Diastereoselectivity in the Staudinger reaction of pentafluorosulfanylaldimines and ketimines

Article abstract of DOI:10.3762/bjoc.9.303

β-Lactams were diastereoselectively formed by the reaction of SF ₅-containing aldimines, or an SF₅-containing ketimine, with benzyloxyketene in a conrotatory ring closure process. Imine formation and cyclization were possible in spite of the acidification of protons on the carbon bound to SF₅. The reactions of the aldimines demonstrated very good 1,2-lk diastereoselectivity, however lack of stereochemical control of the C-N ketimine geometry was reflected in the stereochemistry of the product β-Lactams. Cyclization of imines with a stereogenic center bearing SF ₅ was reflected in the 1,2-lk,lk selectivity of the β-Lactams.

Full text of DOI:10.3762/bjoc.9.303

Diastereoselectivity in the Staudinger reaction of
pentafluorosulfanylaldimines and ketimines
Alexander Penger, Cortney N. von Hahmann, Alexander S. Filatov
and John T. Welch*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 2675–2680.
Department of Chemistry, University at Albany, SUNY, 1400
Washington Ave., Albany, NY 12222, USA
doi:10.3762/bjoc.9.303
Received: 07 August 2013
Accepted: 15 October 2013
Published: 27 November 2013
Email:
John T. Welch* - jwelch@albany.edu
* Corresponding author
This article is part of the Thematic Series "Organo-fluorine chemistry III".
Guest Editor: D. O'Hagan
Keywords:
aldimine; Cornforth transition state; diastereoselectivity; β-lactam;
organo-fluorine; α-pentafluorosulfanyl aldehyde
© 2013 Penger et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
β-Lactams were diastereoselectively formed by the reaction of SF5-containing aldimines, or an SF5-containing ketimine, with
benzyloxyketene in a conrotatory ring closure process. Imine formation and cyclization were possible in spite of the acidification of
protons on the carbon bound to SF5. The reactions of the aldimines demonstrated very good 1,2-lk diastereoselectivity, however
lack of stereochemical control of the C–N ketimine geometry was reflected in the stereochemistry of the product β-lactam. Cycliza-
tion of imines with a stereogenic center bearing SF5 was reflected in the 1,2-lk,lk selectivity of the β-lactam.
Introduction
The pentafluorosulfanyl (SF5) group is one of the few truly new group. This effect can also be seen in the calculated dipole
functional groups to be introduced to synthetic organic chem- moments of 1,1,1-trifluoroethane and pentafluorosulfanyl-
istry in the last 100 years [1-6]. With pseudooctahedral geom- methane of 2.589 and 3.556 Debye respectively [7,8]. When
etry around sulfur, the SF5 group is a unique substituent for the these electronic effects are combined with an occupied volume
medicinal or pharmaceutical chemist. While the electron with- only slightly less than that of a tert-butyl group [3,13], the SF5
drawing properties of the SF5 and CF3 groups are similar [7,8], group can have unanticipated influences on the structure such as
the electronegativity of the SF5 group has been suggested to be anchoring side chain and neighboring hydroxy group conforma-
as high as 3.65 in contrast to 3.36 for CF3 [9]. The inductive σI tions [14,15].
and σR values for SF5, 0.55 and 0.11 [10], contrast with σI and
σR values for CF3 of 0.39 and 0.12 [11,12] illustrating the Given the unique potential of the SF5 group, the rarity of its
increased inductive effect of the SF5 group relative to the CF3 application in medicinal or agrochemical materials may be
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surprising. However, synthesis of aliphatic SF5-containing unstable” [45] with no selective reactions reported. In light of
building blocks (SF5-substituted aromatic compounds are more this precedence the reaction of α-SF5-substituted aldehydes and
accessible) [1,6], is challenging and often beset by confusing re- ketones with amines was particularly intriguing.
activity, largely because of the very properties that make SF5 an
attractive substituent. Studies of aliphatic compounds have α-SF5-Substituted aldehydes and ketones
lagged as a consequence, a fact that is compounded by the lack In this work the SF5-bearing aldehyde 1 was prepared by the
of commercially available aliphatic SF5-containing building addition of SF5Cl to the enol ether 2 instead of the previously
blocks. Although the number of known aliphatic pentafluorosul- described additions to enol acetates [16] (Scheme 1). In earlier
fanylated compounds is constantly increasing [2], this expan- studies, it was found that the yield of SF5Cl addition to enol
sion has not generally been accompanied by applications of acetates was highly dependent upon the purity of the enol
these materials. Among aliphatic SF5-substituted compounds acetate substrate, compounds surprisingly difficult to purify.
