Configuration, relative

Relative configuration is not related to absolute configuration. Two molecules have the same relative configuration about a carbon if they differ by only one substituent and the other substituents are oriented identically about the carbon. Tn an Sn2 reaction, it is the relative configuration that is inverted. 

These two molecules have the same relative configuration, but they have different absolute configurations. 

The relative configuration of this molecule differs from the other two. 

The relative configuration of both adjacent methyl groups and flie cis or trans fusion of the rings A and B still remain open. This problem cannot be solved by means of the HH coupling constants because no protons are attached to the bridgehead carbon atoms. 

Based on this elueidation, an energy-minimized moleeular model ean be ealeulated. Fig. 11 displays the tube, ball-and-stiek and the spaee-filling model of aeanthifolin obtained as a result of this ealeulation by means of eommereial software in order to visualize the angle relations and shape of the moleeule. 

FIGURE 8.27 NMR chemical shift correlations for 1,3-diol acetonides. (From Rychnovsky, S. D., B. N. Rogers, and T. I. Richardson, Accounts of Chemical Research 31 (1998) 9-17.) 

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In polymers made of dis-symmetric monomers, such as, for example, poly(propylene), the stmcture may be irregular and constitutional isomerism can occur as shown in figure C2.1.1(a ). The succession of the relative configurations of the asymmetric centres can also vary between stretches of the chain. Configuration isomerism is characterized by the succession of dyads which are named either meso, if the two asymmetric centres have the same relative configurations, or racemo if the configurations differ (figure C2.1.1(b )). A polymer is called isotactic if it contains only one type of dyad and syndiotactic if the dyad sequence strictly alternates between the meso and racemo fonns. 

Figure C2.1.1. (a) Constitutional isomerism of poly (propylene). The upper chain has a regular constitution. The lower one contains a constitutional defect, (b) Configurational isomerism of poly(propylene). Depending on tire relative configurations of tire asymmetric carbons of two successive monomer units, tire corresponding dyad is eitlier meso or racemo.

The achiral triene chain of (a//-rrans-)-3-demethyl-famesic ester as well as its (6-cis-)-isoiner cyclize in the presence of acids to give the decalol derivative with four chirai centres whose relative configuration is well defined (P.A. Stadler, 1957 A. Escherunoser, 1959 W.S. Johnson, 1968, 1976). A monocyclic diene is formed as an intermediate (G. Stork, 1955). With more complicated 1,5-polyenes, such as squalene, oily mixtures of various cycliz-ation products are obtained. The 18,19-glycol of squalene 2,3-oxide, however, cyclized in modest yield with picric acid catalysis to give a complex tetracyclic natural product with nine chiral centres. Picric acid acts as a protic acid of medium strength whose conjugated base is non-nucleophilic. Such acids activate oxygen functions selectively (K.B. Sharpless, 1970). 

Erythro and three describe the relative configuration (Section 7 5) of two chirality centers within a single molecule... 

Representation of the polymer chain in an extended zigzag conformation as shown m Figure 7 16 reveals several distinct structural possibilities differing with respect to the relative configurations of the carbons that bear the methyl groups... 

Section 7 5 Relative configuration compares the arrangement of atoms m space to some reference The prefix as m as 4 methylcyclohexanol for example describes relative configuration by referencing the orientation of the CH3 group to the OH Absolute configuration is an exact description of the arrangement of atoms m space... 

Section 7 15 Certain polymers such as polypropylene contain chirality centers and the relative configurations of these centers affect the physical properties of... 

Thiol (Section 15 13) Compound of the type RSH or ArSH Th reo (Section 7 11) Term applied to the relative configuration of two stereogenic centers within a molecule The threo stereoisomer has like substituents on opposite sides of a Fischer projection... 

Chemical conversion of compounds to intermediates of known absolute configuration is a method routinely used to determine absolute configuration (86). This is necessary because x-ray analysis is not always possible suitable crystals are required and deterrnination of the absolute configuration of many crystalline molecules caimot be done because of poor resolution. Such poor resolution is usually a function of either molecular instability or the complex nature of the molecule. For example, the relative configuration of the macroHde immunosuppressant FK-506 (105) (Fig. 8), which contains 14 stereocenters, was determined by x-ray crystallographic studies. However, the absolute configuration could only be elucidated by chemical degradation and isolation of L-pipecoUc acid (110) (80). 

Vicinal coupling constants Jhh indicate very clearly the relative configuration of the coupling protons. Their contribution depends, according to the Karplus-Conroy equation... 

The relative configuration of the protons which is deduced from the coupling constant Jab confirms the conformation of this part of the structure of this molecule. On the other hand, the V//// coupling constant of the ethyl group attached to oxygen (7 Hz, Fig. 2.19) reflects equal populations of all stable conformers around the CC bond of this ethyl group. 

Table 2.10. Jhh coupling constants (Hz) and relative configuration The coupling path is shown in bold...

Table 2.12. chemical shifts ([Pg.50]

Table 2.14 summarizes the steps by which molecular structures can be determined using the NMR methods discussed thus far to determine the skeleton structure, relative configuration and conformation of a specific compound. 

One functional group and three substructures given by the H NMR spectrum 8 make up the molecular structure including the relative configuration of the sample compound. 

A natural substance of elemental composition Cis/Z/ Og was isolated from the plant Centaurea chilensis (Compositae). What is the structure and relative configuration of the substance given its H NMR spectrum 10 with (top) and without (bottom) deuterium exchange 10 ... 

Characterisation of the antibiotic monordene (also referred to as radicicol) with the elemental composition Cig/Zz/OgCl isolated from Monosporium bonorden gave the macrolide structure 1. The relative configuration of the H atoms on the two conjugated double bonds (6,7-cis, %,9-trans-) could be deduced from the 60 MHz H NMR spectrum The relative configuration of the C atoms 2-5, which encompass the oxirane ring as a partial structure, has yet to be established. 

The reference compound methyloxirane gives the H NMR spectrum 11a shown with expanded multiplets. What information regarding its relative configuration can be deduced from the expanded H multiplets of monordene displayed in 11b ... 

From the HH COSY contour plot 12a it can be established which cycloadduct has been produced from l-(iV,iV-dimethylamino)-2-methylbuta-1,3-diene and tra 5-P-nitrostyrene. The Jhh coupling constant in the one-dimensional H NMR spectrum 12b can be used to deduce the relative configuration of the adduct. 

Since the olefmic CC double bond is trisubstituted, the relative configuration cannot be determined on the basis of the cis and trans couplings of vicinal alkene protons in the H NMR spectrum. What is the relative configuration given the C NMR spectra 19 ... 

Commercial cyclopentadiene produces the set of NMR spectra 33. In what form does this compound actually exist, and what is its relative configuration ... 

Several shifts and coupling eonstants in the NMR speetra set 34 are so typical that the carbon skeleton can be deduced without any additional information. An NOE difference spectrum gives the relative configuration of the eompound. 

Which CH skeleton can be deduced from the NMR experiments 35 What relative configuration does the H NMR spectrum indicate ... 

No further information is required to identify this compound and its relative configuration from the set of NMR experiments 36 ... 

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