Chiral centres recognising

At the start of the analysis when you have done no more than recognise the FGs and note special features (such as rings) or easy disconnections, note also the number of chiral centres and their relationship to each other. The Prelog-Djerassi lactone 1 is an important intermediate in the synthesis of macrolide antibiotics.1 It has a six-membered lactone ring and a separate carboxylic acid. More to the point, it has four chiral centres la. Three (1-3) are adjacent and one (5) separate. We might say that the three adjacent centres should be easy to control because they are next to each other but that we might have trouble with C-5. Another way to look at it is to say that the three round the six-membered ring (2, 3 and 5) should be easy to control, as the... 

After the advent of combinatorial chemistry and its lack of success, it was recognised that the combinatorial libraries lacked diversity. Hence, new approaches were applied to design compounds that allowed the introduction of many points of diversity, such as chiral centres to mimic natural products.6 These libraries are also called natural product-like libraries.7... 

At the start of the analysis when you have done no more than recognise the FGs and note special features or easy disconnections, note also the number of chiral centres and their position. The Prelog-Djerassi lactone (1) is an important intermediate in the synthesis of macrolide antibiotics. It has a lactone ring, a carboxylic acid, and four chiral centres—three adjacent C2-C4) and one (C6) separate. 

Extensive studies have been carried out on nucleophilic substitution of a-halocarboxylic acid derivatives containing a chiral auxiliary in the carboxylic moiety. Racemisation of the labile chiral centre in the a-position to the carbonyl—induced by additives such as polar solvents, bases or halide salts—allows a high asymmetric induction through a DKR process to be obtained. This methodology has been recently recognised as a powerful synthetic method for asymmetric syntheses of a-heteroatom-substituted carboxylic acid derivatives. 

Care has to be taken in recognising a chiral centre for example, pethidine (Fig. 2.12) may look as if it has a chiral centre at 1 but, in fact, it is symmetrically substituted by the heterocyclic ring. The isomer of ethambutol (see Fig. 2.11) is not a chiral molecule but is called a meso compound since it has a plane of symmetry running... 

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