Cyclohexene oxides, conformational

With a variety of substituted cyclohexene oxides, e.g., the 1-methyl and 3-bromo derivatives, Bellucci et al. demonstrated that the substrates in the catalytic site adopted the conformation shown in Fig. 10.29 for 10.5 (3,4M... 

Thus, all results with cyclohexene oxides are compatible with a configurational and conformational selection by the enzyme, preferential attack at an (S)-configured C-atom, and Zrans-diaxial opening. 

In the synthesis of cyclohexene oxide from cyclohexene shown, this does implicate the less favourable diaxial conformer in the epoxide-forming step. Cyclohexene oxide contains a c/s-fused ring system, the only arrangement possible, since the three-membered ring is necessarily planar (see Section 3.5.2). 

It is impossible for the CO bonds of the product epoxide ring to adopt perfectly axial and equatorial positions. If you make a model of cyclohexene oxide you will see that the ring is a slightly deformed chair—it is more of a half-chair conformation in which four of the carbon atoms are in the same plane (you met this on p. 461). 

The f-bulyl group locks the conformation of the epoxide. Whereas cyclohexene oxide can flip (see above), enabling the nucleophile to attack either of the epoxide carbon atoms, here the ring is conformationally rigid... 

Six-membered rings with more than one sp2 C atom do lose their chair conformation—they become flattened to some degree when there are one or more double bonds included in the ring. Cyclohexene, with just one double bond in the ring, has a half-chair conformation similar to that of its related epoxide, cyclohexene oxide. The usual conformational diagram of cyclohexene is shown below. The barrier for ring inversion of cyclohexene is around 22 kj mol-1 (about half that for cyclohexane ). 

We looked at the conformations of cyclohexenes and cyclohexene oxides in Chapter 18. and we will look again at the stereochemistry of reactions of six-membered rings contai ning double bonds in Chapter 33. 

Simple aliphatic oxiranes can be converted to fluorohydrin with hydrogen fluoride only with great difficulty, but the reaction can be carried out in systems with rigid conformations (e.g., steroids, 9,10-epoxydecalin). Good yields can be attained from cyclopentene and cyclohexene oxides with 42% pyridine-polyhydrogen fluoride and in a series of terpene oxides with an amine-hydrogen fluoride complex. ... 

The question asks for a conformational drawing of the product and indeed that is necessary. The. 3 lactone bridge must be diaxial as that is the only way to reach across and the carboxylate group T.ust attack the iodonium ion from an axial position too. You can regard this as the formation of -. e trans diaxial product as in the opening of a cyclohexene oxide with a nucleophile (pp. 468-70). 

Scheme 4 (a) Cyclohexene oxide adopts a half-chair conformation that exists in two different helicities. (b) The isopropyl substituent of limonene-1,2-epoxide makes one of the two half-chair conformations energetically more favorable. 

Scheme 5.27) [76]. A high yield of the enantiomerically enriched product is obtained, although the enantiomeric excess is very low. The enzymatic hydrolysis of achiral cyclohexene oxide is also an important historical example [77]. Asymmetric bromination of cyclohexene catalysed by a cinchona alkaloid might involve DKR of conformational enantiomers of the olefinic substrate and/or bromonium ion intermediates [78]. 

The conformation of cyclohexene oxide (60) resembles that of its hydrocarbon analogue, bicyclo[4,l,0]heptane (32), and that of cyclohexene (14b), so that both types of thiee-membered ring appear to have the same configuration-determining effect as a double bond. However, this hypothesis does not seem to apply to the pair of molecules a-pinene oxide (61) and a-pinene (39). The four-bond bridge in the latter is planar, whereas (61) shows that in the oxide this is not so however, there is a methyl-to-oxygen non-bonded interaction which may account for this difference. 

The reaction of methyllithium with an epoxide followed by neutralization gives an alcohol. Write the product of the reaction of cyclohexene oxide with methyl lithium, showing the structure in its most stable conformation. 

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. 

Step a The predominant cyclohexene conformer places (i) the /-Bu equatorial and (ii) the allylic-Me pseudoaxial to minimize A1,3 strain, syn-Addition of the H-B bond at the least, after oxidation, to B. 

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