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 (9σS^IV) pentaleno[2,1-a]naphthalen-6-one (III). Modification of this last step leads to 8-methyl-5,9,10-trioxa-9a-thia (S^IV) 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.