α-pentafluorosulfanylated aliphatic carbonyl compounds [16], Since vinyl acetate is the only enol acetate readily accessible for
readily prepared from the corresponding enol acetates or enol this reaction, the commercial availability of high purity
ethers, are especially valuable as starting materials [17-22]. In propenyl and butenyl ethers 2b and 2c rendered these starting
these compounds the profound steric influence of the SF5 group materials highly attractive for the formation of SF5-bearing
[14,15] is accompanied by a dramatic increase in the acidity of aldehydes.
the adjacent α-proton, a phenomenon underlying the abstrac-
tion of protons by methyllithium and a number of different
ylides [16]. The reactions of α-pentafluorosulfanyl carbonyl
compounds are governed by a combination of the substantial
dipole moment and unique steric effects of the octahedral SF5
group. Aliphatic SF5-containing derivatives of biologically
active compounds are not well-known. One exception is the
inclusion of an ω-SF5-substituted amino acid incorporated at the
crucial first and fourth positions of a heptapeptide [23]. When
Scheme 1: Synthesis of 2-pentafluorosulfanylaldehydes by addition of
SF5Cl to enol ethers.
introduced at these positions, the SF5-substituted amino acid
had a strong propensity to direct α-helix formation of even this
short heptapeptide presumably by minimization of unfavourable
hydrophobic interactions.
After addition of SF5Cl to 2, intermediate 3 was typically
formed as a 9:1 mixture of diastereomers. Hydrolysis of 3 was
easily followed by 19F NMR, e.g., the resonance for 3b
Results and Discussion
The use of fluoroalkylimines to form β-lactams has proven appeared approximately 7 ppm upfield from that of the alde-
especially useful in synthesis [24-26] especially in the Ojima hyde 1b. The JH,F coupling constant of 5.0 Hz contrasts with
β-lactam synthon method [27-29] used to prepare docetaxel the JFeq,Fax value of 144 Hz.
analogs [26]. The general utility of the familiar Staudinger reac-
tion of imines to transform readily accessible aldehydes to The ready availability of the α-pentafluorosulfanyl carbonyl
β-lactams has been well reviewed [30-33], yet in spite of this compounds facilitated an effort to dramatically expand the
familiarity, the mechanism of this process remains a topic of utility of these intriguing materials by synthesis and characteri-
interest [34-36].
zation of the corresponding pentafluorosulfanylated β-lactams.
Previously, it has been shown that fluorinated imines can α-SF5-Substituted aldimines and ketimines
undergo a manifold of reactions that are difficult to access with The aldehydes 1 were readily converted to the corresponding
fluorinated aldehydes or ketones [37-40]. While there are many imine 5 in dichloromethane using anhydrous magnesium sulfate
reports of the utility of trifluoroacetaldimines [41-44], there is as a dehydrating agent (Scheme 2). Other desiccants, especially
only a single report of the preparation of N-ethyl 3,3,3-trifluoro- inherently basic materials, such as potassium carbonate lead to
propanaldimine [45], the trifluoromethyl analog of pentafluoro- little imine formation. While the crude imine solution likely
sulfanylacetaldehyde (1a). The imine (3,3,3-trifluoro- contained unreacted amine, attempted separation by silica gel
propanaldimine), in combination with the isomeric trifluoro- chromatography led to extensive decomposition. The product
propenamine, was not formed by the condensation of an amine imine consisted of a single stereoisomer as determined by
with the aldehyde but rather by addition of an amine to 3,3,3- 19F NMR, tentatively assigned as the E-isomer. Similar to the
trifluoropropyne. The imine was described as “extremely formation of 1, it was easy to follow formation of the imine by
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Beilstein J. Org. Chem. 2013, 9, 2675–2680.
