C. Marette et al. / Tetrahedron Letters 47 (2006) 6947–6950
6949
The scope of this reaction to more functionalized com-
Acknowledgements
pounds has been explored taking into account both
the yield and the reaction time (Table 1). One important
aim of this study was to establish a strategy allowing the
isolation of the desired product in the most time efficient
fashion. This has been achieved by a filtration–precipita-
tion process that allows the isolation of a pure product
in less than 30 min leading to an ‘overall production
time’ (reaction + purification) below 2 h.
The authors gratefully thanks the Laboratoires Cyclo-
pharma for Ph.D. fellowship (C.M.) as well as the
CNRS and the University Paul Sabatier for financial
support. We are also indebted to Isochem (group SNPE)
for the precious gift of HBGCÆHCl and to Dr. J. P.
Senet for fruitful discussions.
The coumarin moiety proved stable under these reaction
conditions yielding 75% of the benzoylated product in
40 min (entry 1). This reaction was found especially effi-
cient when applied to polymethoxylated compounds.
The double transprotection was actually successfully
carried out on 4,40-dimethoxybenzophenone moiety also
in 40 min (entry 2). In this case, the presence of an
electron withdrawing group such as the ketone seemed
to enhance both the reaction rate and the overall
efficiency.
References and notes
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In conclusion, we have established a new reactivity of
aryl methyl ether under microwave activation leading
to a very useful and fast transprotection of the phenol
moiety. This reaction proved to be equally efficient on
polymethoxyaryl. In this latter case only transprotection
has been observed; even in the presence of highly acti-
vated aromatic rings, no Friedel–Crafts benzoylation is
observed. The transprotected product was the only one
observed and isolated. Mechanistic studies and develop-
ments of this methodology are currently under
investigations.
´
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