A Chiral Pentenolide-Based Unified Strategy toward Dihydrocorynantheal, Dihydrocorynantheol, Protoemetine, Protoemetinol, and Yohimbane

Article abstract of DOI:10.1021/acs.orglett.7b01573

An organocatalytic cross-aldol reaction of formaldehyde (formalin) with alkyl aldehydes, followed by the Z-selective Horner-Wadsworth-Emmons (HWE) reaction and immediate lactonization, afforded γ-alkylated pentenolides in good overall yields and excellent

Full text of DOI:10.1021/acs.orglett.7b01573

Letter
pubs.acs.org/OrgLett
A Chiral Pentenolide-Based Unified Strategy toward
Dihydrocorynantheal, Dihydrocorynantheol, Protoemetine,
Protoemetinol, and Yohimbane
†
,‡,§
†,‡,§
†,‡
†
,†
Changmin Xie,
Jisheng Luo,
Yan Zhang, Lili Zhu, and Ran Hong*
†CAS Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Road, Shanghai 200032, China
‡
University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China
*
S Supporting Information
ABSTRACT: An organocatalytic cross-aldol reaction of formaldehyde (formalin) with alkyl aldehydes, followed by the Z-
selective Horner−Wadsworth−Emmons (HWE) reaction and immediate lactonization, afforded γ-alkylated pentenolides in good
overall yields and excellent enantioselectivities. Based on this scalable sequence, five quinolizidine alkaloids were synthesized in a
unified and concise manner. The development of an in situ activation of a tertiary amide to improve the efficiency of the
Bischler−Napieraiski (B−N) reaction was also noteworthy due to the generality to sensitive substrates for a variety of target
molecules.
5
δ-Lactones are widely embedded in numerous natural products
transformations being well-documented for decades. The main
challenge in this proposal, however, is the cross-aldol reaction of
formaldehyde to form β-hydroxy aldehyde 3 due to the inherent
low reactivities of the oligomerized formaldehyde, the potential
condensation and self-aldol reactions of the parent aldehyde, and
the oligomerization of the corresponding product (see 3). To our
delight, Boeckman and co-workers reported a highly enantiose-
lective cross-aldol reaction of formaldehyde (formalin) using the
and pharmaceutical intermediates and are versatile building
1
blocks for organic synthesis. Given the wealth of substituent
patterns on the ring system, the development of efficient methods
to synthesize δ-lactones has become a topic of constant interest in
the synthetic community. Among the literature precedents to
access δ-lactones, pentenolide (1) (IUPAC name: 5,6-dihydro-
2H-pyran-2-one) is a common precursor to readily implement a
6
,7
variety of functional groups (Scheme 1a). For instance, new
stereogenic centers can be readily installed through a stereo-
selective Michael addition at C4 and subsequent alkyl or aldol
reactionsatC3. Indeed, itsversatile reactivityhas beenrecognized
Jørgensen−Hayashi catalyst (5a/5b) (Scheme 1c). The
corresponding hemiacetal was further converted into E-
conjugated esters by trapping with a variety of Wittig reagents
in good yields. Interestingly, Hayashi and co-workers applied
organocatalyst 6 to synthesize similar products in higher
2
by the synthetic community in the past several decades. Among
all novel synthetic methods to access chiral pentenolides, ring
8
enantioselectivities. With a secure aldol reaction in excellent
closing metathesis (RCM) has been widely adopted due to the
enantioselectivity, the remaining problem would be the Z-
selectiveolefinationtodeliveradesignedconjugatedester(see4),
which will be readily cyclized under suitable conditions. Here, we
report such an effort to successfully access chiral γ-substituted
pentenolides in high enantioselectivities in a “pot-economy”
3
readily availability of chiral allylic alcohols. We noticed, however,
that the diluted reaction system would be a bottleneck for the
scalability of this protocol. Recently, Liu and co-workers
developed an Evan’s chiral auxiliary-based Michael addition/
elimination cascade to complete the preparation of chiral 5-
9
4
manner, and we further exemplified the short syntheses (5−6
methyl-pentenolide (2a, R = Me). Although the reaction was
total steps) of several biologically interesting alkaloids (vide
infra), whichhavebeentypicallypreparedin>10stepsinprevious
reports.
readily scalable, a catalytic asymmetric variant to access such
versatile chiral lactones is still of great interest.
