Prostereogenic atom

When chloromethylsulfinylmethane is reacted with unsymmetrical ketones in the presence of potassium rm-butoxide in fert-butanol, diastereomeric mixtures of oxiranes are formed, showing that the asymmetric induction at the prostereogenic carbonyl carbon atom is low21. 

Enantioselective addition of diethylzinc to benzaldehyde has been promoted by indole-containing chiral oxazolidines (which are able to use both O and N atoms to effect metal coordination in the transition state), and by chiral o-hydroxyphenyl diaza-phospholidine oxide,and by chiral aziridino alcohols.Enantioselective addition of dialkylzinc to prostereogenic ketones has been promoted using chiral camphorsul-fonamide derivatives. 

The addition criterion may similarly be applied to recognize diastereotopic faces. Methyl a-phenethyl ketone, 58 in Fig. 19 has a chiral center addition clearly gives rise to diastereomers (59a, 59b) the faces of the carbonyl carbon are diastereotopic and the C = 0 group is prochiral. This case is of importance in conjunction with Cram s rule 10). Compounds 60, 62 and 64 also display diastereotopic faces even though the products 61, 63 and 65 are not chiral 60, 62 and 64 have prostereogenic rather than prochiral faces. The C=0 group in 60 is propseudoasymmetric, since C(3) in 61 is a pseudoasymmetric center. a-Phenethyl methyl sulfide (66) displays diastereotopic sides of a molecular plane not due to a double bond 5,24> and may alternatively be considered a case of diastereotopic phantom ligands (unshared pairs on sulfur). This case does involve chirality and the sulfur atom is prochiral. 

Similarly, the nucleophilic substitution at sulfur in [(—)-menthyl] (A)-4-methylbenzenesul-finate by prostereogenic a-metalated nitriles yields the corresponding a-cyano sulfoxides 3 as diastereomeric mixtures. On oxidation these are converted to the corresponding sulfones with a stereogenic center at the a-carbon atom only73. 

This slightly less symmetrical molecule is not chiral but prochiral. The carbon atom is a pro-chiral (or prostereogenic) centre. The plane of the paper is stiU a plane of symmetry, but the yellow plane containing the two H atoms is not and the hydrogen atoms are enantiotopic. They are magnetically equivalent and can be distinguished only by humans, enzymes, and other asymmetric reagents. 

The formation of the lactones exhibits a preference for the hms-product in relation to the ester-bearing carbon atom. If the radical center is prostereogenic. the cyclization generates two new stereogenic centers with excellent trans selectivity. 

Most asymmetric protonations deal with prostereogenic carbon atoms which can be enantiose-lectively protonated by chiral proton sources under strictly kinetic conditions. 

A molecule (anion) containing a prostereogenic carbon atom may result either during the course of a synthesis or by deprotonation of a corresponding stereogenic center e.g., in 1. 

Figure 15.3 illustrates the process of determining the prostereogenicity of divalent sp and trivalent sp and sp atoms. 

Figure 15.4 illustrates the process of determining the prostereogenicity of tetravalent sp and trivalent sp atoms. The rules for breaking the degeneracy of homomorphic ligands of tetravalent sp and sp atoms are given below. 

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