Usual and unusual reactions of cyclohexane-1,2-dione with aryl azides and amines: a structural corrigendum

Article abstract of DOI:10.1039/C6NJ03835B

The reported syntheses of alleged functionalised 1,2,3-triazines from cyclohexane-1,2-dione and aryl azides, in the presence of pyrrolidine and other amines, were repeated. The products do not contain the bicyclic triazine and bicyclic ketone moieties; instead, cyclohexane-fused 4,5-dihydro-1,2,3-triazoles and monocyclic β-ketoamides were obtained, respectively. These corrections are well supported by careful analyses of NMR spectra, IR spectra, elemental analysis and comparison with data which were previously published in the literature. Suitable mechanisms are discussed for the synthesis of the observed compounds.

Full text of DOI:10.1039/C6NJ03835B

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Letter
Usual and unusual reactions of cyclohexane-1,2-dione with aryl
azides and amines: a structural corrigendum†‡
Received 00th December 2016,
Accepted 00th December 2016
Neeraj Singh and Klaus Banert*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The reported syntheses of alleged functionalised 1,2,3-triazines
from cyclohexane-1,2-dione and aryl azides, in the presence of
pyrrolidine and other amines, were repeated. The products do not
contain the bicyclic triazine and bicyclic ketone moieties, instead,
cyclohexane-fused 4,5-dihydro-1,2,3-triazoles and monocyclic β-
ketoamides were obtained, respectively. These corrections are
well supported by careful analyses of NMR spectra, IR spectra,
elemental analysis and comparision with data which were
previously published in literature. Suitable mechanisms are
discussed for the synthesis of the observed compounds.
a)
O
N
OH
NAr
OH
NAr
N
X
X
O
O
N
N
N
1a m
2a h; 2n
amine
base
/THF
/CHCl3
N
H
+
ArN3
O
O
N
NHAr
Ar
N
N2
N
O
The azido group, although absent in natural products, is one of
the most versatile entities, which has found immense
application, not only in synthetic organic chemistry,1 but also
in chemical biology2 and material science.3 Particularly, the
synthesis of triazoles by a cycloaddition reaction between
azides and a C=C/C≡C bond, first observed by A. Michael in
1893,4 intensively investigated and propagated with the
concept of 1,3-dipolar cycloadditions by R. Huisgen,5 and
finally revolutionised by copper(I)-catalysed click chemistry,6 is
one of the most utilised reactions in present research.
N
H
3a m
4a h; 2n
a Ar = Ph; b Ar = p-C6H4-OMe; c Ar = m-C6H4-OMe; d Ar = o-C6H4-OMe;
e Ar = p-C6H4-Me; f Ar = p-C6H4-Br; g Ar = p-C6H4-F; h Ar = m-C6H4-F;
i A = p-C6H4-Cl; j Ar = p-C6H4-CO2Et; k Ar = m-C6H4-Cl; l Ar = 3-pyridyl;
m Ar = 3,5-dimethylphenyl; n Ar = p-C6H4-CF3
b)
O
O
O
O
NR2
NR2
NHR2
ArN3
N
N
N
As part of our current interests in the unusual reactions of
azido compounds,7 we became interested in a report by W. Li
et al., wherein the synthesis of bicyclic 1,2,3-triazines 1a−m
A
Ar
B
and 2a−h, 2n were described, although these compounds were
2a h; 2n
1a m
X
X
named as "highly substituted 1,2,3-triazoles" throughout the
paper including the title (Scheme 1a).8 The products 1a−m
were claimed to be formed from aryl azides, cyclohexane-1,2-
dione and pyrrolidine in THF with 54−95% yield. In the whole
article and in the corresponding Electronic Supplementary
Information, heterocycles 1a−m are always depicted and
addressed without any anion. This gives the impression that
positively charged iminium species were isolated (without
Scheme 1 a) Claimed8 and now revised structures resulting from the reaction of
cyclohexane-1,2-dione with pyrrolidine and diethylamine; b) mechanism
suggested by the authors8 to explain the incorrect structures 1 and 2.
anion!). Adducts 2a−h and 2n were said to result from
cyclohexane-1,2-dione, aryl azides and diethylamine in
chloroform with 51−72% yield. Lower yields of such 1,2,3-
triazines were supposedly observed, when the substrates were
reacted in the presence of other bases, instead of
diethylamine, such as DABCO or DBU, which cannot generate
enamines with ketones. The products 1a−m, 2a−h and 2n were
1
isolated as yellow oils or yellow solids and characterised by H
NMR, 13C NMR as well as by HR-MS. The authors explained the
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formation of compounds 1a−m through formation of enamine Table 1. Comparision of the data of putative triazine 1b and triazole 3b
.
