Synthesis of Semi-tetracycline Backbone Through Diels-Alder Reaction of 2,3-Dimethylbutadiene or Cyclopentadiene With 1,4-Benzoquinones

Two main semi-tetracycline systems were synthesized. The strategy of such synthesis depends on the transformation of aroyl-1,4-benzoquinone to aroylketal-1,4-benzoquinone causes a regiocontrol addition of cyclopentadiene and 2,3-butadiene to the unsubstituted side of the quinone. These ketal adducts were hydrolyzed using the acidic media; as a result deketalization and enolization in one step give the semi-tetracycline system. The structures of these compounds were confirmed by NMR, IR, MS and elemental analysis.