Stereochemical simplification

In summary, modem synthetic methodology allows the stereoselective generation of one, two, or even more stereocenters in a single reaction with or without spatial control by the substrate. The application of transforms to retrosynthetic simplification of stereochemistry requires the selection of transforms on the basis of both structural and stereochemical information in the target and also validation of the corresponding synthetic processes by analysis for both chemical feasibility and stereoselectivity. 

These strategies guide the retrosynthetic conversion of 272 to 278 and the further conversion of 278 via 279 to 282. The r-butyl substituent actuates the clearability of the stereocenters in 279. Further retrosynthetic simplification as dictated by basic FG-, stereochemical and topological strategies then leads from 280 to 281 and to 282, a previously described substance. The successful synthesis followed closely the above outlined retrosynthetic scheme. An enantioselective process was devised for the synthesis of 281 from 282 (see Section 10.12).67, 83... 

The retrosynthetic maneuvers that we have addressed thus far have only resulted in simplification of the vicinal side-chain substituents appended to carbons 8 and 12 we have yet to address the marked stereochemical complexity of the cyclopentane nucleus of... 

It is the object of our research to understand the chemical mechanism of structural and stereochemical regulation in the polymerization reaction as precisely as possible. In order to approach this objective, it is essential to accumulate, step by step, conclusive experimental evidence. The first step to be made experimentally is the simplification and the identification of the initial and final polymerization systems, and the next step is the elucidation of the mode of action of the catalyst on the monomer. 

A further simplification of stereochemical notation for saturated carbon centers is to stretch out the carbon skeleton in the plane of the paper (blackboard, computer screen). Valences of atoms or groups other titan hydrogen are indicated by a bold line if they project into the space in front of the plane and with a dashed line if they project into the space behind the paper (blackboard, computer screen). 

The simplification of stereochemistry results because the two ends of the sugar both now have CH2OH groups so that the possibility of C2 and planar symmetry arises. If we look at the two four-carbon sugars we can establish some important stereochemical correlations. Threose is reduced to... 

Very recently, Pericas reported a new strategy to immobihze trans-4-hydroxypro-line onto an insoluble Merrifield-type polymer by exploiting Cu(I)-catalyzed 1,3-dipolar cycloaddition ( click chemistry ) [42]. The supported catalyst 25 was successfully employed in the a-aminoxylation of ketones and aldehydes (Scheme 8.13). Under the optimized reaction conditions (20mol%/cat, 2 equiv. ketone, DMF, 23 °C, 3 h), the reaction of cyclohexanone with nitrosobenzene catalyzed by 25 gave the product in 60% yield and 98% ee (Scheme 8.13 Equation a). It should be noted that the reaction rates of cyclic ketones with supported catalyst are faster than those reported with (S)-proline. The use of a supported catalyst allowed for a simplification of the work-up procedure, as the product could often be obtained after simple filtration of the catalyst and evaporation of the solvents. Furthermore, 25 was recycled up to three times without any decrease in either the chemical and/or stereochemical efficiency. 

Corey s analysis to) of polycyclic systems and strategic bond disconnections directed by maximizing simplification of intermediates is an example of the definition of new heuristics for skeletal dissection ( 6). This analysis directs the selection of particular bonds for construction. His rules also derive in part from stereochemical considerations ( 7). A further treatment of rules for bond selection via stereochemistry would be valuable. 

Within the last few years n.m.r. spectroscopists have increasingly looked to solvents other than the usual inert materials, such as carbon tetrachloride or deuterochloroform as an aid in the simplification of spectra, in structural elucidation, in the investigation of weak interactions in solution, and in stereochemical studies. 

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