Rational synthesis for all All-homocalixarenes

Article abstract of DOI:10.1002/anie.201206785

Have it all: The synthesis of homocalixarenes of all sizes has been demonstrated using the triple annulation of bis(carbene) complexes with diynes (see scheme). This strategy in principle should allow access to all homocalixarenes. The generality of the synthetic approach is also demonstrated by the preparation of a pyrrole-containing calixarene. Copyright

Full text of DOI:10.1002/anie.201206785

Angewandte
Chemie
DOI: 10.1002/anie.201206785
Synthetic Methods
Rational Synthesis for All All-Homocalixarenes**
Alexander V. Predeus, Vijay Gopalsamuthiram, Richard J. Staples, and William D. Wulff*
Calixarenes and related compounds are established scaffolds
useful as ligands and very popular as building blocks for
molecular design.[1,2] At the same time, the use of these
compounds is virtually always defined by the ability of
researchers to prepare them efficiently in useful quantities,
and in a controlled fashion. As an example, calix[4]arenes,
calix[6]arenes, and calix[8]arenes gained popularity very
quickly after Gutsche et al. developed and perfected one-
step methods for their preparation.[3] Fitting geometry,
namely appropriate ring size and dominant conformation,
are of paramount importance in most macrocycle applica-
tions, yet the overwhelming majority of publications still only
report modifications of the three above-mentioned easily
prepared calixarene cores.
Homocalixarenes have been a subject of scientific interest
for a couple of decades.[1c,4] Whereas calixarenes have only
one carbon atom between the aromatic rings, the presence of
extra linkers between the aromatic rings allows the tuning of
the desired cavity size, symmetry, and conformational mobi-
lity of the molecule. All-homocalixarenes have been defined
by Vçgtle and co-workers as those in which all aromatic rings
are separated by two carbon atoms. All-bis(homocalixarene)s
are those separated by three carbon atoms, All-tris(homoca-
lixarene)s separated by four carbon atoms, etc.[4] We use the
term homocalixarenes to refer to all calixarenes which have
tether lengths greater than one carbon atom for at least one of
the tethers and all-homocalixarenes for all calixarenes in
which each of the tethers is greater than one carbon atom. The
most direct method for the synthesis of all-homocalixarenes is
the Mꢀller–Rçscheisen cyclization which gives mixtures of
calixarenes in low yields.[5,6] All-homocalixarenes have also
been made in a multistep synthesis by sulfur extrusion upon
thermolysis of the corresponding sulfone.[7] Several methods
have been used in the synthesis of all-bis(homocalixarene)s,
including alkylation of bis(bromomethyl) benzenes with
tosylmethyl isocyanide[8] and diethyl malonate,[9] a Claisen
rearrangement,[10] and aryllithium/alkyl halide coupling.[11]
Larger all-homocalixarenes (n ꢀ 4) have been made by the
reaction of a bis(benzyllithium) with a,w-dihalides,[12] and by
the trimerization of a Fischer carbene complex.[13]
Homocalixarenes with heteroatoms in the linkers are
much more widely studied compared to those with all-carbon
linkers. The most popular of these are the bis(homooxaca-
lix[3]arene)s which have enjoyed significant attention from
researchers only after the discovery of a direct method for
their preparation.[14–16]
Earlier reports from our laboratory have shown that the
benzannulation reaction[17] of Fischer carbene complexes can
be used to synthesize paracyclophanes with various tether
lengths.[18] This chemistry was extended to the synthesis of
calix[4]arenes,[19] having various substituents, in a highly
regiocontrolled triple annulation, and later extended to
chiral calix[4]arenes.[20] The purpose of the present work is
to test the viability of this method to serve as a very general
approach to all-homocalixarenes, having tethers with any
number of desired carbon atoms, on a gram scale.
One can envisage that both the homocalix[4]arenes 3 and
homocalix[3]arenes 5 can be prepared by the triple annula-
tion method[19,20] (Scheme 1) involving the formation of two
Scheme 1. Homocalix[4]arenes and homocalix[3]arenes by a triple
annulation of bis(carbene) complexes.
aromatic rings and a macrocyclic ring by two tandem
benzannulations. The key starting material common to these
reactions is the aromatic diyne 1 which should give 5 upon
reaction with the bis(carbene) complex 4 and 3 upon reaction
with the bis(carbene) complex 2. In theory, any size homo-
calixarene could be accessed by this approach by just
programming in the proper number of methylene spacers
between the alkyne and arene ring in the diyne 1 and between
the double bond and the arene ring in the bis(carbene)
complexes 2 or 4.
