Configuration chair

The twist boat form is chiral and there are two enantiomeric conformations. The chirality can be defined in the same way as a cobalt-ethylenediamine ring and designated as S and X. The third form, a boat form cannot be accommodated to form a tris(bidentate) complex and, in fact, such a structure has not yet been reported. Niketii and Woldbye showed that there exist 16 possible conformers of the [M(tn),] system for each of the absolute configurations A and A, in which they adopt the three stable configurations chair andS- and X-skew boat forms (32). 

Haworth formulas are satisfactory for representing configurational relationships in pyranose forms but are uninformative as to carbohydrate conformations X ray crystal lographic studies of a large number of carbohydrates reveal that the six membered pyra nose ring of D glucose adopts a chair conformation... 

The bicyclic compound decahydronaphthalene, or bicyclo[4.4.0]decane, has two fused six-mem-bered rings. It exists in cis and trans forms (see Fig. 1.10), as determined by the configurations at the bridgehead carbon atoms. Both cis- and rran -decahydronaphthaiene can be constructed with two chair conformations. 

Maltose, l- 4- -link in, 998 molecular model of, 998 mutarotation of, 998 structure of, 998 Manicone, synthesis of. 805 Mannich reaction. 915 Mannose, biosynthesis of, 1011 chair conformation of, 126 configuration of, 982 molecular model of, 126 Margarine, manufacture of, 1063 Markovnikov. Vladimir Vassilyevich. 192... 

Diisopropylamino(dimethyl)silyl]-2-propenyl]lithium adds to aromatic and x-branched aldehydes in the presence of anhydrous zinc chloride with essentially complete anti stereoselectiv-ity3s. as expected from the chair-like pericyclic transition state formed by the ( -intermediate. The addition products are not isolated, but after O-silylation, oxidative desilylation with retention of configuration forms the rmft-diols. 

Transmetalation of lithium enolate 1 a (M = Li ) by treatment with tin(II) chloride at — 42 °C generates the tin enolate that reacts with prostereogenic aldehydes at — 78 °C to preferentially produce the opposite aldol diastereomer 3. Diastereoselectivities of this process may be as high as 97 3. This reaction appears to require less exacting conditions since similar results are obtained if one or two equivalents of tin(ll) chloride arc used. The somewhat less reactive tin enolate requires a temperature of —42 C for the reaction to proceed at an acceptable rate. The steric requirements of the tin chloride counterion are probably less than those of the diethyla-luminum ion (vide supra), which has led to the suggestion26 44 that the chair-like transition state I is preferentially adopted26 44. This is consistent with the observed diastereoselective production of aldol product 3, which is of opposite configuration at the / -carbon to the major product obtained from aluminum enolates. 

Lithium and zinc tert-butyl phenylmethyl sulfoxide (1) and A-phenyl imines 2, in which the substituent R is alkenyl or aryl, react at —78 °C over 2 hours with high anti diastereoselection (d.r. >98.5 1.5)6. Shorter reaction times result in poorer yields, due to incomplete reaction. In contrast, the reaction of the sulfoxide anion with benzaldehyde is complete after 5 seconds, but shows poor diastereoselection. When the substituent R1 or R2 of the imine 2 is aliphatic, the substrates exhibit poor chemical reactivity and diastereoselection. The high anti diastereoselection suggests that if a chelated cyclic transition state is involved (E configuration of the imine), then the boat transition state 4 is favored over its chair counterpart 5. 

This monomer may be considered as a compound composed of two cyclohexene rings in the boat configuration. Presumably, the difference in energy between the cyclohexene rings of the monomer and the cyclohexane ring in the polymer, which is free to assume the less strained chair configuration, is responsible for the reactivity. 

D-myo-inositol is a six carbon polyalcohol in a ring structure arranged in a chair configuration. 

FIGURE 6.3. The strain hypothesis for the catalytic reaction of lysozyme. The enzyme is assumed to destabilize the chair geometry by pushing the ground state substrate toward the sofa configuration. This ground-state destabilization effect is supposed to reduce Ag M. 

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