Stereochemistry relative configuration

It should be stressed that this treatment of polymer stereochemistry only deals with relative configurations whether a substituent is "up or down" with respect to that on a neighboring unit. Therefore, the smallest structural unit which contains stereochemical information is the dyad. There are two types of dyad meso (m), where the two chiral centers have like configuration, and racemic /-), where the centers have opposite configuration (Figure 4.1). 

In considering the retrosynthetic analysis of juvabione, two factors draw special attention to the bond between C(4) and C(7). First, this bond establishes the stereochemistry of the molecule. The C(4) and C(7) carbons are stereogenic centers and their relative configuration determines the diastereomeric structure. In a stereocontrolled synthesis, it is necessary to establish the desired stereochemistry at C(4) and C(7). The C(4)-C(7) bond also connects the side chain to the cyclohexene ring. As a cyclohexane derivative is a logical candidate for one key intermediate, the C(4)-C(7) bond is a potential bond disconnection. 

The synthesis in Scheme 13.13 leads diastereospecifically to the erythro stereoisomer. An intramolecular enolate alkylation in Step B gave a bicyclic intermediate. The relative configuration of C(4) and C(7) was established by the hydrogenation in Step C. The hydrogen is added from the less hindered exo face of the bicyclic enone. This reaction is an example of the use of geometric constraints of a ring system to control relative stereochemistry. 

The 29Si resonance is therefore a single narrow line. However for dialkylpolysilanes with two different alkyl groups on each silicon, (RR Si)n, each silicon atom is a chiral center and the resonance for a particular silicon will depend upon the relative stereochemistry of other nearby silicon atoms. For such polymers, a rather symmetrical cluster of peaks is observed (Figure 5). These results are consistent with atactic structures, having a statistical (Bernoullian) distribution of relative configurations.(32,33)... 

As for the stereochemistry, for the case of complete cyclization, besides the usual tacticity (possibly, isotactic or syndiotactic, referred to relative configurations of equivalent stereogenic carbons of subsequent monomeric units), the cis or trans configuration of the 1,3-cycloalkane rings which are present in the polymer main chain also has to be considered.70,74... 

Catalytic reduction of bridgehead enol lactone over Pd/C indicates that, indeed, the syn addition from the exo face of the bridgehead double bound establishes the relative configuration of all substituents [264], Equilibration studies performed in EtONa/EtOH also established that the ratio of the epimers corresponds to an equilibrium mixture. Under mild basic conditions (NajCOj/ EtOH), the product isomerization occurs to a very small extent. The product distribution is best understood by rapid conformational relaxation to one of the two low-eneigy half-chair conformations. The stereochemistry is established at the subsequent protonation step. This takes place with a strong preference for axial protonation from the /I face at carbon 2 to produce the most stable chair conformation (Scheme 14.12). 

As noted by Corey, the relative configuration of the two carbon atoms joining the six- and the five-membered rings is not relevant for the synthesis, since what will determine the stereochemistry of the double bond is the relative configuration of the angular methyl group and the secondary hydroxyl group in 3 (X = OH). 

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

There are four types of possible stereoregular structures for each frans-1,4 polymer of 1,4-disubstituted butadiene (Fig. 13). The stereochemistry of polymers is represented by two kinds of relationship as follows, one of which is the relative configuration between the two repeating monomer units. When all the... 

Stereoselective aldol condensations provide a-substituted /J-hydroxycarbonyl compounds328, and it transpires that H- and 13C-NMR spectroscopy are excellent tools for determining their relative configurations. For example, it has been pointed out329 330 that the vicinal 1H,1H coupling constant is a good indicator of the stereochemistry (see Table 2)331 ... 

The conversion of (4S,5S)-dihydro-5-[(S)-l-hydroxyethyl]-4-trimethylsilyl-2(3//)-furanone (7, see p 416) into the (Z)-alkene 8 served to establish the relative configuration at C-4 and C-5 in 7 on the basis of the established double bond configuration in 8 and the anticipated (anti) stereochemistry of the elimination reaction (see also p474)81. 

The relative configuration at the allene unit of 2, obtained from 1 (see p 424), was unknown. For assignment, 2 was prepared independently from 3 via 4, 5, and 6. On condition that the rearrangement 5 - 6 occurs suprafacially and the desulfurization 6 -> 2 with retention of configuration (see p 473), the configurational assignment of 2 rests solely on the stereochemistry of step 3 - 4. Based on literature precedent the acetylide addition to 3 was assumed to occur from the a-direction to yield 4148. 

In connection with the relative configurational argument as in reference [52], stereochemistry of the compound preferentially hydrolyzed by cellulase was unambiguously determined as (2R,3R). On the other hand, the optical purity was determined by the integration of a CH30 group after derivatization into the corresponding (R)-MTPA ester (6 3.49, q,/= 1.2 Hz) for (2S,3S) isomer (minor) and 3.58 (<5 3.49, q,/= 1.3 Hz) for (2R,3R) isomer. 

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