[*] Dr. A. V. Predeus, Dr. V. Gopalsamuthiram, Dr. R. J. Staples,
Prof. W. D. Wulff
Department of Chemistry, Michigan State University
East Lansing, MI 48824 (USA)
E-mail: wulff@chemistry.msu.edu
Homepage: http://www2.chemistry.msu.edu/faculty/wulff/
myweb26/index.htm
[**] This work was supported by the NSF grant CHE-0750319.
The key diyne 1 can be prepared in high yield and in
a convergent manner by a Suzuki coupling reaction using 2,6-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206785.
Angew. Chem. Int. Ed. 2013, 52, 911 –915
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
911

.
Angewandte
Communications
dibromo-4-methylanisole (6; Scheme 2). The enynes 7b–d
with 3, 5, and 11 methylene spacers, respectively, were
prepared by alkylation of trimethylsilyl acetylene with the
Bestmann–Ohira reagent, dimethyl-1-diazo-2-oxopropyl-
phosphonate, and potassium carbonate.[24]
An attractive feature of the synthesis of the bis(carbene)
complexes 2a–d is that it is convergent with the synthesis of
the diynes 1a–d in that they can be converted to the
corresponding bis(carbene) complexes in two steps via the
bis(trans-vinyl iodide) 11a–d (Table 1). For the preparation of
bis(trans-vinyliodide)s 11 and 13 (Table 2), a variation of
Schwartzꢁs hydrozirconation/iodination sequence reported by
Jacobsen and co-workers was found to be highly beneficial.[25]
The addition of Schwartzꢁs reagent to both alkynes was found
to be very fast in dichloromethane and, more importantly, the
success of the addition was much less sensitive to the quality
of Schwartz reagent. Quenching the reaction mixture with
iodine affords very reproducibly high yields of 11 and 13.
Scheme 2. Preparation of the diynes 1b–d. 9-BBN=9-borabicyclo-
[3.3.1]nonane, TBAF=tetra-n-butylammonium fluoride.
Table 1: Preparation of aryl-tethered bis(carbene) complexes 2.[a]
proper a,w-haloalkene (see the Supporting Information).
Then, using the coupling protocol of Buchwald et al.[21] the 9-
BBN derivatives 8b–d were prepared and then coupled with
6.[22] This Suzuki coupling was performed at 708C and
required freeze-thaw deoxygenation; 2 mol% SPhos and
1 mol% Pd(OAc)2 were used to generate the catalyst, and
potassium triphosphate monohydrate was the base. The
resulting mixture was deprotected with catalytic TBAF in
wet THF to afford good yields of 1b–d in a one-pot
procedure.
Entry
n
11
Yield [%]
2
Yield [%]
1
2
3
4
2
3
5
11a
11b
11c
11d
89
77
80
93
2a
2b
2c
2d
67
79
68
49
There was a desire to extend the substrate scope study to
the all-homocalix[4]arene 3a (n = 2) and the all-homoca-
lix[3]arene 5a (n = 2; see Tables 3 and 4 for structures). The
requisite diyne 1a for this job required special preparation
(Scheme 3) since the appropriate enyne 7a (n = 2) is not as
11
[a] See the Supporting Information for experimental details. Yield is that
of the isolated product. Cp=cyclopentadienyl.
Table 2: Preparation of the alkyl-tethered bis(carbene) complexes 4.[a]
Scheme 3. Preparation of the diyne 1a. DMF=N,N-dimethylform-
amide.
Entry
n
13
Yield [%]
4
Yield [%]
1
2
3
4
2
3
5
13a
13b
13c
13d
90
86
79
90
4a
4b
4c
4d
65
71
75
67
readily available, and the presence of the b-alkyne function in
8a could potentially complicate the Suzuki coupling. As an
alternative, 2,6-diiodo-4-methylanisole (9) was converted into
the dialdehyde 10 in 44% yield using the Heck reaction under
Larockꢁs conditions (in presence of quaternary ammonium
salt and lithium acetate as a base).[23] In the course of this
reaction the double bond of the allylic alcohol is shifted
towards the end of the chain, thus giving the unbranched
bis(aldehyde) as the major product. Finally, 1a was prepared
in 86% yield by treatment of the bis(aldehyde) with the
11
[a] See the Supporting Information for experimental details. Yield is that
of the isolated product.
