The structures of the two isomeric y-lactones resulting from acid-catalyzed dehydration of 2-exo-and 2-endomet hylnorbornene-2-carboxylic acids have been proven by chemical degradation to 2,2-dimethylnorbornane and 7,;-dimethylnorbornane. The mechanism of formation of the lactones is discussed, and the implications of these results for related systems are presented.The relatively rigid geometry of the bridged bicyclic norbornyl system and the fixed positions in space of neighboring transannular atoms have caused these compounds to be used for estimation of proximity effects. Since these conformational features are important in enzymatic catalysis, bridged bicyclic compounds have been used as models for various enzyme systems. Recent examples of this strategy have been published by Loudon and Ryono',* and also by Koshland et al.3 One drawback in the use of these compounds as models is the pronounced tendency of the bicyclo[2.2.l]heptyl system to undergo cationic rearrangement. To a lesser extent this is also true of the bicyclo[2.2.2]octyl system. While this aspect of their behavior has led to many fundamental insights into the nature of cationic processes in general, in cases where the occurrence of such changes went unrecognized some serious misassignments of structure have resulted. Recently we reported, in a preliminary way, some examples of such errors in the bicyclo[2.2.l]heptyl* and bicyclo[2.2.2]octyl system^.^ We report now the complete details of the establishment of these structures of rearrangement products in the first system. An accompanying paper deals with the second system. It is interesting to note that these isomers could not be satisfactorily distinguished by means of predominately modern physical methods, and ultimately we had to resort to chemical logic and chemical methods of a more or less classical type.The particular rearrangement, which is the concern of this paper, is that occurring in cation-induced intramolecular lactone formation. A typical example of intramolecular lactone formation is halolactonization,6 which has been widely used as an adjunct to the Diels-Alder reaction of dienophilic carboxylic acidsT for the separation of exo and endo adducts. The method is based upon the valid assumption that only the endo orientation of the carboxyl group in 1 and 3 is correctly articulated in space for intramolecular cyclization with the olefinic double bond to yield a y-lactone.A priori, I arid 3 could yield the six-membered 2,5-lactones, and in fact occ,isionally claims that they do have been made.
Die Methyl‐norbornencarbonsäure (II) liefert das Hydrierungsprodukt (I) bzw. in einer reversiblen Cyclisierung das Iodlacton (III), das zu (IV) deiodiert wird.
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