Aryl O-Δ^2-glycosides are considered to be 1-protected Δ^2-sugars as well as carbohydrate derivatives of aromatic compounds including aromatic antibiotics. The latter cases are expected to improve the solubility of less soluble compounds. Ferrier reaction is known to produce alkyl O-Δ^2-glycosides from acyl glycals and alcohols. Substitution of alcohols by phenols, however, resulted in poor yields of aryl O-Δ^2-glycosides. Reinvesti-gation of this reaction revealed that the condition using toluene as a solvent and a small amount (〜5mol%) of Lewis acid (BF_3・Et_2O) as a catalyst enabled to provide aryl O-Δ^2-glycosides in good yields. p-Methoxyphenol gave the best result among the phenols tested. This fact showed the great possibility of using the compound for a protection of 1-hydroxy group of Δ^2-sugars because p-methoxyphenyl group can be oxidatively cleaved to yield deprotected sugars. With a larger amount of catalyst and an elevated reaction temperature, O-Δ^2-glycosides rearranged to aryl C-Δ^2-glycosides. Next question to be solved was the method of removing the p-methoxyphenyl protective group from the anomeric center of Δ^2-sugars. Although CAN is complehensively used for such a purpose, it works in the strongly acidic medium and sometimes causes unfavorable side reactions. Silver(II) bis-(hydrogen dipicolinate) [Ag(DPAH)_2]was found to be a suitable agent working under weakly acidic condition and it oxidatively cleaved p-methoxyphenyl group to recover in high yields deprotected sugars without any side reaction. Using the above two methods, an introduction of p-methoxyphenyl group as a protective group for anomeric center of Δ^2-sugars and a mild oxidative deprotection, biologically active dihydropyranones such as osmundalactone (5), phomopsolide B (6), altholactone (7), aspyrone (8), and their stereoisomers were synthesized from carbohydrates.
