Conformational studies. See

All the crystal forces that we treat in this section can be considered in terms of the recognition between a given, reference molecule and the cavity it is to occupy in the crystal. In chiral systems the cavity is clearly of different shape in the d and / crystals, and this generally results in differential incorporation of R and S molecules. However, in molecules containing the sec-butyl group, discrimination is often ineffective. This is because the two enantiomers can assume different conformations with very similar external shapes, and they can then interchangeably enter the same cavity in the crystal. This effect was recognized some time ago (55), and recently its consequences have been studied in detail (56). In the case where two enantiomers may readily replace one another in the crystal, it follows that there is a tendency to conformational disorder (see biphenyl, above), and in many cases, the resolved enantiomers and the racemates are isostructural. 

Conformational studies on monocyclic Cg to Cn ketones have been reviewed with reference to the MM method (184 see also ref. 89). Extensive comparisons of the conformational energies of trans-... 

For complete conformational studies of this intermediate see the original paper, ref. 15a. 

The reduction of thieno[3,2-6]thiophene-2,5-diaIdehyde or dithieno-[3,2-i 2, 3 -d]thiophene-2,6-dialdchyde with potassium in tetra-hydrofuran or 1,2-dimethoxyethane yields the corresponding anion-radicals, the ESR of which give information on the conformation of similar carbonyl compounds.Cation-radicals have also been studied (see Section IV). 

Bicyclo[3,l,0]hexanes.-Hach has concluded his efforts to provide preparative-scale syntheses of pure (+)-thuj-3-one (30), and (-)-isothuj-3-one, and the four derived thujanols, viz. (+)-thujan-3-ol, (-)-isothujan-3-ol, (+)-neothujan-3-ol (31), and (-)-neoisothujan-3-ol (for earlier related work see Vol. 2, pp. 30, 37, 38 Vol, 4, p. 42 Vol. 7, p. 36). ° A conformational study on these four thujanols indicates that they have a slightly developed boat-like conformation, whereas the five-membered ring in (+)-thuj-3-one (30) and (-)-isothuj-3-one is flat, according... 

Some conformational studies of piperazic acids (hexahydropyridazine-3-carboxylic acids) have been reported <1998CSR437>. Piperazic acids are important compounds as they appear as subunit in many natural products (see Sections 8.01.6 and 8.01.12.2). They can be considered as rigid proline equivalents <1998JA80>. The conformation of derivatives of 3,4-dihydrophthalazine-2(l//)-carboxylic acid, a new conformationally restricted analog of phenylalanine, was also studied <1998T165>. 

Microwave spectroscopy has been applied to the study of ring-puckering motions in small-ring compounds, and to the study of conformational isomers. See V. W. Laurie, Accounts Chem. Res., 3, 331 (1970). 

The effect of replacement of the APA nucleus with an ACA nucleus on the susceptibility of the substrate to enzymic hydrolysis depends on the nature of the /3-lactamase 2). It has been richly illustrated in the case of the /3-lactamase of Pseudomonas pyocyanea (31) and analyzed in terms of conformative response (see Section IV,B) in a study of a B. cereus /3-lactamase (75). In that study the role of the nucleus was examined in both catalytic and noncatalytic enzyme-substrate interactions. Little has been published on the effect of modifications within each nucleus, presumably because such modifications lead, as a rule, to the loss of antibiotic activity. 

In this study we also considered the conformational sensitivity of the results. While we usually try to find the conformation with the minimal COSMO energy by a conformational search (see chapter 11.3 for details), we tried a direct AM1/COSMO... 

---