Studies in Xanthone chemistry

Abstract

A series of 3,4-dihydro-9H-xanthen- 1 ,9(2H)-diones has been prepared by the condensation of 2-fluorobenzoyl chlorides with cyclohexane- I ,3-diones followed by Fries rearrangement and cyclisation of the intermediate 3-oxocyclohexenyl
2-fluorobenzoates.
Conversion to spim[ 1 ,2,3,4-tetrahydro-9H-xanthen- 1 ,2'-[ 1 ,3] -dioxolane] -9-one occurred on treatment of the xanthendione with ethane- 1 ,2-diol and trimethylsilyl chloride. 3,4-Dihydro-4-methyl-9H-xanthen- 1 ,9(2H)-dione has been synthesised by quenching the lithium dienolate of the ketal with iodomethane.
Conjugate reduction of the xanthendione has been accomplished using sodium borohydride in pyridine to give 1hydroxy-2,3,4,4a-tetrahydro-9H-Xanthen-9-one whose chemistry has been investigated.
Treatment of the xanthendione with tris(methylthio)methyllithium gave the 1,4-adduct. Subsequent mercuric ion catalysed solvolysis provided methyl 1 -hydroxy-2,3,4,4a-tetrahydro-9H-xanthen-9-one-4a-carboxylate which is the parent nng of the
secalonic acid series of natural products.
4a-Alkyl derivatives have been prepared in excellent yield from the xanthendione and organocyanocuprate reagents. The corresponding Grignard reagents displayed less selectivity in their mode of addition.
Reaction of the 1 -hydroxytetrahydroxanthones with hydroxylamine gave 4-substituted 6,7-dihydro-3-(2-hydroxyphenyl)-2, I -benzisoxazoles resulting from ring cleavage of the xanthone nucleus. Dehydrogenation of these systems with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and with palladium on charcoal catalyst has been explored.
Treatment of 1 -hydroxy-2,3,4,4a-tetrahydro-9H-xanthen-9-ones with lithium diisopropylamide in tetrahydrofuran afforded dianions which provided an entry to 2-functionalised derivatives by quenching with electrophiles.
Acid catalysed treatment of 1hydroxy2,3,4,4a-tetrahydro-9H-xanthen-9-One gave 3,4-dihydro-9H-xanthen-9-one by way of a retro-Michael ring cleavage-bond switch operation. The substituted analogues behaved similarly, although the products were
dependent upon the nature of the substituents present.
The value of some 3,4-dihydro-9H-xanthen-9-ones as dienes in [4+21-cycloaddition reactions has been established with a variety of dienophiles.
The chemistry of some 2-(2-fluorobenzoyl)- I -hydroxy-2,3,4,4a-tetrahydroxanthones has been investigated. Treatment with DBU in toluene led to 4-(2-hydroxybenzoyl)- 9H-xanthen-9-ones, whereas with potassium carbonate in acetone novel 5H,13H-[l]benzopyrano[2,3 -ajxanthen -5, 1 3-diones were obtained. The mechanism of formation for these compounds is discussed.