Copper salt-catalyzed azide-[3 + 2] cycloadditions of ynamides and bis-ynamides

Article abstract of DOI:10.1002/adsc.200600404

A one-pot synthesis of amide-substituted triazoles from alkyl bromides and ynamides is described here along with syntheses of novel bis-ynamidesand their applications in [3 + 2] cycloadditions with azides to construct unique bis-triazoles.

Full text of DOI:10.1002/adsc.200600404

FULL PAPERS
DOI: 10.1002/adsc.200600404
Copper Salt-Catalyzed Azide-[3 + 2] Cycloadditions of
Ynamides and Bis-Ynamides
Xuejun Zhang,a Hongyan Li,a Lingfeng You,a Yu Tang,a and Richard P. Hsunga,*
a
Division of Pharmaceutical Sciences and Department of Chemistry, Rennebohm Hall, 777 Highland Avenue, University
of Wisconsin, Madison, WI 53705, USA
Fax : (+1)-608-262-534; e-mail: rhsung@wisc.edu
Received: August 10, 2006; Accepted: September 19, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A one-pot synthesis of amide-substituted amides and their applications in [3+2] cycloaddi-
triazoles from alkyl bromides and ynamides is de- tions with azides to construct unique bis-triazoles.
scribed here along with syntheses of novel bis-yn-
Keywords: azide-[3 + 2] cycloaddition; bis-triazoles;
bis-ynamides; Cu(I) catalysis; triazoles; ynamides
Introduction
Results and Discussions
The versatility of 1,3-dipolar cycloadditions[1,2] in con- Problems with the Competing Ynamide Hydrolysis
structing synthetically and medicinally useful hetero-
cycles[3] has led us to investigate [3+2] cycloadditions The feasibility of [3+2] cycloadditions of ynamides
of ynamides.[4–8] Specifically, we have examined the with organic azides could be readily established. As
classic Huisgenꢀs azide-[3+2] cycloaddition[9–11] given shown in Scheme 1, the CuSO4·5 H2O-catalyzed cyclo-
its current resurgence facilitated by the “click” addition of terminal ynamide 5 with BnN3 led to
chemistry concept.[12] Recently, Cintrat[13] and we[14] chiral amide-substituted triazole 7 in 82% yield as the
independently communicated azide-[3 + 2] cycload-
ditions employing ynamides in a highly regioselective
manner that favors 1,4-adducts (1+2!3 in Figure 1).
Particularly, we focused on developing tandem azidi-
nation- and hydroazidination-[3+2] cycloadditions of
ynamides, leading to an array of amide-substituted tri-
azoles.[14] We report here details of our work with
copper salt-catalyzed azide-[3+2] cycloadditions of
ynamides that feature a one-pot synthesis of triazoles
from alkyl bromides and constructions of bis-triazoles
from novel bis-ynamides.
Figure 1. Ynamides in [3+2] cycloadditions.
Scheme 1. Huisgenꢀs azide-[3+2] cycloadditions of yn-
amides.
Adv. Synth. Catal. 2006, 348, 2437 – 2442
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2437

Xuejun Zhang et al.
FULL PAPERS
only regioisomer using Fokin–Sharpless conditions.[15]
The catalytic sequence likely proceeds through a
Cu(I) catalytic species generated via reduction of
CuSO4.[15] The high level of regioselectivity is likely a
result of the copper catalysis and steric factors be-
cause of the large oxazolidinone motif.[15,16] However,
there was an unresolved problem in our earlier work.
When ynamides that are more prone to hydrolysis,
such as 9, were used the desired triazole 10 was ob-
tained in only 30% yield with the hydrolysis product
11 being predominant.
Table 1. Scope of one-pot synthesis of triazoles from alkyl
bromides.
To circumvent the hydrolysis problem, an improved
protocol (Conditions B) was developed using
0.2 equivs. CuBr as the source of the Cu(I) catalyst
along with anhydrous CH3CN serving as the solvent
and Et3N as the base and ligand to improve the solu-
bility of CuBr in CH3CN. Under these new conditions,
the yield of triazole 10 was improved to 56% without
noticeable hydrolysis even when CH3CN was not an-
hydrous, although the yield dropped to 44%
(Scheme 1). The improvement was further visible in
the reaction of ynamide 9 with azide 12, which led to
the desired triazole 13 in 80% yield along with intri-
guingly 10% of the debrominated product 14, al-
though 1.0 equiv. of CuBr was used. Under Condi-
tions A, we could only observe a trace amount of 13
with hydrolysis of 9 being the major pathway.
