Gold(I)-catalyzed intermolecular [2+2] cycloadditions between allenamides and alkenes

Article abstract of DOI:10.1002/adsc.201200047

3-(Propa-1,2-dien-1-yl)oxazolidin-2-one works as an efficient two-carbon partner in a variety of intermolecular gold-catalyzed [2+2] cycloadditions to alkenes. The transformation represents a simple and practical entry to highly substituted cyclobutane derivatives and takes place with complete regio- and stereocontrol. Copyright

Full text of DOI:10.1002/adsc.201200047

COMMUNICATIONS
DOI: 10.1002/adsc.201200047
Gold(I)-Catalyzed Intermolecular [2+2]Cycloadditions between
Allenamides and Alkenes
Hꢀlio Faustino,a Paloma Bernal,a Luis Castedo,a Fernando Lꢁpez,b,*
and Josꢀ L. MascareÇasa,*
a
Departamento de Quꢂmica Orgꢃnica, Centro Singular de Investigaciꢁn en Quꢂmica Biolꢁgica y Materiales Moleculares.
Unidad Asociada al CSIC. Universidad de Santiago de Compostela, Avda. de las Ciencias, s/n, 15782 Santiago de
Compostela, Spain
Fax : (+34)-981-595-012; phone: (+34) 981-563-100; e-mail: joseluis.mascarenas@usc.es
Instituto de Quꢂmica Orgꢃnica General, CSIC, C/Juan de la Cierva 3, 28006 Madrid, Spain
b
Fax : (+34)-91-564-4853; phone: (+34)-91-562-2900; e-mail: fernando.lopez@iqog.csic.es
Received: January 17, 2012; Published online: June 5, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200047.
Abstract: 3-(Propa-1,2-dien-1-yl)oxazolidin-2-one
works as an efficient two-carbon partner in a variety
of intermolecular gold-catalyzed [2+2]cycloaddi-
tions to alkenes. The transformation represents
a simple and practical entry to highly substituted
cyclobutane derivatives and takes place with com-
plete regio- and stereocontrol.
of versatility and synthetic potential are the cycload-
ditions between p-unsaturated systems and allenes.[2]
Indeed, a variety of new gold-catalyzed intramolecu-
lar [4+3] and [4+2]cycloadditions of allenes and
dienes,[3,4] as well as [3+2] and [2+2]annulations to
alkenes,[5,6] have been recently developed.
In contrast to these intramolecular cases, the more
challenging intermolecular cycloadditions are ex-
tremely scarce.[7,8] In this context, we recently dis-
closed the first gold-catalyzed intermolecular cycload-
dition of non-activated 1,3-dienes (4C) and allen-
Keywords: allenamides; cycloaddition; cyclobu-
tanes; enamides; gold
AHCTUNGTRENNUNG
amides (2C),[7a] a type of allenic scaffold which is par-
ticularly versatile.[9] This [4 +2]cycloaddition takes
place with good yield and high selectivity using AuCl
During the last decade, homogeneous goldACHTUNTRGNEU(GN I or III) or the cationic gold(I) complex Au1/AgSbF6 as cata-
catalysis has experienced an extraordinary develop- lysts. However, in a number of cases, in addition to
ment.[1] The singular characteristics of gold complexes, the [4+2]adducts (3a, b), we also observed cyclobu-
such as their high carbophilicity and low propensity to tane side products arising from
a competitive
participate in typical metal redox processes, has al- [2+2]cycloaddition between the allene and one of the
AHCTUNGTRENNUNG
lowed the development of a variety of powerful and carbon-carbon double bonds of the diene (4a, b,
unique transformations. Particularly relevant in terms Scheme 1).[10]
Scheme 1. Previously reported [4+2]cycloadditions of allenamides and dienes. Isolation of [2+2]side adducts.
1658
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 1658 – 1664

Gold(I)-Catalyzed Intermolecular [2+2]Cycloadditions between Allenamides and Alkenes
Table 1. Preliminary screening of catalytic activity between 1 and 5a.[a]
Entry
[Au]
Temperature [8C]
6a ([%] yield)
7 ([%] yield)
8a ([%] yield)
1
2
3
4
AuCl
Au1/AgSbF6
Au3
Au4/AgSbF6
Au2/AgSbF6
Au2/AgSbF6
Au2/AgSbF6
r.t.