We envisioned a potentially short and practical approach to
construct the requisite structure (Scheme 1b). In this proposal,
aldol product 3 should be immediately used for olefination to give
Z-isomer 4, which is expected to carry out the ensuing
lactonization to deliver pentenolide 2. To our surprise, this
combination was not fully developed despite the individual
7
Boeckman’s protocol was chosen due to a lower catalyst
loading (5−30 mol % of 5a or 5b) and cheaper catalyst ($60/25 g
Received: May 24, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01573
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Scheme 1. (a) Pentenolide: Reactivities and Synthetic
Methods; (b) Proposed Synthetic Route toward Pentenolide
extracted into the organic layer and then treated with phosphate
10
7 in the presence of NaH (entry 1). The resultant Z-conjugated
ester (see 4) was immediately cyclized to release 5-allyl-
pentenolide 2b in 46% overall yield and 90% ee (entry 1).
When diisopropylethylamine (DIPEA) was used for the
subsequent olefination, a dramatic drop of enantioselectivity
was observed (entry 3 vs 1 and 2). When catalyst 5b (R′ = TES)
was examined, the enantioselectivity was improved (entry 4 vs 2).
Reactions with catalyst 5c (R′ = TBS) or phosphate 8 did not give
better results (entries 5 and 6). A contamination of noncyclized
free alcohol was found during the direct neutral workup
procedure. Therefore, a stoichiometric amount of HCl (aq) was
introduced, providing 2b in 73% yield (entry 7 vs 4). The “three-
step, one-column” procedure was readily scalable while
maintaining a good yield and excellent enantioselectivity (entry
2
; (c) Organocatalysts for the Cross-Aldol Reaction of
Formaldehyde (the Stereochemistry Rationale with 5a
Highlighted in the Box; Also See Ref 7b)
a
8
). The optimized conditions proved to be effective for other
chiral pentenolides 2c−f in good yields and excellent
enantioselectivity (Scheme 2). For instance, chiral pentenolide
2f was readily isolated in 40% overall yield (unoptimized), which
drastically improves the step-economic efficiency of the previous
3a
synthesis.
Scheme 2. Preliminary Exploration on Substrate Scope (50−
200 mg Scale)
a
TMS = trimethylsilyl; TES = triethylsilyl.
of 5) in comparison to Hayashi’s system (30 mol %, $110/1 g of
8
6
). 4-Pentenal was therefore subjected to a buffered biphasic
system in the presence of trimethylsilyl diphenylprolinol 5a
(
entry1, Table1). Thealdolreactionwasperformedat0°Cfor12
h and diluted with toluene. The hemiacetal (not isolated) was
Table 1. Optimization of the Reaction Conditions for the
Preparation of Chiral Pentenolide 2b
Having established a method to access chiral pentenolides, we
sought to devise a new strategy to synthesize bioactive natural
products. Yohimbane is a congener of the yohimbine family, a
class of pentacyclic alkaloids exerting an extensive array of
11
biological activities. Continuing interest in this intriguing
architecture has resulted in dozens of synthetic routes as well as
12,13
biological evaluations over the past six decades.
However, the
previousrouteswerenotsuitableforthesupplyofmaterialsdueto
the generally long synthetic sequences and/or low overall yields.
Recently, Ghosh and co-workers reported a divergent route to
access yohimbane and alloyohimbane via enzymatic desymme-
12e
trization. The total yield of the yohimbane synthesis was
impressively 11%, while the synthetic route still required 13 LLS
(longest linear steps). To quickly establish a library of natural
product-like compounds, an efficient synthesis remains in high
step a:
catalyst
(equiv)
step b: HWE olefination and
ee %
yield
a
b
entry
lactonization
(2b)
(%, 2b)
1
2
3
5a (0.2)
5a (0.2)
5a (0.2)
5b (0.2)
5c (0.2)
5b (0.2)
5b (0.2)
7, NaH, THF, 0 °C
7, NaI, DBU, THF, −78 °C to rt
7, NaI, DIPEA, THF, −78 °C to rt 58.0
90.1
90.2
46
50
14
demand. We proposed pentenolide 2b would be an ideal
c
starting point to reach this objective.
n.d.
