Sofa conformation

We also performed a single-crystal X-ray structure analysis of this lead compound. The solid state structure of this compound depicted in Fig. 3-15 shows a half-boat-like ( sofa ) conformation with the 9-phenanthryl group in a quasi-axial or r/Mf/.v/-flagpole position, and the a, 3-unsaturated exocyclic ester in a s-cis conformation. This cleft-like conformation is advantageous for the creation of centers with a high recognition ability, since one enantiomer fits in better than the other thus leading to selectivity. 

Structural characterization of many quinolizidine derivatives has been established by X-ray diffraction. For example, this technique, in combination with spectroscopic methods, showed that (+)-2-thionosparteine 21 and (+)-2,17-dithionospartine 22 are conformationally rigid and have their lactam and thiolactam groups close to planarity, with the exception of the lactam group in 21, and that rings A and C adopt distorted sofa conformations <2005JST75>. 

Glowka et al. studied two tetrahydro[l,2,4]triazolo[4,3- ]pyrimidine derivatives 66a and 66b by X-ray diffraction <1994JCX375>. In the solid state, both molecules had the same conformation and were found to be packed in similar manner. The triazole rings proved to be planar, and the conjugation of the Jt-electrons with the lone pair of the N-8 atom was assumed to be the reason of the sofa conformation of the saturated six-membered ring. 

Therefore any flexible acetal will undergo conformational changes to permit 2p(0) <-> 2p(C+) stabilizing interaction to intervene in the transition state of its heterolysis. This is also true for pyranosides for which the free energy difference between chair, boat and sofa conformers rarely surpasses 10 kcal/mol. 

Accordingly, the rrans-fused 2,4-dioxodecahydroquinazoline 351T has a biased chair-sofa conformation, whereas the cis isomer consists of an approximately 61 39 conformational mixture of the N(l)-out and N(l)-in forms (351C), in agreement with an early estimate (69T3807). In the... 

In accordance with the H and NMR parameters, the heterocyclic moiety of the -substituted (R = Me, Et, Ph, p-MeQH4, P-CIC6H4) diendo- and diexo-fused 4-oxo-4a,5,6,7,8,8a-hexahydro-lH- and -4a,5,8,8a-tetrahydro-lH-5,8-methanoquinazoline-2-thiones 353 assumes a distorted sofa conformation in which the N(l) —C(=S) —N(3) grouping forms the tip of the sofa [85JCS(P1)2483]. 

Ground-state and excited-state reactions of chiral Meldrum s acid derivatives 39 with the enone function have been reviewed with an emphasis on the facial selectivity in the C=C bond (Figure 2) <1996H(42)861>. Top-face preference, even when it is sterically more hindered than bottom-face attack, is supported by hyperconjugation no —r c=c 39a, whereas bottom-face preference is dominated by steric effects in the sofa conformation of the molecule 39. The trajectory of the attacking reagent plays a balancing role. 

The CD spectra of cis and trans 4-substituted flavans are best explained by assuming a sofa conformation for the heterocyclic ring, with C-2 out of plane, rather than by a half-chair conformation. A similar conformation has been used in a discussion of the CD spectra of naturally occurring flavanols (71T54S9). 

Further, as discussed in Chapter 12, sections B2a and D5c, the sugar ring in site D is distorted to the sofa conformation expected for a carbonium ion. 

The oxathiapinone 8 prepared by an intramolecular Diels-Alder reaction was found to have a sofa conformation from X-ray analysis <2006AXE370, 2006AXE366>. 

X-Ray diffraction determination revealed that in solid state the asymmetric unit cell of (4R,9flR)-4-phenylperhydropyrido[l,2-fl]pyrazine-l,3-dione contains two distinct invertomers 150 and 151, and they formed a head-to-head dimer (09TA1759). The invertomer 150 adopts trans-chair/ sofa conformation with a frans-diaxial arrangement of the N5 lone pair and the H-C9a. The invertomer of 151 adopts cis-chair/sofa conformation with a cis-axial/equatorial arrangement of the N5 lone pair and the H-C9a, with respect to the piperidine ring. 

The gas-phase structure of 1,3,5-trisilacyclohexane was determined by gas electron diffraction (01JMS245) and compared with the results of quantum chemical calculations (98JMS(T)91). The chair conformation, the strongly preferred conformer, is more flattened than cyclohexane due to the intrinsically large Si-C-Si bond angles. By the quantum chemical calculations also the twist and boat conformers were identified to be much less stable than the chair conformation. The chair-to-chair interconversion barrier is 5.5 kcal/mol the transition state was identified as a sofa conformation with an approximately Cs symmetry, similar to the transition state in 173 (cf. Scheme 57). 

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