Conformations of the Five-Membered Rings

The PE spectra of four 2-substituted 1,3-dimethylimidazolidines (58) have been recorded and analysed using AMI and PM3 quantum chemical calculations106. A single broad band is found for the two hn ionizations and the energies of the two mn orbitals are split by only 0.0-0.3 eV indicating little n/n interaction. This is consistent with envelope conformations of the five-membered ring of 58 and an axial-equatorial orientation of the two N-methyl groups. 

Conformations of the five-membered ring D are not clear-cut. Two envelope forms (10) and (ii), and a half-chair conformation (12) [32,33] have been proposed, each for a specific group of compounds. Experimental evidence (spectroscopic and other physical data [34,35,36]), supported by cal-... 

X-ray diffraction data have focused on determining the conformation of the five-membered ring or substituents, or the geometry around the metal atom. Over the past 10 years. X-ray crystallography has been used routinely and most of the new heterocycles synthesized have been examined by this method. Mention of these results is made in the appropriate sections dealing with the synthesis of the heterocycle. 

Rules of the Fischer Projection Mutarotation The Haworth Projection The Mills Projection The Reeves Projection Conformations of the Six-Membered Rings Conformations of the Five-Membered Rings Conformations of the Seven-Membered Rings Conformations of Fused Rings Steric Factors... 

Most studies (x-ray diffraction and NMR) have focused on determining conformation of the five-membered ring or substituents, or geometry about the metal atom. NMR studies have addressed rate of lone-pair inversion of some heteroatoms, such as nitrogen or phosphorus. Occasionally, NMR has been used for structure determination. 

The trans-bromohydrin (125) is reduced directly by nucleophilic displacement of the bromo-substituent, lithium aluminium deuteride giving the 16a-mono-deuterio-17a-alcohol (126). The near-planar conformation of the five-membered ring is unfavourable to epoxide formation, observed under similar conditions in a six-membered ring. ... 

However, the combination of the strained planar conformation of the five-membered ring to any of the six-membered ring conformations exceeds the preset threshold of torsional compatibility of the templates and the combination is rejected (see Fig. 25). 

If the conformation of the five-membered ring is fixed in a bridged (caged) structure, the stereoseiectivity dramaticaily increases, even with UAiH4, as you wili see later in the chapter (p. 862)... 

It can thus be seen that most of the variation in the data (85.9%) is explained by the first principal component, with all but a fraction being explained by the first two components. These two principal components can be plotted as a scatter graph, as shown in Figure 9.33, suggesting that there does indeed seem to be some clustering of the conformations of the five-membered ring in this particular data set. 

MeaAs-CHF-CHF AsMea are mainly those of the major (racemic) isomer, but only for X = Br could the minor product be removed. Nevertheless, good n.m.r. spectra of all the complexes were obtained and were analysed in detail in terms of the preferred conformation of the five-membered rings [e.g. (16a, b)].i A similar reaction of Mn(CO)sI with MeaAs-CFa CHCFCFa AsMea gives (17), the n.m.r. spectra of which show equilibration of the six-membered chair conformations. 

The second site to be altered was the alkyl substituent on the boron center. Initially, it was proposed that the nature of this group should not have a major impact on the level of enantioselection of the cyclopropanation reaction. It is believed that this group simply adopts the pseudoequatorial position of the envelope conformation of the five-membered ring. Several different dioxaborolane ligands were prepared by the same method as that reported earlier. Four novel dioxaborolane additives were prepared with R = Me, Ph, 2-naphthyl and 2,4,6-trimethylphenyl. The enantioselectivities observed for the cyclopropanation reaction are shown in Table I. In all the cases, the enantioselectivities were in the same range as that obtained with R = Bu, except for the 2,4,6-trimethylphenyl substituent. This information suggests that a sterically encumbered substituent on boron may partially prevent the postulated association between the zinc alkoxide and the boron center. In that case, the non-boron-assisted pathway can eventually become competitive. 

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