Stereochemical relation tables

The stereochemical relation table contains in the ith row the Steric relations of atom i to other atoms. The entries in the table consists of pairs of symbols, the first indicating the steric relation and the second indicating the atom concerned. 

For many catalytic cyclopropanations, the stereoselectivity describing the stereochemical relation between substituents at the carbenoid and those at the double bond is not very pronounced. EjZ or syn/anti ratios of ca. 1-3 in favor of the less congested isomer may be considered normal (for examples see Tables 6 and 7). The stereochemical outcome can be expected to be governed by the nature of the olefin, the diazo compound and the catalyst. 

The enzyme also displays a fairly broad tolerance for stereochemically related aldehyde substrates such as a number of sugars and their derivatives larger than or equal to pentoses [47,68,69]. Several natural and unnatural sialic acid derivatives have thus been prepared by replacement of the natural D-manno-configurated substrate with aldose derivatives containing modifications such as epimerization, substitution, or deletion at positions C-2, -4, or -6 (Table 1) [16,22]. Epimerization at C-2, however, is restricted to small polar substituents at strongly decreased reaction rates [63,70]. 

Table 12 shows the equilibrium spreading pressures of each diacid. It is immediately apparent that for three of the diastereomeric pairs there are statistically significant differences. These distinctions relate stereochemical preferences in the spontaneous spreading of (+)- versus meso-monolayers in equilibrium with their respective crystalline phases. However, there appears to be no discernible trend in either the ( )- or meso-ESPs as a function of carbonyl position despite clear trends seen in their monolayer properties in the absence of any bulk crystalline phase. 

Other studies have provided additional data on the relative stabilities of the lithium aldolates 14 and 15 derived from the condensation of dilithium enediolates 13 (Rj = alkyl, aryl) with representative aldehydes (eq. [ 10]) (16). Kinetic aldol ratios were also obtained for comparison in this and related studies (16,17). As summarized in Table 4, the diastereomeric aldol chelates 14a and ISa, derived from the enolate of phenylacetic acid 13 (R = Ph), reach equilibrium after 3 days at 25° C (entries A-D). The percentage of threo diastere-omer 15 increases with the increasing steric bulk of the aldehyde ligand R3 as expected. It is noteworthy that the diastereomeric aldol chelates 14a and 15a (Rj = CH3, C2HS, i-C3H7) do not equilibrate at room temperature over the 3 day period (16). In a related study directed at delineating the stereochemical control elements of the Reformatsky reaction, Kurtev examined the equilibration of both... 

The stereochemical outcome of the cycloaddition to 3-butene-1,2-diol derivatives, cyclic acetals, or to related alkenes that possess an allylic nitrogen substituent such as 4-vinyl-oxazolines or -oxazolidines was also rationalized by this model (162) (Table 6.7). In the latter cases, the A-Boc group instead of the a-oxygen prefers the inside position (Scheme 6.24). 

Optically active aldehydes are available in abundance from amino and hydroxy acids or from carbohydrates, thereby providing a great variety of optically active nitrile oxides via the corresponding oximes. Unfortunately, sufficient 1,4- or 1,3-asymmetric induction in cycloaddition to 1-alkenes or 1,2-disubstituted alkenes has still not been achieved. This represents an interesting problem that will surely be tackled in the years to come. On the other hand, cycloadditions with achiral olefins lead to 1 1 mixtures of diastereoisomers, that on separation furnish pure enantiomers with two or more stereocenters. This process is, of course, related to the separation of racemic mixtures, also leading to both enantiomers with 50% maximum yield for each. There has been a number of applications of this principle in synthesis. Chiral nitrile oxides are stereochemically neutral, and consequently 1,2-induction from achiral alkenes can fully be exploited (see Table 6.10). 

Many of the alkaloids from Veratrum, Zygadenus and related genera are based on the cevane structure 1. The chemistry of the Veratrum alkaloids has been reviewed from time to time both in this treatise (1-3) and elsewhere (4-6). The most recent of these articles was published in 1973, and many new compounds have been discovered since then. Recent years have seen the announcement of many X-ray crystal structure determinations on these stereochemically complex alkaloids. A total synthesis has been reported for only one of the natural products, namely, verticine (7). The work that lead to this notable achievement has been reviewed (8). The pharmacology of both the alkaloids (9-12) and their synthetic derivatives (13) has been reviewed, although not since 1977. This chapter takes a different approach from previous reviews. The chemical reactions used for structure determination and modification are first summarized, but the main part of the chapter is a series of tables which include all the known cevane derivatives, both old and new. This is intended as a reference source for future workers in the field. The literature has been covered to the end of Volume 113 of Chemical Abstracts (1990). 

Diversity in the structure and proportion of pheromone components is mirrored in the diversity of the proteins from the olfactory system. A specialized olfactory system is responsible for distinguishing the pheromone from other odorants in the environment. The high precision of the pheromone olfactory system becomes apparent when we compare closely related species whose pheromones differ in subtle ways. For example, Heliothis species have the same unsaturated aldehyde as the major pheromone component, but their pheromone signals differ in the structure and proportion of minor components (Table 16.1). Another example is seen with the gypsy moth (Lymantria dispar) and nun moth (Lymantria monacha), both of which respond to la. The blend produced by the nun moth consists mostly of lb, which is a powerful behavioral antagonist in the gypsy moth and is behaviorally inactive in the nun moth (Hansen, 1984). Stereochemical features play an important role in the molecular recognition of pheromone components. 

Within the series of compounds containing the (25 ,52 )-dimethylpiper-azine, there is not always a preferred benzhydryl epimer. The epimer with R stereochemistry at this position was certainly preferred for BW373U86 with respect to delta opioid receptor agonism, but for other compounds there was little difference in the activity of the epimers. For example, pyridine containing compounds 35 and 36 have similar potencies at the delta and mu opioid receptors (Table 4). As demonstrated by the activity of related compounds (see Sec. 2.3), the stereochemical features of (+)-BW373U86 are not essential to the delta opioid receptor pharmacophore. 

Aside from the fact that some of the heteroyohimbanes (LXXVII) carry substituents at positions 10 and/or 11, they differ in their configuration at the four centers of assymetry, 3, 15, 30, and 19, to which may be added the possibility of conformational isomerism. Table II summarizes the eight possible types of isomerism, their relations to the corresponding yohimbanes, and the alkaloids known in each type. The solutions of the various stereochemical problems were arrived at by a combination of methods, which follow. 

A summary of the nomenclature used in defining stereochemical configurations and related terms is presented in Table 1. For further rules and definitions, the reader is referred to a publication by the International Union of Pure and Applied Chemistry (lUPAC) (5). 

For the cases examined, die structure of the acetal had a dramatic effect on the stereochemical outcome of the reaction. Thus, allylsilanes (105a-<) all showed syn selectivity. However, the isopropyl case (105d) showed a slight anti preference. This reversal of sdecdvity has been inteipreted as a chtmge in mechanism rather than a steric phenomenon related to the added steric bulk of the diisopropyl acetal. The results of diese experiments are summarized in Table 17. ... 

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