Refined protocols for the preparation of 3-alkoxy-2,5-dihydrofurans, allylic oxidation to β-alkoxybutenolides and short synthesis of (±)-annularin H

The 5-endo cyclization of alpha-allenyl alcohols derived from carbonyl compounds and lithiated alkoxyallenes was reinvestigated by comparing the known reagents KOt-Bu, AgNO3 or AgBF4 with the reagent system AuCl/pyridine. A variety of 3-alkoxy-2,5-dihydrofurans 4 was efficiently prepared, in some cases with high diastereosetectivity. These product compounds were subjected to manganese(III)-catalyzed allylic oxidation which led to beta-alkoxybutenolides with moderate to good yield. Combination of these newly tuned methods allowed for a concise and short synthesis of (+/-)-annularin H.