The synthesis of (2-chloro or 2-bromo)-5,6,8-trihyThe synthesis of (2-chloro or 2-bromo)-5,6,8-trihydroxynaphtho-1,4,-quinone and their use in the predroxynaphtho-1,4,-quinone and their use in the preperation of kermesic acid peration of kermesic acid

摘要

(2-Chloro or 2-bromo-5,6,8-trihydroxynaphtho-1,4-quinone is synthesised in six steps from 2-hydroxy-4-methoxyacetophenone. In the chloro series, 3-chloro-2-hydroxy-5-methoxy acetophenone is methylated to give 3-chloro-2,5-dimethoxyacetophenone which then is converted to methyl 3-(3-chloro-2,5-dimethoxyphenyl)-3-oxopropionate. This compound is reacted with oxalyl chloride and the product cyclised to give methyl 6-chloro-2,5,8-trihydroxynaphtho-1,4-quinone-3-carboxylate, hydrolysis and decarboxylation of which gives 6-chloro-2,6,8-trihydroxynaptho-1,4-quinone. This does not undergo the Diels-Alder reaction due to hydrogen bonding which locks the structure in the undesired tautomeric form. By acetylation-aided tautomerism which is effected with acetic anhydride, 2-chloro-5,6-diacetoxy-8-hydroxynaphtho-1,4-quinone is derived and Diels-Alder cycloaddition with (E) and (Z)-3-methoxycarbonyl-2,4-bis(trimethylsilyloxy)-penta-1,3-dienethengivesmethyl5,6-diacetoxy-3,8-dihydroxy-1-methylanthra-9,10-quinone-2-carboxylate. Hydrolysis results in kermesic acid. Methylation is another way of aiding the desired tautomerism of 6-chloro-2,5,8-trihydroxynaphtho-1,4-quinone which gives 6-chloro-5,8-dihydroxy-2-methoxynaphtho-1,4-quinone and enables a Diels-Alder reaction to take place. Addition reactions at the 7-position of 6-chloro-2,5,8-trihydroxynaphtho-1,4-quinone proceed smoothly with a , b -unsaturated ketones to give, for example, 6-chloro-2,5,8-trihydroxy-3-(1'-methjyl-2'-ethoxycarbonyl-3'-oxobutyl)naphtho-1,4-quinone.