired products in reasonable
yields (Scheme 3, products 24 and 28). Interestingly, different
classes of iodolactone structures all participated in this reductive
coupling reaction with exceptional efficiencies (Scheme 3,
products 30−34).
To explore the versatility of the iodolactone product further,
we examined a range of other nucleophiles (Scheme 4). For
example, reactions of the iodolactone 3 with thiolate,
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the University of Toledo for a startup grant and a grant
from the Summer Research Awards and Fellowship Programs.
We also thank Dr. Yong W. Kim (University of Toledo) for
NMR assistance.
D
DOI: 10.1021/acs.orglett.8b02771
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2016, 52, 6193−6196. (g) Liu, H.; Tan, C.-H. Tetrahedron Lett. 2007,
48, 8220−8222.
REFERENCES
■
(1) For selected recent reviews and examples on aerobic oxidation in
catalysis: (a) Allen, S. E.; Walvoord, R. R.; Padilla-Salinas, R.;
Kozlowski, M. C. Chem. Rev. 2013, 113, 6234−6458. (b) Wang, D.;
Weinstein, A. B.; White, P. B.; Stahl, S. S. Chem. Rev. 2018, 118, 2636−
2679. (c) Campbell, A. N.; Stahl, S. S. Acc. Chem. Res. 2012, 45, 851−
863. (d) McCann, S. D.; Stahl, S. S. Acc. Chem. Res. 2015, 48, 1756−
1766. (e) Jensen, K. H.; Sigman, M. S. Org. Biomol. Chem. 2008, 6,
4083−4088. (f) Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Chem. Rev.
2005, 105, 2329−2363. (g) Shi, Z.; Zhang, C.; Tang, C.; Jiao, N. Chem.
Soc. Rev. 2012, 41, 3381−3430.
(2) For reviews and examples on the aerobic copper-catalyzed
Glaser−Hay couplings: (a) Siemsen, P.; Livingston, R. C.; Diederich, F.
Angew. Chem., Int. Ed. 2000, 39, 2632−2637. (b) Hay, A. S. J. Org.
Chem. 1960, 25, 1275−1276. (c) Hay, A. S., II J. Org. Chem. 1962, 27,
3320−3321.
(3) For selected examples of aerobic copper-catalyzed Chan−Lam
couplings: (a) Vantourout, J. C.; Miras, H. N.; Isidro-Llobet, A.;
Sproules, S.; Watson, A. J. B. J. Am. Chem. Soc. 2017, 139, 4769−4779.
(b) Lam, P. Y. S.; Clark, C. G.; Saubern, S.; Adams, J.; Winters, M. P.;
Chan, D. M. T.; Combs, A. Tetrahedron Lett. 1998, 39, 2941−2944.
(c) Shade, R. E.; Hyde, A. M.; Olsen, J.-C.; Merlic, C. A. J. Am. Chem.
Soc. 2010, 132, 1202−1203. (d) Quach, T. D.; Batey, R. A. Org. Lett.
2003, 5, 4397−4400.
(4) For selected examples and reviews on aerobic copper-catalyzed
C−H functionalizations: (a) King, A. E.; Huffman, L. M.; Casitas, A.;
Costas, M.; Ribas, X.; Stahl, S. S. J. Am. Chem. Soc. 2010, 132, 12068−
12073. (b) Wendlandt, A. E.; Suess, A. M.; Stahl, S. S. Angew. Chem., Int.
Ed. 2011, 50, 11062−11087. (c) Liu, C.; Zhang, H.; Shi, W.; Lei, A.
Chem. Rev. 2011, 111, 1780−1824.
(11) For examples of nonaerobic oxidation of halide salts in
iodolactonizations: (a) Firouzabadi, H.; Iranpoor, N.; Kazemi, S.
Can. J. Chem. 2009, 87, 1675−1681. (b) Firouzabadi, H.; Iranpoor, N.;
Kazemi, S.; Ghaderi, A.; Garzan, A. Adv. Synth. Catal. 2009, 351, 1925−
1932. (c) Higgs, D. E.; Nelen, M. I.; Detty, M. R. Org. Lett. 2001, 3,
349−352. (d) Inamoto, K.; Yamada, T.; Kato, S.; Kikkawa, S.; Kondo,
Y. Tetrahedron 2013, 69, 9192−9199.
(12) Gensch, T.; James, M. J.; Dalton, T.; Glorius, F. Angew. Chem.,
Int. Ed. 2018, 57, 2296−2306. For every product formation, two
photoredox cycles are required. Therefore, the final turnover number is
calculated based on the product yield (excluding the background
reaction) divided by the catalyst loading times two.
