Quasi axial

The reaetion turbine avoids this U-turn and its effieieney penalty. In the reaetion turbine half of the pressure energy is spent aeross the rotor, so there must be a seal around the rotor. With the rotor inlet at its periphery, the diseharge from the rotor may now be ehosen at a redueed diameter radial, or quasi axial, shaft-eoneentrie position. Sinee the diseharge is obviously smaller in diameter, the rotor seal will also be smaller in diameter beeause it only needs to suiTound the diseharge portion of the rotor. As a eonsequenee, seal loss is redueed and shaft thrust deereases as well. Likewise, the diseharge or seeondary nozzle losses are redueed beeause the gas exits at lower veloeity. 

Tetrahydropyranyl ethers have been prepared from the quasi-axial 7a-hydroxyl in a 3)5-acetoxy-A -7a-ol, but in this case enhanced reactivity is due to the adjacent double bond. °... 

It was noted (23) that the NMR spectrum of compound 32 was identical with that published (34) for the third product (assigned structure 29) isolated from the reaction of tetra-O-acetyl-2-hydroxy-D-glucal with acetic anhydride and zinc chloride. The identity of the compounds was fully established and a revised structure proposed for this third product. In the presence of zinc chloride, therefore, epimerization can occur at an allylic site and the quasi-equatorial C-4 acetoxy group in the erythro isomers 27 and 28 can assume the favored quasi-axial orientation (24). 

The enamines derived from cyclohexanones are of particular interest. The pyrrolidine enamine is most frequently used for synthetic applications. The enamine mixture formed from pyrrolidine and 2-methylcyclohexanone is predominantly isomer 17.106 A steric effect is responsible for this preference. Conjugation between the nitrogen atom and the tt orbitals of the double bond favors coplanarity of the bonds that are darkened in the structures. In isomer 17 the methyl group adopts a quasi-axial conformation to avoid steric interaction with the amine substituents.107 A serious nonbonded repulsion (A1,3 strain) in 18 destabilizes this isomer. 

Entry 5 is an example of use of an a-trimethylsilylallyl group to prepare a vinylsilane. The stereochemistry is consistent with a cyclic TS having the trimethylsilyl substituent in a quasi-axial position to avoid interaction with the bridgehead hydrogen of the bicyclic ring. 

We also performed a single-crystal X-ray structure analysis of this lead compound. The solid state structure of this compound depicted in Fig. 3-15 shows a half-boat-like ( sofa ) conformation with the 9-phenanthryl group in a quasi-axial or r/Mf/.v/-flagpole position, and the a, 3-unsaturated exocyclic ester in a s-cis conformation. This cleft-like conformation is advantageous for the creation of centers with a high recognition ability, since one enantiomer fits in better than the other thus leading to selectivity. 

Kametani et al. (163-165) studied the Stevens rearrangement using sodium bis(2-methoxyethoxy)aluminum hydride as the base in dioxane. It became clear from studies using deuterium-labeled or optically active compounds that quasi-axially oriented hydrogens at C-8 and C-14 were independently abstracted by the base, leading to a spirobenzylisoquinoline and an 8-... 

Cyanide binds to a series of Ni complexes with N302 chelate ligands, and EPR spectra of the adducts indicate quasi-axial symmetry with a g pattern typical of low-spin, six-coordinate complexes with axial elongation and with a 2Ay ground state. No direct spin interaction of the unpaired electron with the carbon atom of the cyanide takes place.70... 

A study of the catalytic oxidation of six stereoisomers of 5-cyclohexene-l,2,3,4-tetrol revealed448 that, in all cases, an allylic hydroxyl group is selectively attacked, and a consideration of their favored conformations suggested that quasi-axial groups are selectively dehydrogenated before those that are quasi-equatorial. 

An interesting stereochemical profile of this cyclization is that in the five-membered product structures, substituents PhS and OH groups are placed cis to each other, whereas in six-membered products the placement is trans (Table 6, entry 7). The cis selectivity in the five-membered ring systems is not affected by a and fi-substituents of the alkoxides (entries 2, 3 and 4), indicating that the steric effect is not the dominant factor. Instead, interaction between the oxido and carbene center composes a five- (or seven-) membered transition state 28, which allows the carbene to abstract the nearest quasi axial hydrogen as a hydride to produce a carbonyl intermediate 29, leading to the cyclization products 27 and 30 (Scheme 13, Eq. 1). Similarly, the stereoselective stepwise cyclization of cis- and rra s-2-(3,3-dithiopropyl)cyclohexanol to 2-phenylthio-... 

B. Coordinated Bond Lengths In the Vicinity of the Anomeric Center. In D-aldofuranoses, the Cl-01 bond is quasi-axial in E-E conformers (0.7 E/K) of a-anomers, and Eq- E conformers (1.7 P/7C) of P-anomers (Figure 1). 

QUANTUM MECHANICAL TUNNELING CHEMICAL KINETICS HYDROGEN TUNNELING KINETIC ISOTOPE EFFECTS QUANTUM REQUIREMENT QUANTUM YIELD QUANTUM YIELD FLUQRESCENCE ACTINOMETER Quasi-axial,... 

We explain the selective formation of diastereomer 52 on the basis of conformational arguments. The two likely conformers of 50 should have an essentially planar allyl moiety, whereas the six-membered ring should exist in two quasi-chair conformers with either the isopropyl or the methoxy group in the quasi-axial position. The conformer with the (bulkier) isopropyl group in a quasi-equatorial position is preferred. While access to the terminal allylic carbon (which leads to 51) appears to be unhindered, the quasi-axial substituent at the chiral carbon will interfere with the attack on the internal allylic center, directing the attack to the face opposite to the quasi-axial methoxy function, (- 52). These considerations account for the preference of 51 over 52 as well as for the suppression of the diastereomer. 

Variable temperature studies again show the ring inversion (76T2339). They also indicate that a 2-methyl group shows equal preference for the quasi-axial or quasi-equatorial positions, presumably because there are no other axial substituents to cause 1,3-diaxial interactions. 

In der H 1-Form sind die OH-Gruppen an C-2 und C-3 aquatorial orientiert, an C-1 und C-4 dagegen quasi-aquatorial, wahrend die Hy-droxylgruppen in der IH-Form an C-2 und C-3 axial, an C-1 und C-4 quasi-axial ausgerichtet sind. 

OH quasi-axial > OH quasi-aquatorial mit vicinaler axialer Hydroxyl-gruppe>OH quasi-aquatorial mit vicinaler aquatorialer Hydroxyl-gruppe > OH axial > OH aquatorial. 

Regel 4 Quasi-axiale OH-Gruppen werden bevorzugt vor quasi-aqua-torialen und diese selektiv vor axialen und aquatorialen OH-Gruppen oxydiert. 

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