OXONIUM ION-INITIATED PINACOLIC RING EXPANSION REACTIONS - APPLICATION TO THE ENANTIOSELECTIVE SYNTHESIS OF THE SPIROCYCLIC SESQUITERPENE ETHERS DACTYLOXENE-B AND DACTYLOXENE-C

A convergent synthesis of dactyloxene-B (1) and -C (2) from a common optically active intermediate in seven steps has been achieved. Ozonolysis of(R)-(-)-linalool proceeded regioselectively to deliver a lactol, the benzoate of which when thermolyzed gave dihydrofuran 7. The second building block, levorotatory cyclopentanone 8, was produced by kinetically controlled lipase-induced hydrolysis of chloroacetate 15b. Saponification and oxidation of the less reactive ester was sufficient to provide (-)-8 whose condensation with the cerate of 7 gave rise to 19 and set the stage for implementation of the key structural rearrangement step. When stirred with Dowex resin, 19 was isomerized to four ketones. Following chromatographic separation, 20 and 21 were independently transformed into the target molecules via a two-step sequence. The spectral and optical properties of the two dactyloxenes compared very favorably with those reported earlier.