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 FSO(3)H/SO(2)ClF gives a 4:1 mixture of monoarenium ions 6H(+)()/6aH(+)(). Prolonged reaction times and increased temperature induced fluorosulfonylation at C-14 (6(+)-SO(2)()F), whereas ambient nitration with NO(2)(+)BF(4)(-) occurred at C-12. The 15-ol derivative 7 is cleanly ionized to 11(+)(), providing the first example of an alpha-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.
Laali
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KK
Department of Chemistry, Kent State University, Kent, Ohio 44242, USA. klaali@kent.edu
Okazaki
T
T
Coombs
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MM
D002412:Cations; D007202:Indicators and Reagents; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D010616:Phenanthrenes / Q000138:chemical synthesis* / Q000737:chemistry; D012997:Solvents; D013329:Structure-Activity Relationship
D002412:Cations; D007202:Indicators and Reagents; D010616:Phenanthrenes; D012997:Solvents