Gold(I)-catalyzed synthesis of γ-vinylbutyrolactones by intramolecular oxaallylic alkylation with alcohols

Article abstract of DOI:10.3762/bjoc.7.139

Gold(I)-N-heterocyclic carbene (NHC) complexes proved to be a reliable catalytic system for the direct synthesis of functionalized γ- vinylbutyrolactones by intramolecular oxaallylic alkylation with primary alcohols. Good isolated chemical yields were obt

Full text of DOI:10.3762/bjoc.7.139

Gold(I)-catalyzed synthesis of γ-vinylbutyrolactones
by intramolecular oxaallylic alkylation with alcohols
Michel Chiarucci, Mirko Locritani, Gianpiero Cera and Marco Bandini*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 1198–1204.
Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum –
Università di Bologna, Via Selmi 2, 40126 Bologna, Italy
doi:10.3762/bjoc.7.139
Received: 27 May 2011
Email:
Accepted: 15 August 2011
Published: 01 September 2011
Marco Bandini* - marco.bandini@unibo.it
* Corresponding author
This article is part of the Thematic Series "Gold catalysis for organic
synthesis".
Keywords:
alcohol; butyrolactone; carbene; gold-catalysis; intramolecular
oxaallylic alkylation
Guest Editor: F. D. Toste
© 2011 Chiarucci et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Gold(I)-N-heterocyclic carbene (NHC) complexes proved to be a reliable catalytic system for the direct synthesis of functionalized
γ-vinylbutyrolactones by intramolecular oxaallylic alkylation with primary alcohols. Good isolated chemical yields were obtained
for a range of malonyl and acetate derivatives. The good performance in reagent-grade solvents and the functional group/moisture
tolerance make this catalytic process a promising route for the synthesis of architecturally complex polycyclic structures.
Introduction
Allylic alcohols are highly desirable, readily available, cheap, this context, electrophilic LTM activation of carbon–carbon
and environmental sustainable reaction partners for allylic unsaturations, adjacent to alcoholic moieties (i.e., allylic,
alkylation reactions in the presence of C- as well as X-based (X: benzylic, and propargylic alcohols, usually referred to as π-acti-
heteroatom) nucleophiles [1,2]. Despite their undoubted syn- vated alcohols), deserves a particular mention [10-13].
thetic/economic advantages (i.e., water is the only stoichio-
metric byproduct produced), the intrinsic lower reactivity of As a part of our ongoing interest in the gold-catalyzed allylic
allylic alcohols compared to allyl halides/acetates/carbonates functionalization of C- and heteroatom-based nucleophiles with
generally necessitates harsher reaction conditions and/or the alcohols [14-17], we previously observed the formation of
need for activating agents (i.e., Brønsted or Lewis acids) [3,4]. synthetically useful vinylbutyrolactones [18-22] as minor prod-
ucts in the Friedel–Crafts-type allylic alkylation of arenes [23].
Recently, late-transition metal (LTM) catalysis (i.e., Hg, Pd, Pt, The wide impact of functionalized γ-lactones on the synthesis
Au, and Ru) has received growing attention in organic syn- of naturally occurring compounds [24-26] prompted us to opti-
thesis and enables unprecedented manipulations of unfunction- mize a direct synthesis of vinylbutyrolactones by direct gold ac-
alized hydrocarbons under mild reaction conditions [5-9]. In tivation of allylic alcohols [27-31] with esters [32-37].
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Beilstein J. Org. Chem. 2011, 7, 1198–1204.
Figure 1: Working hypothesis for the present gold-catalyzed oxaallylic alkylation reaction.
In this direction, we targeted malonyl alcohols 1 as a readily Results and Discussion
available class of model acyclic precursors to create chemical At the outset of our investigation, we focused our attention
diversity through an oxaallylic ring-closing reaction (Figure 1). on the non-enolizable allyl alcohol (Z)-1a, as a model
candidate for the intramolecular oxaallyl alkylation. Our
It should be noted that the synthesis of such a class of hetero- choice was dictated by the well-known reluctance of
cyclic compounds has been the subject of several investigations. disubstituted malonyl derivatives to provide vinylbutyro-
Among them, a multi-step synthetic pathway with final TBAF- lactones. This aspect was convincingly highlighted in
promoted cyclization was proposed by Lepore [38] and, almost a recent report by Chen and coworkers that described an
simultaneously, Poli and Prestat described a Pd-catalyzed analogous catalytic approach based on allyl acetate derivatives
Tsuji–Trost-type allylic alkylation procedure to obtain valuable [41].
