4896
M. Curini et al. / Tetrahedron Letters 43 (2002) 4895–4897
Table 1. Carbamate synthesis catalysed by Yb(OTf)3
Acknowledgements
Financial support from Universita` degli Studi di Peru-
gia ‘Progetti d’Ateneo’ is gratefully acknowledged.
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few mL of CH2Cl2 to the reaction mixture precipitated
the catalyst with the consequence of its simple recovery
by filtration in nearly quantitative yield. The catalyst
could be reused several times without any loss of activ-
ity: the reaction to give compound 1 has been in fact
repeated five times through the catalyst washed with
CH2Cl2 and dried at 70°C for 2 h, with the following
yields: 92, 91, 91, 89, and 91%. The filtrate, washed
with a solution of citric acid, afforded after evaporation
the pure carbamate, without the need of further
purification.
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In summary, the method of carbamate synthesis
described herein compares favourably with heretofore
reported methodologies, especially in terms of product
yields, easy work-up procedure, absence of toxic or
hardly removable by-products and simple recovery and
complete recyclability of the catalyst used.
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19. General procedure: To a solution of amine (1.0 mmol) in
dimethyl carbonate (5.0 mmol), Yb(OTf)3 (0.05 mmol)
was added and the resulting mixture stirred for 8 h at
80°C; CH2Cl2 (2 mL) was added at room temperature,
the catalyst so precipitated was filtered under vacuum;