A limitation of native chemical ligation is its intrinsic
reliance on having a cysteine residue at the ligation juncture.
Cysteine is uncommon, comprising only 1.7% of all residues
in proteins.5 Modern peptide synthesis is typically limited
to peptides of e50 residues.3 Hence, most proteins cannot
be prepared by any method that allows for peptides to be
coupled only at cysteine residues.
Scheme 2
The removal of the cysteine limitation by applying a more
general ligation technology would greatly expand the utility
of total protein synthesis. The Staudinger reaction provides
such an alternative. In the Staudinger reaction, a phosphine
is used to reduce an azide to an amine: PR3 + N3R′ + H2O
f OdPR3 + H2NR′ + N2(g).6 This reaction occurs via a
stable intermediate, an iminophosphorane (R3P+--NR′),
which has a nucleophilic nitrogen. Vilarrassa and others have
shown that this nitrogen can be acylated, both in intermo-
lecular and intramolecular reactions.7,8 Hydrolysis of the
resulting amidophosphonium salt gives an amide and phos-
phine oxide. Saxon and Bertozzi have shown that the
phosphine can itself serve as the acyl donor.8a
Recently, we reported the use of the “Staudinger ligation”
to form a peptide.8b In our initial work, an amide bond was
formed from a thioester of phosphinothiol 1 and an azide.
has no reliance upon a cysteine or any other specific residue
at the N-terminus of the peptide fragment. In addition, the
method is tracelesssno residual atoms from the phosphi-
nothiol remain in the peptide product. The model amides
synthesized with this original phosphinothiol are listed in
Table 1.8b Although phosphinothiol 1 does enable amide bond
formation, the isolated yields for these Staudinger ligations
are too low for some applications.
The reaction likely proceeds by the intramolecular re-
arrangement of an iminophosphorane intermediate to give
an amidophosphonium salt, as shown in Scheme 2. This salt
hydrolyzes to leave an amide and o-(diphenylphosphinyl)-
benzenethiol. It is noteworthy that the Staudinger ligation
Staudinger ligation with phosphinothiol 1 occurs through
a transition state with a six-membered ring. We reasoned
Table 1. Yields for Staudinger Ligation with Phosphinothiols 1
and 2a
(5) Only tryptophan (1.3%) is less common. McCaldon, P.; Argos, P.
Proteins 1988, 4, 99-122.
(6) (a) Staudinger, H.; Meyer, J. HelV. Chim. Acta 1919, 2, 635-646.
For comprehensive reviews, see: (b) Gololobov, Yu. G.; Zhmurova, I. N.;
Kasukhin, L. F. Tetrahedron 1981, 37, 437-472. (c) Gololobov, Yu. G.;
Kasukhin, L. F. Tetrahedron 1992, 48, 1353-1406.
(7) For examples, see: (a) Garcia, J.; Urp´ı, F.; Vilarrasa, J. Tetrahedron
Lett. 1984, 25, 4841-4844. (b) Garcia, J.; Vilarrasa, J. Tetrahedron Lett.
1986, 27, 639-640. (c) Urp´ı, F.; Vilarrasa, J. Tetrahedron Lett. 1986, 27,
4623-4624. (d) Bosch, I.; Romea, P.; Urp´ı, F.; Vilarrasa, J. Tetrahedron
Lett. 1993, 34, 4671-4674. (e) Inazu, T.; Kobayashi, K. Synlett. 1993, 869-
870. (f) Molina, P.; Vilaplana, M. J. Synthesis-Stuttgart 1994, 1197-1218.
(g) Bosch, I.; Urp´ı, F.; Vilarrasa, J. J. Chem. Soc., Chem. Commun. 1995,
91-92. (h) Shalev, D. E.; Chiacchiera, S. M.; Radkowsky, A. E.; Kosower,
E. M. J. Org. Chem. 1996, 61, 1689-1701. (i) Bosch, I.; Gonzalez, A.;
Urp´ı, F.; Vilarrasa, J. J. Org. Chem. 1996, 61, 5638-5643. (j) Maunier,
V.; Boullanger, P.; Lafont, D. J. Carbohydr. Res. 1997, 16, 231-235. (k)
Afonso, C. A. M. Synth. Commun. 1998, 28, 261-276. (l) Tang, Z.;
Pelletier, J. C. Tetrahedron Lett. 1998, 39, 4773-4776. (m) Ariza X.; Urp´ı,
F.; Viladomat, C.; Vilarrasa J. Tetrahedron Lett. 1998, 39, 9101-9102.
(n) Mizuno, M.; Muramoto, I.; Kobayashi, K.; Yaginuma, H.; Inazu, T.
Synthesis-Stuttgart 1999, 162-165. (o) Mizuno, M.; Haneda, K.; Iguchi,
R.; Muramoto, I.; Kawakami, T.; Aimoto, S.; Yamamoto, K.; Inazu, T. J.
Am. Chem. Soc. 1999, 121, 284-290. (p) Boullanger P.; Maunier, V.;
Lafont, D. Carbohydr. Res. 2000, 324, 97-106. (q) Velasco, M. D.; Molina,
P.; Fresneda, P. M.; Sanz, M. A. Tetrahedron 2000, 56, 4079-4084. (r)
Malkinson, J. P.; Falconer, R. A.; Toth, I. J. Org. Chem. 2000, 65, 5249-
5252.
(8) (a) Saxon, E.; Bertozzi, C. R. Science 2000, 287, 2007-2010. (b)
Nilsson, B. L.; Kiessling, L. L.; Raines, R. T. Org. Lett. 2000, 2, 1939-
1941. (c) Saxon, E.; Armstrong, J. I.; Bertozzi, C. R. Org. Lett. 2000, 2,
2141-2143.
a Conditions: THF/H2O (3:1); room temperature; 12 h.
10
Org. Lett., Vol. 3, No. 1, 2001