–
–
4
5
51
80
79
–
–
64
34
0
–
–
8
0
0
0
0
À15
r.t.
5
À15
À15
À15
6[b]
7[b,c]
0
0
[a]
Allene 1 (1 equiv.) was added dropwise to a mixture of 5a (3 equiv.) and the gold(I) catalyst (5 mol%) in CH2Cl2 (0.1M)
at À158C, unless otherwise noted; >99% conversions (1H NMR). Yields indicated correspond to isolated products.
Allenamide 1 was added dropwise over 1 h.
[b]
[c]
Reaction carried out with 2 mol% catalyst.
On this basis, and considering the synthetic and me- lectively produced the allene homodimer 7 in 64%
dicinal relevance of the cyclobutane framework,[11] we and 34% yields, respectively (Table 1, entries 3 and
decided to specifically pursue the development of 4).[14] Conversely, the p-acidic cationic phosphite
a
gold-catalyzed intermolecular [2+2]cycloaddi- gold(I) catalyst Au2/AgSbF6 selectively provided the
tion.[12] Herein, we demonstrate that the allenamide desired [2+2]cycloadduct 6a in a moderate 51%
1 participates in a variety of [2+2]cycloadditions yield (Table 1, entry 5). Gratifyingly, running the reac-
with different type of alkenes, in particular with en- tion in the presence of 4 ꢅ molecular sieves, and
amides and styrene derivatives, to provide excellent adding the allenamide to the reaction mixture over
yields of [2+2]adducts of type 6, with complete a period of one hour, led to a significant increase in
regio-, chemo- and stereoselectivity. Our technology the efficiency of the process, which now provided 6a
represents a significant addition to the armoury of in a good 80% yield (Table 1, entry 6). Moreover, the
catalytic cycloaddition methods and provides a partic- catalyst loading could be reduced without affecting
ularly practical, powerful and versatile manner to con- the rate and efficiency of the reaction (Table 1,
struct highly functionalized cyclobutanes.
entry 7). It is important to note that the cycloaddition
In order to assess the viability of a robust and selec- process took place with complete selectivity, since no
tive [2+2]process, we decided to check the reaction other regio- or stereoisomers could be detected by
conditions using as cycloaddition partners the allen- 1H NMR analysis of the crude reaction mixtures.
A
Once having established an optimum catalytic
namely trans-methylstyrene 5a (Table 1). Unfortu- system, the versatility and scope of the process was
nately, using the optimal conditions developed for the evaluated. As shown in Scheme 2, allenamide 1 also
[4+2]cycloaddition, we observed the formation of undergoes the cycloaddition reaction with styrene to
a relatively complex mixture of products (Table 1, en- provide, after just 5 min, the corresponding
tries 1 and 2). Nonetheless, in this mixture we could
ACHTUNGTREN[NGNU 2+2]adduct 6b in 81% yield. Methyl substituents at
identify traces of the desired [2+2]adduct 6a, togeth- the para, meta, and ortho positions of the phenyl ring
er with the homodimer 7 and the acyclic hydroalkeny- of 5 were perfectly tolerated, so the corresponding
lation product 8a.[13] The picolinic acid gold
G
ACHTUNGTREN[NGNU 2+2]adducts (6c–6e) could be isolated in good to ex-
ative Au3 or the biaryl phosphine-based catalyst Au4/ cellent yields (80–96%). The presence of a bromine
AgSbF6, also failed to give the desired adduct, but se- atom at the aromatic ring is also compatible with the
Adv. Synth. Catal. 2012, 354, 1658 – 1664
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1659

Hꢀlio Faustino et al.
COMMUNICATIONS
and 77% yields, the [2+2]cycloadducts 6g and 6h,
which incorporate one quaternary stereocenter. Inter-
estingly, the cycloaddition with the cis-styrene (Z)-5a
led to a 15% yield of a 1:1.2 mixture of 6a and its syn
isomer 6a’.[15] However, the cycloaddition with 1H-
indene provided the [2+2]adduct 6i in an excellent
82% yield. The stereochemical identity of all these
adducts (6a–6i) was determined by NMR analysis.
