Novel Oxa-Cage Compounds: Synthesis, Structures, and the Formation Mechanism of Tetraacetal Oxa-Cages and Convex Tetraquinane Oxa-Cages

Abstract

Several novel tetraacetal oxa-cage compounds Sa-d and convex tetraquinane oxa-cage compounds 16a-d and 1%-d are synthesized from alkylfurans in three steps. Ozonolysis of the cis-endo1,ri-diones 3a-d in dichloromethane at -78 “C followed by reduction with dimethyl sulfide gives the oxa-cages Sa-d in high yields, respectively. The structures of these new tetraacetal oxa-cages are deduced from their spectral data and proven for the first time by X-ray analysis of the crystalline compound Sa. Ozonolysis of 3a-d in dichloromethane at -78 “C followed by treatment with triethylamine gives the convex tetraquinane oxa-cages 16a-d and 17b-d in 85-90% yields, respectively. The structures of these novel convex tetraquinane oxa-cages are finally proven by X-ray analysis of the crystalline compound 16a. Two reaction mechanisms via the common final ozonides are proposed for the formation of these two different types of oxa-cage compounds. The structures of the final ozonides formed by ozonolysis of the norbornene derivatives 3 are deduced to be 9 with endo stereochemistry on the basis of their spectral data and the formation of these two types of oxa-cages from the final ozonides. In reaction with the final ozonides, triethylamine is found to act as a base instead of a reducing agent, a different function from that of dimethyl sulfide. The synthesis of oxa-cages 24 and 25, which possess aromatic substituents directly on the skeleton, has also been accomplished.