Mechanism of the Color Reaction of Deoxycorticosterone with Sulfuric Acid

The mechanism of the color reaction of deoxycorticosterone (1) with sulfuric acid was elucidated. Two rearranged products, 13ξ(or 14ξ)-hydroxy-17β-methyl-18-nor-13ξ, 14ξ, 17α-pregn-4-ene-3, 20-dione (4) and 17β-methyl-18-nor-17α-pregna-4, 13-diene-3, 20-dione (5), were isolated from the reaction mixture of 1 with 97% sulfuric acid at room temperature in 46 and 32% yields, respectively. Proton nuclear magnetic resonance (1H-HMR) spectra of 97% sulfuric acid solutions of 1, 4 and 5 revealed that they are present in the acid as the diprotonated form (7) of 5, showing an absorption maximum at 288 nm. The acid solution of 7 was heated for 90 min at 60°C to give two intermediary species (x-372 and x-436) showing maxiama at 372 and 436 nm, respectively. Dilution of the mixture with ethanol reduced these species and produced a new chromophore (x-600) showing a maximum at 600 nm. From the diluted solution, 17-isopropyl-18-norandrosta-4, 6, 8(14), 13(17)-tetraen-3-one (13) was isolated in 13% yield. Dissolution of 13 in a 2 : 3 mixture of sulfuric acid and ethanol immediately gave x-600, which was identified from its 1H-NMR data as the steroidal carbocation (14) having a hydroxyalkatetraenyl cation moiety. On the other hand, dissolution of 13 in 90% sulfuric acid gave x-372, which was shown to be in acid-base equilibrium with x-600. x-372 was also identified from its 1H-NMR data as the dication (15) having a hydroxyalkenyl cation moiety in ring A and an Alkadienyl cation moiety across rings B, C and D.