(13) For an elegant and recent example of synthetic utilization of
triiodide species to control iodine release: Wappes, E. A.; Fosu, S. C.;
Chopko, T. C.; Nagib, D. A. Angew. Chem., Int. Ed. 2016, 55, 9974−
9978. Our UV−vis studies revealed that a large amount of triiodide
species was already generated at the 5 min interval. The peak intensity
of this species remained the same until the 20th min.
(14) Our UV−vis studies from control and stoichiometric mixing of
Cu(OAc)2 and potassium iodide clearly indicated the formation of a
new triiodide intermediate.
(15) (a) Deng, Y.; Zhang, G.; Qi, X.; Liu, C.; Miller, J. T.; Kropf, A. J.;
Bunel, E. E.; Lan, Y.; Lei, A. Chem. Commun. 2015, 51, 318−321. For
specific examples of Cu(II) insertion into alkenes: (b) Miller, Y.; Miao,
L.; Hosseini, A. S.; Chemler, S. R. J. Am. Chem. Soc. 2012, 134, 12149−
12156. (c) Bovino, M. T.; Liwosz, T. W.; Kendel, N. E.; Miller, Y.;
Tyminska, N.; Zurek, E.; Chemler, S. R. Angew. Chem., Int. Ed. 2014, 53,
6383−6387.
(16) For a review and examples on related halogen abstraction
processes in copper catalysis: (a) Chemler, S. R.; Bovino, M. T. ACS
Catal. 2013, 3, 1076−1091. (b) Bovino, M. T.; Chemler, S. R. Angew.
(5) For selected reviews on photoredox catalysis: (a) Prier, C. K.;
Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322−5363.
(b) Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40,
102−113. (c) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Chem. Rev. 2016,
116, 10035−10074. (d) Romero, N. A.; Nicewicz, D. A. Chem. Rev.
2016, 116, 10075−10166.
̈
Chem., Int. Ed. 2012, 51, 3923−3927. (c) Gottlich, R. Synthesis 2000,
̈
2000, 1561−1564. (d) Heuger, G.; Kalsow, S.; Gottlich, R. Eur. J. Org.
Chem. 2002, 2002, 1848−1854. (e) Wu, F.; Stewart, S.; Ariyarathna, J.
P.; Li, W. ACS Catal. 2018, 8, 1921−1925.
(17) For an elegant study on nucleophilicity impacting the rate of
halocyclization: Ashtekar, K. D.; Vetticatt, M.; Yousefi, R.; Jackson, J.
E.; Borhan, B. J. Am. Chem. Soc. 2016, 138, 8114−8119.
(6) For selected recent reviews and examples on aerobic photoredox
catalysis: (a) Condie, A. G.; Gonzalez-Gomez, J. C.; Stephenson, C. R.
J. J. Am. Chem. Soc. 2010, 132, 1464−1465. (b) Freeman, D. B.; Furst,
L.; Condie, A. G.; Stephenson, C. R. J. Org. Lett. 2012, 14, 94−97.
(c) Rueping, M.; Vila, C.; Koenigs, R. M.; Poscharny, K.; Fabry, D. C.
Chem. Commun. 2011, 47, 2360−2362. (d) Rueping, M.; Zhu, S.;
Koenigs, R. M. Chem. Commun. 2011, 47, 12709−12711. (e) Xuan, J.;
Cheng, Y.; An, J.; Lu, L.-Q.; Zhang, X.-X.; Xiao, W.-J. Chem. Commun.
2011, 47, 8337−8339. (f) Zou, Y.-Q.; Lu, L.-Q.; Fu, L.; Chang, N.-J.;
Rong, J.; Chen, J.-R.; Xiao, W.-J. Angew. Chem., Int. Ed. 2011, 50, 7171−
7175. (g) Zou, Y.-Q.; Chen, J.-R.; Liu, X.-P.; Lu, L.-Q.; Davis, R. L.;
Jørgensen, K. A.; Xiao, W.-J. Angew. Chem. 2012, 124, 808−812.
(h) Zhu, M.; Zheng, N. Synthesis 2011, 2011, 2223−2236. (i) Hari, D.
(18) In addition to the substrate scope presented here, we generally
observed that the photoredox conditions are more robust for tolerance
of functional groups such as nitro, amide, ketone, azide, and aldehyde.
(19) (a) Weix, D. J. Acc. Chem. Res. 2015, 48, 1767−1775. (b) Everson,
D. A.; Weix, D. J. J. Org. Chem. 2014, 79, 4793−4798. (c) Everson, D.
A.; Shrestha, R.; Weix, D. J. J. Am. Chem. Soc. 2010, 132, 920−921.