19F NMR with the resonance attributed to the equatorial fluo- enamine 8. In the case of 7b–d the de of the 1,2-lk to 1,2-ul
rines of the imine 5a (δ 68.8 ppm) appearing upfield of those products was 76%, 84% and 50% respectively. The relative
assigned to the aldehyde 1a (δ 72.4 ppm). In the case of imines proportion of the product mixture that was comprised of
5b–d, the solution also contained between 15–20% of the puta- enamine 8b–d was 10%, 4% and 21% respectively.
tive enamine 6b–d, where, for example, the equatorial fluorine
resonances of 6b appeared at δ 65.5 ppm in contrast to those of In an effort to improve the reaction, the addition of triethyl-
5b that appeared at δ 59.5 ppm. The tendency for the enamine amine to a solution of the acid chloride and imine 5 resulted
resonances to appear downfield of imine resonances was only in decomposition. Excess amine 4 that was present was
confirmed by the preparation of the morpholine enamine of 1a acylated by benzyloxyacetyl chloride to form the corres-
for which the equatorial fluorine resonance is also shifted ponding amide.
downfield [46]. As mentioned above, the preparation of N-ethyl
3,3,3-trifluoropropanaldimine, the Schiff base of 3,3,3-trifluoro- The utility of the ketene–imine cyclization was not limited to
propanal, was accompanied by enamine formation, likely as a aldimines. The addition of SF5Br to the enol acetate of ethyl
consequence of the acidity of the proton α to the trifluoro- pyruvate 9 formed ethyl pentafluorosulfanylpyruvate 11
methyl group [45,47]. Not surprisingly, 5 was relatively reac- (Scheme 3). The ketimine 12, prepared via amine condensation
tive. After careful filtration of the desiccant, the dichloro- with 11, was reacted as described for the aldimines 5 to form
methane solution of the imine was used in further reactions the desired β-lactam 7e.
without purification or separation of unreacted amine or the
enamine side product.
Scheme 3: Preparation of ethyl pentafluorosulfanylpyruvate and for-
mation of the corresponding β-lactam.
Scheme 2: Reaction of pentafluorosulfanylaldimines with benzyloxy-
ketene.
Formation of 7e was accompanied by significantly greater
Ketene imine cycloaddition reactions of
α-SF5-substituted aldimines and ketimines
decomposition of 12 and hence 7e was formed in lower overall
yields. In contrast to the stereoselective formation of 7b–d, the
Dropwise addition of the crude dichloromethane solution of 5 to diastereoselectivity of the Staudinger reactions as determined
benzyloxyacetyl chloride and triethylamine in dichloromethane by 19F NMR was dramatically reduced for 7e to a de of 14%, a
was completed at 0 °C. The solution was then allowed to warm value consistent with the Z/E ratio for 12 of 0.7. While the
to room temperature with stirring overnight. Not surprisingly, isolated yields of purified 7b–e are especially modest, the reac-
the use of the crude solution of the imine led to the formation of tion conditions have not been optimized. But the significance of
a complex mixture where the desired product β-lactam 7 was a these findings lies not only in the difference in reactivity in
minor constituent. However, the yields of 7 are of product puri- comparison with trifluoropropanaldimine but also in the rela-
fied by silica gel chromatography and crystallization. The only tive diastereoselectivity of the ketene–imine cycloaddition reac-
other fluorinated products observed prior to purification were tion in comparison with the other reactions of SF5-bearing alde-
unreacted imine 5 and the tentatively designated N-acyl- hydes [16].
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Beilstein J. Org. Chem. 2013, 9, 2675–2680.
Structural characterization of SF5-containing
β-lactams
Isolated as a single diastereomer, the relative stereochemistry of
7a, the product of the Staudinger reaction of 5a, is shown in
Figure 1. The cis relative stereochemistry of β-lactam is consis-
tent with 1,2-lk conrotatory ring closure of the E-imine 5a as
would be predicted for a reaction with the Bose–Evans ketene
formed from benzyloxyacetyl chloride [30]. The low yield of
β-lactam product is better understood when the reactivity of the
intermediate pentafluorosulfanylated imine and the subse-
quently formed iminium ion are considered. The SF5 group
increases the acidity of the α-proton of the imine 5 and of the
iminium ion intermediate B formed on the initial nucleophilic
attack of the imine on the ketene as illustrated in Scheme 4. The
ring closure step requires bond formation between the iminium
ion carbon and the enolate carbon B to be particularly facile for
the stereoselectivity of the process to be preserved. The ring
Scheme 4: Influence of the SF5 group on the initial attack of the
ketene on the imine nitrogen (A) and on the sense of conrotatory ring
closure (B).
closure process must compete successfully with loss of the group. Previously it was found in single crystal X-ray diffrac-
acidic proton from B to form 8 [16]. Another indication of the tion studies that the pentafluorosulfanyl group [48] is
rapidity of ring closure is the failure to detect the 1,2-ul product. predictably orthogonal to a carbonyl group as shown in A.
