Substituents in equatorial position

For the kinetically controlled formation of 1,3-disubstituted tetrahydro-P-carbolines, placing both substituents in equatorial positions to reduce 1,3-diaxial interactions resulted in the cw-selectivity usually observed in these reactions." Condensation reactions carried out at or below room temperature in the presence of an acid catalyst gave the kinetic product distribution with the cw-diastereomer being the major product observed, as illustrated by the condensation of L-tryptophan methyl ester 41 with benzaldehyde. At higher reaction temperatures, the condensation reaction was reversible and a thermodynamic product distribution was observed. Cis and trans diastereomers were often obtained in nearly equal amounts suggesting that they have similar energies."... 

They correspond to the cross-metathesis of propylene with the neopentyli-dene fragment (Scheme 18), and their relative ratio corresponds to a photograph of the active site as they are formed. Depending on how propylene will approach the carbene, it will generate different metallacyclobutanes, whose stabilities can direct the relative amounts of cross-metathesis (and selfmetathesis) products. This model is based on the following the favoured cross-metathesis product arises from the reaction pathway, in which [1,2]-interactions are avoided and [1,3]-interactions are minimized (here shown with both substituents in equatorial positions) [83]. 

In several cases (including the present example) where diastereoiso-meric aldol products are possible, there is a preference for the formation of the tAreo-diastereoisomer. This stereochemical preference presumably arises because the six-membered cyclic zinc chelate of the ffereo-isomer can exist in a chair conformation with both substituents in equatorial positions. Table I summarizes the results obtained in several aldol condensations performed by the present procedure. 

Slightly twisted-boat conformer with the 3-substituents in equatorial position. 

As an example of spectra of molecules containing the CH2CH2C H chiral fragment, the spectra of (—)-menthone and (-t-)-isomenthone are shown in Figure 5. (—)-Menthone has two bulky substituents and the favored chair conformation is 16, which corresponds to 14 with both substituents in equatorial positions. The observed (-1— +) VCD pattern is that predicted for this fragment geometry. For isomenthone both chair conformations 18 and 19 are populated since, in each, one alkyl substituent is axial and one equatorial. However, the... 

Most sugars occur in the chair conformation that places the largest number of substituents in equatorial positions and is therefore most stable thermodynamically. For D-aldoses this is usually the 4Q conformation ... 

The cyclohexane ring in 56 has a chair conformation with the three trimethylgermyl substituents in equatorial positions. 

Glucose has its substituents on alternating sides of the ring. In drawing the chair conformation, just put all the ring substituents in equatorial positions. 

The anomers of glucose. The hydroxyl group on the anomeric (hemiacetal) carbon is down (axial) in the a anomer and up (equatorial) in the /3 anomer. The /3 anomer of glucose has all its substituents in equatorial positions. 

As mentioned above, usually a chair conformation with the large substituents in equatorial position is typical for the 1,4-substituted cyclohexasilanes. l,l,4,4-tetrakis(trimethylsilyl)-... 

In contrast, the two chair conformers of iran5-l,4-dimethylcyclohexane have different stabilities because one has both methyl substituents in equatorial positions and the other has both methyl groups in axial positions. 

Now let s look at the geometric isomers of l-ierr-butyl-3-methylcyclohexane. Both substituents of the cis isomer are in equatorial positions in one conformer and in axial positions in the other conformer. The conformer with both substituents in equatorial positions is more stable. 

The presence of asymmetric centers along the chain of a hexenyl radical induces diasteroselective cyclization and the main stereoisomer can be predicted from chair or pseudo-chair transition states by keeping the largest substituents in equatorial positions [108]. 

Various other 2,4,6-trisubstituted-l,3,5-trioxanes obtained by cyclotrimerization of the corresponding aldehydes have been reported (cf. Scheme 70 Table 17) common catalysts employed were, besides protic acids, zeolites, bentonitic earth, Lewis acids, heteropoly acids, organic metal oxides, and ion-exchange resins. When X-ray structures were reported, the chair conformer with the substituents in equatorial positions was always found (e.g., <19988417>). Also, the trimerization of acetaldehyde and propionaldehyde in the presence of SO2 has been reported <1983ZOB1787> wherein the aldehydes and SO2 initially form CT complexes. In the case of acetaldehyde, the corresponding enthalpy and entropy of complexation were determined to be —ISkJmoL and —32JmoL respectively. 

The products are the l,3,5-trialkoxy-2,4,6-tri-terf-butyl-l,3,5-triphosphi-nanes 39a, b. In an X-ray crystallographic analysis of 39a the highly distorted chair conformation of the ring skeleton with the tert-butyl substituents in equatorial positions is clearly apparent Two methoxy substituents at P-1 and P-5 occupy approximately equatorial positions while that at P-3 is in an axial position. To examine the reactivity of the new triphosphinanes 39, we have investigated the reactions of 39a with sulfur. The reaction proceeds with a coordination increase at all phosphorus atoms to furnish the 1,3,5-triphosphinane trisulfide 40 (74%) [37]. 

An X-ray crystallographic analysis of 59a revealed a C3-symmetrical structure with the central 1,3,5-triphosphinane ring in the chair conformation and the three sterically demanding terf-butyl substituents in equatorial positions. The tetracyclic system itself forms a cavity that is open to one side. The crystal structure analysis of 60c (Fig. 3) unequivocally confirmed the structure of this... 

When the two methyl substituents are in 1,3 positions, the cw-isomer can have both substituents in equatorial position. In the rran -isomer, one methyl group must always be axial. 

Glucose has its substituents on alternating sides of the ring. In drawing the chair conformation, just put all the ring substituents in equatorial positions. (In the Haworth projection, the —OH on C4 is opposite the —CH2OH on C5, and the — OH on C3 is opposite that on C4.)... 

The more stable chair conformation of a substituted cyclohexane has the greater number of substituents in equatorial positions. (3.5)... 

Upon treatment with acid, 3-methylcyclohexanol undergoes epimerization at the carbon bearing the hydroxyl group, since the hydroxyl will be converted to a good leaving group (H20).The cis diastereomer is expected to be the major product formed since that is the more stable product with both substituents in equatorial positions. 

The X-ray structure reveals a 8-twist conformation for the complex, with the Pr and the R substituents in equatorial positions. However, in solution two isomers are detected, in 9.4 1 ratio, which undergo slow exchange. The major isomer corresponds to that found in the solid state. The minor isomer could be either the X conformer of the major isomer, or a diastereomer. NOESY experiments and analysis of the coupling constants prove it to be the 8 conformation... 

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