Rotativity and stereoselectivity

It is clear that when both properties - rotativity and stereoselectivity - of chemical transformations are taken into account, one finds that stereoaselective, nonstereoselective and stereoselective reactions can lead to mixtures that are either nonrotative or rotative. It follows that nonrotative mixtures can result from stereoaselective, nonstereoselective or stereoselective transformations. It turns out that rotative mixtures also are obtainable in transformations that are stereoaselective, nonstereoselective or stereoselective. 

Finally, in Chapter 18 we present an alternative, universal stereochemical classification of chemical transformations based on (a) overall loss, (b) no loss/gain, and (c) overall gain of chirotopic atoms we label these chirotopoprocesses as chirotopolysis, chirotopomutation and chirotopogenesis, respectively. Further subclassification is carried out using the dual criteria of rotativity (expected optical activity) and stereoselectivity (preferential formation of one stereoisomer over another). We also introduce and define the novel concepts of chiroselectivity and chirospecificity. Finally, the merits of the classification of chirotopoprocesses are discussed, and the stereotopoprocesses and chirotopoprocesses are correlated in relation to the stereotopic molecular faces. 

Astereochirotopolysis (Class 1) may be subclassified into nonrotative astereochirotopolysis (Class lA) and rotative astereochirotopolysis (Class IB), depending cn whether the resulting mixture is nonrotative or rotative, respectively. Nonrotative astereochirotopolysis (Class lA) can be stereoaselective, nonstereoselective, or stereoselective. Where no stereoselectivity is possible, the transformation is deemed stereoaselective (vide supra). In contrast, if stereoselectivity is possible, then the transformation is said to be stereoselective, only if unequal amounts of stereomers are formed in the special instance where stereomers are formed fortuitously in equal amounts, the process is said to be nonstereoselective. Rotative achirostereotopolysis (Class IB) is similarly subclassified into stereoaselective, nonstereoselective and stereoselective categories. 

These reactions bring out important and interesting consequences of the HED classification in the context of facioselectivity and difacioselectivity. Other consequences relating to stereoselectivity, stereospecificity, rotativity, stereotopoprocesses, and chirotopoprocesses, will be given in Volume 3, Chapters 17 and 18. Figure 12.5 represents the 45 quartets that emerge from our analyses. 

In Figure 17.10, transformation 39 and 40 constitute examples of nonrotative astereotopomutation, (Class 3A) the former example is stereoaselective, while the latter one is nonstereoselective. As indicated in this figure, there is no rotative achirostereotopomutation (Class 3B). In Figure 17.11, transformations 41 and 42 exemplify nonrotative nonstereotopomutation (Class 4A) the former transformation is stereoaselective (234=235=236=237) the latter one can be either nonstereoselective (if 240(=241) and 242(=243) are formed in equal amounts) or stereoselective (if 240 and 242 are formed in imequal amounts). Transformations 43 and 44 are examples of rotative nonstereotopomutation (Class 4B). Transformation 43 may be either nonstereoselective (if 246(=247) and 248(=249) are formed in equal amounts) or stereoselective (if 246 and 248 are formed in unequal amounts). Transformation 44 is stereoaselective (252=253=254=255). 

Examples of nonrotative chirostereotopomutation (Class 5A) are embodied in transformations 45 and 46 - the former being stereoaselective (258=259=260=261), and the latter being nonstereoselective (264(=266) is enantiomeric with 265(=267)). Finally, in Figure 17.12, we see transformations 47 and 48 which exemplify rotative chirostereotopomutation (Class 5B) - the former is stereoaselective (270=271=272=273), but the latter can be either nonstereoselective (if 277 and 279 are formed in equal amounts) or stereoselective (if 277 and 279 are formed in unequal amounts). 

In Figure 17.16, all transformations represent examples of rotative chirostereotopogenesis (Class 7B). Transformations 59 and 60 are cases of sp rotative chirostereotopogenesis (As=2 for each compraient in quartet 342-345 and 348-351). Either transformation can be nonstereoselective (if diastereomers 342(=345) and 343(=344) are formed in accidentally-equal amoimts) or stereoselective (if the said diastereomers are formed in unequal amounts). 

Transformations 61 and 62 represent sp rotative chirostereotopogeneses (As=l for each component in quartets 354-357 and 360-363). Each one of these two transformations can be nonstereoselective (if diastereomers 354(=357) and 355(=356) are formed in equal amounts) or stereoselective (if the latter diastereomers are formed in unequal amounts). Transformation 63 is a composite case of sp and sp rotative chirostereotopogeneses (As=2 for each component in quartet 366-369). This transformation is nonstereoselective (if diastereomers 366(=369) and 367(=368) are formed in equal amounts) or stereoselective (if the diastereomers in question are formed in unequal amounts). 

As in the case of stereotopoprocesses in Chapter 17, one can establish a link, between stereoselectivity and rotativity, for chirotopoprocesses as well. 

Of the stereoselective transformations portrayed in Figure 18.5, transformations 24 (118 119+120) and 25 (121+122 123(=125) + 124(=126)) give rise to nonrotative mixtures, whereas transformations 23 (112+113 114(=117)+ 115(=116) and 26 (127+128 129(=132) + 130 (=131)) yield rotative mixtures. Tranformation 24 leads to two nonequimeric achiral products in the case of 25, two achiral diastereomers are generated. In the rotative cases, in each of transformations 23 and 26 one obtains two chiral diastereomers (in unequal amounts). It is to be noted that if, in each of the latter two transformations, the (chiral) diastereomers are accidentally formed in equal amounts, the transformations would be considered nonstereoselective (vide supra). 

Class lA), whereas transformations 32-35 are representative rotative astereochirotopolyses (Class IB). In Classes lA and IB, transformations 27, 32-33 are stereoaselective the remaining transformations - 28-31, 34, and 35 - can be stereoselective or, accidentally, nonstereoselective. 

Astereochirotopomutation (Class 3) is subclassified into nonrotative astereochirotopomutation (Class 3A) and rotative astereochirotopomutation (Class 3B), depending on whether the resulting product mixture is nonrotative or rotative, respectively. Astereochirotopomutations of both subclasses - nonrotative and rotative - can be stereoaselective, nonstereoselective or stereoselective. 

Nonchirotopomutation (Class 4) is subdivided into nonrotative nonchirotopomiitation (Class 4A) and rotative nonchirotopomutation (Class 4B). Each of these subclasses can be also stereoaselective, nonstereoselective or stereoselective. 

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