M. I. de Sairre et al. / Tetrahedron Letters 46 (2005) 2705–2708
2707
11).5d Substrate conversion reached a fairly good value
(65–100%) with most of the b-keto esters and alcohols
employed, but did not exceed 50% when benzyl alcohol
was used (Table 1, entry 10), probably due to the easy
dehydration of this compound to form benzyl ether in
the reaction conditions. As volatile alcohols can also
be removed by distillation during the transesterification
process, gradual addition of new portions of alcohol sig-
nificantly increased substrate conversion. The transeste-
rification of methyl acetoacetate with allylic alcohol and
glycidol produced better results than the corresponding
ethyl derivative (Table 1, entries 1, 2, 4, and 5). The nat-
ure of the R3 group in the alcohols had a significant
effect on the transesterification reactions. Considering
both conversion and isolated yields, primary alcohols
afforded much superior results to those obtained with
secondary and tertiary alcohols (Table 1, entries 7–9).
Allylic alcohol reacted with different b-keto esters (Table
1, entries 1–3), leading to good conversions (65–85%)
and moderate isolated yields (52–58%). These latter re-
sults show this process is very useful, since the transeste-
rification of allylic alcohols is rather difficult due to
successive decarboxylation and rearrangement.8 Good
results were also obtained with glycidol (Table 1, entries
4–6). In the case of niobium(V) oxide, catalyst lifetime
could be maximized due to its easy recovery. In fact,
niobium(V) oxide was recovered and reused at least
three times in our experiments, without appreciable
activity loss, as indicated in the transesterification of n-
butyl alcohol (Table 1, entry 7).
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In summary, we have demonstrated that niobium(V)
oxide serves as an efficient catalyst for the transesterifi-
cation of b-keto esters with several kinds of alcohols,
leading to good conversion and moderate to good iso-
lated product yields. Therefore, it is expected that this
catalyst will find general application for the preparation
of these important compounds in the future.
Acknowledgements
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`
The authors wish to thank the Fundac¸a˜o de Amparo a
Pesquisa do Estado de Sa˜o Paulo (FAPESP), and the
´
Conselho Nacional de Desenvolvimento Cientıfico e
Tecnologico (CNPq) for financial support. The generous
´
gift of niobium oxide samples by CBMM–Companhia
Brasileira de Metalurgia e Minerac¸a˜o is also gratefully
acknowledged.
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Pandey, R. K.; Deshmukh, A. N.; Kumar, P. Synth.
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