Stereoisomers stereochemical relations

What are the stereochemical relations (identical, enantiomers, dia-stereoisomers) of the following four molecules M-P Assign absolute configurations at each stereogenic centre. 

The enantioface and also the configuration (s-trans, s-cis) of the prochiral butadienes involved in the several elementary steps are of crucial importance for the stereocontrol of the cyclo-oligomer formation. Oxidative coupling, for example, can occur between two cA-butadienes, two /rum-butadienes or between cis- and /nmv-butadiene with either the same or the opposite enantioface of the two butadienes involved. The several stereoisomers are exemplified for the [Ni°(butadiene)2L] active catalysts for cyclodimer formation, that are schematically depicted in Fig. 1, together with the related stereoisomers of the ry ri fC1) and bis(r 3) octadienediyl-Ni11 species 2a and 4a, respectively. For each of the individual elementary steps there are several stereochemical pathways, which are exemplified in Fig. 1 for the... 

Both stereoisomers were formed, implying a loss of stereochemical integrity during the formation of the second carbon-carbon bond. When the reaction was conducted on ZnO, surface-related processes affected both the rate and stereochemistry. The effect of various quenchers could be explained as competitive adsorption at active sites, with or without interference with electron transfer. A reaction scheme involving formation of dimer, both in the adsorbed state and in solution, was proposed, the former route being the more important On CdS, the reaction could sometimes be induced in the dark as well because of the presence of acceptor-iike surface states. Neither particle size, surface area, nor crystal structure appeared to significantly influence the dimerization observations parallel to those found in the CdS photoinduced dimerization of N-vinylcarbazole... 

Just as one divides stereoisomers into two sets, enantiomers (Greek enantios = opposite) and diastereomers, so it is convenient to divide heterotopic (non-equivalent) groups or faces into enantiotopic and diastereotopic moieties. Enantiotopic ligands are ligands which find themselves in mirror-image positions whereas diastereotopic ligands are in stereochemically distinct positions not related in mirror-image fashion similar considerations relate to planes of double bonds. 

NMR studies permitting stereochemical assignment to the structurally related festuclavine [351] and its stereoisomers costaclavine, epicostaclavine, and pyroclavine, with emphasis on the analysis of the 9-and 4-methylene proton signals, have also been reported. (220)... 

Obviously, all tris-chelates whose stereochemistry is analyzable in terms of the above model are stereochemically correspondent similarly, compounds of the type ArsZ, such as triarylboranes and triarylcarbenium ions are also stereochemically correspondent. However, the more interesting correspondence is that between the class of tris-chelates and the class of AraZ molecules. Although these two classes of molecules differ enormously in their chemical properties and reactions, the concept of stereochemical correspondence tells us that their stereochemical attributes are necessarily closely related. For example, a tris-chelate and a stereochemically correspondent triarylborane will each have the same number and kinds of stereoisomers, interconvertible by the same number and kinds of rearrangement pathways. 

Figure 6 Structures of the four stereoisomers of sphingosine. Sphingosine has two chiral carbon atoms (C-2 and C-3). The Fischer projection formula of each structure is also shown, with C-1 at the top, to illustrate the D/L and erythro/threo stereochemical nomenclature. C-3 has an erythro orthreo configuration as it relates to C-2, depending on whether the similar groups (amino and hydroxy) are on the same or opposite side of the Fischer projection. D versus L refers to the configuration at C-2 relative to the configuration of D-glyceraldehyde versus L-glyceraldehyde.

The. stereochemistry of vitamin A and related compounds is complex, and a complete. stereochemical analysis is beyond the. scope of this chapter. A brief. summary of some stereochemical features is prc.scnied here as the basis for the characterization of the biochemical actions exerted by this vitamin. Tlte study of the structural relationships among vita-ntin A and its stereoisomers has been complicated by the common use of several numbering. system.s. as. shown below. The first numbering sy.stem (A) is the one currently recommended by the International Union of Pure and Applied Chemistry (lUPAC). The second sy.stem (B) places emphasis on the conjugated tt. system, while the third (C) is used by the USP Dictionary of USAN and International Drug Names. 

The bioactivity of castanospermine and related alkaloids continues to inspire chemists to devise new synthetic routes to the alkaloid, as well as to iimumerable structural, stereochemical and other analogs. The ensuing discussion deals only with reported syntheses of the four natural products 239-242. For syntheses of other reported stereoisomers of the indolizidine-l,6,7,8-tetraols (Fig. 4), the following references should be consulted (-)-l-epicastanosp>ermine (243) (194), ( )-7-epicastanospermine (195) and (15,6S, 75, 87f,8a7 )-7-epicastanospermine (244) (196),... 

The formula of cinnamic acid leads to the view that it should exist in two stereochemical modifications, which bear a relation to each other similar to that shown in the case of fumaric and maleic acids. The stereoisomer melts at 68° and is called allocinnamic acid it has been shown to be the cis modification (140). 

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. 

In Section 7.5, the term relative configuration was used to describe the stereochemical relationship between a single chirality center in one molecule to a chirality center in a different molecule. Relative configuration is also used to describe the way multiple chirality centers within the same molecule are related. The two erythro stereoisomers of 2,3-dihydroxybutanoic acid possess the same relative configuration. The relationship of one chirality center to the other is the same in both, but different from that in the threo stereoisomer. 

Usually the canonicalization and hashing procedures do not distinguish the stereoisomers of a molecule. However, Wipke and Dyott describe the SEMA (Stereochemically Extended Morgan Algorithm), which incorporates information relating to double bonds and tetrahedral stereocenters into the Morgan algorithm. 

The stereochemical rationale put forth for the observed stereoisomers involves examination of four transition states (I-IV). Adducts 52 and 53 would arise from the chair-like conformations I and II respectively (Scheme 2.7). The formation of 52 as the major product was anticipated since the related transition-state conformation II leading to 53 is destabilized by an eclipsing interaction between Ha and Hb. Not surprisingly, the octahydro-quinoline products, 54 and 55, derived from the two boatlike conformations, III and IV, are not detected. 

---