(+)-Chloriolide, a metabolite of the ascomycete Chloridium virescens var. chlamydosporum, was synthesized in 16 linear steps from cellulose as a source of a levoglucosenone that contributed the (Z)-alkene and the R stereocenter. The attachment of a spacer derived from l-lactate gave an ω-hydroxyacetal which was added to the phosphorus ylide Ph3PCCO. The resulting ester ylide was treated with hydrochloric acid to liberate the hemiacetal shown. Addition of sodium hydroxide regenerated the corresponding ylide, which underwent a spontaneous intramolecular Wittig olefination to afford (+)-chloriolide in 65% yield without the necessity of high-dilution conditions. This is the third synthesis of (+)-chloriolide and the first one ever of a macrolide by a ring-closing Wittig olefination of a stabilized phosphorus ylide bearing an ω-hemiacetal. Our synthetic sample exhibited moderate cytotoxicity against cancer cells but no antimicrobial activity against Staphylococcus aureus.
The polyhydroxylated 18-membered lichen macrolide (+)-aspicilin was synthesized in 12 steps and 17% yield (longest linear sequence) starting from d-mannose and (S)-propylene oxide as the source of the stereogenic centers. Key steps were a palladium-catalyzed CX-CZnX Negishi cross-coupling affording an ω-hydroxy hemiacetal which was macrocyclized via a domino addition-Wittig olefination reaction with the cumulated ylide PhPCCO. This synthetic approach also allowed a regioselective glycosylation of 6-OH of aspicilin with d-desosamine, a quick entry to chimeric macrolides with potential antibiotic activity.
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