Conformational analysis twist boat

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

An interesting case of conformational analysis comes to play when we consider a six-membered ring (cyclohexane). There are many conformations that this compound can adopt. You will see them all in your textbook the chair, the boat, the twist-boat. The most stable conformation of cyclohexane is the chair. We call it a chair, because when you draw it, it looks like a chair ... 

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. 

According to the foregoing analysis, conformers such as gauche- and anri-butane or chair and twist-boat cyclohexane would be considered to be diastereomers of each other. However, under most conditions these conformers interconvert so rapidly that butane and cyclohexane are considered to be single species and not mixtures of stereoisomers. When we have to write chemistry books for people living on the outer planets of the solar system, we might have to modify these concepts. 

These cyclohexane-like rings are characterized by the presence of various non-planar conformations chair, boat, and twist conformations. The starting point when dealing with the stereochemistry of these six-membered rings is therefore conformational analysis. The method of choice is NMR spectroscopy as a result the preferred conformer(s) and the axial/equatorial position of attached substituents at the preferred conformer(s) are obtained. 

Configurational and conformational assignment of cis/trans-2-tBu-5-MQ-sulfonyl-l,3-dithiane, based on lH NMR analysis (esp. 3/h-5,h-4,6 = 6.0 Hz), revealed that the trans isomer adopts the chair, and the cis isomer a twist-boat conformation in order to minimize steric and electrostatic repulsive interactions of the axial S02Me group (cf. Scheme 23) (02TL 9369). Chemical equilibration shows the trans isomer to be more stable by AG°323k = — 1.50 0.04 kcal/mol. 

X-Ray crystallography studies on 1,2-diazocines are limited to six derivatives. A single crystal X-ray analysis was performed on 3,8-diphenyl-6-(3-phenylisoxazol-5-yl)-6,7-dihydro-577-[l,2]diazocin-4-one 1 and confirmed the results obtained with theoretical calculations conducted on a model of 1 in which Ar= R= H (vide infra). In fact, the major contribution to the thermodynamic isomer is given by the twist-boat-chair conformer having the substituent in position 6 in a pseudoequatorial orientation relative to the eight-membered ring <2006JOC2480>. 

D-(-f )-Glucose contains the six-membered, pyranose ring. Since the C—O—C bond angle (11 L) is very nearly equal to the tetrahedral angle (109.5°), the pyranose ring should be quite similar to the cyclohexane ring (Sec. 9.14). It should be puckered and, to minimize torsional and van der Waals strain, should exist in chair conformations in preference to twist-boat conformations. X-ray analysis shows this reasoning to be correct. 

The X-ray crystallographic analysis of thyrsiferol 18-acetate (5) revealed a strained tetrahydropyran ring C in a twist-boat conformation so as to avoid 1,3-diaxial interactions between the methyl groups at Cio and Cl 5. Initial biological studies of the natural product by Munro et al did not reveal any significant pharmacological activity [4]. 

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. 

The stereoselectivity in hydrogen-abstraction reactions of carbohydrate radicals differs from the reactions of mono- or disubstituted carbocyclic radicals in some characteristic ways. Generally, the ring oxygen atom leads to pronounced higher axial attack if located next to the radical center. At the same time, it has been observed that some of these carbohydrate C-l radicals prefer a twist boat to a chair conformation44 4b. Furthermore, the large number of substituents present simultaneously make the conformational analysis of axial and equatorial transition states more difficult. 

In 1,1,4,4-tetramethylcyclohexane (50) the rate of ring inversion has been studied by line shape analysis of the methyl resonance at various temperatures. The results are compared with those obtained for the structurally similar acetone peroxide (51). In these two systems (50) and (51), the equilibria are between two identical chair conformers and single coalescence temperatures are observed ( —61°C and — 66°C at 60 Mc./sec., respectively). In duplodithioacetone (52) the more stable twist-boat conformer enters into the equilibrium, and... 

Conformational analysis originated about 100 years ago as the result of the pioneering suggestion by Sachse (1) that six-membered saturated rings are not planar but exist in puckered shapes, such that all the valence angles are tetrahedral. He realized that two forms of cyclohexane free of angle strain are possible (now they are called chair C and twist-boat TB conformers) and... 

The crystal structure analysis of 2 (Fig. 4) revealed a central SiaTea slx-membered ring in a slightly twisted boat conformation. This is the first crystal structure of a compound with an SiaTea ring. Important bond lengths and angles are given in Table 3. 

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