[a]
1.4 equivs. of NaN3, 1.3 equivs. of R-Br, DMSO, room
temperature, 12 h, and then, 1.0 equiv of ynamide,
0.10 equiv of CuSO4·5 H2O, 0.10 equiv of Na ascorbate,
H2O, room temperature, 14 h.
Isolated yields.
1.4 equivs. of NaN3, DMSO, room temperature, 12 h, 1.3
equivs. of R-Br, and then 1.0 equivs. of ynamide, 0.20
equivs. of CuBr, 2.0 equivs. of Et3N, CH3CN, room tem-
perature, 12 h.
Synthetic Scope of Organic Azides
To expand the synthetic scope, we embarked on an
exploration of other organic azides besides BnN3.
However, due to the difficulties in handling organic
azides in general,[17] especially those with low molecu-
lar weights, we ultimately designed a one-pot synthe-
ses of amide-substituted triazoles from alkyl bromides
directly. As shown in Scheme 2, organic azides were
generated in situ by reacting alkyl bromides with
[b]
[c]
NaN3 in DMSO solution,[18,19] and subsequently, tries 1–3, 5–7 and 10) afforded the respective triazoles
copper salt, Na ascorbate, and terminal ynamides 5 in good yields, including allyl bromide (entry 3), prop-
and 15 were added without isolating the intermediate argyl bromide (entry 7), and benzyl bromide (en-
organic azides. Under this one-pot protocol, triazoles tries 5 and 10). A secondary bromide (entry 8) gave
17 and 18 were obtained in excellent yields from 5 slightly lower yield. Many functional groups such as
and 15, respectively.
silyl ether (entry 1), benzyl ether (entry 5), acetal
The scope of this protocol was explored as shown (entry 2), aryl bromide (entries 5 and 10), alkene
in Table 1. In general, all primary bromides (en- (entry 3), and internal alkyne (entry 7) could be toler-
ated.
It is noteworthy that to avoid competing hydrolysis
of ynamides during the cycloaddition, when using
ynamide 9, the desired triazole 13 could be again ob-
tained in 52% yield through the usage of new condi-
tions [see Note c] related to Conditions B illustrated
in Scheme 1 for the cycloaddition (entry 10). In addi-
tion, we found that sulfonyl-substituted ynamides
such as 19 can also be problematic in terms of hydrol-
ysis. Thus, when applicable, the cycloaddition of 19
Scheme 2. One-pot synthesis of triazoles from alkyl bro-
mides.
2438
www.asc.wiley-vch.de
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 2437 – 2442

Azide-[3 + 2] Cycloadditions of Ynamides and Bis-Ynamides
FULL PAPERS
Based on this precedent, we examined cycloaddi-
tions of macrocyclic ynamides, which could be pre-
pared through a Cu(I)-catalyzed intramolecular cou-
pling of amides tethered to alkynyl bromides that we
had reported recently.[20] As a result, novel macrocy-
clic fused triazoles 37 and 38 were successfully isolat-
ed in good yields when their respective macrocyclic
ynamides were heated with BnN3 at 1408C in toluene
in a sealed tube.
Scheme 3. Fused triazoles employing macrocyclic ynamides.
Novel Bis-Triazoles and Bis-Ynamides
With azide-[3+2] cycloadditions of ynamide firmly
with the azide derived from bromide 23 could also be established, we examined two potential approaches
improved to 80% (entry 4 versus 5).
for achieving double cycloadditions to construct novel
bis-triazoles. Two bis-triazoles could be quickly as-
sembled in one-pot with good yields by reacting yn-
amide 19 with di-azides generated in situ from dibro-
mides 39a and 39b and NaN3, thereby providing the
Syntheses of Uniquely Fused Triazoles
In our previous communication,[14] we had demon- feasibility of the first approach (Scheme 4).
strated that the Huisgen azide-[3 + 2] cycloaddition
To explore the scope of the bis-triazole synthesis,
of internal ynamides was also feasible under thermal we achieved the synthesis of novel bis-ynamides 42a
condition. As shown in Scheme 3, n-hexyl-substituted and 42b by coupling the respective dialkynyl bro-
internal ynamide 34 could react with BnN3 at 1408C mides 41a and 41b with Evans auxiliaries in good
in toluene to give 1,4-disubstituted triazole 35 in 58% yields (Scheme 5). Another approach to the bis-yn-
yield with no observable 1,5-adduct 36.
amide synthesis was the coupling of bis-amides with
monoalkynyl bromide, which was demonstrated in
Scheme 5. We successfully synthesized two terminal
bis-ynamides 45 and 46 in reasonable overall yields
through a two-step sequence: 1) coupling of TIPS
substituted alkynyl bromide with bi-sulfonamide 43 or
urethane 44, and 2) the removal of the TIPSgroup
using TBAF. Although bis-ynamides 42a, 42b, 45 and
46 are structurally novel, Diederich[7r] had prepared
some related ynamides.