precursors (i.e., lactams and lactones) of podophyllotoxin
analogs [39,40]. However, to the best of our knowledge, no At this stage, an extended survey of reaction parameters (metal
examples of metal-catalyzed lactonization through direct acti- source, solvent, and temperature) was conducted in order to
vation of allylic alcohols have been described so far.
ascertain the optimal catalytic conditions (Table 1).
Table 1: Optimization of the reaction conditions for the lactonization of 1a.a
Entry
Cat (%)
Solvent
Yield (%)b
(trans:cis)c
1
[P(t-Bu)2o-biphenyl](AuCH3CN)SbF6 (5)
[P(Cy)2o-biphenyl-2,4,6(iPr)3]AuNTf2 (5)
PPh3AuNTf2 (5)
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Toluene
CH3CN
THF
42
nd
2
82
1.5:1
1.1:1
nd
3
52
4
[(PPh3Au)3O]BF4 (2)
AuCl3 (5)
Trace
<20
56
5
nd
6
[biphepAu2Cl2/AgOTf] (2.5)
[dppf(AuNTf2)2] (2.5)
[dppf(AuNTf2)2] (0.5)
IMesAuOTf (5)
1.3:1
1.2:1
1.4:1
2.1:1
nd
7
98
8d
9d,e
10f
11
12
13
14
15
16d,e,g
96
94
IMesAuOTf (5)
Trace
31
IMesAuOTf (5)
1.9:1
nd
IMesAuOTf (5)
Trace
79
IMesAuOTf (5)
1.9:1
nd
AgOTf (5)
DCE
DCE
DCE
35
TsOH (10)
64
1.3:1
nd
IMesAuOTf (5)
Trace
aAll the reactions were carried out under nitrogen atmosphere at 80 °C for 16 h, unless otherwise stated. bIsolated yield after flash chromatography.
cDetermined by GC on the reaction crude. The relative configuration was determined by NOE experiments on the single diastereoisomers separated
by flash chromatography. dUnder no moisture restriction, with reagent-grade solvent. eReaction time: 4 h. fAt room temperature. IMes: 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene. gIn the presence of K2CO3 (1 equiv). nd: not determined.
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Beilstein J. Org. Chem. 2011, 7, 1198–1204.
Initial attempts to perform the lactonization reaction of 1a were In order to confirm that the catalysis was indeed due to the pres-
carried out by means of a silver-free cationic complex ence of gold, a control experiment with AgOTf (5 mol %) was
[P(t-Bu)2o-biphenyl](AuCH3CN)SbF6 (5 mol %). The desired performed on compound 1a. Under comparable reaction condi-
butyrolactone 2a was obtained selectively under reflux in DCE tions (80 °C, 16 h), lactone 2a was isolated in poor yield (35%).
for 16 h (entry 1), although only in low yield. With the less Finally, the hypothetical cocatalysis by Brønsted acids (BA)
bulky triphenylphosphine ligand, the corresponding cationic was verified by means of experimental controls with TsOH
gold(I) complex (i.e., PPh3AuNTf2) led to an increase in the (entry 15), and also in the presence of an acid scavenger (entry
isolated yield up to 52%, although the diastereoselection 16). Here, the desired cyclic compound 2a was obtained in
remained elusive (≈ 1:1, entry 3). After demonstrating that the lower yield (64%) with concomitant substantial decomposition
Au(III) catalysis promoted the cyclization in lower extent of the starting allylic alcohol. Such evidence confirms the
compared to the Au(I) counterpart (entry 5 versus entries 1–3), allylic SN1 mechanism for the present methodology [44].
we also observed that dinuclear [dppf(AuNTf2)2] provided 2a
with almost complete conversion (entry 7). The possibility to The high chemoselectivity guaranteed by the gold catalysts is
reduce the loading of the catalyst (0.5 mol %) further, without worthy of note, as it channels the reaction toward the allylic
the need for moisture restriction, was successfully verified by alkylation mechanism without any contamination deriving from
the isolation of 2a in 96% isolated yield (entry 8). Interestingly, transesterification reactions. This evidence is reasonably ratio-
the diastereoselection of the protocol was slightly improved nalized in terms of the high π-acidity and poor oxophilicity of
(up to 2.1:1) and the reaction time shortened to 4 h, by the Au(I) species [37].