15
d
At the outset, the copper(I)-promoted Michael addition of
pyranone 2b generated lactone 9 in 89% yield with good
diastereoselectivity (ds 12/1) (Scheme 3). The subsequent RCM
4
7, NaI, DBU, THF, 0 °C
7, NaI, DBU, THF, 0 °C
8, NaI, DBU, THF, 0 °C
7, NaI, DBU, THF; HCl
95.6
93.0
92.4
93.2
49
26
30
73
5
6
7
16
reaction proceeded smoothly with the Grubbs-II catalyst (0.5
mol%)todeliverafusedtrans-lactone10inexcellentyield(97%).
The conventional aminolysis and “one-pot” lactamization/
Bischler−Napieraiski (B−N) cyclization/reduction sequence
were adapted to deliver dehydroyohimbane 11 in 26% isolated
yield. A contamination of unidentified side products was
observed, which might be derived from the harsh conditions
(
1.0 equiv), 0 °C
7, NaI, DBU, THF; HCl
1.3 equiv), 0 °C
e
8
5b (0.2)
94.8
68
(
a
Enantiomeric excess (ee) of 2b was determined by chiral HPLC (see
the Supporting Information for details); the absolute configuration was
assigned by Boeckman’s model (ref 7b) and the following synthetic
b
c
d
using phosphorus oxychloride (POCl ), particularly for the
3
17
transformation. Isolated yield. Not determined (n.d.). 4-Pentenal
e
reactive indole moiety. Therefore, a survey of the activation of
was freshly distilled. The reaction was run on a 2-g scale for 48h. TBS
tert-butyldimethylsilyl; THF = tetrahydrofuran; rt = room
temperature; DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene.
=
the tertiary amidewas undertaken. Encouraged byBanwell’s work
18
on the activation of amides, as well as the recent explorations by
B
DOI: 10.1021/acs.orglett.7b01573
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Letter
a
Scheme 3. Synthesis of Yohimbane
yield with a good diastereoselectivity (ds 5/1) (Scheme 4).
Aminolysis with homoveratrylamine and the ensuing cyclization
delivered lactam 15 in a combined 82% yield for two inseparable
diastereomers. The subsequent B−N cyclization with POCl
proceeded smoothly, and the subsequent reduction with NaBH
3
4
afforded the desired cis, trans-product 16 in 79% yield (ds > 20/1
20
forthereductionstep)alongwiththecis,cis-isomerin14%yield.
Cleavage of the terminal alkene to reveal the aldehyde proved to
21
be challenging in the presence of an electron-rich aromatic ring.
After intensive optimization, an interrupted extraction with n-
BuOH after dihydroxylation proved crucial to achieve a high yield
for the water-soluble dihydroxylated product. Thus, catalytic
dihydroxylation in acetone−H O cosolvent and cleavage of the
2
22
corresponding diol with silica-gel-supported NaIO gave
4
protoemetine in 76% yield. The following reduction by NaBH
delivered protoemetinol in quantitative yield.
4
23
Whenhomoveratrylaminewassubstitutedwithtryptamine,the
above synthesis sequence remained effective. Accordingly,
aminolysis of 14 with tryptamine and cyclization with in situ
activation of the primary alcohol afforded lactam 17 in 72% yield
a
ds = diastereoselectivity; TMSCl = trimethylsilyl chloride; TsCl =
para-methylphenylsulfonyl chloride; Tf O = trifluoromethanesulfonic
anhydride; DTBP = 2,6-di-tert-butylpyridine; LiAlH4 = lithium
aluminum hydride.
2
(
a mixture of trans- and cis-isomers). The subsequent B−N
cyclization with Tf O/DTBP and reduction by LiAlH smoothly
2
4
19
the Movassaghi and Huang groups, a combination of Tf O and
2
2
gave the requisite tetracyclic cis, trans-product 18 in 72% yield,
20
,6-di-tert-butylpyridine (DTBP) proved to be effective to
whosestructurewasfurtherconfirmedbyX-raycrystallography.
promote the B−N cyclization in excellent yield. Therefore, a
two-step protocol (conditions c and d) through intermediate 12
was validated to complete the synthesis of 11 in 93% overall yield.
