Regioselective S' Mitsunobu reaction of Morita-Baylis-Hillman alcohols: A facile and stereoselective synthesis of α-alkylidene-ss-hydrazino acid derivatives

Article abstract of DOI:10.3762/bjoc.10.98

A highly regioselective S' Mitsunobu reaction between Morita-Baylis-Hillman (MBH) alcohols, azodicarboxylates, and triphenylphosphine is developed, which provides an easy access to a-alkylidene-ss-hydrazino acid derivatives in high yields and good stereoselectivity. This reaction represents the first direct transformation of MBH alcohols into hydrazines.

Full text of DOI:10.3762/bjoc.10.98

Regioselective SN2' Mitsunobu reaction of
Morita–Baylis–Hillman alcohols:
A facile and stereoselective synthesis of
α-alkylidene-β-hydrazino acid derivatives
Silong Xu*, Jian Shang, Junjie Zhang and Yuhai Tang*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 990–995.
Department of Chemistry, School of Science, Xi’an Jiaotong
University, Xi’an 710049, P. R. China
doi:10.3762/bjoc.10.98
Received: 07 February 2014
Accepted: 10 April 2014
Published: 30 April 2014
Email:
Silong Xu* - silongxu@mail.xjtu.edu.cn; Yuhai Tang* -
tyh57@mail.xjtu.edu.cn
Associate Editor: D. Y.-K. Chen
* Corresponding author
© 2014 Xu et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
azodicarboxylate; hydrazine; Mitsunobu reaction;
Morita–Baylis–Hillman; SN2' reaction
Abstract
A highly regioselective SN2' Mitsunobu reaction between Morita–Baylis–Hillman (MBH) alcohols, azodicarboxylates, and tri-
phenylphosphine is developed, which provides an easy access to α-alkylidene-β-hydrazino acid derivatives in high yields and good
stereoselectivity. This reaction represents the first direct transformation of MBH alcohols into hydrazines.
Introduction
Hydrazines and their derivatives are an important class of com- Morita–Baylis–Hillman (MBH) adducts [9] are a class of
pounds in organic chemistry. They are widely used in the fields unique substrates of great synthetic potential which contain
of pesticides, polymers, dyestuff, and pharmaceutical agents three manipulatable groups, namely, a hydroxy group, a
[1]. They are also versatile building blocks for accessing many carbon–carbon double bond, and an electron-withdrawing
important nitrogen-containing heterocyclic compounds, espe- group. Over the past several decades, a myriad of transforma-
cially pyrazole derivatives [2-7]. Although various methods tions involving MBH adducts have been reported, leading to a
detailing the synthesis of hydrazines have been established [8], wide variety of molecular scaffolds of high diversity and
the development of an efficient synthesis of hydrazines with complexity [10-12]. A number of reports have dealt with the
highly structural diversity from simple starting materials under conversion of the hydroxy group of MBH adducts into useful
mild conditions is still desirable.
functionalities, such as halides [13,14], ethers [15,16], amines
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Beilstein J. Org. Chem. 2014, 10, 990–995.
[17,18], thioethers [19], phosphonates [20,21], alkyl groups Results and Discussion
[22,23], and so on. In this context, however, reports on the In our initial study, MBH alcohol 1a was treated with
conversion of MBH alcohols into hydrazine derivatives are 2.0 equivalents of diethyl azodicarboxylate (2b) and triphenyl-
scanty.
phosphine under very mild conditions (Scheme 2). To our
delight, the reaction was completed in 30 minutes providing the
In 2009, Nair and co-workers [24] reported an interesting reac- anticipated trisubstituted hydrazine 3b in 90% isolated yield
tion of MBH acetates with azodicarboxylates in the presence of with excellent E-selectivity (E/Z > 20:1). To the best of our
PPh3 (Mitsunobu reaction conditions), which gives an efficient knowledge, this reaction represents the first direct conversion of
access to α-alkylidene-β-hydrazino acid derivatives, an impor- MBH alcohols into hydrazines. In addition, the normal SN2
tant precursor for many bioactive compounds [25-30] including Mitsunobu reaction [35,36] product 5, namely, diethyl 1-(2-
β-amino acids [25] (Scheme 1, top). However, the reaction (ethoxycarbonyl)-1-phenylallyl)hydrazine-1,2-dicarboxylate,
exhibited poor chemoselectivity which gave comparable yields could not be detected in the reaction mixture, which suggested a
of tri- and tetrasubstituted hydrazines. For example, the reac- highly regioselective SN2' Mitsunobu process occurred in the
tion of MBH acetate 1a' with diisopropyl azodicarboxylate (2a) reaction (see discussion on mechanism below).