Additionally, the structure of 6a and 6i could be fur-
ther confirmed by X-ray diffraction analysis
(Figure 1).[16]
The requirement of an aromatic substituent at the
alkene was next investigated. Initially, we tested the
feasibility of the cycloaddition of 1 with methylenecy-
clohexene (Table 2, entry 1). Unfortunately, although
the desired cycloadduct 6j was detected, the major
product of the reaction was the acyclic compound 8j,
formally resulting from the hydrofunctionalization of
the allenamide unit with the alkene.[13] Both 6j and 8j
were isolated as an inseparable 1:6 mixture in
a global 78% yield.
The observation of allenamide cyclodimerization
side reactions (to 7) suggested the possibility of using
enamides as alkene components. Gratifyingly, enam-
ides 5k–5n are excellent cycloaddition partners, pro-
viding the corresponding cyclobutanic adducts with
complete selectivity and excellent yields (entries 2–
7).[17] Remarkably, as can be deduced from entries 3–
6, both (E) and (Z) isomers of enamides 5l and 5m
provided, with comparable efficiencies, a single ste-
reoisomer of the cycloadducts 6l and 6m, both featur-
ing a trans disposition of the methyl and carbamoyl
groups. These results clearly point out to a reaction
mechanism involving carbocationic intermediates, as
this could explain the observed loss of stereochemical
Scheme 2.
derivatives.
ACHTUNGTRENNUNG[2+2]Cycloaddition of allenamide 1 and styrene
reaction conditions, although the adduct 6f was isolat- information of the starting Z-enamides. Finally, the
ed in a modest 42% yield. phenylenamide derivative 5n also participated in the
Importantly, the cycloaddition also proceeded with reaction, leading to cycloadduct 6n, which was ob-
1,1-disubstituted alkenes such as 1-phenyl-1-cyclohex- tained with complete regio- and stereoselectivity and
ene and a-methyl styrene to selectively afford, in 74 good yield (entry 7).
Figure 1. X-ray structures of adducts 6a and 6i.[16]
1660
asc.wiley-vch.de
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 1658 – 1664

Gold(I)-Catalyzed Intermolecular [2+2]Cycloadditions between Allenamides and Alkenes
Table 2. Scope of the cycloaddition with other alkenes.[a]
Entry
1
Alkene
5
Products
Yield [%][b]
5j
6j+8j: 78
2
5k
6k: 76
3
4
5
(E)-5l
(Z)-5l
(E)-5m
6l: 91
6l: 96
6m: 73
6
7
(Z)-5m
6m: 71
6n: 78
5n
8
9
5o, R=H
5p, R=Boc
6o: 0; 8o: 75
6p: 33; 8p: 21
10
5q
6q: 94
[a]
Allene 1 (1 equiv.) was added dropwise over 1 h to a mixture of 5 (3 equiv.) and the gold(I) catalyst (2 mol%) in CH2Cl2
(0.1M) at À158C, unless otherwise noted; >99% conversions (1H NMR).
Isolated yields.
[b]
It was recently reported that indoles react with al- functionalization product 8o in 75% yield. However,
ACHTUNGTRENNUNG
lenamides such as 1 in the presence of [Ph3PAu]+ the reaction of its N-Boc analogue 5p afforded a low
complexes to provide hydrofunctionalization products but promising 33% yield of the desired [2+2]cycload-
of type 8o and p.[13b] In consonance, under our catalyt- duct 6p. On the other hand, the reaction with a cyclic
ic conditions, indole 5o reacted to provide the hydro- enamide such as 5q proceeded with excellent yield
Adv. Synth. Catal. 2012, 354, 1658 – 1664
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1661

Hꢀlio Faustino et al.
COMMUNICATIONS
Figure 2. X-ray structures of adducts 6k and 6q.[18]
Scheme 3. Mechanistic rationale for the [2+2]cycloadditions of allenamide 1 and alkenes.
and complete selectivity to provide the 4,6-fused bicy- through attack of the vinyl gold species to the stabi-
clic system 6q in 94% yield. The stereochemical as- lized cation, and elimination of the Au complex,
signment of 6k as well as that of 6q could be success- would yield the final [2+2] adduct of type 6. Alterna-
fully confirmed by X-ray analysis (Figure 2).[18]
tively, when the metal allyl cation intermediate I is at-
From a mechanistic perspective the reaction might tacked by another unit of allenamide 1, a second cat-
proceed through a stepwise cationic pathway such as ionic intermediate of type III could be formed. After
that shown in Scheme 3, at least in the case of the en- a ring closing process, this intermediate could give
amide partners. Thus, activation of the allene by the rise the homodimer adduct 7, observed in certain
Au catalyst would afford an Au-allyl cation species of cases.
