A formal total synthesis of (±)-homogynolide-B
A formal total synthesis of (±)-homogynolide-B, a sesquiterpene containing an α-spiro-β-methylene-γ-butyrolactone moiety spirofused to a bicyclo[4.3.0]nonane framework, is described. Thus, Hagemann’s ester 11 was converted into the allyl alcohol 16 in three steps. One-pot Claisen rearrangement of the allyl alcohol 16 and 2-methoxypropene in the presence of a catalytic amount of propionic acid afforded a 3∶2 epimeric mixture of the ketone 15 and further rearranged product 19. Ozonolysis followed by intramolecular aldol condensation and hydrogenation transformed the enones 15a,b into the key intermediate keto ketals 13a and 13b. Methoxymethylene Wittig reaction followed by bromoacetalisation converted the keto ketal 13a into the radical precursor bromo acetal 22a. The 5-exo-dig radical cyclisation of the bromo acetal 22a, followed by acid catalysed hydrolysis and oxidation, led to the keto spirolactone 12, Greene’s precursor of homogynolide-B. The same sequence transformed the keto ketal 13b into a 3∶2 mixture of the spirolactones 12 and 25, which on equilibration furnished the spirolactone 12. The stereostructure of the keto spirolactone 12 was unambiguously established by single-crystal X-ray diffraction analysis.