PURPOSE: To obtain the titled compound useful as an intermediate for synthesizing a nonnatural-type anthracyclin antibiotic, etc., by reacting an anthro[b]cyclobutene derivative with a dienophile under heating.

CONSTITUTION: An anthro[b]cyclobutene shown by the formula I (R is H, lower alkyl, or lower alkanoyl) is reacted with a dienophile shown by the formula II(X is acetyl, or lower alkoxycarbonyl; Y is H, benzoyloxy, etc.; Z is H, or lower alkyl) usually at 160W220°C to give the desired compound, shown by the formula III. The compound shown by the formula I is a new compound and obtained from o-phthalaldehyde shown by the formula IV and bicyclo[4,2,0]octan- 2,5-dione shown by the formula V as starting raw materials.

EFFECT: An amount of dienophile used is reduced, demethylation process is not required, and the desired compound can be obtained directly.