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4. (a) Matsushita, M.; Maeda, H.; Kodama, M. Tetrahedron
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Universitat de Barcelona, 1995; (c) Bosch, I.; Gonza´lez,
A.; Urp´ı, F. J. Org. Chem. 1996, 61, 5638–5643; (d)
Viladomat, C. Master Thesis; Universitat de Barcelona,
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Lett. 1999, 40, 7515–7517 and references cited therein; (f)
Alternative approach (Fe, NH4Cl, MeOH, ROCOX, son-
ication): Chandrasekhar, S.; Narsihmulu, C. Tetrahedron
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Lett. 1998, 39, 3749–3752. Also see: (b) Ishizuka, T.;
Kunieda, T. Tetrahedron Lett. 1987, 28, 4185–4188; (c)
Bew, S. P.; Bull, S. D.; Davies, S. G. Tetrahedron Lett.
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S.; Katsumura, S. Org. Lett. 2000, 2, 2627–2629.
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10071–10074; (b) Ghosh, A. K.; Shin, D.; Mathivanan, P.
Chem. Commun. 1999, 1025–1026; (c) Bach, T.; Schro¨der,
J. Tetrahedron Lett. 1997, 38, 3707–3710; (d) Curran, T.
P.; Pollastri, M. P.; Abelleira, S. M.; Messier, R. J.;
McCollum, T. A.; Rowe, C. G. Tetrahedron Lett. 1994,
35, 5409–5412; (e) Lago, M. A.; Samanen, J.; Elliott, J. D.
J. Org. Chem. 1992, 57, 3493–3496.
13. By reaction of phosphazenes (or of phosphatriazenes, see
Ref. 12a) as soon as formed with the protection reagent
already present in the flask, instead of by the known
procedure of hydrolysis (of phosphazenes to amines)
followed by protection. When this protection cannot be
performed in situ owing to the reagent incompatibility
with the aqueous medium, when amines may react with
other groups, etc. (see Refs. 10a and 12e), direct conver-
sion of azides to protected amines can be advantageous.
14. Vicinal amino alcohols, or 1,2-amino alcohols, are often
so-called b-amino alcohols but other authors name them
a-amino alcohols, owing to their relationship with a-
amino acids.
6. Greene, T. W.; Wuts, P. G. M. Protective Groups in
Organic Synthesis; Wiley: New York, 1999.
7. (a) Wu, Y.; Shen, X. Tetrahedron: Asymmetry 2000, 11,
4359–4363 and Ref. 3 cited therein; (b) Ziegler, T.; Jurisch,
C. Tetrahedron: Asymmetry 2000, 11, 3403–3418; (c)
Gabriele, B.; Salerno, G.; Brindisi, D.; Costa, M.; Chiu-
soli, G. P. Org. Lett. 2000, 2, 625–627; (d) Tascedda, P.;
Dun˜ach, E. Chem. Commun. 2000, 449–450; (e) Casadei,
M. A.; Feroci, M.; Inesi, A.; Rossi, L.; Sotgiu, G. J. Org.
Chem. 2000, 65, 4759–4761; (f) Tanimori, S.; Kirihata, M.
Tetrahedron Lett. 2000, 41, 6785–6788.
8. For ‘correlations’ among isocyanates, carbamates, ureas,
oxazolidinones and N-Boc-oxazolidinones, from vicinal
amino alcohols, Boc2O, and 4-dimethylaminopyridine
(DMAP), see: (a) Kno¨lker, H.-J.; Braxmeier, T. Tetra-
hedron Lett. 1998, 39, 9407–9410 and references cited
therein. Also see: (b) Gastaldi, S.; Weinreb, S. M.; Stien,
D. J. Org. Chem. 2000, 65, 3239–3240; (c) Kim, Y. H.;
Park, H. S. Synlett 1998, 261–262; (d) Raspoet, G.;
Nguyen, M. T.; McGarraghy, M.; Hegarty, A. F. J. Org.
Chem. 1998, 63, 6878–6885.
9. (a) Annis, D. A.; Helluin, O.; Jacobsen, E. N. Angew.
