The synthesis of methoxy-2-hydroxy-1,4- naphthoquinones and their reaction with aliphatic aldehydes under basic conditions.
Abstract
Although literature reports on the synthesis of variously substituted 2-hydroxy-1 ,4-
naphthoquinones appeared to be reasonable; in our hands difficulty was experienced
in duplicating much of the work.
In order to address this problem, two protocols were used with the one involving
conversion of substituted a-tetralones into hydroxyquinones using a solution of the
tetralones in an oxygenated solution of tertiary butyl alcohol containing potassium
tertiary butoxide, and the other involving Diels-Alder Condensation, oxidation,
pyrolysis and basic hydrolysis of a 2-methoxy-1,4-naphthoquinone 79 into the
corresponding 2-hydroxy analogue 80.
Condensation reaction between 2-hydroxyy-8-methoxy-1,4-naphthoquinone 80 and
caproaldehyde 111, produced the 3-alkenyl analogue 115 which was cyclised to the
corresponding naphtho[2,3-b ]pyrenquinone 116 and eventually reduced to 8-
methoxynaphtho[2,3-b ]pyran-5, 10-dione 1) 7.
Condensation between 2-hydroxynaphthoquinone 80 and 4-dioxolanopentanal under
basic conditions afforded the desired adduct which was cyclised with
dichlorodicyanobenzoquinone, reduced and then hydrolysed with acid to produce 2-
acetyl-3,4-dihydronaphtho[2,3-b ]pyran-5, 10-dione 125. By the employment of an
alternative sequence of events, the corresponding 4-hydroxy analogue of the above 2-
acetylnaphthopyrandione 125 was also prepared for evaluations.
Finally condensation between 2-hydroxy-8-methoxy-1,4-naphthoquinone 80 and 5-
dioxolanohexanal 128 under basic conditions afforded the 3-hexenyldioxolano 129
derivative, which was cyclised to the naphthopyrene 130 but the yields were
disappointing in this instance.