The bis(carbene) complexes 2 and 4 were initially
prepared by a procedure following the Fischer method we
had reported earlier for related bis(carbene) complexes.[19]
912
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 911 –915

Angewandte
Chemie
However, this procedure was plagued with low and unrepro-
ducible yields. It was suspected that the low yields were
a result of Michael-type polymerization reactions or proto-
nation of the bis(vinyllithium) intermediates by traces of
water and, in some cases, the solvent. Thus, after a thorough
search for optimal reaction conditions, an improved synthetic
procedure was developed. Lithium–iodine exchange at
À958C in THF and the subsequent rapid transfer of the
resulting slurry into a chromium carbonyl THF solution at
room temperature was the protocol of choice (Tables 1 and 2).
All complexes but one could be prepared in 65–79% yield,
which constitutes, in many cases, more than a twofold increase
in efficiency compared to previous syntheses of this class of
bis(carbene) complexes.[19,20]
The triple annulation giving the homocalix[4]arenes 3a–d
was found to be more sensitive to the reaction conditions than
would have been expected based on our experience with the
synthesis of calix[4]arenes.[19,20] For example, the reaction of
1b and 2b did not give any detectable products in benzene,
acetonitrile, or dichloromethane. In contrast, reactions in 1,2-
dichloroethane, 1,4-dioxane, and THF all gave the bis(homo-
calix[4]arene) 3b in 25–39% yield, and it was the only product
that was mobile on silica gel (Table 3). This result is to be
compared to a 36% yield for the calix[4]arene 3 (n = 1) in 1,2-
dichloroethane.[19] To our delight, the extension of these
reaction conditions to the other tether lengths afforded all of
the all-homocalix[4]arenes 3a–3d.
still fails, possibly because of the presence of chloroform (see
the Supporting Information). Tetrahydrofuran and 1,4-diox-
ane, in contrast, provide more reproducible and reliable
reactions. Furthermore, 1,4-dioxane can be deoxygenated by
simply purging of the solvent with nitrogen, while the use of
both THF and 1,2-dichloroethane require freeze-thaw deoxy-
genation. Thus, 1,4-dioxane allows greater operational con-
venience and the preparation of 3 on a gram scale. It is also to
be noted that, while increased temperature does not benefit
the reactions with smaller tethers, the yield of 3d increases
from 13 to 18% when the reaction is conducted at 1158C
rather than at 1008C. The macrocycle in 3d consists of a 56-
membered ring.
The triple annulation strategy could be extended to the
synthesis of the homocalix[3]arenes 5 in which two of the
three benzene rings were constructed simultaneously with the
macrocycle (Table 4). Slightly higher yields were observed in
Table 4: Homocalix[3]arenes through the triple annulation of 1 and 4.[a]
Yield [%]
Table 3: Homocalix[4]arenes through the triple annulation of 1 and 2.[a]
5
n
THF
1,4-dioxane
5a
5b
5c
5d
2
3
5
9
17
31
30
28
10
25
11
11[b]
[a] Unless otherwise specified, all reactions were carried out on 1.0 mmol
of 4 with 1.0 equiv of 1 at 0.0025m at 1008C for 24 h. Yield is that of the
isolated product. [b] This reaction was conducted at 1158C.
Yield [%]
1,2-DCE
THF and 1,4-dioxane for the three and five methylene tethers
compared to those for the corresponding homocalix[4]arenes
(Table 3 versus Table 4). However, lower yields were
observed for the smallest macrocycle 5a, and this might be
attributed to steric congestion in the 15-membered macro-
cycle having three aromatic rings when one considers that the
two-rings generated in the coordination sphere of the
chromium will maintain an h6 coordination of a chromium
tricarbonyl to these two rings until the reaction is stopped and
exposed to air. In contrast to 3d, the yield of 5d could not be
improved by increasing the temperature from 100 to 1158C.
