Table
annulation
1 Amino acid salt catalyzed intramolecular Robinson
amino acid salt catalysts were absorbed on silica gel (entry 7).
More importantly, such silica gel absorbed catalysts could be
recovered very easily by filtration or centrifugation/decantation,
and reused for three repeated runs without significant loss of
catalytic activity or enantioselectivity (entry 7, 1st run, 84%
yield, 97% ee; 2nd run, 71% yield, 96% ee; 3rd run, 80% yield,
96% ee). This merit should be attributed to the ionic nature of
these amino acid salt catalysts, making it readily separable
along with the absorbent silica gel from the organic reaction
mixture.12
Entry
Catalyst
Yielda (%)
eeb (%)
In summary, an amino acid salt catalyzed intramolecular
Robinson annulation reaction has been developed, which
provides a simple route to a tricyclic ring structure resembling
the important antibiotic compound platencin. By using silica
gel as absorbent, this type of catalyst is readily recoverable and
reusable. We believe this type of silica gel absorbed amino acid
salt catalyst could find lots of applications in organocatalytic
reactions. Research along these lines, as well as the application
of current methodology to the asymmetric synthesis of
platencin, is currently underway in this laboratory.
1c
2
M = H
= Li
= Li
NRd
23
—
91
54e
38
96e
91
3
4
5
6
= Na
= K
14
17
70
92
= Rb
= Cs
= Cs
50
94
29e
31
96e
97
7
= NnBu4
= NnBu4
= NnBu4
= NnBu4
84f
71g
80h
97f
96g
96h
We thank Professors Shoji Hara and Masanori Yoshida
(Hokkaido University) for stimulating discussions and sharing
with us their results before publication. We also thank
Professor Hans-Ulrich Reissig (Free University of Berlin) for
stimulating discussions. Support of this research has been
provided by NIH (grant number: 5R01GM068433-06).
8e
9e
25
36
90
94
Notes and references
1 For recent reviews, see: (a) Special issue: Organocatalysis, ed.
B. List, Chem. Rev., 2007, 107, pp. 5413–5883;
(b) Enantioselective Organocatalysis, ed. P. I. Dalko, Wiley-VCH,
Weinheim, 2007; (c) Special issue: Asymmetric Organocatalysis,
ed. K. N. Houk and B. List, Acc. Chem. Res., 2004, 37,
pp. 487–631; (d) P. I. Dalko and L. Moisan, Angew. Chem.,
2004, 116, 5248–5286 (Angew. Chem., Int. Ed., 2004, 43,
5138–5175).
2 For outstanding examples, see: (a) H. Ishikawa, T. Suzuki and
Y. Hayashi, Angew. Chem., 2009, 121, 1330–1333 (Angew. Chem.,
Int. Ed., 2009, 48, 1304–1307); (b) D. Enders, M. R. M. Huttl,
J. Runsink, G. Raabe and B. Wendtc, Angew. Chem., 2007, 119,
471–473 (Angew. Chem., Int. Ed., 2007, 46, 467–469);
(c) D. Enders, M. R. M. Huttl, C. Grondal and G. Raabe, Nature,
2006, 441, 861–863.
10e
56
88
11e
12e
36
64
—
NRd
3 (a) P. Li, J. N. Payette and H. Yamamoto, J. Am. Chem. Soc.,
2007, 129, 9534–9535; for a recent review, see; (b) K. Tiefenbacher
and J. Mulzer, Angew. Chem., 2008, 120, 2582–2590 (Angew. Chem.,
Int. Ed., 2008, 47, 2548–2555).
4 For related asymmetric intramolecular Michael additions, see:
(a) G. Stork, C. S. Shiner and J. D. Winkler, J. Am. Chem. Soc.,
1982, 104, 310–312; (b) M. T. H. Fonseca and B. List, Angew.
Chem., 2004, 116, 4048–4050 (Angew. Chem., Int. Ed., 2004, 43,
3958–3960); (c) Y. Hayashi, H. Gotoh, T. Tamura, H. Yamaguchi,
R. Masui and M. Shoji, J. Am. Chem. Soc., 2005, 127,
16028–16029; (d) M. Kikuchi, T. Inagaki and H. Nishiyama,
Synlett, 2007, 1075–1078.
13e
NRd
—
a
b
Isolated yield. Determined by HPLC on OJ–H column. CH2Cl2
c
d
is used as solvent. NR = no reaction. Et2O is used as solvent. 1st
e
f
g
run, 50 mol% silica gel absorbed catalyst is used. 2nd run, using
h
50 mol% recovered silica gel absorbed catalyst from 1st run. 3rd run,
using 50 mol% recovered silica gel absorbed catalyst from 2nd run.
Aromatic substituted alanine derived salt provided the best
results (entries 2–10); leucine derived lithium salt was also
active (entry 11), while tert-leucine (entry 12) and tryptophan
(entry 13) derived lithium salt gave no conversion. Ether-type
solvents were found to give the highest yields and ee’s, while
the enantioselectivity varied when different counter cations
were used (entries 2–7), suggesting that Lewis acid strength
and steric effects of corresponding solvated cations are
important factors for asymmetric induction. The yield of this
reaction was significantly improved (84% yield, 97% ee) when
5 (a) J. Wang, S. Kodali, S. H. Lee, A. Galgoci, R. Painter,
K. Dorso, F. Racine, M. Motyl, L. Hernandez, E. Tinney,
S. L. Colletti, K. Herath, R. Cummings, O. Salazar, I. Gonzalez,
A. Basilio, F. Vicente, O. Genilloud, F. Pelaez, H. Jayasuriya,
K. Young, D. F. Cully and S. B. Singh, Proc. Natl. Acad. Sci. U. S. A.,
2007, 104, 7612–7616; (b) H. Jayasuriya, K. B. Herath, C. Zhang,
D. L. Zink, A. Basilio, O. Genilloud, M. T. Diez, F. Vicente,
I. Gonzalez, O. Salazar, F. Pelaez, R. Cummings, S. Ha, J. Wang
and S. B. Singh, Angew. Chem., 2007, 119, 4768–4772 (Angew. Chem.,
Int. Ed., 2007, 46, 4684–4688).
6 The chiral phosphoramide used is (S)-{3,30-bis[2,4,6-triisopropyl-
phenyl]-1,10-binaphthalen-2,20-diyl}-N-triflyl
phosphoramide,
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5412–5414 | 5413