Assignment of Relative Configurations

The specific rotation [a] can be measured at only a few wavelengths in most laboratories. This is one of the reasons why the significance of the full circular 

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The most simple cases are those in which assignment of relative configuration is achieved by the equilibration of two diastereomers with the configuration of one being known. 

The chemical shifts for hydrogens and methyl groups at C-4 of 5-hydroxy-and 5-amino-A2-1,2,3-triazolines depend on the orientation relative to the hetero substituent at C-5. This has been extensively used for assignment of relative configurations at C-4 and C-5 of variously substituted A2-tria-zolines.216,259 lH-NMR spectra show that 5-alkoxy- and 5-hydroxy-A2-1,2,3-triazolines prefer an envelope conformation218 (63) with the hetero substituent at C-5 pseudoaxial at the flap and the N-l substituent pseudoequatorial, probably because of the anomeric effect. The cis and trans coupling constants in the 5-amino-, 5-hydroxy-, and 5-aIkoxy-A2-l,2,3-triazolines are very constant, being 7.0-9.8 and 2.0-3.4 Hz, respectively.218... 

The advancement of >400 MHz NMR instruments with spin decoupling and Fourier transform software now allows identification of individual olefinic protons of nanogram carotenoids. We have shown two examples (lycopene and capsantin) for which the chemical shifts have been employed in the assignment of relative configuration. As for review of the NMR of carotenoids, Englert in 1981 gave information especially on the position of the cis double bonds in a polyene chain. 

Stereochemical reaction cycles sometimes aid in assignment of relative configurations to compounds and in determination of the stereochemical course of substitution reactions. For example, if mechanistic analogy suggests all reactions of a three-reaction cycle involve the same steric course, then that course (R or/) is determined alone by knowledge of whether three or four chiromers are included in the cyclic (podal or antipodal), and by whether the cycle is trior diligostatic. With any cycle, comparison of the number of reactions and... 

Scheme 3. Assignment of relative configuration of securinine 27 by Saito et al. (26).

Brown B R, MacBride J A H 1964 New methods for assignment of relative configuration to 2,3-trans flavan-3,4-diols. J Chem Soc 3822-3831... 

White, J.D. and Jensen, M.S. (1993) Biomimetic synthesis of a cyclopropane containing eicosanoid from the coral Piexaura homomaUa assignment of relative configuration. /. Am. Chem. Soc., 115, 2970-2971. 

Attempts to assign the relative configuration of the homodimer were complicated due to the extreme instability of the compound. Additionally, due to the increased reactivity of this substrate for homodimerization, there was no noticeable concentration effect between 0.1 and 0.0626 M. 

Determination of relative configuration. Thus, if diastereomeric compounds form complexes with LSR, a number of nuclei differ in their relative orientation with respect to the lanthanide ion. A given nucleus in one diastereomer has different r and 8 values than the corresponding nucleus in the other so that a different LIS is experienced. Using this method, structures of diastereomers can be assigned. For example, the LIS of the phenyl protons is much larger in cw-l-cyclohexyl-2-phenyl-3-azetidinol than in the tram-iso-... 

Chemical modification may also simply be achieved by complex formation with an optically active agent191. For example, the correlation of the configuration of chiral non-racemic phosphate triesters, such as 11 (see p 417)11, with the relative (when compared with ent- ) change in H chemical shift of the methoxy doublet induced by the addition of Eu(hfc)3192 has been used for the assignment of absolute configuration of optically active phosphate triesters (chiral at phosphorus), which were obtained by asymmetric synthesis as indicated. 

The eliminations 1 -> -> 3 and 1 -> -> 6 may serve as tools for assigning the relative configuration of /1-hydroxy acids such as 1. The scheme nicely shows that an exact knowledge of the stereochemical course of the key reaction that translates the relative configuration at the two stereogenic centers into the olefin configuration is of prime importance269. 

The problem of relative configurational assignment was in this case again solved by removal of one chiral unit with known configuration (in step 36 -> 38) and determining the absolute configuration at the one remaining108. 

Problem 5.37 Glyceraldehyde can be converted to lactic acid by the two routes shown below. These results reveal an ambiguity in the assignment of relative d,l configuration. Explain. 

In this Section we shall use the ideas of prochirality in assignment of stereochemical configuration S8) (usually relative — especially meso vs. dl — rather than absolute configuration) and we shall also discuss assignment of prochirality symbol (i.e. recognition of which group is pro-R and which pro-S at a prochiral center). (Recognition of prochiral faces as Re or Si is usually obvious from the stereochemistry of the addition products thereto and will not be discussed here examples are found in Section 5.2). 

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