Chair/twist boat conformation

The spirobis[2,4]-benzodithiepine (90) has been prepared from l,2-bis(chloromethyl)-4,5-dimethylbenzene and sodium trithiocarbonate prepared in situ and a conformational analysis has been carried out using H- and l3C-NMR, X-ray analysis and semi-empirical calculations <95JOC6335). In the solid state it exists in a chair-twisted boat conformations whereas in solution it undergoes two dynamic processes, namely pseudorotation at lower temperatures and ring inversion at higher temperatures. 

An x-ray study of trans-1,8-dinitrosooctahydronaphthyridine (18) reveals a syn, anti-conformation of the nitroso groups. The rings in compound (18) adopt a chair, twist-boat conformation. 

Analysis of the x-ray results revealed that all of the stereocenters for compound 134 were as depicted in Scheme 33. The BC-rings were shown to adopt a chair/twist boat conformation to avoid 1,3-diaxial interactions between the methyl group and the benzyloxy unit. A similar conformation was reported for the C-ring in the case of the trans-fvLStd pyranopyran framework [4]. The configuration of the newly formed stereocenter at C in structure 134 is undoubtedly the consequence of a stereoelectronically controlled addition to the cyclic oxonium intermediate, 133. 

Deviations of the force extension curves from FJC or WLC behavior, for example the kink in the force profile shown in Figure 8, often indicate conformational and configurational transitions along the polymer chain. As shown in Figure 8, Li and co-workers detected a force-induced chair-twist boat conformational transition, which cannot be measured by conventional methods, for a-(l,4)-linked polysaccharides (95). 

Examine the geometry of norcarane. What is the conformation of the cyclohexane ring Choose a name (chair, twist boat, half-chair, etc. see Chapter 5, Problem 4) that accurately describes its shape. The bridgehead hydrogens in norcarane are cis. Do you think a trans stereoisomer is possible Explain. 

In addition to the chair conformation of cyclohexane, a second arrangement called the twist-boat conformation is also nearly free of angle strain. It does, however, have both sleric strain and torsional strain and is about 23 kj/mol (5.5 kcal/mol) higher in energy than the chair conformation. As a result, molecules adopt the twisl-boat geometry only under special circumstances. 

Refer to Figure 24-9 for the chair and twist boat conformations of cyclohexane. 

The [6]radialenes are normally observed to have chair conformations164,168 181,182, although a twist-boat conformation has been observed for a very highly substituted... 

Alternatively, lactols react with benzenesulfinic acid in the presence of CaCl2 to yield the sulfones, again at room temperature [307,309]. In the axial series, the bulk of the sulfone group is such that the 4Cj chair is not always the preferred conformation, and it has been shown that a twist-boat conformer is adopted in at least one instance [305]. Nevertheless, equilibration studies have shown that the sulfonyl group has a small anomeric effect and that the axial anomer is preferred [310],... 

This boat-chair difference is not immediately obtainable from experiment, e.g. the high-temperature conformational isomerization—and then quenching—of cyclohexane does not result in a chair — boat interconversion, but rather chair — twist-boat M. Squillacote, R. S. Sheridan, O. L. Chapman and F. A. L. Anet, 7. Am. Chem. Soc., 97, 3244 (1975). 

Also, 2,2,3,6-tetra-Me-5-Cl-l,3-dioxan, due to 1,3-diaxial interactions, prefers the 2,5-twist-boat form (76BSF563) the same conformation was reported for the stereoisomeric 2-Ph-4-(2 -furyl)-5-N02-6-Me-l,3-dioxanes and 2,2,6-tri-Me-4-(2 -furyl)-5-N02-l,3-dioxanes (75MI2), for 2-Alk-2,4,4-tri-Me-l,3-dioxane derivatives (78KGS1172) and for the cis isomers of 2-OR-4-Me-l,3-dioxane (R = Et, nPr, /Pr, nBu, n-CsHn) (81DOK116). The corresponding trans isomers adopt the chair conformation with di-eq substitution. The isomeric 2-OR-4,4-di-Me-l,3-dioxanes also prefer the 1,4-twisted-boat conformer (81DOK116). 

On the basis of NMR and IR spectra and the kinetics of the quaterniza-tion (see Section 4.2.5), homoberbine stereoisomers 169 are assigned the twist-boat conformation 169 and the pseudo-chair conformation 169, respectively (71T1363). 

According to molecular mechanics calculations, the minimum-energy conformation of the enolate is a twist-boat conformation (because the chair leads to an axial orientation of the t-butyl group). The enolate is convex in shape, with the second ring shielding the lower face of the enolate, and alkylation therefore occurs from the top. 

In general, things are simpler than that, much to our advantage. Within the limits set by the precision of the present estimates, structural features like the chair, boat, or twist-boat conformations of cyclohexane rings, as well as the butane-gawc/ze effects or the cis-tmns isomerism of ethylenic compounds leave no recognizable distinctive trace in zero-point plus heat content energies. Indeed, whatever residual, presently... 

The overall barrier to ring inversion is well established experimentally, and is believed to correspond to the energy difference between chair and half-chair structures. Less certain are the relative energies of chair and twist-boat conformers and the energy of the boat transition structure, although a small range of values for each of these quantities has been established experimentally. Comparison of the experimental data with the results of calculations is provided in Table 8-5. The... 

Cyclohexane minimizes its ring strain by being puckered rather than flat. The two extreme conformations are the more stable chair and the less stable boat. The twist-boat conformer is less stable than the chair by about 23 kJ/mol, but is more stable than the boat. It is formed from the boat by moving one flagpole" to the left and the other to the right. See Fig. 9-5. 

Probtom 9.14 In terms of eclipsing interactions explain why (a) the chair conformation is more stable than the boat conformation and (A) the twist-boat conformation is more stable than the boat conformation. <... 

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