and PPh3 afforded hydrazines 3a and 4a in 51% and 46%
yields, respectively. Inspired by this report, and a pioneering With the encouraging result, the reaction conditions were
SN2' Mitsunobu reaction of MBH alcohols with carboxylic further optimized using the above reaction as a model (Table 1).
acids [31,32], we envisioned that direct treatment of simpler Among several common solvents screened, dichloromethane,
MBH alcohols with azodicarboxylates and PPh3 (Mitsunobu chloroform, and toluene gave comparable yields to that of THF,
conditions) would chemoselectively provide trisubstituted while ethyl acetate emerged as the best solvent, offering an
hydrazines of type 3, via a distinct SN2' Mitsunobu reaction ap- excellent 98% yield (Table 1, entries 2–5). However, polar
proach (Scheme 1, bottom). As part of our interest in exploring solvents such as DMF, ethanol, and acetonitrile were detri-
new reactivities of MBH adducts [33,34], herein we wish to mental to the reaction, giving very low yields (Table 1, entries
report the preliminary results from such an investigation.
6–8). Reducing the amounts of 2b and PPh3 from
Scheme 1: Synthesis of α-alkylidene-β-hydrazino acid derivatives from MBH adducts.
Scheme 2: Initial investigation.
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Beilstein J. Org. Chem. 2014, 10, 990–995.
2.0 equivalents to 1.5 or 1.0 equivalent resulted in substantial
decrease in the yields (Table 1, entries 9 and 10). It was also
found that replacing PPh3 with more nucleophilic PBu3 in the
reaction could shorten the reaction time but led to an inferior
yield (Table 1, entry 11). Therefore PPh3 was chosen as the
preferable phosphine due to its high efficiency and cost-effec-
tiveness.
Table 2: Synthesis of α-alkylidene-β-hydrazino esters 3.a
Entry R1, R2 in 1
2
Time 3, Yieldb E/Zc
(min) (%)
Table 1: Investigation on reaction conditions.a
1
C6H5, Et (1a)
C6H5, Me (1b)
C6H5, Me (1b)
2b 25
2b 25
2c 50
3b, 98
3c, 95
3d, 95
3e, 81
3f, 87
3g, 91
3h, 79
3i, 86
20:1
20:1
20:1
20:1
20:1
20:1
20:1
3:1
2
3
4
4-CH3C6H4, Et (1c) 2b 35
5
2-ClC6H4, Et (1d)
4-ClC6H4, Et (1e)
4-FC6H4, Me (1f)
2b 35
2b 30
2b 30
6
Entry
Solvent
Time (min)
Yieldb (%)
7
1
THF
30
20
40
35
25
24
50
30
25
25
10
90
80
86
88
98
trace
23
39
29
54
82
8
4-NO2C6H4, Et (1g) 2c 40
2
CH2Cl2
CHCl3
toluene
EtOAc
DMF
9
C2H5, Et (1h)
C2H5, Et (1h)
C2H5, Me (1i)
C2H5, Me (1i)
CH3, Et (1j)
CH3, Et (1j)
H, Et (1k)
2a 30
2b 20
2b 25
2a 30
2b 21
2c 40
2b 10
2a 20
2c 25
2b 13
3j, 80
9:1
3
10
11
12
13
14
15
16
17
18
3k, 82
3l, 80
20:1
20:1
13:1
13:1
5:1
4
5
3m, 81
3n, 83
3o, 55
3p, 90
3q, 89
3r, 90
3s, 87
6
7
EtOH
8
CH3CN
EtOAc
EtOAc
EtOAc
–
9c
10d
11e
H, Et (1k)
–
H, Et (1k)
–
H, Me (1l)
–
aDiethyl azodicarboxylate (2b, 0.6 mmol) was slowly added to a solu-
tion of MBH alcohol 1a (0.3 mmol) and PPh3 (0.6 mmol) in the speci-
fied solvent (2 mL) at 0 °C under N2, and then the mixture was allowed
to warm up to room temperature. bIsolated yield. cThe reaction was
conducted using 1.0 equiv of both PPh3 and 2b. d1.5 equiv of PPh3
and 2b were adopted. ePBu3 was used instead of PPh3.
aUnder N2 atmosphere and at 0 °C, to a solution of MBH alcohol 1
(0.3 mmol) and PPh3 (0.6 mmol) in 2 mL ethyl acetate (for entries
9–18, dichloromethane was used as the solvent) was slowly added
azodicarboxylates 2 (0.6 mmol), and the mixture was stirred at room
temperature and monitored by TLC. bIsolated yield. cDetermined by
1H NMR assay.
