Benzoates exciton rule

Benzoates Exciton Rule for Determining die Anomeric Configuration in Saccharides... 

The dibenzoate chirality rule 155, 156 extends the application of the exciton chirality method to molecules containing no suitable chromophore, but, rather two hydroxy groups which can be converted to benzoates or cinnamates. For example, the dibenzoate 1, obtained by benzoy-lation of the ra-diol, from microbial oxidation of ethylbenzene, displays a negative exciton Cotton effect and is hence assigned the 1 S.2R configuration157. 

The absolute configuration of (-)-spiro[4.4]nonane-l,6-dione was determined by application of Horeau s method of optical rotations.[45] In support of the assignment, and with greater certainty, the Exciton Chirality Rule was applied to the bis-p-dimethylamino-benzoate of the cis,trans-diol obtained following reduction of the diketone. The cis,trans-diol is readily distinguished by NMR from the C2-symmetry cis,cis and rrans,rrans-diols. Since the cis,trans-diol dibenzoate from the (-)-dione exhibits a (-) exciton chirality CD, it follows that the absolute configuration of the dione is that shown in Figure 37. The absolute stereochemistry is easily determined by CD. 

To determine the ACs of acyclic 1,2-glycols, the CD exciton chirality method has been applied to their dibenzoates or bis(2-anthroates), which show typical bisignate Cotton effects (see Section 9.04.4) as exemplified in Figures 27 and 28.72,73 Acyclic dibenzoates or bis(2-anthroates) can rotate around the bond connecting two benzoate or 2-anthroate chromophores, and therefore the CD sign depends on the conformational equilibrium. From the data of many examples, general rules were derived as shown in Figures 27 and 28. 

Nakanishi s method is based on the splitting of CD waves when two chromopho-res on a chiral molecule are close in space (the dibenzoate rule). Such is the case with dibromobenzoates of sugars which have a 1,2 or 1,3 relationship. In the first version of the method, saponins were permethylated, methanolyzed with acid and the liberated OH positions were p-bromobenzoylated (44). As terminal sugars are fully methylated and UV transparent, they need not be considered. Branched sugars (two substitutions at least) 3deld di-or tri-benzoates with exciton-split CD curves. The difference in Ae values of the two extrema of split CD curves is directly related to the respective positions of the benzoates (1, 2 eq-eq = 1, 2 eq-ax = 62 1, 2 ax-ax = 6 1, 3 eq-eq = 0 1, 3 eq-ax = 16). The sensitivity of circular dichroism makes the method suitable for microassays. Typical analyses are performed on a 100 pg scale (nanomolar levels). 

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