Supposed
cyclohexane-1,2-dione, pyrrolidine and
1-azido-4-methoxybenzene
product
1b8
from
Product 3b from cyclohexane-1,2-
dione, pyrrolidine and 1-azido-4-
methoxybenzene
A
and subsequent attack of the carbon nucleophile
A
at the
terminal nitrogen of the azide to give intermediate
B, which
then undergoes intramolecular cyclisation (Scheme 1b).
OCH3
Formation of ketones 2a−h and 2n was claimed to be a result
of the hydrolysis of the corresponding compounds of type
However, no such experiment was described to transform the
iminium (salts?) into the ketones
We had serious doubts about the asserted structures for
1.
O
N
N
H
N
1
2.
N
H
compounds 1a−m
,
2a−h and 2n owing to various
Yellow solid; yield (87%); mp N.A.
Yellow solid; yield (41%); mp
103−104 °C; purified by flash
chromatography (Rf = 0.41; SiO2; Et2O
: n-pentane 1:1).
contradictions. Firstly, the mechanism invoked8 is not in
harmony with the general principles of reactivity: Nα of the
azide fragment in intermediate
1H NMR (500 MHz, CDCl3):
1.46
−1.51 (m, 1H), 1.61−1.76 (m, 6H),
δ
=
1H NMR (400 MHz, CDCl3):
δ =
B should attack the more
electrophilic iminium carbon instead of the entropically
disfavoured carbonyl group. Additionally, it is well established
that organic azides regioselectively cycloadd at enamines to
yield 5-amino-4,5-dihydro-1,2,3-triazoles.9 Even the 1,3-dipolar
cycloaddition of phenyl azide at a 2-aminocyclohex-2-en-1-
1.41−1.52 (m, 1H), 1.59−1.76 (m,
6H), 2.16−2.36 (m, 3H), 2.38−2.44
(m, 2H), 2.59−2.64 (m, 2H), 3.75 (s,
3H), 4.78 (m, 1H), 6.81 (d, 3J = 9.2
Hz), 7.41 (d, 3J = 9.2 Hz, 2H).
2.18−2.34 (m, 3H), 2.40−2.44 (m, 2H),
2.62−2.64 (m, 2H), 3.76 (s, 3H),
4.79−4.81 (m, 1H), 6.83 (d, J = 10 Hz,
2H), 7.43 (d, J = 10 Hz, 2H).
13C NMR (125 MHz, CDCl3): δ = 17.1,
24.0, 26.4, 37.3, 45.8, 55.4, 80.6, 83.0,
114.4, 118.7, 133.0, 156.5, 204.0
13C NMR (100 MHz, CDCl3): δ = 17.01
(t, CHCH2), 23.94 (t, NCH2CH2, 2 x
CH2), 26.34 (t, COCH2CH2), 37.24 (t,
one, which is very similar to intermediate A, was reported to
lead to the corresponding five-membered heterocycle.10
Secondly, if compounds 1a−m exist as iminium salts, the
corresponding anion is completely unclear, and it is unlikely
that such polar substances can be purified by silica gel
chromatography as described in the paper8 of W. Li et al.
Furthermore, the fixed geometry of the pyrrolidinium unit in
COCH2), 45.66 (t, NCH2,
2 x CH2),
55.34 (q, OCH3), 80.52 (d, CH), 82.94
(s, NCN), 114.32 (d, C−2'), 118.61 (d,
C−3'), 132.90 (s, C−1'), 156.41 (s,
C−4'), 203.91 (s, C=O).
IR: N.A.
IR (CCl4, cm−1): 1726 (C=O).
HR-MS (ESI): m/z calcd. for C17H23N4O2
[M]+ 315.1816; found: 315.1815
HR-MS (ESI): m/z calcd. for
C17H23N4O2 [M
found: 315.1792
+
H]+ 315.1816;
the asymmetrical product
1 should be responsible for four
different 13C NMR signals (instead of only two signals).11
Therefore, the number of 13C NMR signals turned out to be
incorrect in all 13 cases of the 1,2,3-triazines 1a−m. Finally, the
O
N
OCH3
4'
H
N
N
1
2.75%
OH proton signal is missing in all the H NMR spectra of these
N
H
O
H
N
H
H
N
compounds.
