Concise synthesis of novel oxa-bridged compounds.

[structures: see text] 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 S(N)2 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.

An easy access to gamma-lactone-fused cyclopentanoids.

The tricyclic alpha-keto hemiacetals 3a,b and 8a-d obtained from ruthenium-catalyzed oxidation of tetrahalonorbornyl derivatives possessing a pendant hydroxymethyl group were cleaved using Pb(OAc)(4) or alkaline H(2)O(2) to give gamma-lactone-fused cyclopentane derivatives 5a,b and 9a-d. The alpha-keto hemiacetal 3b has also been elaborated to spiroepoxide derivative 25. The stable hydrate 4 formed from ruthenium-catalyzed oxidation of acrolein adduct 10 furnished an intramolecular hemiacetal 11 upon cleavage with Pb(OAc)(4). The alpha-halo ester moiety in 5a was transformed smoothly in a highly regio- and stereoselective manner to alpha-hydroxy esters through a lactone-assisted intermediate to furnish 18.

Regio- and diastereoselective reduction of nonenolizable alpha-diketones to acyloins mediated by indium metal.

[reaction: see text] Alpha-diketones are efficiently reduced with indium metal in methanol-water in the presence of NH(4)Cl, LiCl, or NaCl to give regio- and diastereoselectively the corresponding acyloins in good to excellent yield. The cleavage of the acyloins under Pb(OAc)(4)/MeOH-PhH condition provides a convenient and regioselective access to highly functionalized cyclopentane carboxaldehydes, potential building blocks in organic syntheses.