Chiral rings, labeling

The compounds in this report usually contain a chirotopic stereogenic carbon ring atom, and were prepared as racemic mixtures. Hypothetically, if the BC conformation prevails, then one can imagine two enantiomers in solution (reference, 5)-BC 9 and (retro-inverso,R)-BC 9-bar. Since this stereochemistry is complicated, it will be helpful if we refer to the descriptor for only one enantiomer. Therefore, in an arbitrary but consistent manner in this report, we will always define the reference ring chirality and label tropicity to be that of the (S)-enantiomer. For example, suppose a racemic mixture of (reference,S)-BC 9 and (retro-inverso,R)-BC 9-bar affords crystals belonging to an achiral space group so that both enantiomers in the racemic compound are present in the crystal lattice. Let us further suppose that dissolution of these crystals will give the same solution-state conformation. We will write that the solid-state (reference,S)-BC 9... 

MSA does not contain any chiral carbon centers. Before the aromatization of the six-membered ring occurs, two prochiral carbons (C-2 and C-4 in the six-carbon intermediate) evolve, each of which loses a hydrogen in the process of the dehydratization/aromatization steps. In addition, C-3 of the six-carbon intermediate forms a chiral center when the ketone is reduced to a hydroxyl by a ketoreductase activity (Fig. 5). The chirality of this hydroxyl carbon is unclear since the intermediate has not been isolated. It is also unknown if this carbon retains its chirality in an eight-carbon intermediate or whether the hydroxyl is eliminated by dehydration prior to the third condensation reaction. The stereospecificity at the prochiral C-2 and C-4 carbons in the reaction intermediates was addressed using chemically synthesized (] )- and (S)-[1- C, 2- H]malonate precursors which were enzymatically converted into CoA derivatives via succinyl CoA transferase [127,128]. Thus, the prochiral methylene in malonyl CoA was replaced by chiral, double-labeled (S)- or (J )-[1- C, 2- H]malonyl CoA substrates in the reaction mixture with 6-MSAS. The condensation is expected to occur with inversion of configuration and the intact methylene... 

FIGURE 9-15 Labeling of Chiral Rings. The rings are numbered arbitrarily Ri through R5. The combination R1-R4 is A, R1-R5 is A, and R2-R5 is A. The notation for this stracture is then AAA-(ethylenedia-minetetraacetato)cobaltate(III). 

Diastereomers that differ at only one chiral carbon are called epimers, ff a ring closure occurs at the epimeric carbon, two possible diastereomers may be formed. These new diastereomers are called anomers. The chiral carbon of an anomer is called the anomeric carbon, Anomers am distinguished by the orientation of their substituents. Glucose forms anomers. When the hydroxyl group on tire anomeric carbon on glucose is oriented in the opposite direction to the methyl group, the anomer is labeled ot when in the same direction, the anomer is 5. 

The explanation for these experimental results, i.e. the lack of label transfer, is that the tetrahedral species (A) resulting from the addition of HSOf to the carbonyl group is capable of epoxidation. Ring closure of (A) is likely to be the rate-determining step in dioxirane formation. This work is important from a synthetic viewpoint, since it is crucial in the development of chiral ketones for the catalytic asymmetric epoxidation and the design of probes of transition state stereoselectivities that the nature of the oxidizing species is understood. 

Draw all possible constitutional and stereoisomers for a compound of molecular formula CeHi2 having a cyclobutane ring and two methyl groups as substituents. Label each compound as chiral or achiral. 

Because many chelate rings are not planar, they can have different conformations in different molecules, even in otherwise identical molecules. In some cases, these different conformations are also chiral. The notation used also requires using two lines to establish the handedness and the lower case labels X and 8. The first line connects the atoms bonded to the metal. In the case of ethylenediamine, this line connects the two nitrogen atoms. The second line connects the two carbon atoms of the ethylenediamine, and the handedness of the two rings is found by the method described in Section 9-3-5 for separate rings. A counterclockwise rotation of the second line is called X (lambda) and a clockwise rotation is called 8 (delta), as shown in Figure 9-16. Complete description of a complex then requires identification of the overall chirality and the chirality of each ring. 

Co(dien)2] can have several forms, two of which are shown below. Identify the A or A chirality of the rings, using all unconnected pairs. Each complex may have three labels. 

An experimental distinction among these three alternatives proved to be difficult, particularly as norcaradienes and cycloheptatrienes usually interconvert easily via valence bond isomerization (norcaradiene - cycloheptatriene EA = 6.5 kcal/mol log A = 11.8) (64) and cycloheptatriene ring inversion (1,3,5-cyclo-heptatriene EA - 6.3 kcal/mol) (Figure 4 ref. 65). Therefore it was necessary to resort to such devices as introducing chirality into the molecules by way of an additional stereochemical label. 

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