Double stereoselection

Double stereoselection chain-end control and site control... 

Trons-divinyl-pyrrolidines and -piperidines were prepared by sequential intermolecular and intramolecular aminations of bis-allylic carbonates (Scheme 9.21) [22aj. Due to double stereoselection, these reactions proceeded with high diastereoselectivity and enantioselectivity. 

Double Stereoselection Chain-End Control and Site Control... 

The same double stereoselection was observed by Young and coworkers when both enantiomers of the glyceraldehyde acetonide were reacted with the lithium enolate of pyroglutamic esters. The (Tf)-enantiomer gave only the (5,5) adduct while the (5)-enantiomer gave a mixture of diastereomers (Scheme 121)576. 

SCHEME 121. Double stereoselection in aldol reaction of pyroglutamic ester576... 

SCHEME 122. Double stereoselection as a function of the metal579 3. Complex aldol reactions... 

Protected a,3-dihydroxy aldehydes have been prepared by oxidation of acetals of a,B-unsaturated aldehydes with osmium tetroxide in the presence of (23), and a remarkable level of enantioselection ee 2 90%) thereby achieved. Oxidation of chiral acetals of a,p-unsaturated aldehydes in which chirality resides in the noncarbonyl moiety with osmium tetroxide-t ydroquinine acetate (or dihydroquitudine acetate) may be regard as a process in which double stereoselection is at work and a high dia-stereoisomeric ratio of products may be obtained. 

Korshin et al. reported an outstanding double stereoselective alkenyla-tion through a 1,5 sulfur-to-carbon translocation on a proline-derived scaffold [169]. 5-Exo-trig cyclization of the radical derived from 181 led to bi-cyclic adduct 182, which collapsed to the open thiyl radical (Scheme 59). The sole stereocontrol exerted by the cyclization step allows for the resulting vinyl translocation to occur entirely stereoselectively. Since the reaction was carried out in the presence of a styryltin derivative, consecutive intermolecular vinylation occurred, leading to bisvinyl compoimd 183 in very high yield (86%). The styrylsulfonyl moiety could be converted to a formyl group. 

In the course of a formal total synthesis of 3-deoxy-D-maimo-2-octulosonic acid (KDO), a chiral (salen)Co(II) complex was used to catalyze a highly double-stereoselective hetero-Diels-Alder reaction between an electron-rich chiral diene and ethyl glyoxylate (Sch. 56) [201]. This reaction was subsequently extended to achiral dienes [202]. 

The matched double stereoselective reaction between the chiral C-phenyl A -( 1-phenylethyl) nitrone and the chiral silyl ketene acetals 558 in acetonitrile/dichloromethane at 20 °C under zinc iodide-catalyzed conditions resulted in only the single diastereomer 559 in 98% yield. Exposure of 559 to acid afforded the isoxazolidinone 560 in 74% yield, together with the chiral auxiliary (li ,2 S )-2-phenylcyclohexanol (Scheme 135) <1997JOC6672>. 

By applying a similar approach, Wu and coworkers [135] observed a highly double-stereoselective hetero Diels-Alder addition between diene 245 and ethyl glyoxylate. This reaction was catalyzed by the (salen)Co° complex 246 (Scheme 13.75). The major adduct 247 is hydroborated to give alcohol 248. Stereochemical inversion is achieved via oxidation and subsequent reduction to 249. Acid methanolysis followed by diol protection furnishes 250. Quenching of the lithium enolate of 250 with phenyl disulfide and subsequent oxidation with A-bromosuccinimide forms, after deprotection, (-f)-KDO (3-deoxy-D-mann6>-oct-2-ulosonic acid) [136a]. 

Hu, Y-J, Huang, X-D, Yao, Z-J, Wu, Y-L, Formal synthesis of 3-deoxy-D-manno-2-octulosonic acid (KDO) via a highly double stereoselective hetero-Diels-Alder reaction directed by a (salen)Co catalyst and chiral diene, J. Org. Chem., 63, 2456-2461, 1998. 

By applying a similar approach, Wu and co-workers [339] have observed a highly double-stereoselective hetero-Diels-Alder addition between diene 192 and ethyl glyoxylate catalyzed... 

This brings us to double stereoselection and reinforcement of the mechanisms. If the site (a)symmetry were to control the orientation of the chain, and if, then, the orientation of the incoming propene is controlled by both the chain and the site, the highest stereoselection is obtained when the two influences reinforce one another. For 1,2-insertion this can be done most effectively for isotactic polymerization, since chain-end control naturally leads to isotactic polymer and this we can reinforce by site control with ligands of the bis(indenyl)ethane type. The chain-end influence of short chains is smaller than that of longer polymer chain and therefore short chain ends lead to lower selectivities. It may also be irrferred that making syndiotactic polymer via a 1,2-insertion mechanism on Ti or Zr complexes is indeed more difficult than making an isotactic polymer, because the two mechanisms now play a counterproductive role. 

In the catalysis shown in Equation (a) a double stereoselection is involved. The formation of the new asymmetric centers in the cyclopropane ring is influenced by the men-thyl group contained in the substrate (15,35,4/ )-menthyl diazoacetate and by the mepy ligand contained in the catalyst. The two influences are referred to as substrate and catalyst control, respectively. With regard to the efficiency it has to be noted that whe-... 

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