3 2,5-DISUBSTITUTED 9-OXABICYCLO[4.2.1]NONANES - TRANSANNULAR O-HETEROCYCLIZATION PRODUCTS OF CYCLOOCTA-1,5-DIENE

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).