type I.[3,4,7] Nucleophilic intermolecular interception of
We are currently further investigating the mecha-
I by the alkene would provide a second cationic inter- nistic aspects of the cycloaddition by theoretical and
mediate II. This would be the regioselectivity-deter- experimental means, and seeking an explanation for
mining step, with the formation of the more stabilized the different behaviour of (Z) and (E)-5a.
bencylic or imonium cation being favoured. At this
In conclusion, we have developed an efficient cata-
À
point, rotation around the sigma C C bond results in lytic [2+2]cycloaddition methodology that provides
the loss of the stereochemical information coming synthetically appealing cyclobutane derivatives in
from the alkene. Finally, a ring closing process a highly or completely selective manner. Work to de-
1662
asc.wiley-vch.de
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 1658 – 1664

Gold(I)-Catalyzed Intermolecular [2+2]Cycloadditions between Allenamides and Alkenes
Chem. Int. Ed. 2011, 50, 11496–11500. For a related Pt-
velop enantioselective variants and to gain a deeper
mechanistic understanding is underway.
catalyzed example, see: c) B. Trillo, F. Lꢁpez, M.
Gulꢂas, L. Castedo, J. L. MascareÇas, Angew. Chem.
2008, 120, 965–968; Angew. Chem. Int. Ed. 2008, 47,
951–954. For other examples, see: d) B. W. Gung, D. T.
Craft, Tetrahedron Lett. 2009, 50 2685–2687; e) B. W.
Gung, D. T. Craft, L. N. Bailey, K. Kirschbaum, Chem.
Eur. J. 2010, 16, 639–644; f) S. Montserrat, G. Ujaque,
F. Lꢁpez, J. L. MascareÇas, A. Lledꢁs, Top. Curr. Chem.
2011, 302, 225–248; g) S. Montserrat, I. Alonso, F.
Lꢁpez, J. L. MascareÇas, A. Lledꢁs, G. Ujaque, Dalton
Trans. 2011, 40, 11095–11105; h) I. Fernꢃndez, J. L.
MascareÇas, Org. Biomol. Chem. 2012, 10, 699–704;
i) I. Fernꢃndez, F. P. Cossio, A. de Cꢁzar, A. Lledꢁs,
J. L. MascareÇas, Chem. Eur. J. 2010, 16, 12147–12157;
j) see also ref.[4a]
Experimental Section
Representative Procedure for the [2+2]Cyclo-
addition of 1 with 5a
To a cooled solution (À158C) of trans-b-methylstyrene (5a,
162 mL, 1.25 mmol), AgSbF6 (2.86 mg, 8.31 mmol) and Au2
(7.31 mg, 8.31 mmol) in a dried Schlenk tube, was slowly
added a solution of allenamide 1 (52 mg, 0.426 mmol) in
CH2Cl2 (1 mL), over 1 hour. The mixture was additionally
stirred at À158C for 5 min and filtered through a short pad
of florisilꢆ, eluting with Et2O. The solvent was evaporated
and the crude mixture was chromatographed to give 6a;
yield: 80 mg (0.30 mmol, 79%).
[4] For intramolecular [4+2] examples, see: a) P. Mauleꢁn,
R. M. Zeldin, A. Z. Gonzꢃlez, F. D. Toste, J. Am.
Chem. Soc. 2009, 131, 6348–6349; b) I. Alonso, B.
Trillo, F. Lꢁpez, S. Montserrat, G. Ujaque, L. Castedo,
A. Lledꢁs, J. L. MascareÇas, J. Am. Chem. Soc. 2009,
131, 13020–13030; c) A. Z. Gonzꢃlez, F. D. Toste, Org.
Lett. 2010, 12, 200–203; d) D. Benitez, E. Tkatchouk,
A. Z. Gonzꢃlez, W. A. Goddard, F. D. Toste, Org. Lett.
2009, 11, 4798–4801.
