Synthetic Studies on Siphonariid Polypropionates: Synthesis and Isomerization of the Caloundrin B Trioxaadamantane Ring System

(1R,3R,5R,7R,8S,9R,10S)-3,5-Diethyl-8,9,10-trimethyl-2,4,6-trioxatricyclo[3.3.1.13,7]decan-1-ol (II), a model of the trioxaadamantane ring system embedded in caloundrin B, was prepared by isomerization of the thermodynamically unstable (9S)-diastereomer (I) in the presence of imidazole. Alternatively, isomerization of I with HF·py or DBU gave the hemiacetal III or its retro-Claisen ester IV, respectively, which represent structural motifs present in the closely related siphonariid polypropionates siphonarin B and baconipyrone C.