Position codes, substituent

The general code for the substituents R and R , as defined for formulae (1) and (2), will be used throughout this Report. For publications in which several substituents R and R have been employed, or where a general class of compounds is implied, R and R will not be specified. Other substituents will be labelled R, R , etc., but their numbering is to avoid ambiguity with R and R , and does not imply similarity of position of substituents or of their nature. 

Vast tabulations of 13C chemical shift data have been assembled in computer searchable form. These databases form the basis for 13C chemical shift prediction algorithms. For the most part, carbon chemical shifts can be calculated using what is referred to as a Hierarchically Ordered Spherical Environment (HOSE) code approach [28]. To calculate a given carbon s chemical shift, the influence of each successive spherical shell is applied to the starting chemical shift for that carbon to calculate its overall chemical shift. Typically, programs will calculate shifts for 3 or 4 layers, beyond which the effects of most substituents are negligible. The spherical layers surrounding the 23-position of strychnine are shown in Fig. 10.8. 

The large amount of work devoted to the synthesis of oligo-p-peptides has created a need for some standardization in nomenclature. The proposals of Seebach 5 will be used throughout this section, i.e. the P-peptides will be named according to their substitution pattern with the superscript representing the position of the substituent on the peptide backbone (Scheme 1) the prefix H to the three-letter code for amino acids has been used throughout to denote a homoamino acid, e.g. HLeu is homoleucine, HVal is homovaline, etc. 

A few things are obvious. Where a thing is located (shown by the number or numbers) has priority over what a thing is (the substituent or substituents). This same locating and identifying code will be used for the benzyl group on the 1-position, but with some extensions which will be explained below. 

The two chair forms shown in Figure 4-11 are, except for the color coding, identical. We can lift this degeneracy by introducing substituents Now the chair with a substituent in the equatorial position is different from its conformer, in which the substituent is axial. The preference for one orientation over the other strongly affects the stereochemistry and reactivity of cyclohexanes. We will describe the consequences of such substitution in the next section. 

---