Like the homocalix[4]arenes 3a–3d, the homocalix[3]arenes
5a–5d are all conformationally mobile and no evidence could
be found for hindered rotation of the benzene rings through
the annulus. However, the pseudosymmetry of the aromatic
protons present in 5c and 5d is broken when the size of the
spacer is reduced to two or three methylenes (5b and 5a; see
the Supporting Information).
3
n
THF
1,4-dioxane
3a
3b
3c
3d
2
3
5
25
25
27
13
35
39
10
0
22
25
17
11
18[b]
[a] Unless otherwise specified, all reactions were carried out on 1.0 mmol
of 2 with 1.0 equiv of 1 at 0.0025m at 1008C for 24 h in THF and 1,4-
dioxane, and for 0.5–1 h in 1,2-DCE. Yield is that of the isolated product.
[b] This reaction was conducted at 1158C. The yield for the reaction run
at 1008C is 13%. 1,2-DCE=1,2-dichloroethane.
The choice of the solvent for the triple annulation reaction
is defined by several factors. While 1,2-dichloromethane gives
the highest yield for tether lengths of two and three (3a and
3b,respectively; Table 3), the yield drops to 10% in case of
n = 5, and the cyclization fails completely when n = 11. It is
also worth noting that only high purity 1,2-dichloroethane
works in this reaction. Even when a lower grade purity solvent
is purified by distillation over calcium hydride, the reaction
Calixarenes and homocalixarenes are characterized by the
presence of a phenol function on each of the benzene rings
Angew. Chem. Int. Ed. 2013, 52, 911 –915
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
913

.
Angewandte
Communications
projecting from the lower rim of the calixarene and these are
crucial for the recognition and binding of substrates, and for
covalent modifications. In an effort to provide access to
unfunctionalized symmetrical homocalixarenes by the triple
annulation methodology, the THP-protected diynes 14b and
14c where prepared and reacted with the corresponding
bis(carbene) complexes 4b and 4c (Scheme 4). The initially
formed homocalix[3]arenes were not isolated but directly
deprotected to give the symmetrical molecules 15b and 15c in
24 and 20% yield, respectively, upon isolation. The reactions
were performed on 10 mmol scale and provided 1.18 g of 15b
and 1.15 g of 15c.
atoms. The method allows full control over the ring size and
symmetry, and its ability to provide the synthesis of a series of
all-homocalixarenes was illustrated for compounds having 2,
3, 5, and 11 methylene linkers between each ring. The method
is also flexible enough to allow the incorporation of a pyrrole
unit in place of one of the benzene rings in the calixarene. The
macrocycles are always isolated as a single product and can be
prepared conveniently on a gram scale. Six of the twelve
homocalixarenes that were prepared were characterized by
single-crystal X-ray analysis.[28]
Received: August 21, 2012
Published online: November 26, 2012
Keywords: annulation · calixarenes · carbenes · macrocycles ·
.
synthetic methods
[1] a) Calixarenes—A Versatile Class of Macrocyclic Compounds
(Eds.: J. Vicens, V. Bohmer), Kluwer, Norwell, MA, 1991;
b) Calixarenes Revisited (Ed.: C. D. Gutsche), Royal Society of
Chemistry, Cambridge, UK, 1998; c) Calixarenes 2001 (Eds.: Z.
Asfari, V. Bohmer, J. Harrowfield, J. Vicens), Kluwer, Norwell,
MA, 2001.
Scheme 4. Synthesis of symmetrical homocalix[3]arenes. THP=tetra-
hydropyran, Ts=4-toluenesulfonyl.
[2] For recent reviews on applications of calixarenes, see: a) F.
Perret, A. W. Coleman, Chem. Commun. 2011, 47, 7303 – 7319;
b) R. Joseph, C. P. Rao, Chem. Rev. 2011, 111, 4658 – 4702; c) B.
Mokhtari, P. Kobra, N. Dalali, Chromatographia 2011, 73, 829 –
847; d) G. A. Evtugyn, E. E. Stoikova, R. V. Shamagsumova,
Russ. Chem. Rev. 2011, 79, 1071 – 1097; e) B. Mokhtari, K.