2'
H
H
H
Thirdly, in case of bicyclic ketones 2a−h and 2n, the 13C NMR
signal of the C−OH bridgehead carbon atom should resonate at
δ ≈ 80−100 ppm.12 However, the 13C NMR data, which were
reported to characterise these triazines,8 did not include any
signal with the expected chemical shift. On the other hand, a
13C NMR signal at δ ≈ 164−165 ppm was observed for all
H
N
N
2.75%
OCH3
3b'
3b
Scheme 2 1D NOESY experiment result to verify the formation of regioisomer 3b
and to exclude 3b'.
products with the claimed structure of the heterocycle 2
.8 But
such a signal is incompatible with this structure and can, for
example, be assigned to a carboxamide unit. Moreover, a
hand, all data are in excellent agreement with the structure of
4,5-dihydro-1H-1,2,3-triazole 3b. Such a compound bears a C2-
symmetric pyrrolidine ring if we assume that rapid inversion of
the nitrogen atom and rotation around the bridgehead-
carbon/nitrogen axis occur. Consequently, only two 13C NMR
signals result from the pyrrolidine unit of 3b; however, the
four corresponding protons at C-2 and C-5 give two different
1
broad singlet at δ ≈ 8.7 ppm was found in all H NMR spectra
of alleged products 2a−h and 2n, and this signal can only be
attributed to an OH or NH group. However, the chemical shift
is unusual for an alcohol and more likely for the NH proton of a
carboxamide unit.
In order to clarify the structures of compounds 1a−m, 2a−h
1
signals in the H NMR spectrum (δ = 2.38−2.44, δ = 2.59−2,64)
and 2n, we first re-investigated the reaction of cyclohexane-
1,2-dione, pyrrolidine and 1-azido-4-methoxybenzene in THF.
After chromatography with silica gel using a 1:1 mixture of
diethyl ether and pentane, the identical substance with the
same NMR data was obtained (Table 1). We also found
thirteen 13C NMR signals, which is not compatible with the
structure of 1b. Therefore, the real product cannot include an
unsymmetrical pyrrolidine ring that leads to four 13C NMR
signals. Furthermore, our IR spectrum of the product indicated
a ketone (1726 cm−1), but a signal of a hydroxy group was
absent. This excludes the structure of 1b as well. On the other
because of the asymmetric (chiral) bicyclic moiety of the
molecule. Our structure assignment was further confirmed by
gCOSY, gHSQCAD and CIGAR (8−3 Hz) NMR experiments.13
Moreover, the 1H NMR spectrum of the compound 3b was
highly compatible with that of a similar cycloaddition product,
which resulted from treatment of 2-morpholinocyclohex-2-
enone with phenyl azide.10 Finally, we conducted 1D NOESY
experiments to confirm the regioselectivity in the formation of
3b and the cis stereochemistry of the bicyclic substructure. No
nuclear Overhauser effect was observed between the
2 | New. J. Chem., 2016, 00, 1-4
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bridgehead proton and 2-H’ of the aryl group, which excludes
the alternative structure 3b’ (Scheme 2). On the other hand,
the strong effect between the pyrrolidine protons (2-H/5-H)
and the bridgehead proton provides evidence of the cis-fused
a)
O
O
O
O
OH
N
N
H
bicyclic structure of 3b
.
Aditionally, we also studied the reaction of cyclohexane-1,2-
ArN3
dione with p-tolyl azide in the presence of pyrrolidine/THF and
isolated the product 3e instead of 1e 13
. A possible mechanism
for the formation of 3a−m is detailed in Scheme 3a. It is well
known in the literature that cyclohexane-1,2-dione and
secondary amines lead to 2-aminocyclohex-2-enones.14
Subsequent 1,3-dipolar cycloaddition to yield 3a-m may
O
O
N
N
Ar
Ar
N
N
N
N
N
N
3a m
C
include intermediate
C or proceed in a synchronous
mechanism.
b)
Moreover, the described8 reaction of cyclohexane-1,2-dione
with 1-azido-4-methoxybenzene and diethylamine in
chloroform was also repeated. Again, we isolated the same
substance, which showed identical NMR data when compared
to the previous report (Table 2).8 However, our additional IR
O
O
O
O
OH
O
H
1. ArN3
2. H
O
O
OH
OH
Ar
NAr
N
Table 2. Comparision of the data of supposed ketone 2b and ketoamide 4b
Supposed product 2b8a from
.
