Diastereomers alkenes

It becomes difficult to decide whether, for example 1 is cis or trans isomer No. two groups are the same and the same is true for the compound II. The Chemical Abstracts Service has proposed an unambiguous system that has been adopted by IUPAC and is based on the preferences of groups. It is also called the E-Z system and applies to alkene diastereomers of all types. 

Single alkene diastereomers are accessible through a Wittig-Homer reaction only if it is performed in two steps (Figure 11.10). A 1 1 mixture of the phosphorylated lithium alkoxides syn- and anti-D is still formed but if the mixture is protonated at this point, the resulting phosphorylated alcohol diastereomers C can usually be separated without difficulty. The suitable diastereomer will be deprotonated with potassium-ferf-butoxide in the second step and then be converted into the stereouniform trans- or cis-alkene E via stereospecific oxaphosphetane formation and fragmentation. 

Detailed studies from these laboratories shed light on the mechanistic intricacies of asymmetric catalytic carbomagnesations, allowing for an understanding of the above trends in regio- and stereoselectivity [6]. Importantly, our mechanistic studies indicate that there is no preference for the formation of either the anti or the syn (EBTHI)Zr-olefin isomers (e.g.,8 anti vs 8 syn) it is only that one metallocene-alkene diastereomer (syn) is more reactive. Our mechanistic studies also indicate that zirconacyclopentane intermediates (i in Scheme 3) do not spontaneously eliminate to the derived zirconocene-alkoxide Zr-Mg ligand exchange is likely a prerequisite for the alkoxide elimination and formation of the terminal alkene. 

This enantioselectivity translates into the outcome of the products resulting from nucleophilic addition to the [(Re)-(jj -alkene)]+ and [(Re)( -aldehyde)] + cations. For example, addition of LiCuMe2 to the alkene diastereomer in (113) results in (116) in >95 5 ratio. 

The electron charge density on atom i is qu Zi is the nuclear charge P and S are the density and overlap matrices, respectively the sum in the first equation is the Mulliken population, i.e., the number of electrons in each atomic orbital p 6,( ) is the electron charge density connectivity hi is the number of hydrogen atoms attached to atom i the electron charge density q k is for the kth hydrogen atom bonded to atom i, and this differentiates cis/trans alkene diastereomers. 

In Section 4.5 we learned to use the terms cis and trans to designate the stereochemistry of alkene diastereomers (cis-trans isomers). These terms are unambiguous, however, only when applied to disubstituted alkenes. If the alkene is trisubstituted or tetrasubstituted, the terms cis and trans are either ambiguous or do not apply at all. Consider the following alkene as an example ... 

A system that works in all cases is based on the priorities of groups in the Cahn-Ingold-Prelog convention (Section 5.7). This system, called the E)- Z) system, applies to alkene diastereomers of all types. 

A number of 2,6-dideoxy-6,6,6-trifluorohexoses were prepared from a six-carbon acetylenic precursor via selective hydroxylation of the derived alkene diastereomers, and differences in stereoselectivity of reduction of a trifluoro-methyl ketone compared with the corresponding methyl ketone are outlined in Chapter 18. Oxidation of methyl 5-0-benzyl-3(2)-deoxy-3(2)-fluoro-a-D-pento-furanosides (DMSO/TFAA) was accompanied by epimerization at the fluori-nated carbon atom a- to the ketone resulting in formation of the corresponding 2-(or 3-) keto derivatives as mixtures of two epimers. Reduction then afforded various mixtures of 2- and 3- fluoro compounds. ... 

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