The absence of 1,2-ul product is consistent with retardation the Cornforth control of the ring closure step of the zwitterion (B in
rate of E/Z imine isomerization by the electron withdrawing Scheme 4) where the SF5 would be antiperiplanar to the
pentafluorosulfanyl group [35]. In the reaction of 12, the E/Z iminium ion would lead to the observed diastereoselectivity
ratio of the imine was reflected very well in the 1,2-diastereo- (Scheme 1). This finding is consistent with dipolar effects being
meric excess of the product β-lactam 7e .
most important in reactions with highly charged transition states
such as B.
Figure 2: The stereochemistry of 7c, 1,2-lk,lk (Si, Si-S), as deter-
mined by single crystal X-ray diffraction studies. Thermal ellipsoids are
set at 50% probability. (a) Complete structure of 7c. (b) PMP
protecting group hidden for clarity.
Figure 1: The 1,2-lk stereochemistry of 7a as determined by single
crystal X-ray diffraction. Thermal ellipsoids are set at 50% probability.
In both structures, consistent with the opposing dipole geom-
etry of the Cornforth transition state, the N–C–C–S torsional
The ketene–imine condensation of 7c is influenced by the pres- angles remain near 170° (169° and 167° for 7a and 7c respect-
ence of the pentafluorosulfanyl group at a stereogenic center. ively).
The 1,2-lk,lk (Si,Si-S) (or (Re,Re-R)) stereochemistry of 7c
(Figure 2) suggests the profound dipole associated with the The 1,2-lk,ul ring closure product may be formed in the reac-
introduction of the SF5 group may influence the diastereoselect- tion of 5c with benzyloxyketene and simply remain undetected,
ivity of ring closure. The initial approach of the ketene (A in but it is clear that the control of diastereofacial selectivity in the
Scheme 4) appears to be influenced by avoidance of unfavor- formation of the principal β-lactam is strongly under the control
able interaction of the ketene with the sterically demanding SF5 of the SF5 group.
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Beilstein J. Org. Chem. 2013, 9, 2675–2680.
4. Winter, R. W.; Dodean, R. A.; Gard, G. L. SF5 synthons: Pathways to
organic derivatives of SF6. In Fluorine-Containing Synthons;
Soloshonok, V. A., Ed.; American Chemical Society: Washington, D.
C., 2005; pp 87–118. doi:10.1021/bk-2005-0911.ch004
5. Gard, G. L. Chim. Oggi 2009, 27, 10–13.
Conclusion
Low molecular weight pentafluorosulfanylated aldehydes 1
were prepared by addition of SF5Cl to enol ethers and the
subsequent acidic hydrolysis of 3. Formation of Schiff base 5 is
problematic but, in contrast to the reactions of the analogous tri-
fluoromethyl compounds, does successfully proceed. Even with
a manifold of possible side reactions, β-lactam formation by the
ketene–imine cycloaddition reaction of 5 occurs, albeit in very
modest yields. The 1,2-lk stereochemistry of the β-lactam 7 was
consistent with rapid cyclization and a failure of the imines 5 to
isomerize. The presence of a pentafluorosulfanylated stere-
ogenic carbon as in 5c, apparently also influences the 2,3-lk
stereochemistry. Optimization of β-lactam synthesis will require
a better understanding of the nature of the competing, undesir-
able reactions and enable utilization of this unique construct in
further synthetic transformations. The product β-lactams are a
useful entrée to the diastereoselective synthesis of pentafluoro-
sulfanyl β-amino acids and suggest a path to the preparation of
more extensively functionalized SF5-containing β-lactams.
6. Kirsch, P. Modern Fluoroorganic Chemistry. Synthesis, Reactivity and
Applications; Wiley-VCH: Weinheim, Germany, 2004.
doi:10.1002/352760393X
7. Brant, P.; Berry, A. D.; DeMarco, R. A.; Carter, F. L.; Fox, W. B.;
Hashmall, J. A. J. Electron Spectrosc. Relat. Phenom. 1981, 22,
119–129. doi:10.1016/0368-2048(81)80021-7
8. True, J. E.; Thomas, T. D.; Winter, R. W.; Gard, G. L. Inorg. Chem.
2003, 42, 4437–4441. doi:10.1021/ic0343298
9. Sæthre, L. J.; Berrah, N.; Bozek, J. D.; Børve, K. J.; Carroll, T. X.;
Kukk, E.; Gard, G. L.; Winter, R.; Thomas, T. D. J. Am. Chem. Soc.