Under the optimized conditions, the scope of the SN2' boxylates, producing the corresponding α-alkylidene-β-
Mitsunobu reaction was examined (Table 2). Variation in the hydrazino esters 3 in high yields (80–90%) and good E/Z selec-
ester alkyl groups of both MBH alcohols and azodicarboxylates tivity (Table 2, entries 9–18). An exception was observed for
had little influence on the reaction; the corresponding the reaction of MBH alcohol 1j with tert-butyl azodicarboxy-
hydrazines with different ester groups were generated in excel- late (2c), which gave 55% yield and a modest E/Z ratio (5:1)
lent yields and stereoselectivity (Table 2, entries 1–3). In addi- (Table 2, entry 14).
tion, a range of aromatic MBH alcohols 1c–g featuring either an
electron-donating or an electron-withdrawing group on the To further investigate the scope, a single case using allenic
benzene ring all worked well, delivering the hydrazines 3e–i in MBH alcohol 6 as the substrate was studied (Scheme 3).
high yields (79–91%) and excellent E/Z selectivity, with an However, under similar conditions, the reaction between
exception of the nitro-substituted 1g giving a moderate E/Z ratio alcohol 6, tert-butyl azodicarboxylate (2c), and PPh3 afforded
(Table 2, entries 4–8). An ortho substituent on the benzene ring an interesting 1H-pyrazole compound 7 in 58% yield. A
of the MBH alcohol was also well tolerated (Table 2, entry 5 vs possible mechanism for the formation of 7 is outlined in
6). Notably, in contrast with Nair’s report [24], it was revealed Supporting Information File 1.
that aliphatic MBH alcohols were excellent candidates in the
reaction. By switching the solvent to dichloromethane, several The above results clearly show that the SN2' Mitsunobu reac-
alkyl MBH alcohols containing ethyl, methyl, or hydrogen tion between MBH alcohols, azodicarboxylates, and PPh3 has a
substituents (1h–l) readily reacted with representative azodicar- broad substrate scope. The reaction is clean and fast, with all
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Beilstein J. Org. Chem. 2014, 10, 990–995.
Conclusion
In conclusion, we have developed a highly regioselective SN2'
Mitsunobu reaction between Morita–Baylis–Hillman (MBH)
alcohols, azodicarboxylates, and triphenylphosphine as an effi-
cient synthetic method for α-alkylidene-β-hydrazino acid
derivatives in high yields and good stereoselectivity. This reac-
tion features additional advantages as mild conditions, wide
substrate scope, and simple starting materials. The reaction
represents the first direct transformation of MBH alcohols into
hydrazines, and constitutes a valuable example of regioselec-
Scheme 3: Synthesis of 1H-pyrazole 7.
the reactions completed in less than one hour. The reaction tive SN2' Mitsunobu reactions. Our future efforts will focus on
exhibits good stereoselectivity (E/Z 3:1 to 20:1) and exclusive the application of the current reaction in the synthesis of
regioselectivity. In all cases, the normal SN2 Mitsunobu prod- nitrogen-containing heterocyclic compounds.
ucts of type 5 could not be detected by using 1H and 13C DEPT
Experimental
NMR analysis. The structure and stereochemistry of all
hydrazines 3 were well identified by 1H, 13C NMR, IR, HRMS, General information
and NOESY analysis for a representative product 3b (for char- All reactions were carried out in nitrogen atmosphere under an-
acterization data, see Supporting Information File 1).
hydrous conditions. Solvents were purified according to stan-
dard procedures. Mortia–Baylis–Hillman (MBH) alcohol 1
A possible mechanism for the SN2' Mitsunobu reaction between were prepared according to literature procedures [46,47].
