Epoxides stereochemical consequences

For example, Cunninghamella elegans oxidizes benzo[a]pyrene to its 7,8-oxide, hydrates the 7,8-oxide to the 7,8-dihydrodiol, and metabolizes this as well as the 9,10-dihydrodiol to diol epoxides " with stereochemical consequences quite similar to those observed for liver microsomes from the 3-methyl-cholanthrene-treated... 

Scheme 27 Stereochemical consequences of intramolecular epoxide opening...

The above-mentioned facts have important consequences on the stereochemical outcome of the kinetic resolution of asymmetrically substituted epoxides. In the majority of kinetic resolutions of esters (e.g. by ester hydrolysis and synthesis using lipases, esterases and proteases) the absolute configuration at the stereogenic centre(s) always remains the same throughout the reaction. In contrast, the enzymatic hydrolysis of epoxides may take place via attack on either carbon of the oxirane ring (Scheme 7) and it is the structure of the substrate and of the enzyme involved which determine the regioselec-tivity of the attack [53, 58-611. As a consequence, the absolute configuration of both the product and substrate from a kinetic resolution of a racemic... 

When two allylic alcohols are contained in a synunetrical molecule, asymmetric epoxidation proceeds with interesting consequences for stereochemical purity. The results were first described for the asymme-... 

As a consequence of the target epoxide s specific stereochemical formation requirements, the aforementioned anhydrosugar conformation forces the hexose out of the stable C4 conformation into the Hq conformation, which is higher in energy and easily attacked by nucleophiles, thus enabling a variety of applications. This particular property of high chenucal reactivity... 

Cyclic sulfates 540a-c are conveniently prepared in good yields by a two-step, one-pot transformation of the appropriate diester la or lb with thionyl chloride followed by ruthenium tetroxide oxidation. They behave like epoxides in that they simultaneously activate and protect adjacent functionalized carbon atoms from nucleophilic attack. As a consequence of their cyclic nature, they render competing elimination processes stereochemically unfavorable [175]. 

The above-mentioned facts have important consequences on the stereochemical course of the kinetic resolution of nonsymmetrically substimted epoxides, hi... 

Although retention of configuration seems to be the more common pathway, inversion has been reported in some cases depending on the substrate structure and the type of enzyme [574,575]. As a consequence, the absolute cOTifiguration of both the product and the substrate from a kinetic resolution of a racemic epoxide has to be determined separately in order to elucidate the stereochemical pathway. As may be deduced from Scheme 2.88, the use of the enantiomeric ratio is only appropriate to describe the enantioselectivity of an epoxide hydrolase as long as its regioselectivity is uniform, i.e., only inversion or retention is taking place, but -values are inapplicable where mixed pathways, i.e., retention and inversion, are detected [576]. For the solution to this stereochemical problem, various methods were proposed [577]. 

The euphane and tirucallane triterpenes differ from the lanostanes only in having the opposite stereochemical dispositions at C13, C14, and Ci7 and are themselves epimeric at C20 384). The stereochemistry is considered to be a consequence of enzymatic cyclization of squalene epoxide (305) from a chair, chair, chair conformation (Scheme 29). The two stereoisomeric forms arise from 17->20 hydrogen shift to the dia-stereotopic faces of the carbonium ion at C20. ... 

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