2001, 123, 10729–10737. doi:10.1021/ja016395j
10.Sheppard, W. A. J. Am. Chem. Soc. 1962, 84, 3072–3076.
doi:10.1021/ja00875a007
11.Taft, R. W., Jr.; Lewis, I. C. J. Am. Chem. Soc. 1959, 81, 5343–5352.
doi:10.1021/ja01529a025
12.Taft, R. W., Jr. J. Phys. Chem. 1960, 64, 1805–1815.
doi:10.1021/j100841a003
13.Anthony, M. Aust. N. Z. J. Med. 1984, 14, 888–895.
doi:10.1111/j.1445-5994.1984.tb03802.x
14.Savoie, P. R.; Higashiya, S.; Lin, J.-H.; Wagle, D. V.; Welch, J. T.
J. Fluorine Chem. 2012, 143, 281–286.
Supporting Information
doi:10.1016/j.jfluchem.2012.06.027
Supporting Information File 1
15.Savoie, P. R.; Welch, J. M.; Higashiya, S.; Welch, J. T.
J. Fluorine Chem. 2013, 148, 1–5. doi:10.1016/j.jfluchem.2013.01.013
16.Ngo, S. C.; Lin, J.-H.; Savoie, P. R.; Hines, E. M.; Pugliese, K. M.;
Welch, J. T. Eur. J. Org. Chem. 2012, 4902–4905.
doi:10.1002/ejoc.201200763
Detailed experimental procedures and spectroscopic data
for 1a–e, 10, 5a–d, 7a–e and 11.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-303-S1.pdf]
17.Winter, R.; Gard, G. L. J. Fluorine Chem. 1994, 66, 109–116.
doi:10.1016/0022-1139(93)03005-7
Supporting Information File 2
X-ray crystallographic data for 7a and 7c, CCDC 937908
and 937909.
18.Ray, N. H. J. Chem. Soc. 1963, 1440–1441.
doi:10.1039/JR9630001440
19.Kleemann, G.; Seppelt, K. Chem. Ber. 1979, 112, 1140–1146.
doi:10.1002/cber.19791120409
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-303-S2.cif]
20.Dolbier, W. R., Jr.; Aït-Mohand, S.; Schertz, T. D.; Sergeeva, T. A.;
Cradlebaugh, J. A.; Mitani, A.; Gard, G. L.; Winter, R. W.;
Thrasher, J. S. J. Fluorine Chem. 2006, 127, 1302–1310.
doi:10.1016/j.jfluchem.2006.05.003
Acknowledgements
We thank Dr. Jin-hong Lin for his assistance in preparing 11.
We gratefully acknowledge National Science Foundation
(CHE0957544) for financial support of this work.
21.Coffman, D. D.; Tullock, C. W. Carbonylic compounds containing the
SF5 function. U.S. Patent US3,102,903, Sept 3, 1963.
22.Winter, R.; Willett, R. D.; Gard, G. L. Inorg. Chem. 1989, 28,
2499–2501. doi:10.1021/ic00311a054
23.Lim, D. S.; Lin, J.-H.; Welch, J. T. Eur. J. Org. Chem. 2012,
3946–3954. doi:10.1002/ejoc.201200327
References
24.Abouabdellah, A.; Bégué, J.-P.; Bonnet-Delpon, D.; Thanh Nga, T. T.
J. Org. Chem. 1997, 62, 8826–8833. doi:10.1021/jo971381a
25.Petrik, V.; Röschenthaler, G.-V.; Cahard, D. Tetrahedron 2011, 67,
3254–3259. doi:10.1016/j.tet.2011.03.001
1. Welch, J. T. Applications of pentafluorosulfanyl substitution in life
sciences research. In Fluorine in Pharmaceutical and Medicinal
Chemistry; Gouverneur, V.; Müller, K., Eds.; Imperial College Press:
London, 2012; pp 175–207.
26.Pepe, A.; Kuznetsova, L.; Sun, L.; Ojima, I. Fluoro-taxoid anticancer
agents. In Fluorine in Medicinal Chemistry and Chemical Biology;
Ojima, I., Ed.; John Wiley & Sons Ltd.: Chichester, 2009; pp 117–139.
doi:10.1002/9781444312096.ch5
2. Altomonte, S.; Zanda, M. J. Fluorine Chem. 2012, 143, 57–93.
doi:10.1016/j.jfluchem.2012.06.030
3. Lentz, D.; Seppelt, K. The –SF5, –SeF5, and –TeF5 groups in organic
chemistry. In Chemistry of Hypervalent Compounds; Akiba, K.-y., Ed.;
Wiley-VCH: New York, 1999; pp 295–323.
