Subject conformational inversion

CGTase is a kind of glycoside hydrolases, and according to the deference in the conformation of the anomeric carbon atom of glycosidic bonds subjected to hydrolysis, there are two kinds of enzyme catalytic mechanisms One is conformation inversion type (Fig. 2.2(a)), and the other is conformation keeping... 

Dynamic NMR spectroscopic measurements indicate, that the free enthalpies of activation of 57 and 59 are 10.2 kcal/mol and 11.9 kcal/mol, respectively. Similar conformational behavior is exhibited by the [2.2](2,5)heterophanes 43, 61, and 62. The thiophene-containing hetero-phanes 61 and 62 8b82) are conformationally rigid up to 200 °C and are not subject to ring inversion, whereas the furanophane 43 is conforma-... 

It is obvious that independently of the rate of exchange the NMR spectrum of a system subject to a dynamic equilibrium depends on the relative concentrations of the components present. These may be expressed as mole fractions or, in the case of intramolecular exchange, as conformer populations. Sometimes the populations are governed by symmetry effects alone and are independent of the experimental conditions. This is always the case for mutual exchange when the process does not alter the chemical identity of a molecule. Examples of such exchange may be drawn from rotations of the t-butyl group in (CH3)3C-C(R1)(R2)(R3), and those about the CO-N bond in N,N-dimethyl-substituted amides (CH3)2N-C(=0)R. Other examples include ring inversions such as those in 1,2-ds-dimethylcyclohexane, 1,3-trans-... 

It has been observed that addition of Lewis acids to the free radical allylation improved the chemical yield [101]. When substrates with a chiral auxiliary were subjected to free radical allylation in the presence of a Lewis acid, the desired allylated products were obtained with high stereoselectivity [94 d]. In these reactions the Lewis acid plays a pivotal role in fixing the conformation of radical intermediates. Recently Sibi indicated that an elevated reaction temperature accelerated inversion of the stereochemistry of the radical-centered carbon giving rise to greater diastereoselectivity (Scheme 12.39) [102]. When enantiomerically pure Lewis acids were employed as chiral auxiliaries enantioselective free radical allylation of sulfones [103] and oxazolidinones [104] were realized. In the latter reaction two contiguous chiral centers were generated successfully in a single operation with excellent stereoselectivity via tandem C-C bond formation both enantiomers can be se-... 

The subject of macrocyclic conformation is complex and will be illustrated here by reference to Kyba s systems. These 11- " and 14-membered phosphorus macrocycles exist in various conformations. The barrier to inversion for phosphines is approximately 146.4kJmol , but some conformations are remarkably stable. For example, 2,6,10-triphenyl-2,6,10-triphosphabi-cyclo[9.4.0]pentadeca-l l(l),12,14-triene (11) has three conformers (a-c in Figure 1). When a xylene solution of (11) is refluxed (135 °C, 1 h), only (a) is obtained when crystallized. The phenyl groups at the 2-P and 10-P remain cis, although the 6-P does invert, but the crystallization process favours conformer (a) over (b). The equilibrium (a) (b) is displaced to the LHS by removal of (a) as it... 

One much-studied subject is the ring inversion of cyclohexane and polymethylcyclo-hexanes, as shown in Table 8. The subject has been reviewed " but, very generally, compounds with flattened chair conformations due to methyl-methyl 1,3-diaxial interactionshave lowered barriers. Simply substituted compounds where rotation about substituted bonds can take place in the boat-twist manifold of conformations have barriers similar to that of cyclohexane, while more highly substituted compounds, where rotation about substituted bonds must take place on the way to the transition state have markedly higher barriers than in cyclohexane. 

Process of ring inversion in molecules 2-8 was subject of several experimental studies by NMR [38, 39] and IR [42-45] spectroscopy. Value of inversion barrier was determined within 4-5 kcal/mol. It was suggested that ring inversion proceeds via boat or twist boat conformation as saddle point. 

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