Prelogs Method

The absolute configuration at sulfur is specified by the Cahn-Ingold-Prelog method with the provision that the unshared electron pah is considered to be the lowest ranking substituent. 

When a molecule has two stereogenic centers, each has its own configuration and can be classified (R) or (5) by the Cahn-Ingold-Prelog method. There are a total of four isomers, since the first center may be (R) or (5) and so may the second. Since a molecule can have only one mirror image, only one of the other three can be the enantiomer of A. This is B [the mirror image of an (R) center is always an (5) center). The compounds C and D are a second pair of enantiomers and the rela-... 

CIP (Cahn, Ingold, Prelog) method, CIP system The CIP sequencing rules establish the conventional ordering of ligands for the unambiguous description of absolute configuration by descriptors such as R, S P, M E, Z. 

Solution This problem deals with configuration, the arrangement of atoms that characterizes a particular stereoisomer. One method of assigning configuration, called the Cahn-Ingold-Prelog Method, is by use of the prefixes R and S. Two steps are involved in this process ... 

Cahn-Ingold-Prelog Method FOR Determining Priority OF Attached Groups... 

The absolute configuration of annotinine shown in I was resolved by Wiesner et al. (76). Three independent methods were used, all of which led to the same conclusion. The ORD-curve of XXVI, the relative stereochemistry of which has been demonstrated, showed a negative rotation maximum at 322 mp, [a] — 3450°. Application of the octant rule led to the absolute configuration shown in structure XXVI. Application of Hudson s lactone rule on annotinine hydrate led to the same conclusion. The Prelog method, when applied to compound VII, which must have the C-11 hydroxyl group cis to the bridge, was in agreement with the other methods. 

Figure 2.11 Using the Cahn-Ingold-Prelog method to assign absolute configuration at a chiral center.

Prelog and Bozieevie applied to this problem a method for the preparation of dieyclic bases of this type, which may be represented by the following summary of generalised formulae. 

Of more direct interest is the preparation of 3-ethylquinuclidine by this method (Prelog, Sogtaric and Gustak (1940)). Ethyl tetrahydropyranyl ketone (X R = tetrahydropyranyl in X to XIV) was condensed wiA ethyl brpmoacetate (XI) to give the hydroxy-acid ester (XII) which was dehydrated by potassium acid sulphate at 90° to ethyl 3-(tetrahydro-pyranyl-4)-pentenate (XIII) and this hydrogenated to tie corresponding... 

A complete synthesis of di-heliotridane has been effected by Prelog and Zalan, 2 using Prelog s general method for the synthesis of dicyclic amines, the proximate, primary material in this case being 1 7-dibromo-4-amino-3-methylheptane hydrobromide,... 

R,S convention (Section 9.5) A method for defining the absolute configuration at chirality centers using the Cahn-IngoId-Prelog sequence rules. 

The Prelog-Djerassi lactone (abbreviated here as P-D lactone) was originally isolated as a degradation product during structural investigations of antibiotics. Its open-chain equivalent 3 is typical of the methyl-branched carbon chains that occur frequently in macrolide and polyether antibiotics. The compound serves as a test case for the development of methods of control of stereochemistry in such polymethylated structures. There have been more than 20 different syntheses of P-D lactone.24 We focus here on some of those that provide enantiomerically pure product, as they illustrate several of the methods for enantioselective synthesis.25... 

It is always advisable to examine the complete molecular topology in the neighborhood of the chiral carbon atom and to confirm the results by employing another analytical method before the final assignment. In conclusion, Prelog s rule does predict the steric course of an asymmetric synthesis carried out with a chiral a-keto ester, and the predictions have been found to be correct in most cases. Indeed, this method has been widely used for determining the absolute configuration of secondary alcohols. 

Demercuradon.1 A recent synthesis of the Prelog-Djcrassi lactone (5), a degradation product of some macrolidc antibiotics, involves mercury(ll) cyclization of 1 to 2. Usual methods of demercuration (NaBH4, Na/ Hg, H2S py) favor formation of the 2-cpimer (6) of 5. The most useful reagent is sodium trithiocarbonate, which gives 5 and 6 in a ratio of 3.5 1. The ratio is 2 1 when Nu2S is used. 

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