Chem., Int. Ed. 1998, 37, 1907–1909; (b) Benedetti, F.;
Berti, F.; Norbedo, S. Tetrahedron Lett. 1998, 39, 7971–
7974 and references cited therein; (c) Kamal, A.; Arifud-
din, M.; Rao, M. V. Tetrahedron: Asymmetry 1999, 10,
4261–4264; (d) Lebel, H.; Jacobsen, E. N. Tetrahedron
Lett. 1999, 40, 7303–7306; (e) Song, C. E.; Oh, C. R.; Roh,
E. J.; Choo, D. J. Chem. Commun. 2000, 1743–1744; (f)
Review: Jacobsen, E. N.; Wu, M. H. In Comprehensive
Asymmetric Catalysis; Jacobsen, E. N.; Pfaltz, A.;
Yamamoto, H., Eds.; Springer: Berlin, 1999; pp. 1309–
1326.
10. For methods based on the azide hydrogenation and
reaction in situ with Boc2O, see: (a) Saito, S.; Nakajima,
H.; Inaba, M.; Moriwake, T. Tetrahedron Lett. 1989, 30,
837–838; (b) Woltering, T. J.; Weiz-Schmidt, G.; Wong,
C.-H. Tetrahedron Lett. 1996, 37, 9033–9036; (c) Kotsuki,
H.; Ohishi, T.; Araki, T. Tetrahedron Lett. 1997, 38,
2129–2132; (d) Lange, M.; Pettersen, A. L.; Undheim, K.
Tetrahedron 1998, 54, 5745–5752. Examples in which
hydrogenation is counterindicated or inefficient: (e)
Yamaguchi, T.; Harada, N.; Ozaki, K.; Hashiyama, T.
Tetrahedron Lett. 1998, 39, 5575–5578 (taxol-related com-
pounds); (f) Pearson, A. J.; Lee, K. J. Org. Chem. 1994,
59, 2304–2313 (ACE inhibitor).
15. (a) Afonso, C. A. M. Tetrahedron Lett. 1995, 36, 8857–
8858; (b) Afonso, C. A. M. Synth. Commun. 1998, 28,
261–276.
16. The carbodiimide arises likely from the reaction of isocy-
anate PhCH(Me)NCO with remaining phosphazene. Since
this carbodiimide is formed in significant amounts when
Boc2O is used, but not with BocON (2-t-butoxycarbonyl-
oxyimino-2-phenylacetonitrile), nor with a previously pre-
pared BocON/DMAP mixture (as a source of DMAP+–
COOtBu), nor with other carboxylating/acylating agents
(Ref. 12e), it can be assumed that the isocyanate comes
from the presence in the reaction medium of CO2 or a
closely related precursor. In fact, Boc2O, a dicarbonate, is
an inherent source of CO2. For an example, see: (a)
Molina, P.; Alajar´ın, M.; Sa´nchez-Andrada, P. Tetra-
hedron Lett. 1993, 34, 5155–5158. For a related reaction
(conversion of poorly nucleophilic amines to isocyanates
with Boc2O/DMAP) and outstanding mechanistic studies,
see: (b) Kno¨lker, H. J.; Braxmeier, T.; Shlechtingen, G.
Angew. Chem., Int. Ed. Engl. 1995, 34, 2497–2500. Review
on DMAP-catalysed acylations: (c) Ragnarsson, U.;
Grehn, L. Acc. Chem. Res. 1998, 31, 494–501. Review on
Boc2O: (d) Encyclopedia of Reagents for Organic Synthe-
sis; Paquette, L. A., Ed.; Wiley: Chichester, 1995; Vol. 3.
For other relevant works, see: (e) Kemp, D. S.; Curran, T.
P. J. Org. Chem. 1988, 53, 5729–5731 (isolation of
R2NCOOCOOtBu in a special case); (f) Saylik, D.; Hor-
vath, M. J.; Elmes, P. S.; Jackson, W. R.; Lovel, C. G.;
Moody, K. J. Org. Chem. 1999, 64, 3940–3946 (iso-
cyanates from primary amines and CO2).
17. In our hands, Boc2O (Fluka, ]99.5% purity, checked by
13C NMR) gave bubbles, clearly seen at rt at concentra-
tions higher than 0.1 M, when mixed with 2 equiv. of
DMAP and also (but more smoothly) with 1:1 DMAP–
Me3P or with 2 equiv. of Me3P in anhydrous THF. The
nucleophile-catalysed decomposition of Boc2O, in general,
might be summarised as in Scheme 5 (further possible
reactions of intermediates with Nu, or between species
such as Nu+–COO− and Nu+–COOtBu, are not included
for the sake of simplification), with preferred courses
depending on the features of each substrate, temperature,
11. Review
on
phosphazenes
(iminophosphoranes):
Gololobov, Y. G.; Kasukhin, L. F. Tetrahedron 1992, 48,
1353–1406.