With these bis-ynamides in hand, we examined the
second approach to bis-triazoles (Scheme 6). Ther-
mally driven [3 + 2] cycloadditions of internal bis-
ynamides 42a and 42b with BnN3 succeeded in giving
bis-triazoles 47a and 47b in moderate yields, while the
Scheme 4. One-pot syntheses of bistriazoles from dibro-
mides.
Scheme 5. Preparations of novel bis-ynamides.
Adv. Synth. Catal. 2006, 348, 2437 – 2442
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
2439

Xuejun Zhang et al.
FULL PAPERS
Scheme 6. Syntheses of bis-triazoles from bis-ynamides.
Triazole 10 with Conditions B
Fokin–Sharpless copper salt-catalyzed cycloadditions
of terminal bis-ynamides 45 and 46 proceeded well to
To a vial charged with ynamide 9 (15.0 mg, 0.070 mmol)
afford bis-triazoles 48 and 49, respectively, in good were added CuBr (1.8 mg, 0.0035 mmol, 0.20 equivs.), BnN3
(13.0 mL,
0.098 mmol,
1.40 equivs.),
19.5 mL
Et3N
yields.
(0.14 mmol, 14.1 mg, 2.0 equivs.) and CH3CN (1 mL). The
reaction mixture was then vigorously stirred at room tem-
perature for 12 h under nitrogen. After TLC showed that
ynamide 9 was all consumed, the reaction mixture was dilut-
ed with H2O (4 mL) and extracted with EtOAc (3”5 mL).
The combined organic layers were washed with saturated
aqueous NaCl and dried over Na2SO4. Removal of solvent
under reduced pressure gave crude triazole 10, which was
purified via silica gel flash column chromatography with
EtOAc/hexane as gradient eluent to yield the pure triazole
10 as a white solid; yield: 12.6 mg (56%).
Conclusions
We have described here a one-pot regioselective syn-
thesis of amide-substituted triazoles from alkyl bro-
mides and ynamides, and the synthesis of novel bis-
ynamides and their applications to synthesize unique
bis-triazoles.
General Procedure for Triazoles in Table 1
Experimental Section
A stock solution of 0.5M NaN3 in DMSO was prepared by
stirring at room temperature for 3 h. To a vial charged with
1.40 equivs. of 0.5M NaN3 in DMSO solution was added
1.30 equivs. of an alkyl bromide. After the mixture had been
stirred for 12 h, and TLC showed that alkyl bromide was all
consumed, H2O (1/5 the total volume of DMSO used
above) was added followed by 1.00 equiv. of the correspond-
ing ynamide, 0.10 equiv. of CuSO4·5 H2O and 0.10 equiv. of
sodium ascorbate. After TLC showed the starting ynamide
was all consumed, the reaction mixture was poured into
dilute aqueous NH4OH, and then extracted by EtOAc (3”
equal volumes). The combined organic layers were washed
with saturated aqueous NaCl and dried over Na2SO4. Re-
moval of solvent under reduced pressure gave crude tria-
zole, which was purified via silica gel column flash chroma-
tography with EtOAc/hexane as gradient eluent.
Triazole 7a with Conditions A
To a vial charged with ynamide 5 (50.0 mg, 0.267 mmol)
were
0.050 equivs.), Na ascorbate (2.70 mg, 0.0134 mmol,
0.050 equivs.), benzyl azide (37.0 mL, 0.294 mmol,
added
CuSO4·5 H2O
(3.3 mg,
0.0134 mmol,
1.10 equivs.), and tert-butyl alcohol (0.5 mL). The mixture
was stirred at room temperature for 2 min before H2O
(1 mL) was added. The reaction mixture was then vigorously
stirred at room temperature for 12 h under nitrogen. After
TLC showed that ynamide 5 was all consumed, the reaction
mixture was diluted with H2O (8 mL) and extracted with
EtOAc (3”10 mL). The combined organic layers were
washed with saturated aqueous NaCl and dried over
Na2SO4. Removal of solvent under reduced pressure gave
crude triazole 7a, which was purified via silica gel flash
column chromatography with EtOAc/hexane as gradient
eluent to yield the pure triazole 7a as a white solid; yield:
70.0 mg (82%).
