Ring compounds stereochemical analysis

The cyclic ether (219) was converted into (220), which underwent rearrangement to (221) in a manner analogous to the celebrated norcaradiene ring-walk stereochemical analysis of the rearrangement indicated predominant inversion of configuration for both the photolytic and thermal processes and so symmetry considerations of the Woodward-Hoffman type may not be relevant to these (or at least the thermal) processes.559 Esters of pinocarveol were converted into myrtenyl compounds by CuOAc.560 cis- and rrans-Verbenyl and verbanyl acetates,561 562 related compounds563 and 2-methylverbanone564 have been synthesized for evaluation as attractants for cockroaches. 

Neither rw-l,2-dimethylcyclopropane nor c -l,2-dimethylcyclohexane can be resolved, because the cyclopropane is a meso compound (Chapter 4, p. 168), and the cyclohexane flips into its mirror image. However, both 1,2-dimethyl-cyclopropane and /rawi-l,2-dimethylcyclohexane can be resolved they are not superimposable on their mirror images. The point is that in a practical sense the planar cyclopropanes and nonplanar cyclohexanes behave in the same way. This finding has important consequences. In deciding questions of stereochemistry, we can treat the decidedly nonplanar 1,2-dimethylcyclohexanes as if they were planar. Indeed, all cyclohexanes can be treated as planar for the purposes of stereochemical analysis, because the planar forms represent the average positions of ring atoms in the rapid chair-chair interconversions. 

PROBLEM 10.45 In Section 10.2b (p. 414), we considered two possible mechanisms for the addition of Br2 to alkenes. Ultimately, a stereochemical experiment that used a ring compound was used to decide the issue in favor of a mechanism in which addition proceeded through a bromonium ion rather than an open carbocation. Use a detailed stereochemical analysis to show how the experimental results shown below are accommodated by an intermediate bromonium ion but not by an open carbocation. 

Treatment of compounds 44 with a catalytic amount of sodium ethoxide in ethanol led to a smooth intramolecular reaction, which resulted in the formation of y-lactams 45 in good yields [45] (Scheme 35). The stereochemical course of this ring expansion was unambiguously established by an X-ray analysis of product 45 [45]. Clearly an intramolecular Sn2 reaction has taken place. 

The above examples of the unique possibilities of stereochemically fixed aromatic rings in multibridged, multilayered and multistepped hydrocarbon compounds show that these classes of compounds are most appropriate for the study of intramolecular, steric and electronic interactions, from which too little quantitative relations are known. Again, it follows that further synthetic approaches are not only possible, but also necessary for a more accurate differentiation and analysis of the frequently overlapping electronic and steric effects. More progress in this field is sure to be expected, above all when synthetic and spectroscopic chemists, and physicists adhere to interdisciplinary, collaborative work. 

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