Two synthetic approaches have been investigated for the syntheses of model
angucyclinones related to ochromycinone. The first involves a
Diels–Alder/Friedel–Crafts strategy in which the
Diels–Alder adduct formed between dimethyl acetylenedicarboxylate and
3-ethenyl-5,5-dimethylcyclohex-2-en-1- one was converted into
6,6-dimethyl-8-oxo-5,6,7,8-tetrahydronaphthalene-1,2-dicarboxylic anhydride,
which was then reacted with benzene in a Friedel–Crafts reaction.
Acid-catalysed cyclization of the Friedel–Crafts products gave
3,3-dimethyl-3,4-dihydrobenz[a]anthracene-1,7,12(2H)-trione
(3) in poor yield. Angucyclinones related to (3) are formed (in
40–50% overall yield) by aromatization of the adduct formed
between the appropriate 1,4-naphthoquinone and
3-[(E)-2-methoxyethenyl]-
5,5-dimethylcyclohex-2-en-1-one (this dienone reacts with itself by a
Diels–Alder process to yield an adduct which decomposes to
3,3,8,8-tetramethyl-3,4,7,8-tetrahydroanthracene-1,6(2H,5H)-dione).
When the substituted 1,4-naphthoquinone is unsymmetrical, a boron triacetate
assisted Diels–Alder reaction gives a single regioisomer (e.g. X-ray
investigations indicate that
8-hydroxy-3,3-dimethyl-3,4-dihydrobenz[a]anthracene-1,7,12(2H)-trione
is the product from 5-hydroxy-1,4-naphthoquinone). An X-ray structural study
of the Diels–Alder adduct in the above reaction confirms the operation
of an endo cyclization.
