Cycloalkanes, diastereomers

R Stereoisomers (Section 4.2) are compounds whose atoms are connected in the same order but with a different geometry. Among the kinds of stereoisomers we ve seen are enantiomers, diastereomers, and cis-trans isomers (both in alkenes and in cycloalkanes). Actually, cis-trans isomers are just another kind of diastereomers because they are non-mirror-image stereoisomers. 

Tsuji has reported the cyclization/distannylation of bis( 1,3-dienes) that form bis(functionalized) 1,2-dialkenyl-cycloalkanes, although the scope of the transformation was quite limited.In one example, reaction of ( , )-6,6-bis(ethoxycarbonyl)-l,3,8,10-undecatetraene and hexamethyldistannane (1.2equiv.) catalyzed by Pd(DBA)2 in toluene at room temperature for 20h gave /ra/ i--( ,Z)-l,2-bis-[2-(trimethylstannyl)vinyl]cyclopentane trans- E,Z)-108] in 90% yield as a single regioisomer and diastereomer (Equation (69)). 

To draw all possible stereoisomers, remember that a disubstituted cycloalkane can have two substituents on the same side of the ring (ds isomer, labeled A) or on opposite sides of the ring (trans isomer, labeled B). These compounds are stereoisomers but not mirror images of each other, making them diastereomers. A and B are two of the four possible stereoisomers. 

Draw all possible stereoisomers for each cycloalkane. Label pairs of enantiomers and diastereomers. Label any meso compound. 

Cis and trans isomers of cycloalkanes furnish us with another example of stereoisomers that are diastereomers. Consider the following two compounds ... 

Stereoisomers Compounds that have the same molecular formula, the same connectivity of their atoms, but a different orientation of their atoms in space. The term "stereoisomer"includes cis,trans isomers in cycloalkanes and alkenes as well as enantiomers, diastereomers, meso compounds, and atropisomers. Conformational isomers are also stereoisomers, whether they are isolable or not. 

Two additional brominated cycloalkanes are currently used as additive BFRs. l,2-dibromo-4-(l,2-dibromoethyl)cyclohexane (TBECH) and 1,2,5,6 tetrabromocyclooctane (TBCO) are produced for use in textiles, paints, and plastics. TBECH can exist as four diastereomers (designated a, fl, y, and 8), and TBCO can exist as two diastereomers. Chiral separations of TBECH and TBCO enantiomers have not yet been reported. These BFRs exhibit some of the same analytical challenges as HBCDs, and LC—MS approaches offer the most promise for the development of robust methods. 

It is instructive to compare the stereoisomers of 2-bromo-3-chlorobutane with those of a cychc analog, l-bromo-2-chlorocyclobutane (Figure 5-9). In both cases, there are four stereoisomers RJt, S,S, R,S, and 5,/f. In the cyclic compound, however, the stereoisomeric relation of the first pair to the second is easily recognized One pair has cis stereochemistry, the other trans. Cis and trans isomers (Section 4-1) in cycloalkanes are in fact diastereomers. 

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