MBH alcohols 1, azodicarboxylates 2, and PPh3 is depicted in Benzyl 2-(hydroxymethyl)buta-2,3-dienoate (6) is a known
Scheme 4. Initially, the addition of PPh3 to azodicarboxylates 2 compound and was prepared according to a literature [48].
generates the Huisgen zwitterion intermediate 8 [37,38]. Subse- Reagents from commercial sources were used without further
quent proton transfer and phosphonium migration between 8 purification. 1H and 13C NMR spectra were recorded on a
and the MBH alcohol 1 produce an oxophosphonium intermedi- Bruker AV 400 spectrometer in CDCl3 with tetramethylsilane
ate 9 and a hydrazine anionic species 10 [36]. Finally, an expe- (TMS) as the internal standard. High resolution ESI mass
dient SN2' attack of species 10 on 9, probably facilitated by the spectra were acquired with an IonSpec QFT-ESI instrument. IR
ester group of 9 [31,32], leads to the formation of hydrazines 3 spectra were recorded on a Nicolet Avatar 330 FTIR spectrom-
and phosphine oxide. The alternative SN2 displacement of the eter (in KBr). Column chromatography was performed on silica
oxophosphonium moiety of 9 by the species 10 may be retarded gel (200–300 mesh) using a mixture of petroleum ether (bp
by steric hindrance. Recently, SN2' Mitsunobu reactions 60–90 °C)/ethyl acetate as the eluant.
[31,32,39-45] have attracted considerable interest from the
General procedure for the synthesis of
organic chemistry community due to their great synthetic poten-
tial being complementary to the Mitsunobu reactions. This hydrazines 3
report accordingly adds to a new valuable example of SN2' Under N2 atmosphere and at 0 °C, to a stirred solution of MBH
Mitsunobu reactions.
alcohols 1 (0.3 mmol) and PPh3 (0.6 mmol) in EtOAc or
CH2Cl2 (2 mL) in a Schlenk tube (25 mL) was slowly added
azodicarboxylates 2 (0.6 mmol) over 5 minutes by the means of
a microsyringe. The resulting reaction mixture was allowed to
warm up to room temperature and stirred until the MBH alco-
hols 1 were completely consumed, as monitored by TLC. The
solvent was removed under reduced pressure and the residue
was purified by column chromatography on silica gel (gradient
eluant: petroleum ether/ethyl acetate 9:1–3:1) to afford the
hydrazines 3.
Synthesis of 1H-pyrazole 7
Under N2 atmosphere and at room temperature, PPh3 (157 mg,
0.6 mmol) was slowly added to a stirred solution of allenic
MBH alcohol 6 (61 mg, 0.3 mmol) and di-tert-butyl azodicar-
boxylate 2c (138 mg, 0.6 mmol) in CH2Cl2 (2 mL) in a Schlenk
Scheme 4: A plausible mechanism for the formations of 3.
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Beilstein J. Org. Chem. 2014, 10, 990–995.
14.Das, B.; Banerjee, J.; Ravindranath, N.; Venkataiah, B.
tube (25 mL). The resulting reaction mixture was stirred for
1 hour at that time the alcohol 6 was disappeared as monitored
by TLC. The solvent was removed using a rotatory evaporator
under reduced pressure. The residue was then directly subject to
column chromatography on silica gel (eluant: petroleum ether/
ethyl acetate 9:1) to afford the 1H-pyrazole compound 7 in
73 mg, 58% yield, as slightly yellow oil.
Tetrahedron Lett. 2004, 45, 2425–2426.
doi:10.1016/j.tetlet.2004.01.101
15.Shanmugam, P.; Rajasingh, P. Tetrahedron 2004, 60, 9283–9295.
doi:10.1016/j.tet.2004.07.067
16.Das, B.; Majhi, A.; Banerjee, J. Tetrahedron Lett. 2006, 47, 7619–7623.
doi:10.1016/j.tetlet.2006.08.060
17.Rajesh, S.; Banerji, B.; Iqbal, J. J. Org. Chem. 2002, 67, 7852–7857.
doi:10.1021/jo010981d
18.Ciclosi, M.; Fava, C.; Galeazzi, R.; Orena, M.; Sepulveda-Arques, J.
Tetrahedron Lett. 2002, 43, 2199–2202.
