M. Amjad, D. W. Knight / Tetrahedron Letters 45 (2004) 539–541
541
quite variable and particularly poor with a combination
of a carbamate and an alkyl-substituted alkyne function.
Significantly, Barluenga found that a combination of an
N-methane sulfonyl group and an alkylalkyne acceptor
group led to a complex mixture when such substrates
were exposed to conditions similar to those used in the
present study. We have no certain explanation for this,
although it is possible that we were fortunate to use
N-toluene sulfonyl groups wherein perhaps the addi-
tional electron withdrawing power of the aryl group is
sufficient to tip the balance in favour of suitably rapid
N–H bond cleavage, as the cyclisation proceeds. Most
likely, future applications will reveal a complementarity
between the two methods when these are applied to
more elaborate substrates. One notable feature of the
2. Bew, S. P.; Knight, D. W. J. Chem. Soc., Chem. Commun.
996, 1007–1008; El-Taeb, G. M. M.; Evans, A. B.;
Knight, D. W.; Jones, S. Tetrahedron Lett. 2001, 42, 5945–
948.
1
5
3
. Knight, D. W.; Redfern, A. L.; Gilmore, J. J. Chem. Soc.,
Perkin Trans. 1 2001, 2874–2883; and 2002, 622–628.
. Areadi, A.; Cacchi, S.; Fabrizi, G.; Marinelli, F.; Moro, L.
Synlett 1999, 1432–1434; See also Buckle, D. R.; Rock-
well, C. J. M. J. Chem. Soc., Perkin Trans. 1 1985, 2443–
2446.
4
5. For a recent contribution to this methodology, See Ma,
C.; Yu, S.; He, X.; Liu, X.; Cook, J. M. Tetrahedron Lett.
2
000, 41, 2781–2785.
6
. For exceptions, see, for example, Burns, B.; Grigg, R.;
Sridharan, V.; Stevenson, P.; Sukirthalingam, S.; Wor-
akun, T. Tetrahedron Lett. 1989, 30, 1135–1138; Luo,
F.-T.; Wang, R.-T. Heterocycles 1991, 32, 2365–2372;
Grigg, R.; Sridharan, V.; Zhang, J. Tetrahedron Lett.
7
Barluenga method is that it can be applied successfully
to unprotected 2-alkynylanilines to give directly 60–75%
yields of 2-substituted-3-iodoindoles (2-Ph and 2-Bu).
However, N-tosyl groups are relatively easy to remove
from indoles, as we have recently pointed out in a report
1999, 40, 8277–8280; Kondo, Y.; Shiga, F.; Murata, N.;
Sakamoto, T.; Yamanaka, H. Tetrahedron 1994, 50,
11803–11812.
7. Barluenga, J.; Trincado, M.; Rubio, E.; Gonz ꢀa lez, J. M.
Angew. Chem. 2003, 115, 2508–2511; Angew. Chem., Int.
Ed. 2003, 42, 2406–2409.
11
of a new method for achieving this using thioglycolate.
8
. For a review of electrophile-driven 5-endo cyclisations,
see Knight, D. W. Prog. Heterocycl. Chem. 2002, 14, 19–
Acknowledgements
5
1.
9
. Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron
Lett. 1975, 4467–4470; Sonogashira, K. In Comp. Org.
Synth.; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford,
We thank Professor Thomas Wirth for very helpful
comments, Cardiff University for partial support of this
project and the EPSRC Mass Spectrometry Service,
University College, Swansea for the provision of high
resolution mass spectrometric data.
1
991; Vol. 3; pp 521–549.
1
0. Koci ꢀe nski, P. J. Protecting Groups. In: Foundations of
Organic Chemistry Series; Georg Thieme: Stuttgart, 1994;
For recent modifications, see Maia, H. L. S.; Monteiro, L.
S.; Sebastiao, J. J. Org. Chem. 2001, 1967–1970 (Mg–
MeOH or electrolysis); Chakrabarty, M.; Ghosh, N.;
Khasnobis, S.; Chakrabarty, M.. Synth. Commun. 2002,
32, 265–272 (DBU–MeOH).
References and Notes
1
. Li, J. J.; Gribble, G. W. Palladium in Heterocyclic
Chemistry. In: Tetrahedron Organic Chemistry Series;
Pergamon: Oxford, 2000; Vol. 20.