Stereo triad

In two studies toward the total synthesis of natural products it could be shown that the a,jS-unsaturated esters derived from the vinylogous Mukaiyama aldol reactions can be further functionalized into advanced intermediates. The C1-C7 segment of oleandolide commences with the VMAR of aldehyde 68 derived from the Roche ester. The so-generated stereo-triad was protected as PMB ether and the ester 76 was reduced to the allylic alcohol. Sharpless asym-... 

In a series of pubhcations Warren et al. demonstrated the usefulness of 1,3-chirality transfer in the context of Type M rearrangements to synthesize 1,4,5-stereo triads with -configured double bonds (Scheme 90) [209-211]. Based on their successful work on diol syntheses with 1,4-related stereogenic centers across E-configured double bonds [212,213] involving the Mislow-Evans rearrangement (Sect. 3.5), the authors developed a procedure for the stereoselective synthesis of E-homoallyhc sulfides. 

Addition of crotyl metal reagents to aldehydes bearing a stereogenic center a to the carbonyl (51 Scheme 4) has been used as a strategy for the controlled synthesis of the stereo triads. Two of the four possible diastereomers (52) and (53) are available from the addition of an ( )-crotyl metal reagent to (51)... 

Reaction of crotylchromium with a-methyl-p,7-unsaturated aldehyde (87) afforded (88) as the major diastereomer. The other Cram product (89), which is expected to arise from an antiperiplanar transition state (46 Scheme 3), is obtained from a BFs-catalyzed tributylcrotylstaimane addition. The remaining members of the stereo triad can be accessed by inversion of the C-2 hydroxy (i.e. 88 to 91 and 89 to 90)... 

Woerpel has recently reported a tandem double asymmetric aldol/C=0 reduction sequence that diastereoselectively affords propionate stereo-triads and -pentads commonly found in polyketide-derived natural products (Scheme 8-2) [14], When the lithium enolate of propiophenone is treated with excess aldehyde, the expected aldolates 30/31 are formed however, following warming to ambient temperature a mono-protected diol 34 can be isolated. In a powerful demonstration of the method, treatment of 3-pentanone with 1.3 equiv of LDA and excess benzaldehyde yielded product in corporating five new stereocenters in 81% as an 86 5 5 3 mixture of diastereomers (Eq. (8.8)). A series of elegant experiments have shown that under the condition that the reaction is conducted, the aldol addition reaction is rapidly reversible with an irreversible intramolecular Tischenko reduction serving as the stereochemically determining step (32 34, Scheme 8-2). 

Almost every step in the literature routes required chromatography with very few to no crystalline intermediates being reported. The crystallization of the final product was not defined in any of the publications. All the routes recognized the stereo-triad present in... 

Polypropionate chains can be described and their synthesis can be planned in terms of stereotriads, stereotetrads, stereopentads, and so on. The term stereo-tri-ad or in a broader sense the concept of stereo- -ad was introduced by R. W. Hoffmann and has become accepted within the scientific community. " Thus, the family of stereo-triads possesses 8 members, stereotetrads are accordingly... 

Fig. 21.—Structure of the 6-fold anhydrous curdlan III (19) helix, (a) Stereo view of a full turn of the parallel triple helix. The three strands are distinguished by thin bonds, open bonds, and filled bonds, respectively. In addition to intrachain hydrogen bonds, the triplex shows a triad of 2-OH - 0-2 interchain hydrogen bonds around the helix axis (vertical line) at intervals of 2.94 A. (b) A c-axis projection of the unit cell contents illustrates how the 6-0H - 0-4 hydrogen bonds between triple helices stabilize the crystalline lattice.

Rh138 was almost the same (almost atactic, slightly syndiotactic) as the tacticity of those obtained with conventional radical initiators such as AIBN under similar conditions. The triad ratio of rr.mr.mm as determined by 13C NMR is usually 58 38 4 and does not change even with the use of chiral and/or bulky ligands.103116 These results may exclude a coordination mechanism and suggest a radical nature. However, the stereochemical structure alone is not strong evidence for the radical polymerization because, for example, group-transfer polymerization, basically via an anionic mechanism, results in a stereo structure of PMMA similar to those for free radical processes.263... 

Figure 7.71 illustrates the change of the glass transition for poly(styrene-co-acrylonitrile) as a function of the run number, defined in Fig. 7.70. The branches of the Barton equation (mA > mg, and mA < mB) are symmetric to the line which extends to the alternating copolymer. This treatment of T can also be applied to triads, and one can use the other equations of Fig. 7.69 as base, but with an increase in complexity. Stereo-specific copolymers consisting of meso and racemic dyads can be treated if the samples of different tacticity have different glass transitions [32].

Poly(vinyl alcohol) is another example where relating the melting temperature and stereoisomerism is difficult.(51,52) High degrees of structural regularity have not been achieved with either the isotactic or syndiotactic polymer. The determinations of A//u and A5u suffer from the same problem.(51,52,52a) The level of stereo regularity, based on triads, is in the order of 75%. The available evidence indicates that the melting temperatures of the isotactic and syndiotactic polymers are comparable to one another. 

These correlation studies indicate a dominating influence of the chiral enolate 15 versus the chirality of the aldehyde. In the case of (S)-phenyl-propionaldehyde 28 the chirality of enolate 15 overrides the directing effect of the aldehyde chirality, still producing a 2.5 1 ratio in favor of the 6(R),7(S),8(R)-stereochemical triad. The use of the (R)-phenylpropionalde-hyde 31 forms exclusively the 6(R),7(S),8(R)-stereo chemical triad in a 40 1 ratio. In this case the matched pair is the (S)/(R)-combination of chirality. The (R)-enantiomer of the enolate reverses the stereochemical outcome. 

When the relative amounts of various stereosequences can be measured by nmr, it is possible to discern what statistical relationships exist among the various structural features and thus learn something about the stereo-regulating processes occurring during polymer formation. In such considerations, stationary probabilities (e.g., P(m), P(mr), etc.) may be used to represent the relative amounts of dyads, triads and n-add placements and conditional probabilities (e.g., P(m/m), P(r/mm), etc.) may be used to represent the probabilities that m or r placements follow particular sequences in the polymer. The expression P(m/r), for example, refers to the probability that a meso placement follows a racemic placement in a polymer chain. 

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