Isomers subdivision

The stereoselective cyclopropanation of chiral alkenes can be divided into two classes cyclic and acyclic alkenes. Furthermore, within each class, a subdivision exists involving those that contain a proximal basic group that can direct the cyclopropanation reaction of zinc carbenoids and the others that do not. The discrimination of reactivity between alkenes that possess a proximal basic group and those that do not was first highhghted early on when Simmons and Smith noticed that the cyclopropanation of l-(o-methoxyphenyl)-l-propene was more efficient than that of the related meta and para isomers (equation 46). ... 

Table 6 is a compilation of data for numbers of benzenoid isomers with h > 15. They are incomplete in the sense that, for each h, the list of n values is not complete. Furthermore, in many cases the subdivisions into normal and essentially disconnected benzenoids are unknown. 

The focussing on formulas for single coronoids is continued in this chapter. It starts with a treatment of sequences of associated formulas for extremal single coronoids. This study leads to another subdivision of the extremal coronoids ground forms and higher members. The pertinent sections can be considered as a preparation to the enumeration of coronoid isomers, which is treated in the subsequent chapter. Also the process of building-up (Sect. 6.5) is highly relevant to the enumeration of isomers. 

Admittedly only the axial and epimeric pairs can be reasonably easily separated from each other further subdivision is difficult and more is said about this in connection with the urinary 17-ketosteroids. Even the 3j5,6a-isomer (epiandrosterone) cannot be separated from the important J ,3iS-compound (dehydroepiandrosterone) using normal TLC. As mentioned in the preceding section, complex formation on silver nitrate-containing layers helps in this instance. Many synthetic Cig -steroids contain a 17a-alkyl as well as the 17j8-hydroxyl group the former reduces the polarity in the order — CH3, — CgHg, — CH=CH2 equals — CsCH, — C3H7. 

Suzuki (1989), and a thorough diemotaxonomic study has proposed the subdivision of SAPs into five categories (I-V) that seem specific to the orders of Echinoidea (Suzuki and Yoshino, 1992). In the family Toxopneustidae, the species Tripneustes gratilla is unusual because it is the only one whose SAPs contain ortho, meta, and para isomers of bromophenylalanine (Yoshino et al., 1989 Suzuki and Yoshino, 1992). 

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