Quasiequatorial conformation

Analytical Properties Resolution of several enantiomers of polycyclic aromatic hydrocarbons, for example, chrysene 5,6-epoxide, dibenz[a,h]anthracene 5,6-epoxide, 7-methyl benz[a]anthracene 5,6-epoxide resolution of barbiturates, mephenytoin, benzodiazepinones, and succinimides direct separation of some mono-ol and diol enantiomers of phenanthrene, benz[a]anthrene, and chrysene ionically bonded to silica gel, this phase provides resolution of enantiomers of c/s-dihydroidiols of unsubstituted and methyl- and bromo-substituted benz[a]anthracene derivatives having hydroxyl groups that adopt quasiequatorial-quasiaxial and quasiaxial-quasiequatorial conformation Reference 31-35... 

The fact that with acetal 1 (R1 = H R2 = CH3) a lower stereoselectivity is observed than with the acetals where R1 = Ft or C6II5 suggests that the bulkiness of the substituent at the acetal center also plays an important role in fixing the conformation of the transition state. With 1 bearing a hydrogen atom at the acetal center (R1 = II), the acetyl group is allowed to occupy the quasiequatorial position (3B) and the addition reaction therefore proceeds with no or only a weak chelation control. The same presumably holds for the elyoxal monoacetal 1 (Ri = R2 = M). 

For example, the rigid, planar flavone 84 is completely tasteless. Phyllodul-cin (77) and the flavanones 79 and 80, on the other hand, can exist in a conformational equilibrium between a planar form (with the B-ring in the quasiequatorial disposition) and a bent form (with the B-ring assuming a quasiaxial orientation). Fig. 23,i presents the best binding to the receptor site. 

Quasiaxial and quasiequatorial substituents at C-6 in conformationally rigid lithium enolates such as (33a) have a very small effect on the stereoselectivity of the reaction. - In the case of the lithium... 

On reduction of 3-alkylcyclopentanones with lithium aluminum hydride the formation of a.v-3-methylcyclopentanol and CM,crs-3.4-dimethylcyclopentanol show that significant effects can be transmitted from the 3-position. These results have been discussed in terms of half-chair conformations in which the alkyl groups occupy quasiequatorial positions92. 

Using X-ray crystallography James [50] has however found that the compound 61 in crystalline state does not exist in the 4 conformation (61e in Fig. 2.19) but in the B2, 5 conformation 6182,5, as shown in Fig. 2.20 with the methylpyridinium group oriented quasiequatorially. 

Figure 4 shows several conformations designated quasiequatorial (essentially equivalent to intra), i.e., the ring N-C bond inside the inter-planar fold, coplanar, and quasiaxial (essentially equivalent to extra). Several workers claimed a preference for an extra or quasiaxial CPZ HCl conformation. Fronza and colleagues also claimed that the intra ring N-substituent bond is favored for phenothiazine but not for phenothiazine derivatives. In solids the orientation of the N-C bond was found to be closer to intra than extra, and probably closer to a flatter set of ring N bonds. 

In addition to being quasiequatorial, the conformation of the side chain of CPZ HCl is quite similar in DjO and CDCI3, being 80% tram at the Cp-Cy and roughly 10% at the C -Cp bonds. The conformation of the side chain in dimethylsulfoxide is quite different. Since CPZ base in CDCI3... 

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