2-Diazopropane and 2-methyl-1,4-naphthoquinone combine to give 3a,9a-dihydro-3,3,9a-trimethyl-benz[f]indazole-4,9(3H)-dione (1) which reacts further with 2-diazopropane to give 3a,9a-dihydro-3,3,3′,3′,9a-pentamethylspiro[9H-benz[f]indazole-9,2′-oxirane]-4(3H)-one (15) with unusual ease. Standard methods failed to determine the structure of this compound, but a novel combination of ¹³C-{¹H-selective} and ¹H-{¹H} nuclear Overhauser enhancement experiments conducted using difference techniques established both the structure and the conformation. The normal adduct (1) differs from lower homologues in its stability to acetate ion in methanol, but potassium hydroxide induces loss of nitrogen and the formation of 2-isopropenyl-3-methyl-1,4-naphthoquinone (7); if not isolated promptly this reacts further with alkali to give 3,3′-di-isopropenyl-2,2′-ethylenedi-1,4-naphthoquinone (6a). At lower concentrations of alkali the adduct (1) forms an intermediate carbanion that traps oxygen to afford 2-isopropenyl-3-methyl-1,4-naphthoquinone 2,3-epoxide (14). All these reactions are of types new in this series. The adduct (3) from 2-t-butyl-1,4-naphthoquinone and 2-diazopropane is so hindered that it does not react even with potassium hydroxide.