employing the carbene-based gold complex IMesAuCl/AgOTf
(5 mol %, entry 9) [42,43]. Therefore, by addressing With the optimal catalytic systems in hand (IMesAuOTf or
NHCAuOTf as the optimal catalytic system, the impact of the [dppf(AuNTf2)2], DCE, 80 °C), we verified the generality of
reaction media on the chemical output of the process was the method by subjecting a range of malonyl alcohols 1b–j to
investigated (entries 10–13). Here, although 2a was also the gold-catalyzed lactonization (Table 2).
isolated in good yield in reagent-grade THF (yield = 79%,
dr = 1.9:1, entry 13), DCE was employed as the solvent of The impact of the carbon–carbon double bond configuration on
choice.
both chemical and stereochemical outputs of the process was
Table 2: Proving the scope of the gold-catalyzed intramolecular allylation of 1.a
Entry
1
1
Catalytic system
Product
Yield (%)b
trans:cisc
(E)-1a
A
72
1.3:1
2a
2b
2
(Z)-1b
A
95
1:1
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Beilstein J. Org. Chem. 2011, 7, 1198–1204.
Table 2: Proving the scope of the gold-catalyzed intramolecular allylation of 1.a (continued)
3
4
5
6
(Z)-1c
(Z)-1d
(Z)-1e
(Z)-1f
A
66
1.1:1
2c
2d
2e
2f
A
67
1.4:1
A
85
1:1
A, B
94, 35
1.5:1, 1.5:1
7
(Z)-1g
Ad, Be
63, 12
1:1.4, 1:1.4
2g
2h
8
(Z)-1h
(Z)-1i
(Z)-1j
A
54
45
93
3.2:1
1.4:1
1.1:1
9
A
2i
2j
10
Af
aAll the reactions were carried out in reagent-grade solvents under air (80 °C, 0.3 M). Catalytic systems: A = IMesAuCl/AgOTf (5 mol %), DCE, 80 °C,
7–9 h. B = [dppf(AuNTf2)2] (2.5 mol %), THF, 80 °C, 16 h. bIsolated yield after flash chromatography. cDetermined by GC on the reaction crude.
dDihydronaphthalene derived from undesired Friedel–Crafts alkylation (yield = 14%). eA considerable amount of Friedel–Crafts dihydronaphthalene
(yield = 67%) was isolated [23]. f10 mol % of catalyst was used.
initially investigated. Here, by subjecting (E)-1a to the reaction entry 1) and similar diastereomeric ratio. Here, although the
conditions A (i.e., IMesAuCl/AgOTf, DCE, 80 °C) the corres- impact of the C─C double bond configuration on the stereo-
ponding lactone 2a was isolated in comparable yield (72%, chemical outcome of SN2′-type gold-catalyzed intramolecular
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Beilstein J. Org. Chem. 2011, 7, 1198–1204.
O- [45] and N-alkylations [37,46] with allylic alcohols was the methodology proved to be adaptable allowing a double
demonstrated, we consider it likely that an allylic SN1 mecha- lactonization event with 1j, hence providing spiro-lactone 2j in
nism is involved in the present methodology, due to the similar 93% yield.
optical outcomes obtained in the presence of BA metal-free
catalysts (entry 15, Table 1).
Apart from the generality on malonyl substrates, we decided to
explore the applicability of the present methodology to less
Then, enolizable substrates carrying different malonyl residues reactive monoester analogs [47]. In this context, readily avail-
(1b–e) were taken into account. In all cases the cyclization able alcohols 3a,b were subjected to cyclization in the presence
occurred smoothly leading to the disappearance of the acyclic of the gold catalytic system A. In both cases lactones 4a,b were
precursors within 7–9 h reaction time (entries 2–5). Interest- isolated in good to excellent yields (93 and 75%, respectively,
ingly, in this case no appreciable differences in reaction rate Scheme 1).
were observed between substrates carrying labile and nonlabile
ester alkyl groups.