N
Product 4b15b
Product
4b
from
N
N2
cyclohexane-1,2-dione,
cyclohexane-1,2-dione,
diethylamine and 1-
azido-4-
E
D
diethylamine and 1-azido-4-
methoxybenzene
O
NHAr
methoxybenzene
N2
O
4a h, 4n
Scheme 3 a) Mechanism of formation of triazoles 3a m; b) formation of keto-
amides 4a h and 4n.
spectrum indicated the carbonyl groups of a ketone (1728
cm−1) and an amide (1679 cm−1), and an NH signal at 3341 cm−1
can also be assigned to an amide. These substructures are
supported by the 1H NMR data, which include a broad
exchangeable NH Proton at δ = 8.63, and by two carbonyl
carbon signals in the 13C NMR spectrum (ketone δ = 217.11,
amide δ = 164.23). Most importantly, the HR-MS data and the
elemental analysis excluded the claimed8 formula C13H15N3O3
(or C13H16N3O3 for M+H) and provided evidence for the real
formula C13H15NO3. Therefore, it is obvious that the formation
of the product is accompanied by loss of dinitrogen, and the
structure 2b is wrong. We came to the conclusion that this
Yellow solid; yield (71%); mp
N.A.
mp 139 °C15e
Yellow
(47%); mp 137 °C;
purified by flash
solid;
yield
chromatography (Rf
=
0.41; SiO2; Et2O).
1H NMR (500 MHz, CDCl3): δ =
1.83−1.89 (m, 1H), 2.06−2.11
(m, 1H), 2.31−2.44 (m, 4H),
3.13 (t, J = 10 Hz, 1H), 3.78 (s,
3H), 6.85 (d, J = 10 Hz, 2H),
7.44 (d, J = 10 Hz, 2H), 8.60
(br., 1H).
1H NMR (400 MHz,
1H NMR (400 MHz,
CDCl3):
δ
=
1.80−1.95
CDCl3): δ = 1.81−1.92
(m, 1H), 2.05−2.15 (m,
1H), 2.30−2.50 (m, 4H),
3.18 (t, 1H), 3.80 (s, 3H),
6.82 (d, 2H), 7.42 (d,
2H), 8.64 (s, 1H).
(m, 1H), 2.05−2.14 (m,
1H), 2.30−2.45 (m, 4H),
3.14 (t, 3J = 9.4 Hz), 3.79
(s, 3H), 6.85 (d, 3J = 8.8
Hz, 2H, 7.44 (d, 3J = 8.8
Hz), 8.63 (br. s, 1H, NH).
13C NMR (100 MHz,
13C NMR (125 MHz, CDCl3): δ =
20.2, 25.7, 39.1, 54.5, 55.5,
114.1, 121.5, 130.9, 156.3,
164.2, 217.0
N.A.
CDCl3):
δ = 20.21 (t,
CH2), 25.73 (t, CH2),
39.11 (t, CH2), 54.48 (d,
CH), 55.44 (q, OCH3),
114.05 (d, C−2' or C−3'),
121.50 (d, C−2' or C−3'),
130.83 (s, Cquat), 156.30
(s, Cquat), 164.23 (s,
CONH), 217.11 (C=O).
structure has to be corrected to the β-ketoamide 4b, which is a
known15b,e compound. Our data of 4b are identical with those
that were previously published (Table 2).15b,e Especially, the
triplet signal of 2-H (δ = 3.14, J = 9.4 Hz) is highly characteristic
in all 1H NMR spectra of products of type
4. We also
Anal. calcd. for N.A.
Anal.15e
C13H15NO3:
calcd.
66.94,
for
H
C
Anal.
C13H15NO3:
calcd.
66.94,
for
H
synthesised compound 4e from cyclohexane-1,2-dione and p-
tolyl azide in the presence of diethylamine in chloroform.13
Furthermore, we prepared 4e for comparision, additionally,
from ethyl 2-oxocyclopentanecarboxylate and p-toluidine.13
A quick literature search for compounds like 4b revealed
C
C
6.48, N 6.00; found:
67.3, H 6.3, N 6.9
N.A.
6.48, N 6.00; found: C
66.31, H 6.39, N 5.97
IR: N.A.
IR (CHCl3, cm−1): 1728
(C=O), 1679 (CONH),
3341 (NH).
