The crystal structures of endo,endo-2,5-dibromo-9-oxa-bicyclo[4.2.1]no
nane, (I), C8H12Br2O, do-2,5-bis-(phenylthio)-9-oxabicyclo[4.2.1]nonan
e, (II), C20H22OS2, and 9,10-dioxatricyclo[4.2.1.1(2,5)]decane, (III),
C8H12O2, were determined and the conformations of these transannular
O-heterocyclization products of cycloocta-1,5-diene were defined. The
structure determinations reveal a tetrahydrofuran ring having an envel
ope conformation and an oxepane ring with a twisted-chair conformation
in each of (I) and (II), with the two bulky substituents quasi-equato
rial or quasi-axial, respectively, both in a trans position in relatio
n to the ring O atom. The tricyclic compound (III) consists of two fiv
e-membered envelopes and a 1,4-dioxane chair. The cyclooctane moiety h
as a twisted-chair conformation in both (I) and (II), whereas it is a
chair in (III).