Pourabdoliah, N. Dallali, J. Radioanal. Nucl. Chem. 2011, 287,
921 – 934; f) B. Mokhtari, K. Pourabdollah, N. Dalali, J. Inclusion
Phenom. Macrocyclic Chem. 2011, 69, 1 – 55; g) F. Sansone, L.
Baldini, A. Casnati, R. Ungaro, New J. Chem. 2010, 34, 2715 –
2728; h) A. Dondoni, A. Marra, Chem. Rev. 2010, 110, 4949 –
4977; i) B. Mokhtari, K. Pourabdollah, N. Dalali, J. Coord. Chem.
2011, 64, 743 – 794; j) W. Sliwa, T. Girek, J. Inclusion Phenom.
Macrocyclic Chem. 2010, 66, 15 – 41; k) I. Leray, B. Valeur, Eur.
J. Inorg. Chem. 2009, 3525 – 3535; l) E. A. Shokova, V. V.
Kovalev, Russ. J. Org. Chem. 2009, 45, 1275 – 1314; m) O.
Danylyuk, K. Suwinska, Chem. Commun. 2009, 5799 – 5813;
n) D. Coquiꢂre, S. Le Gac, U. Darbost, O. Seneque, I. Jabin, O.
Reinaud, Org. Biomol. Chem. 2009, 7, 2485 – 2500; o) J. S. Kim,
S. Y. Lee, J. Yoon, J. Vicens, Chem. Commun. 2009, 4791 – 4802;
p) A. de Fatima, S. A. Fernandes, A. A. Sabino, Curr. Drug
Discovery Technol. 2009, 6, 151 – 170; q) S. Sameni, C. Jeunesse,
D. Matt, J. Harrowfield, Chem. Soc. Rev. 2009, 38, 2117 – 2146;
r) B. S. Creaven, D. F. Donlon, J. McGinley, Coord. Chem. Rev.
2009, 253, 893 – 962; s) P. K. Lo, M. S. Wong, Sensors 2008, 8,
5313 – 5335; t) D. M. Homden, C. Redshaw, Chem. Rev. 2008,
108, 5086 – 5130.
Calix[4]pyrroles are widely studied and their attractive
properties have been used for a variety of applications
including as optical sensors for anions.[26] To put our method
to a final test, the previously unknown pyrrole/calixarene
hybrid 17 with linkers of different lengths (three and five
methylene groups) was prepared from the protected diyne
16b in 18% yield (Scheme 5). The Boc group was removed by
heating the macrocycle as a thin film in an inert atmosphere at
1808C,[27] thus affording the homocalix[3]arene 18 in 88%
yield. Interestingly, even though the triple annulation was
performed at 1008C, 18 was not detected in the initial reaction
mixture (ꢁ 5%). The air-sensitive macrocycle 18 was pre-
pared on a 200 mg scale, thus allowing us to grow a single
crystal and perform X-ray analysis.[28]
[3] a) C. D. Gutsche, M. Iqbal, Org. Synth. Coll. Vol. 8 1993, 75;
b) C. D. Gutsche, B. Dhawan, M. Leonis, D. Stewart, Org. Synth.
Coll. Vol. 8 1993, 77; c) C. D. Gutsche, J. H. Munch, Org. Synth.
Coll. Vol. 8 1993, 80.
[4] S. Ibach, V. Prautzsch, F. Vçgtle, C. Chartroux, K. Gloe, Acc.
Chem. Res. 1999, 32, 729.
Scheme 5. Synthesis of a pyrrole/benzene calix[3]arene hybrid. Boc=
tert-butoxycarbonyl.
[5] a) F. Vçgtle, J. Schmitz, M. Nieger, Chem. Ber. 1992, 125, 2523;
b) J. Schmitz, F. Vçgtle, M. Nieger, K. Gloe, H. Stepman, O.
Heitzsch, H.-J. Buschmann, W. Hasse, K. Cammann, Chem. Ber.
1993, 126, 2483.
[6] Higher yields (34%) can be obtained if parts of the calixarene
are pre-fabricated: M. Tashiro, T. Yamato, J. Org. Chem. 1981,
46, 1543.
