The syntheses of some naturally derived naphthoquinones
The synthesis of the naphthalene core of the ansamycin antibiotics, 8-acetyl-3-acetylamino-5,7-dihydroxy-1,4-naphthoquinone from benzoquinone by means of simple reactions including, Diels-Alder adduct formation, mild acetylation, oxime formation and Beckmann rearrangement in five steps with an overall yield of 22% is described in Chapter 1. The synthesis of the naturally occurring naphthoquinone derivative, possessing anti tumour and antiprotozoal properties, bikaverin is described in Chapter 2. Starting from vanillin the key intermediate 2-(2'-benzyloxy-6'-methyl-4'-methoxybenzoyl)-1,4,5,6,8-pentamethoxynaphthalene was prepared in six simple steps in an overall yield of 18%. This key intermediate was converted into bikaverin utilizing two independent routes. In the first route the benzyl group was removed from the key intermediate by hydrogenolysis followed by oxidative spiro ring formation , with 2,3-dichloro-5,6-dicyanobenzoquinone. After effecting xanthone ring formation and removal of two methyl groups with lithium iodide, bikaverin was produced in six steps in an overall yield of 32%. In the second route the key intermediate was first oxidised by silver (II) oxide this was followed by removal of the benzyl group and two methyl groups peri- to the quinone with boron trichloride, which led to spontaneous spiro ring formation, ultimately bikaverin was produced in three steps in an overall of 34%. The syntheses of the naturally occurring product ventiloquinone E and its trans-isomer as well as an isomer of the naturally occurring ventiloquinone J and its trans-isomer are described in Chapter 3. Starting from 1,2,4,5,8-pentamethoxynaphthalene, the synthesis of which has been described in Chapter 2, ventiloquinone E was prepared in nine steps in an overall yield of 7%. Similarly an isomer of ventiloquinone J was also prepared from 1,2,4,5,8-pentamethoxynaphthalene in ten steps in an overall yield of 6%. In both cases a mixture of cis-and trans-isomers was obtained, a successful resolution of both mixtures was accomplished by thin layer chromatography. By two other methods the trans-isomer of ventiloquinone E could be prepared in either nine steps in an overall yield of 23% or in six steps with an overall yield of 30% starting from the same pentamethoxynaphthalene.
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