Persistent Carbocations from Bay Region Methoxy-Substituted Cyclopenta[<i>a</i>]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 <b>6</b>, its 15-ol <b>7</b>, and the dimer <b>10</b>, 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 <b>6</b> with FSO<sub>3</sub>H/SO<sub>2</sub>ClF gives a 4:1 mixture of monoarenium ions <b>6H</b><b><sup>+</sup></b><sup></sup>/<b>6aH</b><b><sup>+</sup></b><sup></sup>. Prolonged reaction times and increased temperature induced fluorosulfonylation at C-14 (<b>6</b><sup>+</sup>-<b>SO</b><b><sub>2</sub></b><b>F</b>), whereas ambient nitration with NO<sub>2</sub><sup>+</sup>BF<sub>4</sub><sup>-</sup> occurred at C-12. The 15-ol derivative <b>7</b> is cleanly ionized to <b>11</b><b><sup>+</sup></b><sup></sup>, providing the first example of an α-phenanthrene-substituted carbocation from phenanthrene C-1 position. Contrasting behavior of the D-ring methyl-substituted <b>9</b> and the C-11 methoxy-substituted <b>10</b> dimers is remarkable in that unlike <b>9</b> which is readily cleaved to produce the monomeric arenium ion <b>3H</b><b><sup>+</sup></b><sup></sup>, <b>10</b> 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 <b>7</b> underscores the importance of <b>11</b><b><sup>+</sup></b><sup></sup>. 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.