Isomers ring substituents

All the ring substituents m p D glucopyranose are equatorial m the most stable chair conformation Only the anomenc hydroxyl group is axial m the a isomer all the other substituents are equatorial... 

A cycloalkane is a saturated cyclic hydrocarbon with the general formula C H2 . In contrast to open-chain alkanes, where nearly free rotation occurs around C, -C bonds, rotation is greatly reduced in cycloalkanes. Disubstituted cycloalkanes can therefore exist as cis-trans isomers. The cis isomer has both substituents on the same face of the ring the trans isomer has substituents on opposite faces. Cis-trans isomers are just one kind of stereoisomers—isomers... 

Principles and Characteristics Traditional analytical approaches include off-line characterisation of isolated components, and the use of several chromatographic separations, each optimised for a specific spectroscopic detector. Neither LC-NMR nor LC-MS alone can always provide complete structure determinations. For example, MS may fail in assigning an unequivocal structure for positional isomers of substituents on an aromatic ring, whereas NMR is silent for structural moieties lacking NMR resonances. Often both techniques are needed. 

The terms cis and trans are used to describe the configuration, which is considered to be the spatial sequence about the double bond or the spatial sequence relative to a ring system. The cis isomer has substituents on the same side of the double bond or ring system (Latin cis = on this side), whereas the trans isomer has substituents on opposite sides (Latin trans = across). 

If the lH-azepine bears a ring substituent then a mixture of two isomeric 2-azabicyclohep-tadienes, (38) and (39), is possible, corresponding to the two allowed electrocyclic processes indicated in Scheme 1 (paths a and b, respectively). In fact, the ratio of the two isomers varies with the position of the substituent. For example, the 3-methylazepine (37 R = 3-Me) affords a 1 1 mixture of the 4- and 7-methyl-2-azabicycloheptadienes, whereas the 4-methylazepine (37 R = 4-Me) yields the 5- and 6-methyl derivatives in the ratio of 1.5 1 (B-69MI5600). 

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