Synthesis and Hydrolysis of a <i>cis</i>-Chlorohydrin Derived from a Benzo[<i>a</i>]pyrene 7,8-Diol 9,10-Epoxide

(±)-7β,8α-Dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydrobenzo[<i>a</i>]pyrene (<b>DE</b><b>-</b><b>1</b>) undergoes reaction with anhydrous HCl in dioxane to yield predominantly (∼94%) a single chlorohydrin. This chlorohydrin was assigned structure <b>9</b>, in which the chloro goup at C-10 is located cis to the C-9 hydroxyl group, on the basis of its ¹H NMR spectrum. This result is in contrast to the reaction of a diastereomeric benzo[<i>a</i>]pyrene 7,8-diol 9,10-epoxide (<b>DE</b><b>-</b><b>2</b>) with HCl, which yields only <i>trans</i>-chlorohydrin <b>8</b>. The hydrolysis of <i>cis</i>-chlorohydrin <b>9</b> in 10:90 dioxane−water solutions yields the same ratio of tetrols (∼89% cis/11% trans) as that formed by acid-catalyzed hydrolysis of <b>DE</b><b>-</b><b>1</b>. This result again contrasts with the hydrolysis of <i>trans</i>-chlorohydrin <b>8</b>, which undergoes hydrolysis to give tetrols in a ratio different from that from acid-catalyzed hydrolysis of <b>DE</b><b>-</b><b>2</b>. A marked common ion rate depression in the hydrolysis of <i>cis</i>-chlorohydrin <b>9</b> is observed, which shows that hydrolysis proceeds via an intermediate carbocation that has a sufficient lifetime to be trapped by external chloride ion. The observation that <b>DE</b><b>-</b><b>1</b> reacts with HCl to give mainly the <i>cis</i>-chlorohydrin is rationalized by quantum chemical calculations that suggest that the <i>cis</i>-chlorohydrin is more stable than the epimeric <i>trans</i>-chlorohydrin.