Helicity, propellers, chiral cages

Helices are chiral objects often encountered in nature, for example helical shells. The absolute configuration of these objects is designated P (plus) and M (minus) for right- and left-handed helices, respectively. A number of chemical structures resort to helicity, the most famous example being the class of molecules known as helicenes. Thus, the compound shown in diagram XXXVIII is (/ )-( + )-hexahelicene [33]. 

A particular case of helicity is that displayed by molecular propellers [47], aptly described by this picturesque name. An example of a 3-bladed propeller structure is provided by tri-o-thymotide, the (— )-enantiomer of which has now been shown to be the left-handed form (M configuration) (diagram XXXIX) [48]. 

The molecular helices and propellers discussed above contain no center of chirality, and the P and M nomenclature is thus the only way of describing their absolute configuration. This nomenclature, however, is also applicable to some series of chiral compounds which display several centers of chirality. As will be discussed in Section 6, the presence in a molecule of two or more centers of chirality usually implies the existence of several stereoisomers, but steric reasons may reduce down to two the possible number of stereoisomeric forms. Thus, 2,3-epoxycyclohexanone contains two asymmetric carbon atoms, but for steric reasons only two stereoisomers, namely the (2S 3S)-(—)- and the (2/ 3/J)-( + )-enantiomer, exist the former is depicted in diagram XL [49]. 

Chiral cages are such examples of molecules containing several centers of chirality but existing only in two stereoisomeric (enantiomeric) forms the absolute configuration of which can be described according to helicity rules. Thus, the two enantiomers of 4,9-twistadiene are the (lS 3S 6S 8S)-( + )- and (l/ 3/J 6/J 87 )-(—)-isomer, 

The cage-shaped compounds discussed above belong to point group D2. The term gyrochiral has been proposed in order to describe all chiral but not asymmetric structures [52]. 

---