Theexcellentstereoselectivity(ds>20/1)isattributedtotheboat
conformation adapted for α-hydride delivery. Hydrogenation of
the alkene proceeded smoothly to deliver yohimbane, and the
characterizationwasconsistentwiththereporteddata. Thetotal
synthesis described here with 50% overall yield from simple 4-
pentenal represents the most efficient route reported so far.
The rapid synthesis of chiral pentenolide 2b encouraged us to
construct other quinolizidine alkaloids that were previously
accessed by other synthetic sequences. A 2-g scale synthesis of 2e
was secured with high enantioselectivity (95.2% ee). The
following copper(I)-promoted allylation of pentenolide 2e
afforded the adducts 14 (trans- and cis-isomers) in 84% combined
Following the above procedure for the oxidative cleavage of the
terminal alkene, dihydrocorynantheal was obtained in good
22
yield. The subsequent reduction of aldehyde with LiAlH resulted
4
23
in dihydrocorynantheol in 77% yield, and the spectral data were
identical with the previous literature report.
12
In summary, we devised a unified approach to access intriguing
alkaloids fromγ-alkylatedpentenolides, whichwereenabledbyan
organocatalytic cross-aldol reaction, achieving good yields and
24
excellent enantioselectivities. The new protecting-group-free
syntheses improved the overall efficiency compared to other
known synthetic routes. Compared with the conventional B−N
cyclization, the newly developed “two-step” procedure, including
tosylation/lactamization and B−N cyclization/reduction, was
practical for sensitive substrates (e.g., the N1-unprotected indole
a
Scheme 4. Concise Syntheses of Dihydrocorynantheal, Dihydrocorynantheol, Protoemetinol, and Protoemetine
a
NMO = N-methylmorpholine N-oxide.
C
DOI: 10.1021/acs.orglett.7b01573
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Organic Letters
Letter
derivative) and thus can further complement a strategy to
construct more complex quinolizidine alkaloids.
2016, 57, 2567. (e) Meninno, S.; Lattanzi, A. Chem. Rec. 2016, 16, 2016.
(f) Vellalath, S.; Romo, D. Angew. Chem., Int. Ed. 2016, 55, 13934.
(6) (a) Marigo, M.; Wabnitz, T. C.; Fielenbach, D.; Jørgensen, K. A.
Angew. Chem., Int. Ed. 2005, 44, 794. (b) Marigo, M.; Fielenbach, D.;
Braunton, A.; Kjærsgaard, A.; Jørgensen, K. A. Angew. Chem., Int. Ed.
2005, 44, 3703. (c) Hayashi, Y.; Gotoh, H.; Hayashi, T.; Shoji, M. Angew.
Chem., Int. Ed. 2005, 44, 4212.
ASSOCIATED CONTENT
■
*
S
Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b01573.
(7) (a) Boeckman, R. K., Jr.; Miller, J. R. Org. Lett. 2009, 11, 4544.
(b) Boeckman, R. K., Jr.; Biegasiewicz, K. F.; Tusch, D. J.; Miller, J. R. J.
Full experimental details and analytical data including
NMR spectra and X-ray diffraction of compounds 10, 11,
and 18 (PDF)
Org. Chem. 2015, 80, 4030.
(8) Yasui, Y.; Benohoud, M.; Sato, I.; Hayashi, Y. Chem. Lett. 2014, 43,
5
(
56.
9) Hayashi, Y. Chem. Sci. 2016, 7, 866.
Crystallographic data (ZIP)
(10) Ando, K. Tetrahedron Lett. 1995, 36, 4105.
(11) (a) Szantay, C.; Blasko, G.; Honty, K.; Domye, G. The Alkaloids;
AUTHOR INFORMATION
Academic Press: New York, 1986; Vol. 27, pp 131−268. (b) Baxter, E.
W.; Marino, P. S. In Alkaloids: Chemical and Biological Perspectives;
Pelletier, S. W., Ed.; Springer-Verlag: New York, 1992; Vol. 8, pp 197−
319.
■
*
Corresponding Author
rhong@sioc.ac.cn
(
12) (a) Meyers, A. I.; Highsmith, T. K.; Buonora, P. T. J. Org. Chem.