Thermal Cycloadditions
To a sealed tube charged with an ynamide (1.00 equiv.) in
toluene solution was added BnN3 (1.30 equivs.). The reac-
tion mixture was heated at 1408C for 14 h. Removal of tolu-
ene under reduced pressure gave the crude triazole, which
was purified via silica gel flash column chromatography with
2440
www.asc.wiley-vch.de
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 2437 – 2442

Azide-[3 + 2] Cycloadditions of Ynamides and Bis-Ynamides
FULL PAPERS
using EtOAc/hexane as gradient eluent to yield the pure tri-
azole.
Charcterization data for triazoles and bis-triazoles pre-
pared can be found in the Supporting Information.
[7] For recent reports on ynamides in the last three years,
see: a) J. R. Dunetz, R. L. Danheiser, J. Am. Chem.
Soc. 2005, 127, 5776; b) N. Riddell, K. Villeneuve, W.
Tam, Org. Lett. 2005, 7, 3681; c) Y. Zhang, Tetrahedron
Lett. 2005, 46, 6483; d) M. F. Martinez-Esperon, D. Ro-
driguez, L. Castedo, C. Saµ, Org. Lett. 2005, 7, 2213;
e) M. Bendikov, H. M. Duong, E. Bolanos, F. Wudl,
Org. Lett. 2005, 7, 783; f) F. Marion, J. Coulomb, C.
Courillon, L. Fensterbank, M. Malacria, Org. Lett.
2004, 6, 1509; g) M. Rosillo, G. Domínguez, L. Casarru-
bios, U. Amador, J. PØrez-Castells, J. Org. Chem. 2004,
69, 2084; h) S. Couty, B. LiØgault, C. Meyer, J. Cossy,
Org. Lett. 2004, 6, 2511; i) D. Rodríguez, L. Castedo, C.
Saµ. Synlett 2004, 783; j) D. Rodríguez, L. Castedo, C.
Saµ, Synlett 2004, 377; k) S. Hirano, R. Tanaka, H.
Urabe, F. Sato, Org. Lett. 2004, 6, 727; l) M. Klein, B.
Kçnig, Tetrahedron 2004, 60, 1087; m) F. Marion, C.
Courillon, M. Malacria, Org. Lett. 2003, 5, 5095; n) B.
Witulski, C. Alayrac, L. Tevzaadze-Saeftel, Angew.
Chem. Int. Ed. 2003, 42, 4257; o) R. Tanaka, S. Hirano,
H. Urabe, F. Sato, Org. Lett. 2003, 5, 67; p) B. Witulski,
J. Lumtscher, U. Bergstrꢂber, Synlett 2003, 708; q) S.
Naud, J.-C. Cintrat, Synthesis 2003, 1391; r) F. De-
nonne, P. Paul Seiler, F. Diederich, Helv. Chim. Acta
2003, 86, 3096; s) see ref.[4] for chemistry before 2003.
[8] For our work on ynamides since 2003, see: a) K. C. M.
Kurtz, R. P. Hsung, Y. Zhang, Org. Lett. 2006, 8, 231;
b) K. C. M. Kurtz, M. O. Frederick, J. A. Mulder, R. P.
Hsung, Tetrahedron 2006, 62, 3928; c) Y. Zhang, R. P.
Hsung, X. Zhang, J. Huang, B. W. Slafer, A. Davis,
Org. Lett. 2005, 7, 1047; d) M. R. Tracey, Y. Zhang,
M. O. Frederick, J. A. Mulder, R. P. Hsung, Org. Lett.
2004, 6, 2209; e) L. Shen, R. P. Hsung, Tetrahedron
Lett. 2003, 44, 9353; f) M. O. Frederick, R. P. Hsung,
R. H. Lambeth, J. A. Mulder, M. R. Tracey, Org. Lett.
2003, 5, 2663; g) J. A. Mulder, K. C. M. Kurtz, R. P.
Hsung, H. A. Coverdale, M. O. Frederick, L. Shen,
C. A. Zificsak, Org. Lett. 2003, 5, 1547.
Acknowledgements
The authors thank NIH-NIGMS [GM066055], UW-Madison,
and the UW-Cancer Center for financial support.
