Catalytic enantioselective intermolecular cycloaddition of 2-diazo-3,6-diketoester-derived carbonyl ylides with alkynes and styrenes using chiral dirhodium(II) carboxylates

Article abstract of DOI:10.1021/ol8013733

(Chemical Equation Presented) Dirhodium(II) tetrakis[N- tetrachlorophthaloyl-(S)-tert-leucinate], Rh₂(S-TCPTTL)₄, is an exceptionally effective catalyst for enantioselective tandem carbonyl ylide formation-cycloaddition reactions of

Full text of DOI:10.1021/ol8013733

ORGANIC
LETTERS
2008
Vol. 10, No. 16
3603-3606
Catalytic Enantioselective Intermolecular
Cycloaddition of
2-Diazo-3,6-diketoester-Derived Carbonyl
Ylides with Alkynes and Styrenes Using
Chiral Dirhodium(II) Carboxylates
Naoyuki Shimada, Masahiro Anada, Seiichi Nakamura, Hisanori Nambu,
Hideyuki Tsutsui, and Shunichi Hashimoto*
Faculty of Pharmaceutical Sciences, Hokkaido UniVersity, Sapporo 060-0812, Japan
hsmt@pharm.hokudai.ac.jp
Received June 18, 2008
ABSTRACT
Dirhodium(II) tetrakis[N-tetrachlorophthaloyl-(S)-tert-leucinate], Rh2(S-TCPTTL)4, is an exceptionally effective catalyst for enantioselective tandem
carbonyl ylide formation-cycloaddition reactions of 2-diazo-3,6-diketoesters with arylacetylene, alkoxyacetylene, and styrene dipolarophiles,
providing cycloadducts in good to high yields and with enantioselectivities of up to 99% ee as well as with perfect exo diastereoselectivity
for styrenes.
The dirhodium(II) complex-catalyzed tandem cyclic carbonyl
ylide formation-1,3-dipolar cycloaddition reaction sequence
extensively studied by the Padwa group represents one of
the most efficient methods for the rapid assembly of complex
oxapolycyclic systems containing embedded di- or tetrahy-
drofuran rings.1 The exceptional power of the carbonyl ylide
cycloaddition strategy has recently been demonstrated by a
growing number of syntheses of diverse natural products2
such as zaragozic acids,2a,b nemorensic acids,2c colchicine,2d
aspidophytine,2e pseudolaric acid,2f polygalolides,2g 3H-
epivincamine,2h,i indicol,2j and platensimycin.2k Conse-
quently, the development of an enantioselective version of
this sequence catalyzed by chiral dirhodium(II) complexes
(2) For recent reports on the total syntheses of natural products via
carbonyl ylide cycloaddition strategy, see: (a) Nakamura, S.; Hirata, Y.;
Kurosaki, T.; Anada, M.; Kataoka, O.; Kitagaki, S.; Hashimoto, S. Angew.
Chem., Int. Ed. 2003, 42, 5351–5355. (b) Hirata, Y.; Nakamura, S.;
Watanabe, N.; Kataoka, O.; Kurosaki, T.; Anada, M.; Kitagaki, S.; Shiro,
M.; Hashimoto, S. Chem. Eur. J. 2006, 12, 8898–8925. (c) Hodgson, D. M.;
Strat, F. L.; Avery, T. D.; Donohue, A. C.; Bru¨ckl, T. J. Org. Chem. 2004,
69, 8796–8803. (d) Graening, T.; Bette, V.; Neudo¨rfl, J.; Lex, J.; Schmalz,
H.-G. Org. Lett. 2005, 7, 4317–4320. (e) Mej´ıa-Oneto, J. M.; Padwa, A.
Org. Lett. 2006, 8, 3275–3278. (f) Geng, Z.; Chen, B.; Chiu, P. Angew.
Chem., Int. Ed. 2006, 45, 6197–6201. (g) Nakamura, S.; Sugano, Y.;
Kikuchi, F.; Hashimoto, S. Angew. Chem., Int. Ed. 2006, 45, 6532–6535.
(h) England, D. B.; Padwa, A. Org. Lett. 2007, 9, 3249–3252. (i) England,
D. B.; Padwa, A. J Org. Chem. 2008, 73, 2792–2802. (j) Lam, S. K.; Chiu,
P. Chem. Eur. J. 2007, 13, 9589–9599. (k) Kim, C. H.; Jang, K. P.; Choi,
(1) For books and reviews, see: (a) Padwa, A.; Weingarten, M. D. Chem.
