As chemists proceeded to synthesize more complicated structures, they developed
more satisfactory protective groups and more effective methods for the formation
and cleavage of protected compounds. At fi rst a tetrahydropyranyl acetal was prepared,
4 by an acid-catalyzed reaction with dihydropyran, to protect a hydroxyl group.
The acetal is readily cleaved by mild acid hydrolysis, but formation of this acetal
introduces a new stereogenic center. Formation of the 4-methoxytetrahydropyranyl
ketal5 eliminates this problem.