In some specific cases, both catalytic systems were tested and a
direct comparison of performances can be made. Clear evi-
dence was gained for the higher activity of the catalytic system
A in the expected oxaallylic alkylation process. As an example,
when multiple reactive channels were available (i.e., lactoniza-
tion and Friedel–Crafts-type alkylation, 1g) dppf-based species
Scheme 1: Gold-catalyzed synthesis of γ-lactones 4 from the corres-
ponding monoesters 3.
(catalytic system B) led to a complex mixture of crude reaction
products (entry 7), while carbene–gold complex provided
mainly the butyrolactone 2g. Moreover, the methodology
proved to be tolerant toward several functional groups/atoms at Finally, the 1,3-ketoester 3c was also subjected to the opti-
the methylene carbon atom of the malonyl derivative. In par- mized conditions, but a complex reaction mixture was observed
ticular, 3,5-trans-3-bromo-γ-vinylbutyrolactone 2h was isolated with concomitant decomposition of the starting material.
with 54% yield and in 76:24 diastereoisomeric ratio (entry 8). A
protected carbonyl moiety was also tested leading to the corres- The mechanistic proposal for the formation of γ-vinylbutyro-
ponding lactone 2i in moderate yield (45%, entry 9). Finally, lactones 2 is depicted in Scheme 2. As previously mentioned,
Scheme 2: Mechanistic sketch of the gold-promoted oxaallylic alkylation reaction.
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4. Hartwig, J. F. Allylic Substitution. In Organotransition Metal Chemistry:
From Bonding to Catalysis; University Science Books: Sausalito, CA,
2010; pp 967–1014.
the formation of an allylic cationic species (II) is assumed,
upon coordination of the gold catalyst to the allylic alcohol (I).
In Scheme 2, the possible coordination modes for [Au+] to the
allylic alcohol are reported. As a matter of fact, although we
have previously demonstrated the C=C···Au interaction in the
presence of allylic alcohols [16], a concomitant [Au]···OH
contact cannot be ruled out [48,49]. Subsequently, the direct
nucleophilic attack by the carboxylate unit would lead to an
oxonium intermediate III [50,51] that, after dealkylation,
resulted in the final lactone 2. Control experiments have been
performed to indentify the presence of a Brønsted acid cocatal-
ysis in the ring-closing procedure (see [52] and entry 15 in
Table 1). Regeneration of the active cationic gold species or
assistance in the formation of the reactive allylic carbocation
intermediate II are key steps in which the Brønsted cocatalysis
could be exerted [52]. Finally, the mandatory role of enol
tautomer (or gold–enolate intermediates) [53-55] in the nucleo-
philic attack was excluded; non-enolizable compounds being
suitable candidates for the cyclization reaction.
5. Nevado, C.; Echavarren, A. M. Synthesis 2005, 167.
doi:10.1055/s-2005-861781
6. Widenhoefer, R. A.; Han, X. Eur. J. Org. Chem. 2006, 4555.
doi:10.1002/ejoc.200600399
7. Fürstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007, 46, 3410.
doi:10.1002/anie.200604335
8. Chianese, A. R.; Lee, S. J.; Gagné, M. R. Angew. Chem., Int. Ed.
2007, 46, 4042. doi:10.1002/anie.200603954
9. Abu Sohel, S. M.; Liu, R.-S. Chem. Soc. Rev. 2009, 38, 2269.
doi:10.1039/b807499m
10.Defieber, C.; Ariger, M. A.; Moriel, P.; Carreira, E. M.
Angew. Chem., Int. Ed. 2007, 46, 3139. doi:10.1002/anie.200700159
11.Tanaka, S.; Seki, T.; Kitamura, M. Angew. Chem., Int. Ed. 2009, 48,
8948. doi:10.1002/anie.200904671
12.Yamamoto, H.; Ho, E.; Namba, K.; Imagawa, H.; Nishizawa, M.
Chem.–Eur. J. 2010, 16, 11271. doi:10.1002/chem.201001656
13.Miyata, K.; Kutsana, H.; Kawakami, S.; Kitamura, M.
Angew. Chem., Int. Ed. 2011, 50, 4649. doi:10.1002/anie.201100772
14.Bandini, M.; Eichholzer, A. Angew. Chem., Int. Ed. 2009, 48, 9533.
doi:10.1002/anie.200904388
15.Bandini, M.; Eichholzer, A.; Gualandi, A.; Quinto, T.; Savoia, D.