991, 56, 2960. (b) Hua, D. H.; Bharathi, S. N.; Panangadan, J. A. K.;
Tsujimoto, A. J.Org. Chem. 1991,56,6998. (c)Aube,J.;Ghosh, S.;Tanol,
ORCID
1
Ran Hong: 0000-0002-7602-539X
Author Contributions
§C.X. and J.L. contributed equally.
́
M. J. Am. Chem. Soc. 1994, 116, 9009. (d) Bergmeier, S. C.; Seth, P. P. J.
Org. Chem. 1999, 64, 3237. (e) Ghosh, A. K.; Sarkar, A. Eur. J. Org. Chem.
2
016, 2016, 6001 and references of racemic synthesis cited therein.
Notes
(13) Noren-Muller, A.; Reis-Correa, I., Jr.; Prinz, H.; Rosenbaum, C.;
Saxena, K.; Schwalbe, H. J.; Vestweber, D.; Cagna, G.; Schunk, S.;
Schwarz, O.; Schiewe, H.; Waldmann, H. Proc. Natl. Acad. Sci. U. S. A.
̈
̈
̂
The authors declare no competing financial interest.
2
(
006, 103, 10606.
14) For selected approaches, see: (a) Martin, S. F.; Rueger, H.;
Williamson, S. A.; Grzejszczak, S. J. Am. Chem. Soc. 1987, 109, 6124.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science Foundation
of China (21290184 and 21672246 to R.H., 21302208 to L.Z.),
the Strategic Priority Research Program (XDB20000000), and
the Key Research Program of Frontier Sciences (QYZDY-SSW-
SLH026) of the Chinese Academy of Sciences are greatly
appreciated. We also thank Jie Sun (SIOC) for X-ray analysis and
Yuping Zhang (Shanghai Institute of Technology) for exper-
imental assistance.
(
(
(
b) Kaoudi, T.; Miranda, L. D.; Zard, S. Z. Org. Lett. 2001, 3, 3125.
c) Mergott, D. J.; Zuend, S. J.; Jacobsen, E. N. Org. Lett. 2008, 10, 745.
d) Franzen, J.; Fisher, A. Angew. Chem., Int. Ed. 2009, 48, 787. (e) Wu, X.
́
Y.; Dai, X. Y.; Nie, L. L.; Fang, H. H.; Chen, J.; Ren, Z. J.; Cao, W. G.;
Zhao, G. Chem. Commun. 2010, 46, 2733. (f) Li, L.; Aibibula, P.; Jia, Q.;
Jia, Y. Org. Lett. 2017, 19, 2642. (g) Wang, X.; Xia, D.; Qin, W.; Zhou, R.;
Zhou, X.; Zhou, Q.; Liu, W.; Dai, X.; Wang, H.; Wang, S.; Tan, L.; Zhang,
D.; Song, H.; Liu, X.-Y.; Qin, Y. Chem. 2017, 2, 803.
(
15) Lipshutz, B. H.; Ellsworth, E. L.; Dimock, S. H.; Smith, R. A. J. J.
Am. Chem. Soc. 1990, 112, 4404.
16) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1,
53.
17) (a) Bischler, A.; Napieralski, B. Ber. Dtsch. Chem. Ges. 1893, 26,
903. (b) Whaley, W. M.; Govindachari, T. R. Org. React. 1951, 6, 74.
c) Heravi, M. M.; Khaghaninejad, S.; Nazari, N. Adv. Heterocycl. Chem.
014, 112, 183.
18) Banwell, M. G.; Bissett, B. D.; Busato, S.; Cowden, C. J.; Hockless,
REFERENCES
■
(
9
(
1
(
2
(
(
1) (a) Davies-Coleman, M. T.; Rivett, D. E. A. In Progress in the
Chemistry of Organic Natural Products; Herz, W., Grisebach, H., Kirby, G.
W., Tamm, C., Eds.; Springer-Verlag: New York, 1989; Vol. 55, pp 1−35.
b) Collett, L. A.; Davis-Coleman, M. T.; Rivett, D. E. A. In Progress in the
Chemistry of Organic Natural Products; Herz, W., Falk, H., Kirby, G. W.,
Moore, R. E., Tamm, C., Eds.; Springer-Verlag: New York, 1998; Vol. 75,
pp 181−209. (c) Tian, W.-S.; Shi, Y. Prog. Chem. 2010, 22, 537.
d) Janecki, T. Natural Lactones and Lactams. Synthesis Occurrence and
Biological Activity; Wiley-VCH: Weinheim, 2014.