[5] For intramolecular [3+2] examples, see: a) X. Huang,
L. Zhang, J. Am. Chem. Soc. 2007, 129, 6398–6399;
b) A. Buzas, F. Gagosz, J. Am. Chem. Soc. 2006, 128,
12614–12615. For a Pt-catalyzed intramolecular [3+
2]cycloaddition, see: c) G. Zhang, V. J. Catalano, L.
Zhang, J. Am. Chem. Soc. 2007, 129, 11358–11359.
[6] For intramolecular [2+2] examples, see: a) M. R.
Luzung, P. Mauleꢁn, F. D. Toste, J. Am. Chem. Soc.
2007, 129, 12402–12403; b) L. Zhang, J. Am. Chem.
Soc. 2005, 127, 16804–16805; c) H. Teller, S. Flugge, R.
Goddard, A. Fꢇrstner, Angew. Chem. 2010, 122, 1993–
1997; Angew. Chem. Int. Ed. 2010, 49, 1949–1953; d) M.
Alcarazo, T. Stork, A. Anoop, W. Thiel, A. Fꢇrstner,
Angew. Chem. 2010, 122, 2596–2600; Angew. Chem.
Int. Ed. 2010, 49, 2542–2546; e) A. Z. Gonzꢃlez, D. Be-
nitez, E. Tkatchouk, W. A. Goddard, F. D. Toste, J.
Am. Chem. Soc. 2011, 133, 5500–5507.
[7] a) H. Faustino, F. Lꢁpez, L. Castedo, J. L. MascareÇas,
Chem. Sci. 2011, 2, 633–637. For related [4+2]cycload-
ditions with allenyl ethers, see: b) G. Wang, Y. Zou, Z.
Li, Q. Wang, A. Goeke. Adv. Synth. Catal. 2011, 353,
550–556. For a thermal [4+2]cycloaddition of tosyl al-
lenamides and dienes, see: c) A. G. Lohse, R. P. Hsung,
Org. Lett. 2009, 11, 3430–3433.
[8] For a relevant intermolecular platinum-catalyzed [3+
2]cycloaddition between allenyl silyl ethers and al-
kenes, in which [2+2]adducts were also detected as
minor side products, see: H. Kusama, M. Ebisawa, H.
Funami, N. Iwasawa, J. Am. Chem. Soc. 2009, 131,
16352–16353.
Acknowledgements
This work was supported by the Spanish MEC (SAF2007-
61015, SAF2010-20822-C02) and Consolider-Ingenio 2010
(CSD2007-00006), the ERDF, CSIC and Xunta de Galicia
(INCITE09 209 122 PR and GRC2010/12). HF and PB ac-
knowledge the Fundażo para a CiÞncia e a Tecnologia-Por-
tugal for a PhD Grant SFRH/BD/60214/2009 and the Span-
ish MICINN for a FPI fellowship, respectively. Ana Gimeno
is gratefully acknowledged for preliminary experiments with
other type of alkenes. Johnson-Matthey is acknowledged for
a gift of metals.
References
[1] For recent reviews, see: a) A. S. K. Hashmi, Chem. Rev.
2007, 107, 3180–3211; b) D. J. Gorin, B. D. Sherry, F. D.
Toste, Chem. Rev. 2008, 108, 3351–3378; c) A. Arcadi,
Chem. Rev. 2008, 108, 3266–3325; d) A. Fꢇrstner,
Chem. Soc. Rev. 2009, 38, 3208–3221; e) A. Corma, A.
Leyva-Pꢀrez, M. J. Sabater, Chem. Rev. 2011, 111,
1657–1712; f) E. Jimꢀnez-NfflÇez, A. M. Echavarren,
Chem. Rev. 2008, 108, 3326–3350; g) N. Krause, C.
Winter, Chem. Rev. 2011, 111, 1994–2009; h) B.-L. Lu,
L. Dai, M. Shi, Chem. Soc. Rev. 2012, 41, 3318–3339.
[2] For reviews including gold-catalyzed cycloadditions of
allenes, see: a) F. Lꢁpez, J. L. MascareÇas, Beilstein J.