Supporting Information
doi:10.1016/S0040-4039(02)00233-2
19.Liu, Y.; Xu, D.; Xu, Z.; Zhang, Y. Heteroat. Chem. 2008, 19, 188–198.
doi:10.1002/hc.20394
Supporting Information File 1
Experimental details on the synthesis of all hydrazines 3
and 1H-pyrazole 7, full characterization data and 1H, 13C,
DEPT NMR spectra for all compounds 3 and 7, and a
mechanistic rationale for the formation of 7.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-98-S1.pdf]
20.Janecki, T.; Bodalski, R. Synthesis 1990, 799–801.
doi:10.1055/s-1990-27019
21.Basavaiah, D.; Pandiaraju, S. Tetrahedron 1996, 52, 2261–2268.
doi:10.1016/0040-4020(95)01055-6
22.Basavaiah, D.; Sarma, P. K. S.; Bhavani, A. K. D.
J. Chem. Soc., Chem. Commun. 1994, 1091–1092.
doi:10.1039/c39940001091
23.Basavaiah, D.; Pandiaraju, S. Tetrahedron Lett. 1995, 36, 757–758.
doi:10.1016/0040-4039(94)02360-N
Acknowledgements
24.Jose, A.; Paul, R. R.; Mohan, R.; Mathew, S. C.; Biju, A. T.; Suresh, E.;
Nair, V. Synthesis 2009, 1829–1833. doi:10.1055/s-0028-1088063
25.Liu, M.; Sibi, M. P. Tetrahedron 2002, 58, 7991–8035.
doi:10.1016/S0040-4020(02)00991-2
This work was supported by the National Natural Science Foun-
dation of China (No. 21305107) and the Fundamental Research
Funds for the Central Universities (No. 08143076). We grate-
fully thank Professor Zhengjie He (Nankai University) for the
help with the HRMS measurements and the long-term support.
26.Abouzid, K. A. M.; El-Abhar, H. S. Arch. Pharmacal Res. 2003, 26,
1–8. doi:10.1007/BF03179922
27.Pareek, A. K.; Joseph, P. E.; Seth, D. S. Biomed. Pharmacol. J. 2010,
3, 385–390.
References
1. Licandro, E.; Perdicchia, D. Eur. J. Org. Chem. 2004, 665–675.
28.Hansen, T.; Alst, T.; Havelkova, M.; Strøm, M. B. J. Med. Chem. 2010,
53, 595–606. doi:10.1021/jm901052r
29.Sadowsky, J. D.; Murray, J. K.; Tomita, Y.; Gellman, S. H.
ChemBioChem 2007, 8, 903–916. doi:10.1002/cbic.200600546
30.Sabatino, D.; Proulx, C.; Pohankova, P.; Ong, H.; Lubell, W. D.
J. Am. Chem. Soc. 2011, 133, 12493–12506. doi:10.1021/ja203007u
31.Charette, A. B.; Côté, B. Tetrahedron Lett. 1993, 34, 6833–6836.
doi:10.1016/S0040-4039(00)91807-0
doi:10.1002/ejoc.200300416
2. LaLonde, R. L.; Wang, Z. J.; Mba, M.; Lackner, A. D.; Toste, F. D.
Angew. Chem., Int. Ed. 2010, 49, 598–601.
doi:10.1002/anie.200905000
3. Suzuki, Y.; Naoe, S.; Oishi, S.; Fujii, N.; Ohno, H. Org. Lett. 2012, 14,
326–329. doi:10.1021/ol203072u
32.Charette, A. B.; Cote, B.; Monroc, S.; Prescott, S. J. Org. Chem. 1995,
60, 6888–6894. doi:10.1021/jo00126a046
4. Fernández, M.; Reyes, E.; Vicario, J. L.; Badía, D.; Carrillo, L.
Adv. Synth. Catal. 2012, 354, 371–376. doi:10.1002/adsc.201100722
5. Hashimoto, T.; Takiguchi, Y.; Maruoka, K. J. Am. Chem. Soc. 2013,
135, 11473–11476. doi:10.1021/ja405444c
33.Xu, S.; Chen, R.; Qin, Z.; Wu, G.; He, Z. Org. Lett. 2012, 14, 996–999.
doi:10.1021/ol2032569
34.Chen, R.; Xu, S.; Wang, L.; Tang, Y.; He, Z. Chem. Commun. 2013,
49, 3543–3545. doi:10.1039/c3cc41419a
6. Dumeunier, R.; Lamberth, C.; Trah, S. Synlett 2013, 24, 1150–1154.
doi:10.1055/s-0033-1338433
35.Dow, R. L.; Kelly, R. C.; Schletter, I.; Wierenga, W. Synth. Commun.
