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 n itration study of the methoxy-substituted hydrocarbon 6, its 15-ol 7, and t he dimer 10, in order to evaluate OMe substituent effects on directing elec trophilic attack and on charge delocalization. mode/conformational aspects in the resulting carbocations. It is found that the C-ll methoxy group dire cts the electrophilic attack to C-12 and C-14. Thus protonation of 6 with F SO3H/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. T he 15-ol-derivative 7 is cleanly ionized to 11(+), providing the first exam ple of an alpha -phenanthrene-substituted carbocation from phenanthrene C-l position. Contrasting behavior of the D-ring methyl-substituted 9 and the C-ll methoxy-substituteb 10 dimers is remarkable in that unlike 9 which is readily cleaved to produce the monomeric arenium ion 3H(+), 10 is diprotona ted at the two C-12 sites and at C-12/C-14 in each unit. The latter dicatio n-dimer exists as a mixture of diastereomers. Reactivity of 7 underscores t he 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 (epoxidat ion) followed by ring opening to give the biologically active 15-ol as a ma jor metabolite.