The oxymercuration of 1,4-TT-methylene-As-cyclohexene-2-T-carboxylic acid yields two diastereomeric hydroxychloromercurials. Their configurations cannot be specified on the basis of information now available with certainty. The oxymercuration of 1,4-71-methylene-A5-~~clohexene-2-J-carboxylic acid also yields two diastereomers, which are designated as 6-~-hydroxy-5-J-chloromercuri-1,4-~-methyenecycohexane-2--carboxylic acid, ?-lactone and 6-J-hydroxy-5-~-chloromercuri-1,4-~T-methyenecycohexane-2--dicarboxyic acid, 7-lactone because the former is more easily deoxymercurated than the latter and because their dipole moments match relatively the corresponding moments of the analogous dicarboxylic acid r-lactones, the configurations of which have been proved chemically. This is the first instance where two diastereomers of opposite configuration have been shown to be formed in a n oxymercuration.During a recent study of the oxymercuration of various substituted [2.2.1]-bicycloheptenes (1) an anomaly has become apparent. The [2.2.1]-bicycloheptenes behave differently from other cyclic alkenes because the oxymercuration is acid catalyzed and the oxyn~ercurials have the TJ (trans) configuration. The exception is the isolable oxymercurial 111-C from 1,4-~~-methylene-A5-cyclohexene-2,3-JJ-dicarboxylic acid. The che~_llical behavior of 111-C shows it to be 5-J-hydroxy-6-J-chloromercuri-l,4-TT-methylene~~clohexane-2,3-JJ-dicarboxylic acid, 7-lactone. But if the reaction proceeds through an ionic intermediate, a t least one other diastereomer (probably 5-J-hydroxy-6-T-chloromercuri-1,4-~~zmethylenecyclohexane-2,3-JJ-dicarboxylic acid, y-lactone, IV-C, ought t o be formed. However, this diastereomer, IV-C, has the same solubility characteristics as 111-C. Its main chemical difference from 111-C, a higher resistance toward deoxymercuration, would not be apparent if it were a contaminant of low concentration. Consequently its presence in small-amount might be missed, especially if traces of other diastereomers were present.