Org. Chem. 2011, 7, 1075–1094; b) H. C. Shen, Tetrahe-
dron 2008, 64, 7847–7870; c) F. Lꢁpez, J. L. MascareÇas,
Chem. Eur. J. 2011, 17, 418–428. For recent reviews on
allene chemistry, see: d) Cumulenes and Allenes, in:
Science of Synthesis: Houben-Weyl Methods of Molecu-
lar Transformations, Vol. 44 (Ed.: N. Krause), Georg
Thieme Verlag, Stuttgart, 2008; e) S. Ma, Acc. Chem.
Res. 2009, 42, 1679–1688; f) see also ref.[1g]
[9] For a review on the synthetic utility of allenamides,
see: L.-L. Wei, H. Xiong, R. P. Hsung, Acc. Chem. Res.
2003, 36, 773.
[10] For other types of [2+2]cycloadditions employing dif-
ferent types of allenamides, see: a) M. Kimura, Y.
Horino, Y. Wakamiya, T. Okayima, Y. Tamaru, J. Am.
Chem. Soc. 1997, 119, 10869. For a relevant gold-cata-
[3] For intramolecular [4+3] examples, see: a) B. Trillo, F.
Lꢁpez, S. Montserrat, G. Ujaque, L. Castedo, A.
Lledꢁs, J. L. MascareÇas, Chem. Eur. J. 2009, 15, 3336–
3339; b) I. Alonso, H. Faustino, F. Lꢁpez, J. L. Mascare-
Ças, Angew. Chem. 2011, 123, 11698–11702; Angew.
Adv. Synth. Catal. 2012, 354, 1658 – 1664
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1663

Hꢀlio Faustino et al.
COMMUNICATIONS
lyzed [2+2]cycloaddition of alkynes and alkenes, see:
b) V. Lꢁpez-Carrillo, A. M. Echavarren J. Am. Chem.
Soc. 2010, 132, 9292–9294. For a review on [2+2]cyclo-
additions of allenes, see: c) B. Alcaide, P. Almendros,
C. Aragoncillo, Chem. Soc. Rev. 2010, 39, 783–816. For
a [2+2]cycloaddition of allenes to alkenes promoted
by Ru complexes, see: d) M. Gulꢂas, A. Collado, B.
Trillo, F. Lꢁpez, E. OÇate, E.; M. A. Esteruelas, J. L.
MascareÇas J. Am. Chem. Soc. 2011, 133, 7660–7663.
[11] a) J. C. Namyslo, D. E. Kaufmann, Chem. Rev. 2003,
103, 1485–1538. For a review on bioactive cyclobutane
alkaloids, see: b) V. M. Dembitsky, J. Nat. Med. 2008,
62, 1–33.
[12] While refining this work, Chen et al. reported a gold-
catalyzed intermolecular cycloaddition of tosyl allena-
mides. This publication urged us to disclose our own re-
sults in this area, see: X.-X. Li, L.-L. Zhu, W. Zhou, Z.
Chen, Org. Lett. 2011, 14, 436–439.
[13] The gold-catalyzed hydrofunctionalization of allena-
mides with indoles, aromatic nucleophiles or amines
provides products related to 8: a) A. W. Hill, M. R. J.
Elsegood, M. C. Kimber, J. Org. Chem. 2010, 75, 5406;
b) M. C. Kimber, Org. Lett. 2010, 12, 1128.
[16] CCDC 863034 (6a) and CCDC 863036 (6i) contain sup-
plementary crystallographic data for these compounds.
These data can be obtained free of charge from The
Cambridge
Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[17] Cycloaddition of allenamide 5k could be carried out
using 1.5 equiv. of enamide, albeit 6k is isolated in
a slightly lower yield (56%). Additionally, performing
the addition of the enamide in one portion did not sig-
nificantly affect the efficiency of the reaction, since 6k
could be isolated in a comparable 63% yield.
[18] CCDC 863038 (6k) and CCDC 863037 (6q) contain
supplementary crystallographic data for these com-
pounds. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[14] ACHTUNGTRENNUNG[2+2]Homodimer 7 could be further characterized by
X-ray analysis. CCDC 863035 contains the supplemen-
tary crystallographic data for this compound. These
data can be obtained free of charge from The
Cambridge
Crystallographic Data
Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[15] This reaction provided a complex mixture of products
together with a 15% yield of a 1:1.2 mixture of 6a and
6a’.
1664
asc.wiley-vch.de
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 1658 – 1664