Persistent Carbocations from Bay Region Methoxy-Substituted Cyclopenta[a]phenanthrene and Its Derivatives. A Structure/Reactivity Study

Using 500 MHz NMR, we have carried out a stable ion protonation and model nitration study of the methoxy-substituted hydrocarbon 6, its 15-ol 7, and the dimer 10, in order to evaluate OMe substituent effects on directing electrophilic attack and on charge delocalization mode/conformational aspects in the resulting carbocations. It is found that the C-11 methoxy group directs the electrophilic attack to C-12 and C-14. Thus protonation of 6 with FSO₃H/SO₂ClF gives a 4:1 mixture of monoarenium ions 6H⁺/6aH⁺. Prolonged reaction times and increased temperature induced fluorosulfonylation at C-14 (6⁺-SO₂F), whereas ambient nitration with NO₂⁺BF₄^- occurred at C-12. The 15-ol derivative 7 is cleanly ionized to 11⁺, providing the first example of an α-phenanthrene-substituted carbocation from phenanthrene C-1 position. Contrasting behavior of the D-ring methyl-substituted 9 and the C-11 methoxy-substituted 10 dimers is remarkable in that unlike 9 which is readily cleaved to produce the monomeric arenium ion 3H⁺, 10 is diprotonated at the two C-12 sites and at C-12/C-14 in each unit. The latter dication−dimer exists as a mixture of diastereomers. Reactivity of 7 underscores the importance of 11⁺. Attack at the C-14 ring junction is in concert with the proposal that electrophilic oxygen would attack at C-14/C-15 (epoxidation) followed by ring opening to give the biologically active 15-ol as a major metabolite.