Synthesis of pyrano[3,2-c]pyranthiones; characteristic shifts in the 1H nuclear magnetic resonance spectra of thiones

Abstract

The behaviour of boron sulphide and silicon sulphide as sulphurization reagents varies according to batch and source. Silicon sulphide has now been used to convert 2-methylpyrano[3,2-c][1]benzopyran-4,5-dione (I) into 2-methyl-4H-thioxo-4-pyrano[3,2-c][1]benzopyran-5-one (II), characterised by further conversion into 8-methyl-5,10-dioxa-9,9a-dithia (9σSIV) pentaleno[2,1-a]naphthalen-6-one (III). Modification of this last step leads to 8-methyl-5,9,10-trioxa-9a-thia (SIV) pentaleno[2,1-a]naphthalen-6-one (IV), identical with a substance previously obtained from a different reaction and tentatively assigned this structure.4-Methylpyrano[3,2-c][1]benzopyran-2,5-dione (VI) was attacked only with difficulty by silicon sulphide but boron sulphide smoothly converted it into 4-methyl-2-thioxo-2H-pyrano[3,2-c][1]benzopyran-5-one (VII). In the first place, the thione group was located by stereochemical arguments, supported by the conversion of the lower homologue (VIII), pyrano[3,2-c][1]benzopyran-2,5-dione, under the same conditions into a mixture including pyrano[3,2-c][1]benzopyran-2,5-dithione (IX). This assignment was confirmed by a study of the 1H n.m.r. shift differences between pyrones and the corresponding pyranthiones which gave unequivocal evidence favouring structure (VII). The method may have a wider utility.Derivatives of pyrano[3,2-c][1]benzopyran-2,5-dione are accessible in very mild conditions by condensing the requisite 4-hydroxycoumarin with ethyl acetoacetate by means of potassium carbonate instead of an acid catalyst.