Stereochemical projections

These four hybrid orbitals may then combine with four other atomic orbitals to form four bonding and four anti-bonding molecular orbitals. Thus, suggest the shape of the tetrahydride of carbon. Draw it using a stereochemical projection. 

Zig-zag projection A stereochemical projection in which the main chain of an acyclic compound is drawn in the plane of the paper with 180° torsion angles, with substituents above the plane drawn with bold or solid wedges, and hashed lines for substituents behind the plane. 

FIGURE 3.4 Stereochemical projection of a generic diacylglycerol illustrating the stereochemical numbering (sn) system and the chiral center at the sn-2 carbon. Each molecule, although identical in formula, is a stereoisomer (specifically an enantiomer, a nonsuperim-posable mirror image). Rj and Rj represent substituent aUcyl chains. 

Figure 1.6 Computer-generated stereochemical projections for flavan-3-ol diastereoisomers. EGCG, epi-gallocatechin gallate GCG, gallocatechin gallate. Three dimensional structures computed by Professor David Lewis, School of Biomedical and Molecular Sciences, University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom.

We mentioned in Section 7 6 that the d l system of stereochemical notation while outdated for most purposes is still widely used for carbohydrates and amino acids Likewise Fischer projections find their major application m these same two families of compounds... 

Make a molecular model corresponding to the stereochem istry of the Fischer projection of 2 phenyl 2 butanol shown in the equation and verify that it has the R configuration... 

L Ammo acid (Section 27 2) A descnption of the stereochem istry at the a carbon atom of a chiral ammo acid The Fis cher projection of an a amino acid has the ammo group on the left when the carbon chain is vertical with the carboxyl group at the top... 

Fischer projection (Section 7 7) Method for representing stereochemical relationships The four bonds to a chirality... 

To verify that the Fischer projection has the R configuration at its chirality center, rotate the three-dimensional r epresentation so that the lowest-ranked atom (H) points away from you. Be careful to maintain the proper stereochemical relationships during the operation. 

R,S stereochemical designations (Section 9.5) can be assigned to the chirality center in a Fischer projection by following three steps, as shown in Worked Example 25.1. 

Newman projection (Section 3.6) A means of indicating stereochemical relationships between substituent groups on neighboring carbons. The carbon-carbon bond is viewed end-on, and the carbons are indicated by a circle. Bonds radiating from the center of Ihe circle are attached to the front carbon, and bonds radiating from the edge of the circle are attached to the rear carbon. 

The issue of stereochemistry, on the other hand, is more ambiguous. A priori, an aldol condensation between compounds 3 and 4 could proceed with little or no selectivity for a particular aldol dia-stereoisomer. For the desired C-7 epimer (compound 2) to be produced preferentially, the crucial aldol condensation between compounds 3 and 4 would have to exhibit Cram-Felkin-Anh selectivity22 23 (see 3 + 4 - 2, Scheme 9). In light of observations made during the course of Kishi s lasalocid A synthesis,12 there was good reason to believe that the preferred stereochemical course for the projected aldol reaction between intermediates 3 and 4 would be consistent with a Cram-Felkin-Anh model. Thus, on the basis of the lasalocid A precedent, it was anticipated that compound 2 would emerge as the major product from an aldol coupling of intermediates 3 and 4. 

Based on information accrued during the stereochemical elucidation, macrolactone 85 was identified as a viable synthetic intermediate (Scheme 12). The authors were cognizant of the potential challenges that could arise. First, the required formation of a trisubstituted alkene in a projected Horner-Emmons macrocyclization was without strong precedent. Also, this strategy would necessitate a stereoselective reduction of the Cl5 ketone, which was predicted to be feasible based on MM2 calculations. 

This is the conformation from which the reaction can take place. The don-ble bond is being formed between the front carbon and the back carbon, and this Newman projection shows ns the stereochemical ontcome (look carefnlly at the dotted ovals, which are drawn to help you see the ontcome more clearly) ... 

You need to get into the habit of drawing Newman projections so that you can determine the stereoisomer that is expected from an E2 reaction. If you are rusty on Newman Projections, you should go back and review the first two sections in Chapter 6 in this book. Then come back to here, and try to use Newman projections to determine the stereochemical outcome of the following reactions. 

Figure 4 Sketch of two possible stereochemical arrangements for a chiral monomer. P represents the polymer chain, R represents a vinyl substitutent on a carbon, H represents hydrogen, (a) Linear sketch showing one conformation and two configurations (bracketed and unbracketed). The apex of bonds is a tetrahedrally bonded carbon atom (solid and dashed circles), (b) Newman projection of the same monomer showing the free rotation about the C-C bond.

Figure 1 Stereochemical drawing and Fischer projection of an L-a-amino acid, where R is the side chain of the amino acid.

In the third model (finite chain with different terminal groups) no reflection symmetry element exists in the Fischer projection. The individual macromolecules are, therefore, chiral and all the tertiary atoms are asymmetric and different. The stereochemical notation for a single chain, depending on the priority order of the end groups, can be R, R2, R. . . R -2, R -i, Rn or R, R2, R3... 

Throughout this document, stereochemical formulae for polymer chains are shown as Fischer projections rotated through 90°, i.e., displayed horizontally rather than vertically,... 

The use of rotated Fischer projections has been retained in the present edition in order to provide a link with, and an explanation of, the bulk of existing published polymer literature, although the present common practice [4] is to depict main-chain bonds in planar, extended zigzag (all-trans) conformations, together with a stereochemical representation of side-groups at tetrahedrally-bonded atoms. 

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