Conformations Stereoisomers chair

The following are representations of two forms of glucose The six membered ring is known to exist in a chair conformation in each form Draw clear representations of the most stable con formation of each Are they two different conformations of the same molecule or are they stereoisomers Which substituents (if any) occupy axial sites ... 

Examine the geometry of norcarane. What is the conformation of the cyclohexane ring Choose a name (chair, twist boat, half-chair, etc. see Chapter 5, Problem 4) that accurately describes its shape. The bridgehead hydrogens in norcarane are cis. Do you think a trans stereoisomer is possible Explain. 

Ketones react with alcohols to yield products called acetals. Why does the all-cis isomer of 4- cvf-butyl-l13-cyclohexanediol react readily with acetone and an acid cataty st to form an acetal while other stereoisomers do not react In formulating your answer, draw the more stable chair conformations of all four stereoisomers and the product acetal. Use molecular models for help. 

The stereoselectivity achieved in the synthesis in Scheme 13.12 is the result of a preferred conformation for the base-catalyzed oxy-Cope rearrangement in Step B. Although the intermediate used in Step B was a mixture of stereoisomers, both gave predominantly the desired relative stereochemistry at C(4) and C(7). The stereoselectivity is based on the preferred chair conformation for the TS of the oxy-Cope rearrangement. 

The stereochemical outcome of the reaction of 1-43, formed from 1-40 by desily-lation, can be explained by assuming a pseudoequatorial orientation of the epoxide moiety in a pseudo-chair-chairlike transition state 1-44 which, after being attacked by the phenolic oxygen, furnishes the correct trans-fused stereoisomer 1-41 (Scheme 1.12). The conformation 1-45, which would lead to 1-46 seems to be disfavored. 

Stereoisomers adopt a chair conformation with equatorial substituents at carbon (forms A and B, respectively), dioxaphosphorinane oxide, and... 

NMR data of 1,3,5-azoxaphosphorinanes (8) are analyzed in several references [73ZC( 13)291 74MI1]. The comparison of spin-spin coupling constants for 1,3,5-azoxaphosphorinanes with those for 1,3,5-dioxaphosphorinanes made it possible to establish the conformation of substituents in 1,3,5-azoxaphosphorinanes (83MI1). All four stereoisomers adopt a chair conformation with equatorial phenyl groups, including that at phosphorus. 

In the tetra-bridged phosphocavitands, the preorganized structure is imposed by the fixed boat-chair conformation of the four fused eight-mem-bered rings. Inwards (i) and outwards (o) configurations are defined relatively to the endo and exo orientations of the P=X bonds (X=0, S, electron pair), and six different stereoisomers arise from the equatorial or axial orientation of the substituents on the phosphorus atoms (Scheme 3). 

However, in a cyclohexane system we also need to consider the conformational mobility that generates two different chair forms of the ring (see Section 3.3.2). Let us consider 3-methylcyclohexanecarboxylic acid. This has two chiral centres, and thus there are four configurational stereoisomers. These are the enantiomeric forms of the trans and cis isomers. 

On the basis of NMR and IR spectra and the kinetics of the quaterniza-tion (see Section 4.2.5), homoberbine stereoisomers 169 are assigned the twist-boat conformation 169 and the pseudo-chair conformation 169, respectively (71T1363). 

Among quinolizidine alkaloids, sparteine and its stereoisomers have been studied in detail by X-ray analysis (42-50). It was demonstrated that proper conformation was not reorganized in monohydrates (42), diperchlorates (43), or methyliodides of a-isosparteine (11) (53). Unlike in the case of a-isosparteine, in spareteine diperchlorate rings C/D appear to have a boat-chair conformation (44-46). On the basis of spectroscopy data a cis conformation for sparteine methyliodide (12) was proposed (57,52). However, radiographic examination (53) of this compound showed it to have the trans conformation (13). 

Like the 1,4-dimethyl derivatives, trans-1,2-dimethylcyclohexane has a lower heat of combustion (see Table 3.2) and is more stable than cis-1,2-dimethylcyclohexane. The cis stereoisomer has two chair conformations of equal energy, each containing one axial and one equatorial methyl group. 

Among the most important of the bicyclic hydrocarbons are the two stereoisomeric bicyclo[4.4.0]decanes, called cis- and frans-decalin. The hydrogen atoms at the ring junctions are on the same side in c/s-decalin and on opposite sides in mms-decalin. Both rings adopt the chair conformation in each stereoisomer. 

The two pyrrolopiperidine stereoisomers 42a and 42c were isolated in the individual form(m.p., 139-140and 155-157°C, respectively). Analysis of the spin-spin coupling constants in H-NMR spectra indicates that the piperidine conformation is a half-chair with a transdiequatorial arrangement of methyl groups at C4 and C7. 

When comparing two stereoisomeric cyclohexane derivatives, the more stable stereoisomer is the one with the greater number of its substituents in equatorial orientations. Rewrite the structures as chair conformations to see which substituents are axial and which are equatorial. 

According to the foregoing analysis, conformers such as gauche- and anri-butane or chair and twist-boat cyclohexane would be considered to be diastereomers of each other. However, under most conditions these conformers interconvert so rapidly that butane and cyclohexane are considered to be single species and not mixtures of stereoisomers. When we have to write chemistry books for people living on the outer planets of the solar system, we might have to modify these concepts. 

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