Concise Synthesis of Novel Oxa-Bridged Compounds

A stereoselective strategy for the replacement of the 1,2-dihaloalkene bridge of tetrahalonorbornyl
derivatives by an oxygen bridge involving stepwise controlled oxidation, cleavage of the dione thus
formed, and reiterative intramolecular SN2 displacement of two bridgehead halogens is devised.
The synthesis of four highly strained pentacyclic bis-oxa-bridged derivatives 10, 27, 28, and 29
with interesting structural variations is presented. The two oxa bridges are syn to each other,
separated by central cyclohexane and cycloheptane rings in 10 and 27, while they are anti to each
other and are separated by central cyclopentane and furan rings in 28 and 29. In the case of the
highly symmetric bis-oxa-bridged derivative 10 the two syn oxa bridges constrain the central
cyclohexane ring into the boat form. The endo,anti,endo 2:1 bis adducts of 1,2,3,4-tetrahalo-5,5-dimethoxycyclopenta-1,3-diene with cyclopentadiene were prepared for the first time, while the
reactivites of previously unexplored bis adducts derived from furan and cycloheptatriene were
revealed.