ChemCatChem 2010, 2, 661. doi:10.1002/cctc.201000077
16.Bandini, M.; Monari, M.; Romaniello, A.; Tragni, M. Chem.–Eur. J.
2010, 16, 14272. doi:10.1002/chem.201002606
Conclusions
In conclusion, we have documented an unprecedented example
of gold-catalyzed lactonization with primary allylic alcohols.
Cationic NHCAu carbene gold complexes allowed the prepar-
ation of a range of functionalized malonyl esters by direct acti-
vation of the allylic alcohol by gold. The methodology appears
highly chemoselective toward the allylic lactonization, with the
possibility to extend the protocol also to acetate derivatives.
17.Bandini, M.; Gualandi, A.; Monari, M.; Romaniello, A.; Savoia, D.;
Tragni, M. J. Organomet. Chem. 2011, 696, 338.
doi:10.1016/j.jorganchem.2010.09.065
18.Hon, Y.-S.; Chen, H.-F.; Kao, C.-Y.; Luo, C.-Z. Tetrahedron 2010, 66,
8468. doi:10.1016/j.tet.2010.08.035
19.Park, B. R.; Kim, S. H.; Kim, Y. M.; Kim, J. N. Tetrahedron Lett. 2011,
52, 1700. doi:10.1016/j.tetlet.2011.01.153
20.He, H.; Dai, L.-X.; You, S.-L. Org. Biomol. Chem. 2010, 8, 3207.
doi:10.1039/b924770j
Supporting Information
21.Csuk, R.; Barthel, A.; Schwarz, S.; Kommera, H.; Paschke, R.
Bioorg. Med. Chem. 2010, 18, 2549. doi:10.1016/j.bmc.2010.02.042
22.Elford, T. G.; Hall, D. G. Synthesis 2010, 893.
doi:10.1055/s-0029-1218664
Supporting Information File 1
Experimental details and characterization of the synthesized
compounds.
23.Bandini, M.; Eichholzer, A.; Kotrusz, P.; Tragni, M.; Troisi, S.;
Umani-Ronchi, A. Adv. Synth. Catal. 2009, 351, 319.
doi:10.1002/adsc.200800628
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-139-S1.pdf]
24.Ward, R. S. Recent Advances in the Chemistry of Lignans. In Studies
in Natural Products Chemistry, Volume 24, Bioactive Natural Products,
Part E; Rahman, A.-u., Ed.; Elsevier: Amsterdam, 2000; pp 739–798.
25.Ward, R. S. Nat. Prod. Rep. 1999, 16, 75. doi:10.1039/a705992b
26.Saleem, M.; Kim, H. J.; Ali, M. S.; Lee, Y. S. Nat. Prod. Rep. 2005, 22,
696. doi:10.1039/b514045p
Acknowledgment
Acknowledgment is made to Progetto FIRB “Futuro in Ricerca”
Innovative sustainable synthetic methodologies for C–H acti-
vation processes (MIUR, Rome), Università di Bologna.
27.Gorin, D. J.; Toste, F. D. Nature 2007, 446, 395.
doi:10.1038/nature05592
References
28.Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180. doi:10.1021/cr000436x
29.Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239.
doi:10.1021/cr068434l
1. Bandini, M.; Tragni, M. Org. Biomol. Chem. 2009, 7, 1501.
doi:10.1039/b823217b
30.Jiménez-Núñez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326.
doi:10.1021/cr0684319
2. Bandini, M. Angew. Chem., Int. Ed. 2011, 50, 994.
doi:10.1002/anie.201006522
31.Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108, 3351.
doi:10.1021/cr068430g
And references therein.
3. Kimura, M.; Tamaru, Y. Mini-Rev. Org. Chem. 2009, 6, 392.
doi:10.2174/157019309789371631
32.Aponick, A.; Biannic, B. Synthesis 2008, 3356.
doi:10.1055/s-0028-1083160
1203

Beilstein J. Org. Chem. 2011, 7, 1198–1204.