HR-MS (ESI): m/z calcd. for
HR-MS (ESI): m/z calcd.
that all the compounds 4a−h
are known in literature,15 only 4n
C13H16N3O3 [M]+ 262.1192;
for C13H15NNaO3 [M +
is unknown.16 The 1H NMR and 13C NMR spectra of these
literature-known compounds are in perfect harmony with our
own results and with the data which were reported8 for the
found: 262.1196
Na]+ 256.0944; found:
256.0934
alleged 1,2,3-triazines
2. However, we cannot easily explain
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the strong difference in the HR-MS data of the real products
4
Fokin and K. B. Sharpless, Angew. Chem. Int. Ed., 2002, 41,
and the claimed substances
authors8 include impurities with higher molecular masses.
Scheme 3b represents the possible mechanism leading to 7. (a) K. Weigand, N. Singh, M. Hagedorn and K. Banert, Org.
the formation of -ketoamides 4a Amine-induced Chem. Front. 10.1039/c6qo00625f; (b) K. Banert, M. Hagedorn,
and 4n 17
deprotonation of cyclohexane-1,2-dione,18 which mainly exists C. Hemeltjen, A. Ihle, K. Weigand and H. Priebe, ARKIVOC,
2
. Perhaps, the substances of the 2597−2599; (c) C. W. Tornøe, C. Christensen and M. Meldal, J.
Org. Chem. 2002, 67, 3057−3064.
β
−h
.
as 2-hydroxycyclohex-2-enone, and subsequent regioselective 2016,
stepwise or synchronous 1,3-dipolar cycloaddition followed by 2361−2375.
reprotonation lead to triazole . The cleavage of the weak 8. (a) X. Xu, Z. Shi and W. Li, New. J. Chem., 2016, 40
N−N bond generates the zwitterion
with simultaneous ring contraction to form
and 4n. Thus formation of an enamine is not necessary. This is 9. (a) R. Fusco, G. Bianchetti and D. Pocar, Gazz. Chim. Ital.
compatible with the fact that is also produced in the 1961, 91, 849−865; (b) R. Fusco, G. Biancheꢀ, D. Pocar and R.
presence of other bases, such as DABCO or DBU, which cannot Ugo, Chem. Ber., 1963, 96, 802−812; (c) G. Biancheꢀ, D. Pocar,
react with ketones to give enamines. P. D. Croce and A. Vigevani, Chem. Ber., 1965, 98, 2715−2724;
v
,
338−361; (c) K. Banert, Synthesis, 2016, 48
,
D
,
E
, that loses dinitrogen 6559−6563; (b) Z. Shi, W. Li, X. Xu and X. Zhou, Faming Zhuanli
-ketoamides 4a−h Shenqing, 2016, CN 105646455 A 20160608.
β
4
In summary, we have undoubtedly demonstrated that the (d) R. Huisgen, G. Szeimies and L. Möbius, Chem. Ber. 1967,
reaction of cyclohexane-1,2-dione and aryl azides in the 100, 2494−2507; (e) G. Bianchetti, P. D. Croce and D. Pocar,
presence of secondary amines does not lead to bicyclic 1,2,3- Tetrahedron Lett., 1965, 2039−2041.
triazines
5-amino-4,5-dihydro-1H-1,2,3-triazoles
oxocyclopentanecarboxamides , respectively. The corrected 11. For NMR data of other comparable iminium salts, see: (a)
structures of and are in complete agreement with our H. Guan, M. Iimura, M. P. Magee, J. R. Norton and G. Zhu, J.
spectroscopic data, and in case of known compounds , these Am. Chem. Soc., 2005, 127, 7805−7814; (b) C. Rabiller, J. P.
data are identical with those of published15 product Renou and G. J. Martin, J. Chem. Soc., Perkin Trans. 2., 1977,
characterisations. The formation of corresponds to the well 536−541.
known 1,3-dipolar cycloaddition chemistry of enamines and 12. For 13C NMR data of comparable heterocycles with N and O
organic azides,9 whereas surprising generation of
includes atoms, see: K. Banert, J. Lehmann, H. Quast, G. Meichsner, D.
the decay of bicyclic 4,5-dihydro-1H-1,2,3-triazole Regnat and B. Seiferling, J. Chem. Soc., Perkin Trans. 2., 2002,
intermediate with carbocyclic ring contraction by migration of 126−134.
an acyl group. Compounds of type can more easily be 13. See Electronic Supporting Information (ESI) for further
1
and
2. Instead, we isolated the cyclohexane-fused 10. G. I. Polozov and I. G. Tishchenko, Vestn. Akad. Nauk Bel.
3
and the 2- SSR, Ser. Khim. Nauk, 1978, 62−69.