(d) Everson, D. A.; Jones, B. A.; Weix, D. J. J. Am. Chem. Soc. 2012, 134,
6146−6159. (e) Biswas, S.; Weix, D. J. J. Am. Chem. Soc. 2013, 135,
16192−16197. (f) Anka-Lufford, L. L.; Huihui, K. M. M.; Gower, N. J.;
Ackerman, L. K. G.; Weix, D. J. Chem. - Eur. J. 2016, 22, 11564−11567.
(20) (a) Wang, S.; Qian, Q.; Gong, H. Org. Lett. 2012, 14, 3352−3355.
(b) Wang, X.; Wang, S.; Xue, W.; Gong, H. J. Am. Chem. Soc. 2015, 137,
11562−11565. (c) Yu, X.; Yang, T.; Wang, S.; Xu, H.; Gong, H. Org.
Lett. 2011, 13, 2138−2141. (d) Wu, F.; Lu, W.; Qian, Q.; Ren, Q.;
Gong, H. Org. Lett. 2012, 14, 3044−3047. (e) Chen, H.; Jia, X.; Yu, Y.;
Qian, Q.; Gong, H. Angew. Chem., Int. Ed. 2017, 56, 13103−13106.
(21) (a) Poremba, K. E.; Kadunce, N. T.; Suzuki, N.; Cherney, A. H.;
Reisman, S. E. J. Am. Chem. Soc. 2017, 139, 5684−5687. (b) Kadunce,
N. T.; Reisman, S. E. J. Am. Chem. Soc. 2015, 137, 10480−10483.
(c) Cherney, A. H.; Kadunce, N. T.; Reisman, S. E. J. Am. Chem. Soc.
2013, 135, 7442−7445.
̈
P.; Konig, B. Org. Lett. 2011, 13, 3852−3855.
(7) (a) Evans, R. W.; Zbieg, J. R.; Zhu, S.; Li, W.; MacMillan, D. W. C.
J. Am. Chem. Soc. 2013, 135, 16074−16077. (b) Alom, N.-E.; Wu, F.; Li,
W. Org. Lett. 2017, 19, 930−933. (c) Alom, N.-E.; Rina, Y. A.; Li, W.
Org. Lett. 2017, 19, 6204−6207.
(8) (a) Denmark, S. E.; Kuester, W. E.; Burk, M. T. Angew. Chem., Int.
Ed. 2012, 51, 10938−10953. (b) Dowle, M. D.; Davies, D. I. Chem. Soc.
Rev. 1979, 8, 171−197. (c) Ranganathan, S.; Muraleedharan, K. M.;
Vaish, N. K.; Jayaraman, N. Tetrahedron 2004, 60, 5273−5308.
(9) (a) Danishefsky, S.; Schuda, P. F.; Kitahara, T.; Etheredge, S. J. J.
Am. Chem. Soc. 1977, 99, 6066−6075. (b) Zhou, Q.; Snider, B. B. Org.
Lett. 2008, 10, 1401−1404. (c) Corey, E. J.; Weinshenker, N. M.;
Schaaf, T. K.; Huber, W. J. Am. Chem. Soc. 1969, 91, 5675−5677.
(10) (a) Bartlett, P. A.; Richardson, D. P.; Myerson, J. J. Am. Chem.
Soc. 1978, 100, 3950−3952. (b) Haas, J.; Piguel, S.; Wirth, T. Org. Lett.
2002, 4, 297−300. (c) Haas, J.; Bissmire, S.; Wirth, T. Chem. - Eur. J.
2005, 11, 5777−5785. (d) Meng, C.; Liu, Z.; Liu, Y.; Wang, Q. Org.
Biomol. Chem. 2015, 13, 6766−6772. (e) Pels, K.; Dragojlovic, V.
Beilstein J. Org. Chem. 2009, 5, No. 75, DOI: 10.3762/bjoc.5.75.
(f) Kang, Y.-B.; Chen, X.-M.; Yao, C.-Z.; Ning, X.-S. Chem. Commun.
(22) For a previous synthesis of product 36, please see reference:
Roth, S.; Stark, C. B. W. Chem. Commun. 2008, 6411−6413.
(23) For related synthesis of product 37 and 38: (a) Hemric, B. N.;
Shen, K.; Wang, Q. J. Am. Chem. Soc. 2016, 138, 5813−5816. (b) Shen,
K.; Wang, Q. J. Am. Chem. Soc. 2017, 139, 13110−13116.
E
DOI: 10.1021/acs.orglett.8b02771
Org. Lett. XXXX, XXX, XXX−XXX