Reduction enantiomeric distinctions

More recently, Doris et al. have described the reductive ring-opening of a-keto epoxides [16]. In this manner, p-hydroxy ketones can be obtained in high yields. The synthesis of enantiomerically pure compounds can easily be realized. The titanocene] 111) reagents are distinctly superior to samarium diiodide, which is also known to induce this transformation. 

The electrochemical reduction proceeds via a radical anion, a subsequently formed diradical dianion and a tetraanion formed after intramolecular rearrangement. The CD spectra of the two enantiomeric forms of the diradical dianion, as shown in Fig. 5.32, are distinctly different. 

Guette and Horeau were able to derive a mathematical expression that relates the molar fraction of each product to the observed enantiomeric purities. The expression shown in Equation 6.1 is based on a more recent version developed by Kagan [7, 9] that apphes to the case where a racemic substance is partially converted into two distinct products (corresponding to diastereomers 12 and 13 in the camphor reduction). The molar fractions Xj (unreacted starting material),... 

Quantitative Distribution of Adducts as a Function of Dienophile Stereochemistry. For accurate quantitation of the isomer distribution in the products of cycloaddition to each of the dienophiles 5—8, the entire mixtures of the four stereoisomeric products in each instance were first subjected to sequential Q-deacetylation and periodate oxidation to afford a mixture of two aldehydo esters 29 and 30, which upon reduction with LiAlH afforded trans-2-norbornene-5,6-dimethanol as an unequal mixture of the two enantiomers (only the 5S,6g enantiomer is shown). NMR analysis of the mixture of 29 and 30 showed distinctive resonances for the CH3O and CHO groups in exo and endo orientations, permitting accurate determination of the endo/exo ratio of the products in the mixture. The observed specific rotation of the diol, in comparison with that (+23 ) determined for the enantiomerically pure 5S,6S diol 26 (and its enantiomer), provided a quantitative measure of the si.re diastereofacial selectivity. 

Several other mechanistically distinct metal-catalyzed dearomatization procedures have been reported, and almost all involve phenol or naphthol derivatives undergoing dearomatization via intramolecular transformations. Intramolecular Pd- and Rh-catalyzed C4-arylation and alkylation of /)ara-substituted phenols has been used to construct compounds of general structure 82 (Fig. 15.1) [86]. These reactions rely on generation of electrophilic aryl or alkyl o-metal complex intermediates that participate in tandem C4 metalation-reductive elimination with an attached phenol. Ruthenium- and Pt-catalyzed reactions of naphthalenes and alkynes deliver spirocyclic products such as 83 [87, 88]. An asymmetric intramolecular naphthalene dearomatization catalyzed by Pd(0)-phosphine complexes has been used to prepare carbazole derivatives 84 in good enantiomeric excess from l-(AI-2-bromophenyl)aminonaphthalene precursors [89]. 

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