Chiral centers terms Links

Tertiary carbon atoms along the chain have been defined as asymmetric (22-25, 34-37), pseudoasymmetric (6, 10, 38-40), stereoisomeric centers (30, 31), and diasteric centers (41). The first two terms put the accent on chirality and are linked to the use of models of finite and infinite length, respectively the last two consider only phenomena of stereoisomerism. Note the relationship between these last definitions and Mislow s and Siegel s recent discussion (42), where the two concepts—stereoisomerism (or stereogenicity) and chirality—are clearly distinguished. The tertiary carbon atoms of vinyl polymers are always stereogenic whether they are chinotopic or achirotopic (42) depends on stmctural features and also on the type of model chosen (43). 

Three consecutive 3,3 -linked pyrroles were designed in 183 and 184. These compounds presented two chirality axes and a fixed chirality center. If the barriers about the axes were high enough, four diastereomers should be observed by NMR. The two axes are not equivalent in terms of rotational barriers since in one case one finds two flanking substituents and in the other case one finds three flanking substituents. Obviously in the case of the axis with only two flanking substituents, the barrier is too low to be... 

Scheme 23 shows how four possible diastereomers can arise from the combination of two sp -carbon centers C-1 and C-2 in a donor component 23-1 and an acceptor component 23-2. Species 23-3 and 23-4 are two diastereomers and 23-5 and 23-6 are their enantiomers.The problem of simple diastereoselection is the control of the diastereomer ratio 23-3-1-23-5/23-4-1-23-6. The enantiocontrol of 23-3 vs 23-5 or of 23-4 vs 23-6 cannot be achieved by simple diastereoselection in this case an external source of chirality has to be applied, for instance a chiral catalyst or the incorporation of stereogenic units in one of the components. Simple diastereoselection can be exerted in terms of closed and open transition states, depending on the mutual interaction of the termini X and Do, respectively. If these termini are linked via a six-membered chelate, a closed ( Zimmerman-Traxler ) transition state 23-7 with synperiplanar olefinic units is formed. On the other hand, if the termini have a repulsive interaction an open transition state 23-8 with an antiperiplanar arrangement of the olefinic units is adopted. Efficient stereocontrol via Zimmerman-Traxler transition states 24-1 to 24-4 is observed in aldol-type and allylborane carbonyl additions (Scheme 24). The crucial stereo differentiating interaction is the diaxial repulsion between Rax and R, which must be kept as low as possible. Only small substituents (nor-...

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