ReV. 1996, 96, 223–269. (b) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern
Catalytic Methods for Organic Synthesis with Diazo Compounds; Wiley-
Interscience: New York, 1998; Chapter 7. (c) Hodgson, D. M.; Pierard,
F. Y. T. M.; Stupple, P. A. Chem. Soc. ReV. 2001, 30, 50–61. (d) Mehta,
G.; Muthusamy, S. Tetrahedron 2002, 58, 9477–9504. (e) McMills, M. C.;
Wright, D. In Synthetic Applications of 1,3-Dipolar Cycloaddition Chemistry
Toward Heterocycles and Natural Products; Padwa, A., Pearson, W. H.,
Eds.; John Wiley & Sons: Hoboken, 2003; Chapter 4. (f) Savizky, R. M.;
Austin, D. J. In Modern Rhodium-Catalyzed Organic Reactions; Evans,
P. A., Ed.; Wiley-VCH: Weinheim, 2005; Chapter 19. (g) Padwa, A. HelV.
Chim. Acta 2005, 88, 1357–1374. (h) Nair, V.; Suja, T. D. Tetrahedron
2007, 63, 12247–12275.
S. Y.; Chung, Y. K.; Lee, E. Angew. Chem., Int. Ed. 2008, 47, 4009–4011
.
10.1021/ol8013733 CCC: $40.75
Published on Web 07/11/2008
2008 American Chemical Society

has become a challenging objective. In this process, a prime
requirement for asymmetric induction is the use of chiral
dirhodium(II) catalyst-associated carbonyl ylide intermediates
in the cycloaddition step, because catalyst-free carbonyl
ylides are achiral.3 Hodgson and co-workers were the first
to demonstrate high levels of asymmetric induction (up to
90% ee) in intramolecular cycloadditions of carbonyl ylides
derived from unsaturated R-diazo-ꢀ-ketoesters using binaph-
tholphosphate catalyst Rh2(R-DDBNP)4 (3) (Figure 1).4 We
lecular process but also for the intermolecular reaction of
R-diazo-ꢀ-ketoester with DMAD.4b,c Later extensive studies
by the same group demonstrated that a complex blend of
electronic effects from the dipole and dipolarophile, together
with the nature of the catalyst, contribute to the origin of
asymmetric induction.4e,9
With respect to the dipole reactivity of cyclic carbonyl
ylides derived from R-diazocarbonyl compounds, it is
documented that the most dominant interaction in the case
of R-diazo ketone is between the HOMO of the carbonyl
ylide and the LUMO of electron-deficient dipolarophiles,10
whereas the most favorable interaction in the case of R-diazo-
ꢀ-ketoester is between the LUMO of the carbonyl ylide and
the HOMO of more electron-rich dipolarophiles.11 Recently,
Hodgson and co-workers reported enantioselective intermo-
lecular cycloadditions of 2-diazo-3,6-diketoester-derived
carbonyl ylides with dipolarophiles that do not contain
electron-withdrawing substituents on the π bond, using Rh2-
(S-DOSP)4 (2)12 and Rh2(R-DDBNP)4 (3).13 In this process,
high levels of asymmetric induction (up to 92% ee) were
only achieved when strained alkene dipolarophiles such as
norbornene and norbornadiene were used. As a logical
extension of our previous studies, we addressed the issue of
the scope of the reaction with respect to the dipolarophile
component. Herein, we report the successful examples of
enantioselective carbonyl ylide cycloadditions of 2-diazo-
3,6-diketoester derivatives with arylacetylene, alkoxyacety-
lene, and styrene dipolarophiles, in which Rh2(S-TCPTTL)4
(1h),14a,c the chlorinated analogue of Rh2(S-PTTL)4 (1c),15
has emerged as the catalyst of choice for achieving enanti-
oselectivities of up to 99% ee.
Figure 1. Chiral dirhodium(II) complexes.
reported the first successful example of intermolecular
cycloadditions of R-diazoketone-derived carbonyl ylides
with dimethyl acetylenedicarboxylate (DMAD), in which
Rh2(S-BPTV)4 (1e) proved to be the catalyst of choice for
achieving enantioselectivities of up to 92% ee.5,6 However,
the reactions have serious limitations in that only highly
electron-deficient dipolarophiles such as DMAD6,7 and
p-nitrobenzaldehyde8 have met with any real success.
