Experiments on the synthesis of tetracycline. Part XIV. Closure of ring B by base-catalysed photocyclisation

The photocyclisation of 4-[2-(dioxolan-2-yl)benzyl]-2-phenylnaphtho[1,8-bc]furan-5-one (I) to the naphthacenofuran (II) is catalysed by strong base. The simpler 2-[2-(dioxolan-2-yl)benzyl]-1,4-naphthoquinone (V) was not efficiently photocyclised to the naphthacenequinone (VI). Extension of the base-catalysed photocyclisation procedure provided an efficient route to the fully substituted linear tetracyclic acetal (IX) from the naphthofuran (VIII). An alternative theory for the photocyclisation of acetals of the type (I) to the linear tetracycline-type acetals (II) is advanced.