Quasi-equatorial

Beta- and 3a-equatorial hydroxyl groups of the 5a- and 5j5-series respectively, as well as the quasi-equatorial 3/ -hydroxyl in A -enes, are highly reactive. Most of the protective groups considered in this review can be readily prepared with these alcohols. 

It was noted (23) that the NMR spectrum of compound 32 was identical with that published (34) for the third product (assigned structure 29) isolated from the reaction of tetra-O-acetyl-2-hydroxy-D-glucal with acetic anhydride and zinc chloride. The identity of the compounds was fully established and a revised structure proposed for this third product. In the presence of zinc chloride, therefore, epimerization can occur at an allylic site and the quasi-equatorial C-4 acetoxy group in the erythro isomers 27 and 28 can assume the favored quasi-axial orientation (24). 

A study of the catalytic oxidation of six stereoisomers of 5-cyclohexene-l,2,3,4-tetrol revealed448 that, in all cases, an allylic hydroxyl group is selectively attacked, and a consideration of their favored conformations suggested that quasi-axial groups are selectively dehydrogenated before those that are quasi-equatorial. 

The stereochemistry of products 27 (Table 6, entries 2, 3, and 4) can be explained distinctly on the same basis that alkyl substituents R1, R2 are positioned in quasi equatorial positions in the transition structure. Exclusive formation of the all-cis isomer in the reaction of cyclohexyl acetate (entry 5) is further evidence to support the oxido-carbene interaction in a double-chaired bicyclic structure 28, as depicted in Scheme 13, Eq. 1. 

F2i2121 Z = 4 Dx = 1.53 R not reported for 1,074 intensities. The D-ribofuranosyl group has the °E conformation (tp = 3°, q = 31 pm), with the cyanomethyl group quasi-equatorial. The results were correlated with H-n.m.r. data obtained for the compound in CDC13 solution (using the Karplus equations). 

For the planar DKP ring, the /3-carbon atom of an L-amino acid is located on the left-hand side, as shown in Figure 5(a). For the boat conformation, the /3-carbon atom of an L-amino acid is in the axial position (Figure 5(b)). For the bowsprit-boat form, the /3-carbon atom of an L-amino acid is in the equatorial position (Figure 5(c)). Finally, for the twist-boat form to exist, the /3-carbon atom of an L-amino acid must adopt a quasi-equatorial position (Figure 5(d)). 

We explain the selective formation of diastereomer 52 on the basis of conformational arguments. The two likely conformers of 50 should have an essentially planar allyl moiety, whereas the six-membered ring should exist in two quasi-chair conformers with either the isopropyl or the methoxy group in the quasi-axial position. The conformer with the (bulkier) isopropyl group in a quasi-equatorial position is preferred. While access to the terminal allylic carbon (which leads to 51) appears to be unhindered, the quasi-axial substituent at the chiral carbon will interfere with the attack on the internal allylic center, directing the attack to the face opposite to the quasi-axial methoxy function, (- 52). These considerations account for the preference of 51 over 52 as well as for the suppression of the diastereomer. 

Variable temperature studies again show the ring inversion (76T2339). They also indicate that a 2-methyl group shows equal preference for the quasi-axial or quasi-equatorial positions, presumably because there are no other axial substituents to cause 1,3-diaxial interactions. 

As the quasi-axial hydrogens are in more favorable positions than are the corresponding quasi-equatorial (e ) hydrogens at C3 and C6, the formation of hydroperoxides corresponding to the alcohols 22,26 and 18,21 should be favored over those corresponding to 23,27 and 17,20, respectively. This is the case, as the product distribution... 

A number of studies have now been made, notably by Buchanan and coworkers, with the object of determining the position of the equilibrium between the various pairs of interconvertible epoxides the findings are summarized in Chart I. For the dianhydro compounds, the epoxides that contain the free hydroxyl group quasi-axial, and that also have the possibility of a polar interaction between the epoxide ring and the 1,6-anhydro bridge, are clearly less stable than those in which it is quasi-equatorial, although the relative importance of these two factors is uncertain. The situation is less clear for the monocyclic epoxides. The half-chair conformations indicated are considered to be favored on the basis that the alkyl... 

Anhydro compounds can similarly be analyzed from JU2 values. The chemical shift of H-l appears to be the most generally useful parameter, as it lies in the range t 4.95-5.43 when the proton is equatorial or quasi-equatorial, and has a value of r 5.82 for H-I in methyl... 

The above criteria apply in the case of isolated hydroxyl groups but when additional polar substituents are placed in the vicinity of the substrate hydroxyl the oxidation rate can be expected to change. Allylic hydroxyls are generally oxidized more rapidly than their saturated counterparts. Burstein and Rin-gold have studied the chromic acid oxidation of steroidal allylic alcohols in some detail and have found that the quasi-equatorial 3)3-isomer is oxidized more... 

Fusion of an oxirane ring to a pyranose ring also deforms it, and thereby lowers its stability. The composition of 2,3-anhydro-D-mannose in aqueous solution,165 as determined by g.l.c. of the trimethylsilyl derivatives, is 23 7 65 5. This is remarkably similar to the composition of a solution of 2,3-O-isopropylidene-L-rhamnose. For 2,3-anhydro-D-allose, the ratios are166 41 12 5 42 (or 41 5 12 42). In this case, although the proportion of furanose forms is substantial, there is no clear preponderance of the //-furanose form, presumably because OH-1 and OH-2 are trans but OH-1 is quasi-equatorial by contrast, in the (preponderant)... 

The five-inembered ring has the E3 conformation, with a tendency towards the 3T2 form the Cremer-Pople puckering parameters89,70 are q= 42.0 pm and 4>2 = 102.98°, and the asymmetry parameters after Duax and coworkers91 are AC, = 8.4° for the E3 conformation and AC2 = 12.9° for the 3T2 conformation. The acetoxyl group on C-2 and the methyl group on C-4 are linked quasi-equatorially, whereas the AcO-3... 

The 9-methyl group on 6,7,8,9-tetrahydro- and 1,2,3,4,6,7,8,9-octahydro-117/-pyrido[2,l-6]quinazolin-l 1-ones occupies a quasi-axial position due to the 1-3 allyl-type strain, which would develop between the 9-methyl group and the neighboring C(11)=0 carbonyl group if the methyl group were in a quasi-equatorial position (87JHC1045). 

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). 

Figure 12-15 Nonplanar cyclobutane conformation with a substituent R in the less hindered, quasi-equatorial position. The dihedral angle between the two halves of the bent ring usually is 25° to 30°, that is, a 25° to 30° deviation from planarity.

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