Furthermore, Hodgson and co-workers reported that Rh2(S-
BPTV)4 is less effective not only for the foregoing intramo-
Patterned after the original work of Hodgson,13 we initially
explored the reaction of tert-butyl 2-diazo-3,6-dioxohep-
tanoate (4a) with phenylacetylene (5a) (3 equiv) in benzo-
trifluoride at 23 °C using two classes of dirhodium(II)
carboxylate catalysts (1 mol %), which incorporate N-
(8) Tsutsui, H.; Shimada, N.; Abe, T.; Anada, M.; Nakajima, M.;
Nakamura, S.; Nambu, H.; Hashimoto, S. AdV. Synth. Catal. 2007, 349,
521–526
.
(9) (a) Hodgson, D. M.; Glen, R.; Redgrave, A. J. Tetrahedron Lett.
2002, 43, 3927–3930. (b) Hodgson, D. M.; Glen, R.; Grant, G. H.; Redgrave,
(3) Suga and co-workers developed a conceptually different approach
and demonstrated highly enantioselective 1,3-dipolar cycloadditions of
2-benzopyrylium-4-olates with a variety of dipolarophiles using chiral Lewis
acid catalysts: (a) Suga, H.; Inoue, K.; Inoue, S.; Kakehi, A. J. Am. Chem.
Soc. 2002, 124, 14836–14837. (b) Suga, H.; Inoue, K.; Inoue, S.; Kakehi,
A.; Shiro, M. J. Org. Chem. 2005, 70, 47–56.
A. J. J. Org. Chem. 2003, 68, 581–586.
(10) Padwa, A.; Fryxell, G. E.; Zhi, L. J. Am. Chem. Soc. 1990, 112,
3100–3109.
(11) Koyama, H.; Ball, R. G.; Berger, G. D. Tetrahedron Lett. 1994,
35, 9185–9188.
(12) (a) Davies, H. M. L. Aldrichm. Acta 1997, 30, 107–114. (b) Davies,
H. M. L. Eur. J. Org. Chem. 1999, 2459–2469.
(4) (a) Hodgson, D. M.; Stupple, P. A.; Johnstone, C. Tetrahedron Lett.
1997, 38, 6471–6472. (b) Hodgson, D. M.; Stupple, P. A.; Johnstone, C.
Chem. Commun. 1999, 2185–2186. (c) Hodgson, D. M.; Stupple, P. A.;
Pierard, F. Y. T. M.; Labande, A. H.; Johnstone, C. Chem. Eur. J. 2001, 7,
4465–4476. (d) Hodgson, D. M.; Labande, A. H.; Pierard, F. Y. T. M. Synlett
2003, 59–62. (e) Hodgson, D. M.; Labande, A. H.; Pierard, F. Y. T. M.;
(13) (a) Hodgson, D. M.; Labande, A. H.; Glen, R.; Redgrave, A. J.
Tetrahedron: Asymmetry 2003, 14, 921–924. (b) Hodgson, D. M.; Bru¨ckl,
T.; Glen, R.; Labande, A. H.; Selden, D. A.; Dossetter, A. G.; Redgrave,
A. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5450–5454.
(14) For the effective use of 1g,h in enantioselective aminations and
aromatic C-H insertions, see: (a) Yamawaki, M.; Tsutsui, H.; Kitagaki,
S.; Anada, M.; Hashimoto, S. Tetrahedron Lett. 2002, 43, 9561–9564. (b)
Tsutsui, H.; Yamaguchi, Y.; Kitagaki, S.; Nakamura, S.; Anada, M.;
Hashimoto, S. Tetrahedron: Asymmetry 2003, 14, 817–821. (c) Tanaka,
M.; Kurosaki, Y.; Washio, T.; Anada, M.; Hashimoto, S. Tetrahedron Lett.
2007, 48, 8799–8802. (d) Anada, M.; Tanaka, M.; Washio, T.; Yamawaki,
M.; Abe, T.; Hashimoto, S. Org. Lett. 2007, 9, 4559–4562.
(15) (a) Watanabe, N.; Ogawa, T.; Ohtake, Y.; Ikegami, S.; Hashimoto,
S. Synlett 1996, 85–86. (b) Saito, H.; Oishi, H.; Kitagaki, S.; Nakamura,
S.; Anada, M.; Hashimoto, S. Org. Lett. 2002, 4, 3887–3890. (c) Minami,
K.; Saito, H.; Tsutsui, H.; Nambu, H.; Anada, M.; Hashimoto, S. AdV. Synth.
