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Regioselective decomposition of 11-substituted-16α,17α-methyleneazopregna-1,4-dien- 3, 20–dione I proceeded smoothly through thermal decomposition in the presence of BF3·Et2O or HClO4 with high yields. I was converted to 11-substituted-16-methylpregna-1, 4, 16(17)-trien-3, 20-dione II by the thermal decomposition, while with HClO4 as a catalyst, 11-substituted-16α, 17α-methylenepregna-1, 4-dien-3, 20-dione III was the main product. However, when BF3·Et2O was used as catalyst, a mixture of II and III (1 : 3) was obtained.
Regioselective decomposition of 11-substituted-16α, 17α-methyleneazopregna-1,4-dien-3, 20-dione I proceeded smoothly through thermal decomposition in the presence of BF3 · Et2O or HClO4 with high yields. I was converted to 11-substituted Dione II by the thermal decomposition, while with HClO4 as a catalyst, 11-substituted-16α, 17α-methylenepregna-1, 4-dien- 3, 20-dione III was the main product. However, when BF3 · Et2O was used as a catalyst, a mixture of II and III (1: 3) was obtained.