Pseudoaxial bonds

Pseudoaxial bond (a ), pseudoequatorial (e ) Bonds from atoms directly attached to the doubly bonded atoms in a monounsaturated six-membered ring are termed pseudoequatorial and pseudoaxial depending on whether the angles that they make with the best plane through the six ring atoms is near 0° or 90,° respectively. 

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. 

For this mechanism to be correct, it is also necessary for the dioxaborolane to exist in conformation B with the two -COaiPr units pseudoaxial. In any other conformation of the dioxaborolane, or if other C-COaiPr bond rotational isomers are considered, the ester and aldehydic oxygen atoms are too far removed to interact. It should be noted further that reasonable transition states for C-C bond formation are not accessible if the aldehyde is symmetrically disposed with respect to the dioxaborolane system. Clockwise rotation about the B-O bond as indicated in B moves the aldehyde nonbonding lone pair away from the proximate ester carbonyl and leads to the favored transition state A. 

In the corresponding nonfluorinated 4,5-dialkylsultines, the sultine ring adopts a half-chair conformation with the S=0 bond in a pseudoaxial orientation 50 (cf. Scheme 12) <20030EJ4911>. Again, hyperconjugative interactions within the sulfinyl moiety (the anomeric effect ) were found to be responsible for the conformational preference. 

Absolute configuration of PGA 1 (21)at C-4 follows similarly from the sign of the Cotton effects, determined by the right-handed helicity due to allylic pseudoaxial substitution by a C-C bond. 

It is possible to distinguish axial and equatorial hydroxy groups in chroman-4-ols on the basis of their chemical shifts in DMSO (79BCJ2163). The pseudoaxial OH proton resonates at a significantly higher field the different behaviour is attributed to solute-solvent hydrogen bonding. 

Shine and co-workers239,240 have reported the synthesis and full NMR characterization of a series of alkoxysulfonium ion perchlorates including ions 85 derived from cis- and fraws-substituted cyclohexanols. The X-ray structural study of four salts showed that the orientation of the S O bond is always pseudoaxial. The optically active (—)-86 ion and similar systems have been synthesized.241 1H and 13C NMR measurements indicate that sulfur has a trigonal bipyramidal geometry due to the S—O intramolecular interaction in the axial position. 

Conformational analysis of some 9-chloro- and 9-bromo-6,7,8,9-tetrahy-dro-4//-pyrido[ 1,2-a]pyrimidin-4-ones 31 and their 9,9-dichloro and 9,9-dibromo derivatives was also carried out by I3C NMR spectroscopy (83JHC619). The halogen atoms in the 9-chloro and 9-bromo derivatives 31 (R = H) in the predominantly half-chair conformation occupy the pseudoaxial position. This conformer is probably stabilized by a favorable orbital interaction, while the other one, with a presudoequatorial halogen atom, is destabilized by the unfavorable dipole-dipole interaction between the 9-halogen and C(9a)=N(l) bonds. The methyl group in the 6-methyl derivatives in predominantly half-chair conformations is in the pseudoaxial position (83JHC619). 

Figure 4.11). This reaction, a Claisen rearrangement, transfers the PEP-derived side-chain so that it becomes directly bonded to the carbocycle, and so builds up the basic carbon skeleton of phenylalanine and tyrosine. The reaction is catalysed in nature by the enzyme chorismate mutase, and, although it can also occur thermally, the rate increases some 106-fold in the presence of the enzyme. The enzyme achieves this by binding the pseudoaxial conformer of chorismic acid, allowing a transition state with chairlike geometry to develop. 

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. 

The conformations of /raor-fused cyclohexathiazolidines 35a and 35b were determined by H NMR spectroscopy <1996ACS1036>. From the anisotropy effect of the S=0 bond it is obvious that, of the two trans conformations, 35a (5H-3a 2.60 and 6H 7a 4.57 ppm) is the one with pseudoaxial sulfinyl oxygen. Similar consideration has been made for... 

In all these structures the Sn-C-C=C angle is close to optimum for g-tc interaction between the C(allyl)-Sn g bond and the n system. In the two crystallographically independent molecules of cyclopenten-2-yl-triphenylstannane 197 and cyclohepten-2-yltriphenylstannane 199, the stannyl substituent takes up a pseudoaxial orientation. Although cyclohex-2-enyltriphenylstannane 198 exists in the solid state with the stannyl substituent in a pseudoequatorial orientation (Sn-C-C=C 120°), it exists in solution predominantly in a pseudoaxial conformation. Analysis of the structural parameters in 198 and 199 provides some tentative structural evidence for the presence of the o-Jt interaction in these compounds the Sn-C(allyl) bond distances, which are 2.189(5) and 2.182(5) A, respectively, are slightly longer than the normal Sn-C(aliphatic) distance, which is typically of the order of 2.13 A, consistent with the expected structural effects of electron donation from the Gc Sn orbital into the n orbital. 

---