Quasi-axial bonds

Tetrahydropyranyl ethers have been prepared from the quasi-axial 7a-hydroxyl in a 3)5-acetoxy-A -7a-ol, but in this case enhanced reactivity is due to the adjacent double bond. °... 

The enamines derived from cyclohexanones are of particular interest. The pyrrolidine enamine is most frequently used for synthetic applications. The enamine mixture formed from pyrrolidine and 2-methylcyclohexanone is predominantly isomer 17.106 A steric effect is responsible for this preference. Conjugation between the nitrogen atom and the tt orbitals of the double bond favors coplanarity of the bonds that are darkened in the structures. In isomer 17 the methyl group adopts a quasi-axial conformation to avoid steric interaction with the amine substituents.107 A serious nonbonded repulsion (A1,3 strain) in 18 destabilizes this isomer. 

B. Coordinated Bond Lengths In the Vicinity of the Anomeric Center. In D-aldofuranoses, the Cl-01 bond is quasi-axial in E-E conformers (0.7 E/K) of a-anomers, and Eq- E conformers (1.7 P/7C) of P-anomers (Figure 1). 

It has been concluded that some secondary chroman-4-oIs adopt a half-chair conformation in which a quasi-axial OH group, which absorbs at ca. 3618 cm-1, is predominant over a quasi-equatorial hydroxyl, which shows two peaks (3622 and 3600 cm-1) (74BCJ509). A study of more complex tertiary chromanols has indicated that the cis epimer (94) exhibits peaks at 3619 cm-1 (free OH group) and 3604 cm-1 (bonded OH). The trans epimer (95) shows only one band (3620 cm-1) which is also associated with a hydrogen bonded hydroxyl group, though now in a quasi-axial orientation (81JCS(P2)944). 

Fig. 11. Central skeleton of the W2(02CNMe2)6 molecule dotted lines indicate the long, quasi-axial W-O bonds. Reproduced with permission from Ref. 100. Copyright 1982 American Chemical Society.

The H-NMR studies of 23 and 24 indicated that their conformations were different (Fig. 1). Compound 23 adopts the usual half-chair form with the quasi-axial azido group, while the amino compound 24 exists in the boat-like form with the gwasZ-equatorial amino group. The latter form 24 might be due to the interaction such as hydrogen bonding between the C-l amino and C-2 hydroxy groups. 

Most furanoses prefer the envelope conformation and it appears that a quasi-equatorial exocyclic side chain and a quasi-axial C(l)—0(1) bond (anomeric effect) are equally important stabilising factors (Figure 1.8). 

The conformational effects arising from the endoanomeric effect are for furanoses much less profound and as a result relatively little research has been performed in this area. The puckering of the furanose ring of an a and a P anomer usually adjusts the anomeric substituent in a quasi-axial orientation and hence both anomers experience a similar stereoelectronic effect. On the other hand, the conformational preference of the exocyclic C—O bond is controlled by the exoanomeric effect in the usual way. 

The X-ray structures of two derivatives of 3,6-dihydro- 1,2-thiazine 1-oxide 137 indicate (cf. Scheme 51) (02AX165, 02AX198) the six-membered ring to be a half-chair conformer with the S=0 bond in an quasi-axial position. 

There exist two simple rationales to explain the observed direction of the dia-stereoselective bond activation in 7/Fe+, i.e., more pronounced loss of H2 from 7a/Fe+ in comparison to 7b/Fe+. At first, one can safely assume that the reaction proceeds via insertion of the docked Fe+ in a terminal C-H bond to form a six-membered ring. Depending on the relative stereochemistry at C(3) and C(4), the eliminations of H2 and HD, respectively, therefore involve quasi-axial or quasi-equatorial orientations of the methyl substituents in the intermediates eq- and ax-lOa of course, similar considerations apply to the associated transition structures (TSs). By analogy to conventional arguments of conformational analysis, an equatorial position of the methyl group is assumed to be preferred, thereby accounting for the experimentally observed H2/HD ratios. Thus, for the stereoisomer shown in Scheme 8, both the KIE and the equatorial position of the methyl substituent favor loss of H2, whereas the SE favors loss of HD from 7b/Fe+. However, for the latter this path is impeded by the operation of a kinetic isotope effect that slows down activation of a C-D bond. Secondly, one arrives at... 

Exo cycloalkylations have been used to synthesize ct5-1-decalones. For example, treatment of 2-methyl-3(4-tosyloxybutyl)cyclohexanone with sodium t-pentylate in benzene gave c/j-9-methyl-l-deca-lone (50) in 60% yield. Also, as shown in Scheme 28, conjugate addition-cycloalkylation was employed to synthesize a cw-fused decalone related to the sesquiterpene, ( )-valerane. Apparently, in these cases, the enolate intermediate adopts a conformation having the 4-bromobutyl side chain quasi-axial, and C—C bond formation occurs via equatorial attack to give initially a twist-boat conformation of the product. 

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