Catal. 2005, 347, 1483–1487. (d) Tsutsui, H.; Abe, T.; Nakamura, S.; Anada,
M.; Hashimoto, S. Chem. Pharm. Bull. 2005, 53, 1366–1368.
´
Expo´sito Castro, M. A. J. Org. Chem. 2003, 68, 6153–6159. (f) Hodgson,
D. M.; Selden, D. A.; Dossetter, A. G. Tetrahedron: Asymmetry 2003, 14,
3841–3849.
(5) For seminal works on enantioselective intermolecular cycloadditon
of carbonyl ylides using chiral dirhodium(II) catalyst, see: (a) Doyle, M. P.;
Forbes, D. C. Chem. ReV. 1998, 98, 911–935. (b) Suga, H.; Ishida, H.;
Ibata, T. Tetrahedron Lett. 1998, 39, 3165–3166
(6) Kitagaki, S.; Anada, M.; Kataoka, O.; Matsuno, K.; Umeda, C.;
Watanabe, N.; Hashimoto, S. J. Am. Chem. Soc. 1999, 121, 1417–1418
.
.
(7) We reported high levels of enantioselection (up to 93% ee) in the
intermolecular cycloaddition of ester-derived carbonyl ylides with DMAD
using Rh2(S-PTTL)4 (1c): Kitagaki, S.; Yasugahira, M.; Anada, M.;
Nakajima, M.; Hashimoto, S. Tetrahedron Lett. 2000, 41, 5931–5935
.
3604
Org. Lett., Vol. 10, No. 16, 2008

phthaloyl- and N-benzene-fused-phthaloyl-(S)-amino acids
as the bridging ligands (Table 1, entries 1-6).16 While a
profile demonstrated that catalysis with Rh2(S-TCPTTL)4
performed exceptionally well over a wide temperature range
(entries 9 and 10), which was in stark contrast to that with
Rh2(S-PTTL)4 (60 °C, 32% yield, 56% ee; 100 °C, 14%
yield, 28% ee). Surprisingly, only 15% and 12% drops in
enantioselection and product yield, respectively, were ob-
served at 100 °C, suggesting that Rh2(S-TCPTTL)4 remains
even more strongly associated with the carbonyl ylide than
Rh2(S-PTTL)4. From frontier molecular orbital (FMO)
analysis for the reaction in the absence of a catalyst, the
dominant interaction is between the LUMO of the carbonyl
ylide derived from 4a and the HOMO of dipolarophile 5a,
which predicts the regiochemistry exactly as observed.20
Aside from the stereochemical outcome,17 these results
suggest that cycloaddition would occur on the opposite face
of the carbonyl ylide to the catalyst in concert with
dissociation of the catalyst as proposed by Hodgson.4c,13b
With the superiority of Rh2(S-TCPTTL)4 as a catalyst
verified, we then investigated the reactions of a range of
2-diazo-3,6-diketoester derivatives and arylacetylene dipo-
larophiles. The reaction of phenyl-substituted diazodike-
toester 4b with phenylacetylene (5a) afforded cycloadduct
6b in 64% yield with 98% ee (Table 2, entry 1). The reaction
Table 1. Enantioselective Intermolecular Cycloaddition of 4a
with 5a Catalyzed by 1a-ha
entry
Rh(II) catalyst
T (°C)
yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
Rh2(S-PTA)4 (1a)
Rh2(S-PTV)4 (1b)
23
23
23
23
23
23
23
23
60
100
35
45
56
53
60
65
80
82
80
70
70
80
89
75
82
87
92
97
95
82
Rh2(S-PTTL)4 (1c)
Rh2(S-BPTA)4 (1d)
Rh2(S-BPTV)4 (1e)
Rh2(S-BPTTL)4 (1f)
Rh2(S-TFPTTL)4 (1g)
Rh2(S-TCPTTL)4 (1h)
Rh2(S-TCPTTL)4 (1h)
Rh2(S-TCPTTL)4 (1h)
10
a All reactions were carried out as follows: a solution of 4a (48.0 mg,
0.2 mmol) and 5a (3 equiv) in CF3C6H5 (1 mL) was added over 1 h to a
solution of Rh(II) catalyst (1 mol %) in CF3C6H5 (1 mL) at the indicated
temperature. b Isolated yield. c Determined by HPLC.