In conclusion, the reaction of bis(carbene) complexes with
bis(alkyne)s in a triple annulation provides direct access to
homocalixarenes with three and four aromatic rings. Through
proper choice of tether length, these macrocycles were
assembled with ring sizes ranging from 15 to 56 carbon
914
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 911 –915

Angewandte
Chemie
[7] M. Tashiro, A. Tsuge, T. Sawada, T. Makishima, S. Horie, T.
Arimura, S. Mataka, T. Yamoto, J. Org. Chem. 1990, 55, 2404.
[8] T. Yamato, L. K. Doamekpor, K. Koizumi, K. Kishi, M.
Haraguchi, M. Tashiro, Liebigs Ann. 1995, 1259.
[20] a) V. Gopalsamuthiram, A. V. Predeus, R. H. Huang, W. D.
Wulff, J. Am. Chem. Soc. 2009, 131, 18018 – 18019; b) V.
Gopalsamuthiram, R. H. Huang, W. D. Wulff, Chem. Commun.
2010, 46, 8213 – 8215.
[9] a) F. Vçgtle, M. Zuber, Synthesis 1972, 543; b) C. Meiners, M.
Nieger, F. Vçgtle, Liebigs Ann. 1996, 297.
[10] H. Uzawa, K. Hiratani, N. Minoura, T. Takahashi, Chem. Lett.
1998, 307.
[11] D. H. Burns, J. D. Miller, J. Santana, J. Org. Chem. 1993, 58, 6526.
[12] A calix[3]arene with n = 5 has been reported in 1% yield: R. B.
Bates, S. Gangwar, V. V. Kane, K. Suvannachut, S. R. Taylor, J.
Org. Chem. 1991, 56, 1696.
[21] S. D. Walker, T. E. Barder, J. R. Martinelli, S. L. Buchwald,
Angew. Chem. 2004, 116, 1907; Angew. Chem. Int. Ed. 2004, 43,
1871.
[22] The compound 6 can be readily prepared from the commercially
available 2,6-dibromo-4-methylphenol.
[23] R. C. Larock, W.-Y. Leung, S. Stolz-Dunn, Tetrahedron Lett.
1989, 30, 6629.
[24] J. Pietruszka, S. Witt, Synthesis 2006, 4266.
[13] This reaction is not general, but a calix[3]arene with n = 6 was
generated as a minor product in up to 19% yield: H. Wang,
W. D. Wulff, J. Am. Chem. Soc. 1998, 120, 10573.
[25] a) M. Van der Sande, H.-J. Gais, Chem. Eur. J. 2007, 13, 1784;
b) D. E. Chavez, E. N. Jacobsen, Angew. Chem. 2001, 113, 3779;
Angew. Chem. Int. Ed. 2001, 40, 3667.
[14] For a review of their synthesis, see: E. A. Shokova, V. V.
Kovalev, Russ. J. Org. Chem. 2004, 40, 607 – 643.
[15] For a review of their receptor properties, see: E. A. Shokova,
V. V. Kovalev, Russ. J. Org. Chem. 2004, 40, 1547 – 1571.
[16] For a general review, see: K. Cottet, P. M. Marcos, P. J. Cragg,
Beilstein J. Org. Chem. 2012, 8, 201 – 226.
[17] M. L. Waters, W. D. Wulff, Org. React. 2008, 70, 121 – 623.
[18] See reference [13] and: a) H. Wang, W. D. Wulff, J. Am. Chem.
Soc. 2000, 122, 9862 – 9863; b) H. Wang, J. Hunag, W. D. Wulff,
A. L. Rheingold, J. Am. Chem. Soc. 2003, 125, 8980 – 8981.
[19] V. Gopalsamuthiram, W. D. Wulff, J. Am. Chem. Soc. 2004, 126,
13936.
[26] a) P. A. Gale, P. Anzenbacker, Jr., J. L. Sessler, Coord. Chem.
Rev. 2001, 222, 57 – 102; b) P. Anzenbacher, Jr., R. Nishiyabu,
M. A. Palacios, Coord. Chem. Rev. 2006, 250, 2929 – 2938.
[27] V. H. Rawal, M. P. Cava, Tetrahedron Lett. 1985, 26, 6141.
[28] CCDC 897168 – 897173 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Please see the Supporting
Information for additional details regarding the X-ray structures
obtained for the compounds presented herein.
Angew. Chem. Int. Ed. 2013, 52, 911 –915
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
915