1981, 11, 43–53. doi:10.1080/00397918108064281
36.Swamy, K. C. K.; Kumar, N. N. B.; Balaraman, E.; Kumar, K. V. P. P.
Chem. Rev. 2009, 109, 2551–2651. doi:10.1021/cr800278z
37.Brunn, E.; Huisgen, R. Angew. Chem., Int. Ed. Engl. 1969, 8, 513–515.
doi:10.1002/anie.196905131
7. Bouvet, S.; Moreau, X.; Coeffard, V.; Greck, C. J. Org. Chem. 2013,
78, 427–437. doi:10.1021/jo302320v
8. Ragnarsson, U. Chem. Soc. Rev. 2001, 30, 205–213.
doi:10.1039/b010091a
9. Morita, K.; Suzuki, Z.; Hirose, H. Bull. Chem. Soc. Jpn. 1968, 41, 2815.
doi:10.1246/bcsj.41.2815
38.Nair, V.; Biju, A. T.; Mathew, S. C.; Babu, B. P. Chem.–Asian J. 2008,
3, 810–820. doi:10.1002/asia.200700341
10.Basavaiah, D.; Reddy, B. S.; Badsara, S. S. Chem. Rev. 2010, 110,
5447–5674. doi:10.1021/cr900291g
39.Higashino, M.; Ikeda, N.; Shinada, T.; Sakaguchi, K.; Ohfune, Y.
Tetrahedron Lett. 2011, 52, 422–425. doi:10.1016/j.tetlet.2010.11.080
40.Berrée, F.; Gernigon, N.; Hercouet, A.; Lin, C. H.; Carboni, B.
Eur. J. Org. Chem. 2009, 329–333. doi:10.1002/ejoc.200800965
41.Grove, J. J. C.; van Heerden, P. S.; Ferreira, D.;
Bezuidenhoudt, B. C. B. Tetrahedron Lett. 2010, 51, 57–59.
doi:10.1016/j.tetlet.2009.10.080
11.Basavaiah, D.; Veeraraghavaiah, G. Chem. Soc. Rev. 2012, 41,
68–78. doi:10.1039/c1cs15174f
12.Wei, Y.; Shi, M. Chem. Rev. 2013, 113, 6659–6690.
doi:10.1021/cr300192h
13.Basavaiah, D.; Bhavani, A. K. D.; Pandiaraju, S.; Sarma, P. K. S.
Synlett 1995, 243–244. doi:10.1055/s-1995-4929
994

Beilstein J. Org. Chem. 2014, 10, 990–995.
42.Yoshimura, Y.; Asami, K.; Imamichi, T.; Okuda, T.; Shiraki, K.;
Takahata, H. J. Org. Chem. 2010, 75, 4161–4171.
doi:10.1021/jo100556u
43.Bisegger, P.; Manov, N.; Bienz, S. Tetrahedron 2008, 64, 7531–7536.
doi:10.1016/j.tet.2008.05.119
44.Zhu, S.; Hudson, T. H.; Kyle, D. E.; Lin, A. J. J. Med. Chem. 2002, 45,
3491–3496. doi:10.1021/jm020104f
45.Kwon, Y.-U.; Lee, C.; Chung, S.-K. J. Org. Chem. 2002, 67,
3327–3338. doi:10.1021/jo016237a
46.Yu, C.; Liu, B.; Hu, L. J. Org. Chem. 2001, 66, 5413–5418.
doi:10.1021/jo015628m
47.Deng, J.; Hu, X.-P.; Huang, J.-D.; Yu, S.-B.; Wang, D.-Y.; Duan, Z.-C.;
Zheng, Z. J. Org. Chem. 2008, 73, 2015–2017. doi:10.1021/jo702510m
48.Hue, B. T. B.; Dijkink, J.; Kuiper, S.; van Schaik, S.;
van Maarseveen, J. H.; Hiemstra, H. Eur. J. Org. Chem. 2006,
127–137. doi:10.1002/ejoc.200500609
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