Diels–Alder adduct of levoglucosenone and isoprene in the syntheses of the key synthon for loganin

Based on the hemiketal obtained by the carbonyl eneketalization of the Diels–Alder adduct of levoglucosenone and isoprene with acetaldehyde, two methods have been suggested for incorporating the carboxy group of loganin at the pyran ring. The first method involved one-pot conversion employing ozonolytic cleavage of the double bond, ozonide reduction, in situ aldehyde oxidation and esterification. An alternative five-step sequential conversion of the ketal, with isolation and identification of intermediate compounds, involved ketalization, Wagner vic-hydroxylation of the double bond, periodate cleavage of diols, in situ oxidation of the aldehyde to an acid, and esterification.