References
[1] For general reviews, see: a) I. Coldham, R. Hufton,
Chem. Rev. 2005, 105, 2765; b) K. Rueck-Braun,
T. H. E. Freysoldt, F. Wierschem, Chem. Soc. Rev. 2005,
34, 507; c) A. I. Kotyatkina, V. N. Zhabinsky, V. A.
Khripach, Russ. Chem. Rev. 2005, 34, 507; d) M. Har-
mata, P. Rashatasakhon, Tetrahedron 2003, 59, 2371;
e) H. M. L. Davies, in: Advances in Cycloaddition,
(Ed.: M. Harmata), JAI Press, Greenwich, 1998, Vol. 5,
pp. 119–164; f) K. V. Gothelf, K. A. Jørgensen, Chem.
Rev. 1998, 98, 863; g) F. G. West, in: Advances in Cyclo-
addition, (Ed.: M. Lautens), JAI Press, Greenwich,
1997, Vol. 4, p. 1; h) J. H. Rigby, F. C. Pigge, Org.
React. 1997, 51, 351; i) A. Padwa, in: Comprehensive
Organic Synthesis (Ed.: B. M. Trost), Pergamon Press,
Oxford, 1991, Vol. 4, pp. 1069–1109.
[2] For related reviews, see: a) J. A. Varela, C. Saµ, Chem.
Rev. 2003, 103, 3787; b) M. Rubin, A. W. Sromek, V.
Gervorgyan, Synlett 2003, 2265; c) C. Aubert, O. Bui-
sine, M. Malacria, Chem. Rev. 2002, 102, 813; d) S.
Saito, Y. Yamamoto, Chem. Rev. 2000, 100, 2901.
[3] For a review, see: W.-Q. Fan, A. R. Katritzky, in: Com-
prehensive Heterocyclic Chemistry, (Eds.: A. R. Katritz-
ky, C. W. Rees, E. F. V. Scriven), Pergamon Press,
Oxford, 1996, Vol. 4, pp. 101–126.
[4] For reviews on ynamides, see: a) C. A. Zificsak, J. A.
Mulder, R. P. Hsung, C. Rameshkumar, L.-L. Wei, Tet-
rahedron 2001, 57, 7575; b) J. A. Mulder, K. C. M.
Kurtz, R. P. Hsung, Synlett 2003, 1379; c) A. R. Katritz-
ky, R. Jiang, S. K. Singh, Heterocycles 2004, 63, 1455;
d) M. R. Tracey, R. P. Hsung, J. Antoline, K. C. M.
Kurtz, L. Shen, B. W. Slafer, Y. Zhang, in: Science of
Synthesis Houben-Weyl Methods of Molecular Transfor-
mations, (Ed.: S. M. Weinreb), Georg Thieme Verlag,
Stuttgart, Chapter 21.4, 2005.
[9] a) R. Huisgen, Angew Chem. 1963, 75, 604; b) R. Huis-
gen, G. Szeimies, L. Moebiu, Chem. Ber. 1967, 100,
2494; c) R. Huisgen, in: 1,3-Dipolar Cycloaddition
Chemistry, (Ed.: A. Padwa), Pergamon Press, Oxford,
1984, pp. 1–176.
[10] For recent reviews on azide-alkyne cycloadditions, see:
a) V. D. Bock, H. Hiemstra, J. H. Van Maarseveen, Eur.
J. Org. Chem. 2006, 51; b) A. R. Katritzky, Y. Zhang,
S. K. Singh, Heterocycles. 2003, 60, 1225; c) S. T. Abu-
Orabi, Molecule. 2002, 7, 302; d) H. C. Kolb, M. G.
Finn, K. B. Sharpless, Angew. Chem. Int. Ed. 2001, 40,
2004.
[11] For reviews on chemistry of organic azides, see: a) S.
Brꢂse, C. Gil, K. Knepper, V. Zimmermann, Angew.
Chem. Int. Ed. 2005. 44, 5188; b) C.-K. Sha, A. K. Mo-
hanakrishnan, Chemistry of Heterocyclic Compounds,
(Eds.: A. Padwa, W. H. Pearson), John Wiley, New
York, 2002, Vol. 59, Chapter 9, pp. 623–679.