33.Aponick, A.; Li, C.-Y.; Palmes, J. A. Org. Lett. 2009, 11, 121.
doi:10.1021/ol802491m
License and Terms
34.Rao, W.; Chan, P. W. H. Org. Biomol. Chem. 2008, 6, 2426.
doi:10.1039/b805067h
This is an Open Access article under the terms of the
Creative Commons Attribution License
35.Aponick, A.; Li, C.-Y.; Biannic, B. Org. Lett. 2008, 10, 669.
doi:10.1021/ol703002p
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
36.Aponick, A.; Biannic, B.; Jong, M. R. Chem. Commun. 2010, 46, 6849.
doi:10.1039/c0cc01961e
37.Mukherjee, P.; Widenhoefer, R. A. Org. Lett. 2011, 13, 1334.
doi:10.1021/ol103175w
38.Silvestri, M. A.; He, C.; Khoram, A.; Lepore, S. D. Tetrahedron Lett.
2006, 47, 1625. doi:10.1016/j.tetlet.2005.12.114
39.Vitale, M.; Prestat, G.; Lopes, D.; Madec, D.; Poli, G. Synlett 2006,
2231. doi:10.1055/s-2006-949651
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(http://www.beilstein-journals.org/bjoc)
40.Vitale, M.; Prestat, G.; Lopes, D.; Madec, D.; Kammerer, C.; Poli, G.;
Girnita, L. J. Org. Chem. 2008, 73, 5795. doi:10.1021/jo800707q
And references therein.
The definitive version of this article is the electronic one
which can be found at:
41.Wang, Y.-H.; Zhu, L.-L.; Zhang, Y.-X.; Chen, Z. Chem. Commun. 2010,
46, 577. doi:10.1039/b913348h
doi:10.3762/bjoc.7.139
42.Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776.
doi:10.1039/b711132k
43.Nolan, S. P. Acc. Chem. Res. 2011, 44, 91. doi:10.1021/ar1000764
44.Different Lewis acids were tested (10 mol %), furnishing 2a in lower
yields in comparison to IMesAuOTf and [dppf(AuNTf2)2]. Zn(OTf)2:
traces, [codPt(OTf)2]: 65% (dr = 1.7:1), In(OTf)3: 87% (dr = 1.1:1),
Bi(OTf)3: 80% (dr = 1.9:1).
45.Aponick, A.; Biannic, B. Org. Lett. 2011, 13, 1330.
doi:10.1021/ol200203k
46.Mukherjee, P.; Widenhoefer, R. A. Org. Lett. 2010, 12, 1184.
doi:10.1021/ol902923e
47.Gierasch, T. M.; Shi, Z.; Verdine, G. L. Org. Lett. 2003, 5, 621.
doi:10.1021/ol027116f
The usefulness of compounds of 4-type in obtaining libraries of
diversity oriented-synthesis has been documented.
48.Georgy, M.; Boucard, V.; Campagne, J.-M. J. Am. Chem. Soc. 2005,
127, 14180. doi:10.1021/ja0534147
49.Georgy, M.; Boucard, V.; Debleds, O.; Dal Zotto, C.; Campagne, J.-M.
Tetrahedron 2009, 65, 1758. doi:10.1016/j.tet.2008.12.051
50.Liu, L.-P.; Xu, B.; Mashuta, M. S.; Hammond, G. B. J. Am. Chem. Soc.
2008, 130, 17642. doi:10.1021/ja806685j
51.Liu, L.-P.; Hammond, G. B. Chem.–Asian J. 2009, 4, 1230.
doi:10.1002/asia.200900091
52.In this context, when K2CO3 (1 equiv) was used as a scavenger, the
desired product 2a was obtained only in trace amount. Interestingly,
the use of stoichiometric amounts of TFA led to the formation of the
O-allyl trifluoroacetate derivative exclusively (yield = 63%).
53.Yao, X.; Li, C.-J. J. Am. Chem. Soc. 2004, 126, 6884.
doi:10.1021/ja0482637
54.Hashmi, A. S. K.; Schäfer, S.; Wölfle, M.; Gil, C. D.; Fischer, P.;
Laguna, A.; Blanco, M. C.; Gimeno, M. C. Angew. Chem., Int. Ed.
2007, 46, 6184. doi:10.1002/anie.200701521
55.Hashmi, A. S. K. Angew. Chem., Int. Ed. 2010, 49, 5232.
doi:10.1002/anie.200907078
1204