analysis of 6e (ESIw). All three newly formed chiral centers
possess the R configuration. It is worth pointing out that the
nitro group lies anti to the aryl group, occupying the less sterically
hindered position. This supports the proposed epimerization of
the center a to the nitro group towards the thermodynamically
more stable product.
H. Kim, Y. Park and J. Hong, Angew. Chem., Int. Ed., 2009, 48,
7577–7581; (k) M. Davi and H. Lebel, Org. Lett., 2009, 11, 41–44;
(l) J. Brioche, G. Masson and J. Zhu, Org. Lett., 2010, 12,
1432–1435; (m) S. De Sarkar, S. Grimme and A. Studer, J. Am.
Chem. Soc., 2010, 132, 1190–1191; (n) S. De Sarkar and A. Studer,
Org. Lett., 2010, 12, 1992–1995; (o) S. De Sarkar and A. Studer,
Angew. Chem., Int. Ed., 2010, 49, 9266–9269; (p) C. A. Rose and
K. Zeitler, Org. Lett., 2010, 12, 4552–4555; (q) J. H. Park,
S. V. Bhilare and S. W. Youn, Org. Lett., 2011, 13, 2228–2231;
(r) B. E. Maki, E. V. Patterson, C. J. Cramer and K. A. Scheidt,
Org. Lett., 2011, 13, 3942–3945; (s) M. Rueping, H. Sunden,
L. Hubener and E. Sugiono, Chem. Commun., 2012, 48, 2201–2203.
3 A. Quintard, A. Alexakis and C. Mazet, Angew. Chem., Int. Ed.,
2011, 50, 2354–2358.
4 (a) Y. Hayashi, T. Itoh and H. Ishikawa, Angew. Chem., Int. Ed.,
2011, 50, 3920–3924; (b) S.-L. Zhang, H.-X. Xie, J. Zhu, H. Li,
X.-S. Zhang, J. Li and W. Wang, Nat. Commun., 2011, 2, 211.
5 For the activation of propargyl aldehydes: (a) S. B. Jones,
B. Simmons and D. W. C. MacMillan, J. Am. Chem. Soc., 2009,
131, 13606–13607; (b) R. R. Knowles, J. Carpenter, S. B. Blakey,
A. Kayano, I. K. Mangion, C. J. Sinz and D. W. C. MacMillan,
Chem. Sci., 2011, 2, 308–311.
In summary, we have succeeded in developing a cascade
reaction combining a catalytic oxidation with an organocatalytic
enantioselective domino process. For a complex one pot reaction
sequence we chose a prolinol catalyzed iminium–allenamine
cascade as an example since it would be a good demonstration
for the viability of this combined process.
We were able to show for the first time that the use of
substrate-selective TPAP/NMO redox system is not only
compatible with diarylprolinolsilyl ether catalysts, but also
with different reactive functionalities and sensitive intermediates
and chromene products. Thus, the in situ formed propargyl
aldehydes could be further elaborated in the enantioselective
domino process. The oxidative cycle in this combined catalytic
procedure prevents the necessary purification or distillation step
associated with the use of aldehydes in organocatalysis and
should in particular be useful for reactions involving sensitive
aldehydes as substrates. Hence, the reported procedure repre-
sents a valuable alternative to previously described procedures
which do employ aldehydes and stoichiometric amounts of
oxidants such as IBX or DDQ.
6 (a) X. Zhang, S. Zhang and W. Wang, Angew. Chem., Int. Ed.,
2010, 49, 1481–1484; (b) C. Liu, X. Zhang, R. Wang and W. Wang,
Org. Lett., 2010, 12, 4948–4951; (c) J. Aleman, A. Nunez,
L. Marzo, V. Marcos, C. Alvarado and J. L. Garcıa Ruano,
Chem.–Eur. J., 2010, 16, 9453–9456.
7 S. V. Ley, J. Norman, W. P. Griffith and S. P. Marsden, Synthesis,
1994, 639–666.
8 (a) M. Marigo, T. C. Wabnitz, D. Fielenbach and K. A. Jørgensen,
Angew. Chem., Int. Ed., 2005, 44, 794–797; (b) Y. Hayashi,
H. Gotoh, T. Hayashi and M. Shoji, Angew. Chem., Int. Ed.,
2005, 44, 4212–4215; Reviews on prolinol TMS-ether catalysis:
(c) C. Palomo and A. Mielgo, Angew. Chem., Int. Ed., 2006, 45,
7876–7880; (d) A. Mielgo and C. Palomo, Chem.–Asian. J., 2008, 3,
922–948.
9 For a review, see: B. A. Keay and P. W. Dibble, in Comprehensive
Heterocyclic Chemistry II, ed. A. R. Katritzky, C. W. Rees and
E. F. V. Scriven, Oxford, Pergamon, 1996, vol. 2, pp. 395–436.
10 Reviews on organcatalytic synthesis of biologically active molecules:
(a) E. Marques-Lopez, R. P. Herrera and M. Christmann, Nat. Prod.
Rep., 2010, 27, 1138–1167; (b) R. M. de Figueiredo and
M. Christmann, Eur. J. Org. Chem., 2007, 2575–2600.
11 (a) M. Rueping, E. Sugiono and E. Merino, Angew. Chem., Int.
