Synthesis and Hydrolysis of a cis-Chlorohydrin Derived from a Benzo[a]pyrene 7,8-Diol 9,10-Epoxide
journal contributionposted on 12.11.2004 by Lanxuan Doan, Haruhiko Yagi, Donald M. Jerina, Dale L. Whalen
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(±)-7β,8α-Dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (DE-1) undergoes reaction with anhydrous HCl in dioxane to yield predominantly (∼94%) a single chlorohydrin. This chlorohydrin was assigned structure 9, 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[a]pyrene 7,8-diol 9,10-epoxide (DE-2) with HCl, which yields only trans-chlorohydrin 8. The hydrolysis of cis-chlorohydrin 9 in 10:90 dioxane−water solutions yields the same ratio of tetrols (∼89% cis/11% trans) as that formed by acid-catalyzed hydrolysis of DE-1. This result again contrasts with the hydrolysis of trans-chlorohydrin 8, which undergoes hydrolysis to give tetrols in a ratio different from that from acid-catalyzed hydrolysis of DE-2. A marked common ion rate depression in the hydrolysis of cis-chlorohydrin 9 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 DE-1 reacts with HCl to give mainly the cis-chlorohydrin is rationalized by quantum chemical calculations that suggest that the cis-chlorohydrin is more stable than the epimeric trans-chlorohydrin.