B. Fohlisch et A. Radl, A DIFLUORO-SUBSTITUTED OXYALLYL INTERMEDIATE IN [4+3]CYCLOADDITION WITH CYCLOPENTADIENE AND FURAN, Journal of fluorine chemistry, 90(1), 1998, pp. 23-28
1-Bromo-1,3-difluoropropan-2-one (2) reacts with cyclopentadiene, fura
n and 2,5-dimethylfuran in 2,2,2-trifluoroethanol/sodium 2,2,2-trifluo
roethoxide to form 2,4-difluorobicyclo[ 3.2.1]oct-6-en-3-one (7b) and
the 8-oxa analogues (7a, 7c). The endo orientation of the fluoro subst
ituents at C-2 and C-4 indicates a concerted [4 + 3] cycloaddition of
a W-configurated difluoro-oxyallyl intermediate (5) via a compact tran
sition state. (C) 1998 Elsevier Science S.A. All rights reserved.