Ed., 2008, 47, 3046–3049; (b) M. Rueping, E. Sugiono and
E. Merino, Chem.–Eur. J., 2008, 14, 6329–6332; (c) M. Rueping,
E. Merino and E. Sugiono, Adv. Synth. Catal., 2008, 350,
2127–2131; (d) M. Rueping, U. Uria, M.-Y. Lin and
I. Atodiresei, J. Am. Chem. Soc., 2011, 133, 3732–3735;
(e) M. Rueping and M.-Y. Lin, Chem.–Eur. J., 2010, 16,
4169–4172.
12 (a) H. Gotoh, H. Ishikawa and Y. Hayashi, Org. Lett., 2007, 9,
5307–5309; (b) C. Palomo, A. Landa, A. Mielgo, M. Oiarbide,
A. Puente and S. Vera, Angew. Chem., Int. Ed., 2007, 46,
8431–8435; (c) L. Zu, H. Xie, H. Li, J. Wang and W. Wang,
Adv. Synth. Catal., 2007, 349, 2660–2664; (d) Y. Wang, P. Li,
X. Liang, T. Y. Zhang and J. Ye, Chem. Commun., 2008,
1232–1234.
Important 4H-chromene derivatives could be synthesized by
this operationally simple cascade, under mild conditions, in
good yields and with excellent enantioselectivities. By applying
a subsequent domino iminium–enamine cascade, we were
able to prepare new complex tricyclic 4H-chromenes. In this
sequential cascade, four bonds were formed and three chiral
centers were controlled, yielding highly functionalized struc-
tures from simple starting materials (i.e. a propargyl alcohol,
a cinnamaldehyde and a nitrostyrene derivative). This first
unification of metal catalyzed oxidation with organocatalysis
expands the scope of asymmetric covalent catalysis and combined
catalytic procedures and we are confident that future develop-
ments will soon arise.
The authors acknowledge financial support by European
Research Council (ERC starting grant).
Notes and references
1 Reviews: (a) D. Enders, C. Grondal and M. R. M. Huttl, Angew.
Chem., Int. Ed., 2007, 46, 1570–1581; (b) A. Dondoni and
A. Massi, Angew. Chem., Int. Ed., 2008, 47, 4638–4660;
(c) A. N. Alba, X. Companyo, M. Viciano and R. Rios, Curr.
Org. Chem., 2009, 13, 1432–1474; (d) C. Grondal, M. Jeanty and
D. Enders, Nat. Chem., 2010, 2, 167–178.
2 Review on tandem oxidation processes: (a) R. J. K. Taylor,
M. Reid, J. Foot and S. A. Raw, Acc. Chem. Res., 2005, 38,
851–869; Selected examples of tandem oxidation processes:
(b) M. F. Oswald, S. A. Raw and R. J. K. Taylor, Org. Lett.,
2004, 6, 3997–4000; (c) S. A. Raw, C. D. Wilfred and
R. J. K. Taylor, Org. Biomol. Chem., 2004, 2, 788–796;
(d) M. F. Oswald, S. A. Raw and R. J. K. Taylor, Chem. Commun.,
2005, 2253–2255; (e) T. Ngouansavanh and J. Zhu, Angew. Chem.,
Int. Ed., 2006, 45, 3495–3497; (f) B. E. Maki, A. Chan,
E. M. Phillips and K. A. Scheidt, Org. Lett., 2007, 9, 371–374;
(g) M. I. Hall, S. J. Pridmore and J. M. J. Williams, Adv. Synth.
Catal., 2008, 350, 1975–1978; (h) B. E. Maki and K. A. Scheidt,
Org. Lett., 2008, 10, 4331–4334; (i) K. Zeitler and C. A. Rose,
J. Org. Chem., 2009, 74, 1759–1762; (j) H. Kim, Y. Park, J. Hong,
13 For Michael/aldol condensation reaction from our group:
(a) M. Rueping, A. Kuenkel, F. Tato and J. W. Bats, Angew.
Chem., Int. Ed., 2009, 48, 3699–3702; (b) M. Rueping, A. Kuenkel
and R. Frohlich, Chem.–Eur. J., 2010, 16, 4173–4176.
14 Selected publications: (a) D. Enders, M. R. M. Huttl, C. Grondal
and G. Raabe, Nature, 2006, 441, 861–863; (b) Z. Fang-Lin,
A.-W. Xu, Y.-F. Gong, M.-H. Wei and X.-L. Yang, Chem.–Eur.
J., 2009, 15, 6815–6818; (c) D. Enders, M. Jeanty and J. W. Bats,
Synlett, 2009, 3175–3178; (d) P. Kotame, B.-C. Hong and
J.-H. Liao, Tetrahedron Lett., 2009, 50, 704–707; (e) D. Enders,
C. Wang, M. Mukanova and A. Greb, Chem. Commun., 2010, 46,
2447–2449; (f) B.-C. Hong, N. S. Dange, C.-S. Hsu and J.-H. Liao,
Org. Lett., 2010, 12, 4812–4815; (g) B.-C. Hong, P. Kotame,
C.-W. Tsai and J.-H. Liao, Org. Lett., 2010, 12, 776–779;
(h) M. Rueping, K. Haack, W. Ieawsuwan, H. Sunden,
M. Blanco and F. R. Schoepke, Chem. Commun., 2011, 47,
3828–3830.
c
3408 Chem. Commun., 2012, 48, 3406–3408
This journal is The Royal Society of Chemistry 2012