4
3
4
4
3
4
a
4
prepared by treatment of alkyl 2-oxocyclopentanecarboxylates details.
with anilines,19 in other cases, however, the new ring- 14. (a) Y. Matsumoto, R. Tsuzuki, A. Matsuhisa, K. Takayama, T.
contraction reaction of cycloalkane-1,2-diones and azides may Yoden, W. Uchida, M. Asano, S. Fujita, I. Yanagisawa and T.
be useful in organic synthesis.
Fujikura, Chem. Pharm. Bull., 1996, 44, 103−114; (b) U.
Kuckländer and B. Schneider, Chem. Ber., 1986, 119
,
Acknowledgements
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Food Chem., 2001, 49, 5383−5390; (d) N. S. Kozlov and V. N.
9, 2254−2256.
The authors would like to thank Dr. Manfred Hagedorn for the Kovaleva, Zh. Org. Khim., 1973,
measurement of CIGAR and 1D NOESY NMR spectra, and Ms. 15. For an alternative synthesis of 4a, see: (a) M. Presset, Y.
Jana Buschmann for providing the starting compounds.
Coquerel and J. Rodriguez, J. Org. Chem., 2009, 74, 415−418;
for 4b: (b) T. L. Andreana, S. S. Y. Cho, J. M. Graham, T. F.
Gregory, H. R. Howard, Jr., B. E. Kornberg, S. S. Nikam and D. A.
Pflum, PCT Int. Appl., 2004, WO 2004026864; for 4c: (c) M. M.
Notes and references
1. (a) Organic Azides, Syntheses and Applications, ed. S. Bräse Sanchez Duque, O. Baslé, N. Isambert, A. Gaudel-Siri, Y.
and K. Banert, Wiley, Chichester, 2010; (b) H. Tanimoto and K. Génisson, J.-C. Plaquevent, J. Rodriguez and T. Constantieux,
Kakiuchi, Nat. Prod. Commun., 2013,
Bräse, C. Gil, K. Knepper and V. Zimmermann, Angew. Chem. Pianka, J. Chem. Soc., 1947, 1420−1421; for 4b
Int. Ed., 2005, 44, 5188−5240; (d) E. F. V. Scriven and K. R. J. Brown, F. W. S. Carver and B. L. Hollingsworth, J. Chem.
8, 1021−1034; (c) S. Org. Lett., 2011, 13, 3296−3299; for 4d: (d) H. C. Barany and M.
,
4e and 4f: (e)
Turnbull, Chem. Rev., 1988, 88, 297−368.
Soc., 1961, 4295−4298; for 4g: (f) F. D. Popp, W. R. Schleigh, P.
M. Froehlich, R. J. Dubois and A. C. Casey, J. Org. Chem., 1968,
33, 833−834; for 4h: (g) S. Jaroch, H. Rehwinkel, P. Hölscher, D.
Sülzle, M. Hillmann, G. A. Burton and F. M. McDonald, PCT Int.
Appl., 1999, WO 9941240.
2. X. Zhang and Y. Zhang, Molecules, 2013, 18, 7145−7159.
3. J.-F. Lutz, Angew. Chem. Int. Ed., 2007, 46, 1018−1025.
4. A. Michael, J. Prakt. Chem. 1893, 48, 94−95.
5. (a) R. Huisgen, Angew. Chem. Int. Ed. Engl., 1963,
565−598; (b) R. Huisgen, Angew. Chem. Int. Ed. Engl., 1963,
633−645.
2
,
2,
16. For a related compound, see: K. Mohanan, Y. Coquerel and
J. Rodriguez, Org. Lett., 2012, 14, 4686−4689.
6. (a) H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem. Int. 17. We would also like to suggest that compounds 4a
−h and
Ed., 2001, 40, 2004−2021; (b) V. V. Rostovtsev, L. G. Green, V. V. 4n, should be stable in their anti conformation with an
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COMMUNICATION
intramolecular H-bond; for other examples, see: D. Mailhol, M.
M. Sanchez Duque, W. Raimondi, D. Bonne, T. Constantieux, Y.
Coquerel and J. Rodriguez, Adv. Synth. Catal., 2012, 354
3523−3532.
,
18. A. Feigenbaum, J.-P. Pete and A. Poquet-Dhimane,
Tetrahedron Lett., 1988, 29, 73−74.
19. (a) D. H. Johnson, J. Chem. Soc., 1958, 1624−1628; (b) B. K.
Blount, W. H. Perkin, Jr. and S. G. P. Plant, J. Chem. Soc., 1929,
1975−1987.
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