Table 2. Enantioselective Intermolecular Cycloaddition of 4a-e
with 5a-g Catalyzed by Rh2(S-TCPTTL)4 (1h)a
uniform sense of asymmetric induction and perfect regiose-
lectivity were observed in all cases, the ee values were
dependent on the catalyst. Clearly, the best catalysts were
Rh2(S-PTTL)4 (1c) and Rh2(S-BPTTL)4 (1f), which are
characterized by a bulky tert-butyl group, as they provided
cycloadduct 6a17 in 56% and 65% yields with 89% and 87%
ee, respectively (entries 3 and 6). It is interesting to note
that the substituents of the amino acids more markedly
influenced enantioselectivity than the choice of N-phthaloyl-
or N-benzene-fused-phthaloyl groups. It is also noteworthy
that dirhodium(II) catalysts 1d-f characterized by an exten-
sion of the phthalimido wall with one additional benzene
ring provided even higher yields than the respective parent
dirhodium(II) catalysts 1a-c (entries 4-6 vs 1-3). We then
evaluated the performance of Rh2(S-TFPTTL)4 (1g)14b,d and
Rh2(S-TCPTTL)4 (1h),14a,c fluorinated and chlorinated ana-
logues of Rh2(S-PTTL)4, which could bring about an electron
deficiency on the rhodium(II) center (entries 7 and 8).18
Gratifyingly, the halogenated catalysts noticeably improved
the enantioselectivity and product yield obtained with Rh2(S-
PTTL)4, 97% ee with Rh2(S-TCPTTL)4 being the highest
achievement (entry 8). A survey of solvents with Rh2(S-
TCPTTL)4 revealed that benzotrifluoride was the optimal
solvent for this transformation in terms of both product yield
and enantioselectivity.19 An examination of the temperature
diazodiketoester
R1
dipolarophile
R2
product
entry
yieldb(%) eec(%)
1d
2
3
4
5
6
7
8
9
4b C6H5
4a Me
4a Me
4a Me
4a Me
4a Me
4a Me
4b C6H5
4c 4-ClC6H4
5a C6H5
5b 4-MeOC6H4 6c
5c 4-MeC6H4
5d 4-BrC6H4
5e 3-MeOC6H4 6f
5f 2-MeOC6H4 6g
5g EtO
5g EtO
5g EtO
5g EtO
6b
64
80
77
70
64
53
93
85
84
75
60
98
91
97
88
92
92
85
89
88
92
94
6d
6e
6h
6i
6j
6k
6l
10 4d 4-MeC6H4
11 4e 4-MeOC6H4 5g EtO
a All reactions were performed on a 0.2 mmol scale with 3 equiv of
dipolarophile. b Isolated yield. c Determined by HPLC. d 5 equiv of
phenylacetylene (5a) was used.
of 4a with phenylacetylenes bearing methoxy, methyl, and
bromo substituents at the para position also gave high
enantioselectivities (88-97% ee, entries 2-4).21 A high
enantioselectivity (92% ee) was maintained with m- or
o-methoxyphenylacetylenes, though a slight decrease in
(16) Hodgson and co-workers reported that the reaction of 4a with 5a
(10 equiv) under the influence of 1 mol % of Rh2(R-DDBNP)4 (3) in hexane
at 25 °C provided cycloadduct 6a in 41% yield with 61% ee. See ref 13.
(17) The preferred absolute stereochemistry of cycloadducts was not
determined.
(19) The product yields and enantioselectivities obtained with other
solvents are as follows: toluene, 73% yield, 95% ee; benzene, 76% yield,
91% ee; hexanes, 63% yield, 90% ee; Et2O, 23% yield, 72% ee; CH2Cl2,
14% yield, 40% ee.
(18) Recently, dirhodium(II) tetrakis[N-tetrachlorophthaloyl-(S)-(1-ada-
mantyl)glycinate], Rh2(S-TCPTAD)4, was developed by Reddy and Davies:
Reddy, R. P.; Davies, H. M. L. Org. Lett. 2006, 8, 5013–5016.
Org. Lett., Vol. 10, No. 16, 2008
3605

product yield was observed (entries 5 and 6). The reaction
of 4a,b with ethyl ethynyl ether (5g) proceeded smoothly to
provide cycloadducts 6h,i in high yields with 85% and 89%
ee, respectively (entries 7 and 8). The use of 5g was found
to allow for variation of para-substituents in aryl-substituted
diazodiketoesters 4c-e (88-94% ee, entries 9-11). To the
best of our knowledge, this is the first example of the use of
alkoxyacetylene as a dipolarophile in enantioselective car-
bonyl ylide cycloaddition reactions.