[12] For some recent examples, see: a) V. D. Bock, R. Per-
ciaccante, T. P. Jansen, H. Hiemstra, J. H. van Maar-
seveen, Org. Lett. 2006, 919; b) F. Francesca Pagliai, T.
Pirali, E. D. Grosso, R. D. Brisco, G. C. Tron, G. Sorba,
A. A. Genazzani, J. Med. Chem. 2006, 49, 467; c) V. O.
Rodionov, V. V. Fokin, M. G. Finn, Angew. Chem. Int.
[5] For the synthesis of ynamides, see: a) M. O. Frederick,
J. A. Mulder, M. R. Tracey, R. P. Hsung, J. Huang,
K. C. M. Kurtz, L. Shen, C. J. Douglas, J. Am. Chem.
Soc. 2003, 125, 2368; b) Y. Zhang, R. P. Hsung, M. R.
Tracey, K. C. M. Kurtz, E. L. Vera, Org. Lett. 2004, 6,
1151; see also: c) J. R. Dunetz, R. L. Danheiser, Org.
Lett. 2003, 5, 4011; d) S. Couty, M. Barbazanges, C.
Meyer, J. Cossy, Synlett 2005, 906.
[6] For a special issue dedicated to the chemistry of yna-
mides, see: Tetrahedron-Symposium-In-Print: “Chemis-
try of Electron-Deficient Ynamines and Ynamides.”
Tetrahedron 2006, 62, Issue No. 16.
Adv. Synth. Catal. 2006, 348, 2437 – 2442
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
2441

Xuejun Zhang et al.
FULL PAPERS
Ed. 2005, 44, 2210; d) L. Zhang, X. Chen, P. Xue,
H. H. Y. Sun, I. D. Williams, K. B. Sharpless, V. V.
Fokin, G. Jia, J. Am .Chem. Soc., 2005, 127, 15998;
e) F. Himo, T. Lovell, R. Higraf, V. V. Rostovstsev, L.
Noodleman, K. B. Sharpless, V. V. Fokin, J. Am. Chem.
Soc . 2005, 127, 210; f) J. F. Billing, U. Nilsson, J. Org.
Chem. 2005, 70, 4847; g) V. Gracias, D. Darczak, A. F.
Gasiecki, S. W. Djuric, Tetrahedron. Lett. 2005, 46,
9053; h) H. Yanai, T. Taguchi, Tetrahedron. Lett. 2005,
46, 8639; i) C. Chowdhury, S. B. Mandal, B. Achari, Tet-
rahedron. Lett. 2005, 46, 8531; j) Y. Angell, K. Burgess,
J. Org. Chem. 2005, 70, 9595.
A. K. Feldman, A. K. Nugent, C. J. Hawker, A. Scheel,
B. Voit, J. Pyun, J. M. J. Frechet, K. B. Sharpless, V. V.
Fokin, Angew. Chem. Int. Ed. 2004, 43, 3928; c) Q.
Wang, T. R. Chan, R. Hilgraf, K. B. Sharpless, V. V.
Fokin, J. Am. Chem. Soc. 2003, 125, 3192; d) A. K.
Feldman, B. Colasson, V. V. Fokin, Org. Lett. 2004, 6,
3897.
[16] a) D. J. Hlasta, J. A. Ackerman, J. Org. Chem. 1994, 59,
6184; b) S. J. Howell, N. Spencer, D. Philp, Tetrahedron
2001, 57, 4945; c) J. Chen, J. Rebek, Jr., Org. Lett.
2002, 4, 327.
[17] E. F. V. Scriven, K. Turnbull, Chem. Rev. 1988, 88, 297.
[18] S. G. Alvarez, M. T. Alvarez, Synthesis 1997, 413.
[19] K. Kacprzak, Synlett 2005, 943.
[20] X. Zhang, Y. Zhang, J. Huang, R. P. Hsung, K. C. M.
Kurtz, J. Oppenheimer, M. E. Petersen, I. K. Sagama-
nova, M. R. Tracey, J. Org. Chem. 2006. 71, 4170.
[13] M. IJsselstijin, J.-C. Cintrat, Tetrahedron 2006, 62, 3837.
[14] X. Zhang, R. P. Hsung, L. You, Organic Biomol. Chem.
2006, 4, 2679.
[15] a) V. Rostovtsev, L. G. Green, V. V. Fokin, K. B. Sharp-
less, Angew. Chem. Int. Ed. 2002, 41, 2596; b) P. Wu,
2442
www.asc.wiley-vch.de
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 2437 – 2442