By Ernest L. Eliel, Samuel H. Wilen

A part of a sequence which supplies in-depth assurance of natural stereochemistry, this quantity contains dialogue of aldol diastereoselection lower than thermodynamic and kinetic regulate, enantioselection in aldol condensations regarding chiral reactants, purposes of molecular mechanics calculations to natural chemistry, and masses extra. The members target to collect the disciplines of floor chemistry and stereochemistry to illustrate that the homes of stereoisomers can be worthy in extending the knowledge of the vulnerable but vital molecular forces that function in floor monolayers.

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Extra info for Topics in Stereochemistry, Volume 21

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8 ppm that were attributed to the cis and trans oxaphosphetanes, respectively. 3 (zJc-c-p = 15 Hz). In each diastereomer, C, shows the large lJc-p coupling constant expected for an equatorially oriented substituent in a trigonal bipyramidal phosphorus environment by analogy to the precedents cited above (36, 37). No other 13Cdata were reported from this series. A number of other oxaphosphetanes have now been studied using natural abundance ,C NMR methods, and several representative examples are summarized in Table 5 (21a,b, 39).

2, f). The last item ( 5 ) may explain why the independent betaine generation control experiments gave contradictory results. If betaines are high-energy species that lie above the saddle point leading from the ylide to the oxaphosphetane, then experiments that deliberately generate betaines (Schemes 3-5) may encounter pathways for stereochemical equilibration that are not accessible to typical Wittig reactions. The chronology of events that resulted in generalizations 1-5 will not be presented in detail because this topic is extensively discussed in another account (6).

One version of this technique depends on resolution of the 31Psignals of oxaphosphetane diastereomers using high-field NMR instruments. If two distinct signals can be resolved for 31 and 32 from a Wittig reaction performed at -40°C or below, then the kinetic diastereomer ratio can be determined. If this ratio differs from the empirical Z: E ratio of alkene products, then stereochemical equilibration of intermediates is established. In some cases, the ratio changes well below the temperature threshold for Wittig decomposition to the alkene, and it is possible to monitor equilibration (“stereochemical drift”) directly by using NMR methods.

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