Pseudoequatorial position

The stereochemical features of the Claisen rearrangement are very similar to those described for the Cope rearrangement, and reliable stereochemical predictions can be made on the basis of the preference for a chairlike transition state. The major product has the -configuration at the newly formed double bond because of the preference for placing the larger substituent in the pseudoequatorial position in the transition state. ... 

Similar to cyclohexanones, substituted cyclopentanones also adopt a conformation with the substituents in a sterically favorable position. In the case of 2-substituted cyclopentanones 1 the substituent occupies a pseudoequatorial position and the diastereoselectivity of nucleophilic addition reactions to 1 is determined by the relative importance of the interactions leading to predominant fra s(equatorial) or cw(axial) attack of the nucleophile. When the nucleophile approaches from the cis side, steric interaction with the substituent at C-2 is encountered. On the other hand, according to Felkin, significant torsional strain between the pseudoaxial C-2—H bond and the incipient bond occurs if the nucleophile approaches the carbonyl group from the trans side. 

Ternay and coworkers examined the conformations of thioxanthene-10-oxide (186) and related compounds. They found that oxygen preferred the pseudoequatorial position, 186e, but that the amount of pseudoaxial conformer, 186a, increased when the oxygen was complexed with iodine monochloride or trifluoroacetic acid171. 

As one would expect, the tri-substituted eis-trans-2,4-diaryl-3-dimethylaminothietanes (187c,d) were shown by NMR to have all three substituents in pseudoequatorial positions with the remaining hydrogens in axial positions202. 

Cyclization of an organolithium tethered to a suitably positioned carbon-carbon jt-bond is a thermodynamically favorable process that proceeds in a totally regioselective exo-fashion with a high degree of stereocontrol via a transition state in which the lithium atom is intramolecularly coordinated with the remote rc-bond.9 The stereochemical outcome of the cyclization of a substituted 5-hexenyllithium follows from the preference of the substituent to occupy a pseudoequatorial position in the chair-like transition state depicted below.7... 

Entry 13 shows cyclization with concomitant loss of the benzyloxycarbonyl group. The TS for this reaction is 5-exo with conformation determined by the pseudoequatorial position of the methyl group. 

The conformation bearing the substituent at the asymmetric center in a pseudoequatorial position (R1) is more stable than that bearing the substituent... 

It is to be noted that in the allyl vinyl ether (where R, > R2) the newly formed double bond has the E configuration. This is because that in the chair-like transition state, there is preference for placing the larger group in the pseudoequatorial position. 

If the transition state has the chair conformation 9, groups in the terminal positions of each allyl moiety can occupy pseudoaxial or pseudoequatorial positions. We therefore calculated the structures with a single methyl subsituent the results are shown in Fig. 6. 

The greater stability of the anti isomer is attributed to the pseudoequatorial position of the methyl group in the chair-like chelate. With larger substituent groups, the thermodynamic preference for the anti isomer is still greater.10 11 Thermodynamic equilibration can be used to control product composition if one of the desired stereoisomers is significantly more stable than the other. 

Data on the apparent dipole moment and H NMR spectra of thianthrene-5-oxide 24 under different conditions of solvent and temperature support the rapid conformational equilibrium of 24, which flaps between two limit boat conformations with the sulfoxide group in a pseudoaxial or pseudoequatorial position, respectively. Variable-temperature... 

H NMR spectra have established the interconversion barrier of /ra r-thianthrene 5,10-dioxides and confirmed that the conformational equilibrium of rfr-thianthrene 5,10-dioxides is strongly displaced toward the conformation with both sulfinyl groups in the pseudoequatorial position <2005JCX13450>. 

Alkenyl nitrones, having the alkene connected to the nitrone nitrogen atom, have been used in another approach to intramolecular reactions (231-235). Holmes and co-workers have this method for the synthesis of the alkaloid (—)-indolizidine 209B 137 (210,231). The alkenyl nitrone 134, was obtained from the chiral hydroxylamine 133 and an aldehyde. In the intramolecular 1,3-dipolar cycloaddition, 135 was formed as the only isomer (Scheme 12.45). The diastereofacial selectivity was controlled by the favored conformation of the cyclohexane-like transition state in which the pentyl group was in a pseudoequatorial position, as indicated by 134. Further transformation of 135 led to the desired product 137. 

For 9-phenylamino-6,7,8,9-tetrahydro-4//-pyrido[l,2-a]pyrimidin-4-ones 32 (R = NHPh R1 = H R2 = H, COOH) the half-chair conformation with a pseudoaxial 9-phenylamino group was the predominant one. But with the 6-methyl-9-phenylamino derivatives 32 (R = NHPh, R1 = Me) the cis-trans ratio was near 1 1 in the imine form. The presence of a methyl group on the nitrogen atom of the 9-anilino group 32 (R = NMePh) increased the amount of the cis form which contained the 9-substituent in a pseudoequatorial position [85JCS(P1)1015]. The 6-methyl group was in a pseudoaxial position in all tautomers. 

The approach for the enantioselective aldol reaction based on oxazolidinones like 22 and 23 is called Evans asymmetric aldol reaction.14 Conversion of an oxazolidinone amide into the corresponding lithium or boron enolates yields the Z-stereoisomers exclusively. Reaction of the Z-enolate 24 and the carbonyl compound 6 proceeds via the cyclic transition state 25, in which the oxazolidinone carbonyl oxygen and both ring oxygens have an anti conformation because of dipole interactions. The back of the enolate is shielded by the benzyl group thus the aldehyde forms the six-membered transition state 25 by approaching from the front with the larger carbonyl substituent in pseudoequatorial position. The... 

Single crystal X-ray diffraction data for disubstituted 1,3,2-dioxathiolane V-oxides 12 (R = c-C6Hn) <1995AXC129>, 12 (R = Ph), 13, and 14 <1996AXC739> revealed half-chair (envelope) conformations of the five-membered cycles with the S=0 group in a pseudoaxial position and other substituents in pseudoequatorial positions. 

The diastereoselectivity of the reduction depicted in Figure 17.53 is determined when the hydroxylated radical A is reduced to the hydroxylated organosodium compound B. For steric reasons, the OH group assumes a pseudoequatorial position in the trivalent and moderately pyramidalized C atom of the radical center of the cyclohexyl ring of intermediate A. Consequently, the unpaired electron at that C atom occupies a pseudoaxially oriented AO. This preferred geometry is fixed with the second electron transfer. It gives rise to the organosodium compound B. B isomerizes immediately to afford the equatorial sodium alkoxide. 

Tetronates derived from l,3-divinyl-2-cyclopentanol were employed to study the possibility of a differentiation of enantiotopic or diastereotopic double bonds in their [2 + 2]-photocydoaddition [140]. It was found that tetronate 148 underwent a selective [2 + 2]-photocydoaddition (r.r. = 75/25) at one of the two possible double bonds to deliver product 149 in 67% yield (Scheme 6.52). The reaction was analyzed regarding the preferred conformations of the cyclopentanol, with the notion that the tetronate resides in a pseudoequatorial position, and the vinyl group in a pseudoaxial position of the envelope conformation. Intermediate 149 served as starting material for the first total synthesis of the tetracydic sesquiterpene (T)-punctaporonin C (150) [141]. 

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