27.Ojima, I. β-Lactam synthon method: enantiomerically pure β-lactams as
synthetic intermediates. In Organic Chemistry of β-lactams;
Georg, G. I., Ed.; VCH: New York, NY, 1993; pp 197–255.
2679

Beilstein J. Org. Chem. 2013, 9, 2675–2680.
28.Kamath, A.; Ojima, I. Tetrahedron 2012, 68, 10640–10664.
doi:10.1016/j.tet.2012.07.090
License and Terms
29.Ojima, I.; Zuniga, E. S.; Seitz, J. D. Top. Heterocycl. Chem. 2013, 30,
1–63. doi:10.1007/7081_2012_86
This is an Open Access article under the terms of the
Creative Commons Attribution License
30.Georg, G. I.; Ravikumar, V. T. Stereocontrolled ketene-imine
cycloaddition reactions. In Organic Chemistry of β-lactams;
Georg, G. I., Ed.; VCH: New York, NY, 1993; pp 295–368.
31.Palomo, C.; Aizpurua, J. M.; Ganboa, I.; Oiarbide, M.
Eur. J. Org. Chem. 1999, 3223–3235.
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
doi:10.1002/(SICI)1099-0690(199912)1999:12<3223::AID-EJOC3223>
3.0.CO;2-1
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
32.Palomo, C.; Aizpurua, J. M.; Ganboa, I.; Oiarbide, M.
Curr. Med. Chem. 2004, 11, 1837–1872.
(http://www.beilstein-journals.org/bjoc)
doi:10.2174/0929867043364900
33.Thompson, S.; Coyne, A. G.; Knipe, P. C.; Smith, M. D.
Chem. Soc. Rev. 2011, 40, 4217–4231. doi:10.1039/c1cs15022g
34.Cossio, F. P.; Arrieta, A.; Sierra, M. A. Acc. Chem. Res. 2008, 41,
925–936. doi:10.1021/ar800033j
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.9.303
35.Jiao, L.; Liang, Y.; Xu, J. J. Am. Chem. Soc. 2006, 128, 6060–6069.
doi:10.1021/ja056711k
36.Xu, J. Tetrahedron 2012, 68, 10696–10747.
doi:10.1016/j.tet.2012.04.007
37.Fustero, S.; Jiménez, D.; Sanz-Cervera, J. F.; Sánchez-Roselló, M.;
Esteban, E.; Simón-Fuentes, A. Org. Lett. 2005, 7, 3433–3436.
doi:10.1021/ol050791f
38.Zanda, M.; Bravo, P.; Volonterio, A. Stereoselective synthesis of
β-fluoroalkyl β-amino alcohol units. In Asymmetric Fluoroorganic
Chemistry; Ramachandran, P. V., Ed.; American Chemical Society:
Washington, D.C., 2000; Vol. 746, pp 127–141.
doi:10.1021/bk-2000-0746.ch010
39.Welch, J. T.; De Corte, B.; De Kimpe, N. J. Org. Chem. 1990, 55,
4981–4983. doi:10.1021/jo00304a002
40.Welch, J. T.; Seper, K. W. J. Org. Chem. 1988, 53, 2991–2999.
doi:10.1021/jo00248a017
41.Carroccia, L.; Fioravanti, S.; Pellacani, L.; Tardella, P. A. Synthesis
2010, 4096–4100. doi:10.1055/s-0030-1258280
42.Nie, J.; Guo, H.-C.; Cahard, D.; Ma, J.-A. Chem. Rev. 2011, 111,
455–529. doi:10.1021/cr100166a
43.Magueur, G.; Legros, J.; Meyer, F.; Ourévitch, M.; Crousse, B.;
Bonnet-Delpon, D. Eur. J. Org. Chem. 2005, 1258–1265.
doi:10.1002/ejoc.200400719
44.Crousse, B.; Narizuka, S.; Bonnet-Delpon, D.; Bégué, J.-P. Synlett
2001, 679–681. doi:10.1055/s-2001-13364
45.Stepanova, N. P.; Lebedev, V. B.; Orlova, N. A.; Turbanova, E. S.;
Petrov, A. A. Zh. Org. Khim. 1988, 24, 692–699.
46.Welch, J. T.; Penger, A. unpublished results.
47.Yamazaki, T.; Kobayashi, R.; Kitazume, T.; Kubota, T. J. Org. Chem.
2006, 71, 2499–2502. doi:10.1021/jo052434o
48.Welch, J. T.; Zhong, L.; Filatov, A. S. unpublished results.
2680