To expand the scope of this catalytic process, we also
examined styrene derivatives as dipolarophiles. Apart from
enantiocontrol, exo/endo diastereocontrol has remained a
major challenge in enantioselective carbonyl ylide cycload-
ditions with styrene dipolarophiles. Thus, we were gratified
to find that the reaction of methyl-substituted diazodiketoester
4a with styrene (7a, 3 equiv) produced cycloadduct 8a in
89% yield with perfect exo diastereoselectivity and 97% ee
(Table 3, entry 1).22 Furthermore, the reaction with phenyl-
formation-cycloaddition (entry 2). Aside from complete exo
diastereoselectivity, the highest enantioselectivity (99% ee)
was consistently observed regardless of the nature of para-
substituents in styrenes (entries 3-6). While an excellent
result was also obtained for m-methoxystyrene (7f) (entry
7), the reaction with o-methoxystyrene (7g) slightly dimin-
ished both the product yield and enantioselectivity (entry 8).
In summary, we have demonstrated that Rh2(S-TCPTTL)4
is an exceptionally effective catalyst for LUMO-controlled
carbonyl ylide cycloadditions of 2-diazo-3,6-diketoester
derivatives with arylacetylene, alkoxyacetylene, and styrene
dipolarophiles, in which high levels of asymmetric induction
(up to 99% ee) as well as perfect exo diastereoselectivity
for styrenes have been achieved.23,24 It is also noteworthy
that Rh2(S-DOSP)4, Rh2(R-DDBNP)4, and Rh2(S-TCPTTL)4
can complement each other in this type of carbonyl ylide
cycloaddition. Further studies on the scope of the reaction
as well as mechanistic and stereochemical studies are
currently in progress.
Acknowledgment. This research was supported in part
by a Grant-in-Aid for Scientific Research on Priority Areas
17035002 and 18032002 from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT). We thank
S. Oka, M. Kiuchi, and T. Hirose of the Center for
Instrumental Analysis at Hokkaido University for technical
assistance in MS and elemental analyses.
Table 3. Enantioselective Intermolecular Cycloaddition of 4a
and 4b with 7a-g Catalyzed by Rh2(S-TCPTTL)4 (1h)a
Supporting Information Available: Experimental details
and spectroscopic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
diazodiketoester dipolarophile
Ar
7a C6H5
7a C6H5
product
entry
R
yieldb (%) eec (%)
OL8013733
1
2
3
4
5
6
7
8
4a
4b
4b
4b
4b
4b
4b
4b
Me
8a
8b
89
85
94
90
80
73
88
75
97
99
99
99
99
99
98
90
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
(20) For the orbital correlation diagram, see the Supporting Information
(21) The use of p-nitrophenylacetylene as a strongly electron-deficient
dipolarophile resulted in 17% yield of cycloadduct with 72% ee
.
7b 4-MeOC6H4 8c
7c 4-MeC6H4 8d
7d 4-BrC6H4
7e 4-CF3C6H4 8f
7f 3-MeOC6H4 8g
7g 2-MeOC6H4 8h
.
8e
(22) Hodgson and co-workers reported that the reaction of 4a with 7a
(10 equiv) under Rh2(OAc)4 catalysis in CH2Cl2 at 20 °C afforded a 2:1
mixture of exo and endo cycloadducts in 53% yield, while the use of Rh2(S-
DOSP)4 (2) in hexane at 20 °C increased exo diastereoselectivity (exo:
endo ) 10:1, 73% yield) and provided 61% ee for exo cycloadduct 8a. See
a All reactions were performed on a 0.2 mmol scale with 3 equiv of
ref 13b
(23) Rh2(S-TCPTTL)4 was less effective for the reaction of 4a with
DMAD, giving the corresponding cycloadduct in 60% yield with 40% ee
.
dipolarophile. b Isolated yield. c Determined by HPLC.
.
(24) Very recently, Suga and co-workers have reported highly enanti-
oselective dipole-LUMO/dipolarophile-HOMO controlled cycloadditions
between 2-benzopyrylium-4-olates and vinyl ether derivatives using chiral
Lewis acid catalysts: Suga, H.; Ishimoto, D.; Higuchi, S.; Ohtsuka, M.;
Arikawa, T.; Tsuchida, T.; Kakehi, A.; Baba, T. Org. Lett. 2007, 9, 4359–
substituted diazodiketoester 4b exhibited enantioselectivity
of 99% ee, which is the highest ever recorded for chiral
dirhodium(II) complex-catalyzed tandem carbonyl ylide
4362
.
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Org. Lett., Vol. 10, No. 16, 2008