Chirality without Stereogenic Carbon

A stereogenic carbon atom (chiral center, chiral atom, asymmetric atom) is bound to four unlike groups and thus generates chirality. Note that a molecule may possess a molecular chirality without having a stereogenic center. 

We have previously seen how geometric isomerism can exist around both the carbon-carbon double bond in an alkene and the carbon-nitrogen double bond in, for example, an oxime (Chapter 4), and we have now seen how an alkylidenecyclohexane such as 6 can be chiral without the need for a stereogenic carbon. Can one therefore obtain a chiral molecule in which the alkene part of 6 is replaced by, say, an oxime Oxime 8 has four different groups in the key positions. The nature of the oxime group, in particular the lone pair on N, makes it impossible to construct a stable chiral oxime with only two different groups. 

The chiral bicyclic guanidine 170 acts as a Bronsted base catalyst. The hydroxy group controls diastereo- and enantioselectivities in the direct aldol reaction of 5H-oxazol-4-ones (169) with aldehydes (62). The aldol products (171) are easily converted into amides or esters without loss of enantiopurity thus, this method provides synthetically usefiil a,p-dihydroxycarboxylates bearing a chiral quaternary stereogenic center at the a-carbon atom (Scheme 28.18) [88]. 

On the other hand, acyclic diastereomers are not necessarily compounds without any cyclic structural element, but the fragment connecting the stereogenic centers is acyclic. For example, a bicyclo[2.2.1]heptane derivative with a chiral side-chain attached to a nonbridgehead carbon belongs to this second category of acyclic diastereomers. 

Although four is the maximum possible number of isomers when the compound has two stereogenic centers (chiral compounds without a chiral carbon, or with one chiral carbon and another type of stereogenic center, also follow the rules described... 

Although we have shown that 4 is chiral, and that it has no stereogenic centre, it is not without symmetry. This is best seen with the aid of a Newman projection of 4, which is shown in 5. A two-fold (C2) axis of symmetry exists as shown this passes through the central carbon, C(2), and bisects the right angle between the two chlorine atoms (and likewise the two hydrogen atoms). Rotation about this axis by 180° gives an identical molecule. Because of the axis of symmetry, 4 cannot be said to be asymmetric, and similar situations are found in certain other chiral molecules. This has had implications for stereochemical nomenclature. In particular, to avoid confusion the term asymmetric carbon is now little used even for an sp3 hybridized carbon that carries four... 

Whereas allyl enol carbonates are prochiral starting materials, the fi-keto esters are frequently chiral but can be used as racemic compounds because the stereogenic center vanishes in the course of the decarboxylative allylic alkylation, precisely during the formation of the enolate. In general, both enantiomers of chiral fi-keto esters 91 react without enantiodifferentiation with the palladium(O) catalyst to give the enolate, a course of the reaction that was termed stereoablative... 

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