2) (a) Collins, I. J. Chem. Soc., Perkin Trans. 1 1999, 1377. (b) Boucard,
V.; Broustal, G.; Campagne, J. M. Eur. J. Org. Chem. 2007, 2007, 225.
(
D. C. R.; Holman, J. W.; Read, R. W.; Wu, A. W. J. Chem. Soc., Chem.
Commun. 1995, 2551.
(
(
19) (a) Movassaghi, M.; Hill, M. D. Org. Lett. 2008, 10, 3485. (b) Xiao,
K.-J.; Luo, J.-M.; Ye, K.-Y.; Wang, Y.; Huang, P.-Q. Angew. Chem., Int. Ed.
010,49,3037.(c)Xiao, K.-J.;Luo,J.-M.;Xia, X.-E.;Wang, Y.;Huang, P.-
(
2
(
2
c) Marco, J. A.; Carda, M.; Murga, J.; Falomir, E. Tetrahedron 2007, 63,
Q. Chem. - Eur. J. 2013, 19, 13075. (d) White, K. L.; Mewald, M.;
Movassaghi, M. J. Org. Chem. 2015, 80, 7403. (e) Huang, P.-Q.; Huang,
Y.-H.; Geng, H.; Ye, J.-L. Sci. Rep. 2016, 6, e28801. (f) Huang, P.-Q.;
Huang, Y.-H.; Wang, S.-R. Org. Chem. Front. 2017, 4, 431 and references
cited therein.
̌
929. (d) Spulak
́
, M.; Ghavre, M.; Pour, M. Tetrahedron Lett. 2017, 58,
2
63.
(
3) For selected examples, see: (a) Liao, X.; Wu, Y.; De Brabander, J. K.
Angew. Chem., Int. Ed. 2003, 42, 1648. (b) Izzo, I.; Gaudio, P. D.; Monica,
C. D.; Squitieri, M.; De Riccardis, F. Heterocycles 2003, 60, 2057.
c) Yanai, H.; Takahashi, A.; Taguchi, T. Tetrahedron 2007, 63, 12149.
d) Hamel, C.; Prusov, E. V.; Gertsch, J.; Schweizer, W. B.; Altmann, K.-
H. Angew. Chem., Int. Ed. 2008, 47, 10081. (e) Garza, V. J.; Krische, M. J. J.
Am. Chem. Soc. 2016, 138, 3655.
4) Huang, X.; Song, L.; Xu, J.; Zhu, G.; Liu, B. Angew. Chem., Int. Ed.
013, 52, 952.
5) Selected recent monograph and reviews, see: (a) Comprehensive
Enantioselective Organocatalysis: Catalysts, Reactions, and Applications;
Dalko, P. I., Ed.; Wiley-VCH: 2013. (b) Abbasov, M. E.; Romo, D. Nat.
Prod. Rep. 2014, 31, 1318. (c)Sun, B.-F. Tetrahedron Lett. 2015, 56, 2133.
(
(
20) See Supporting Information for details.
21) Li, J.; Wang, T.; Yu, P.; Peterson, A.; Weber, R.; Soerens, D.;
(
(
Grubisha, D.; Bennett, D.; Cook, J. M. J. Am. Chem. Soc. 1999, 121, 6998.
(
(
Bah, J.; Wohlfarth, A.; Franzen
S.;Deiana, L.;Tseggai, A.;Cor
(
22) Zhong, Y.-L.; Shing, T. K. M. J. Org. Chem. 1997, 62, 2622.
23) (a) Sun, X.; Ma, D. Chem. - Asian J. 2011, 6, 2158. (b) Zhang, W.;
(
2
(
́
, J. Chem. - Eur. J. 2011, 17, 13814. (c) Lin,
dova, A. Eur.J. Org.Chem. 2012,2012,398.
24) (a) Hoffmann, R. W. Synthesis 2006, 2006, 3531. (b) Young, I. S.;
́
Baran, P. S. Nat. Chem. 2009, 1, 193.
(
d) Pantaine, L.; Moreau, X.; Coeffard, V.; Greck, C. Tetrahedron Lett.
D
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