Cis interconversion

The dynamic NMR (DNMR) spectroscopy has been used in studies of stereomutations of non-symmetrical di-Schiff bases [18].39 It was shown that the hindered Schiff bases exist in DMSO in two chiral conformations. The presence of a pair of conformers being in equilibrium was explained by the existence of two stereogenic axes a g (aligned to Cl—N8 bond) and a 7 (aligned to C6—N7 bond) due to restricted rotation around two Ar—N bonds. The trans to cis interconversion as well as enantio- or diastereoisomerisation barriers for the compounds studied have been established using line shape analysis. 

Fig. 2.1 Tetrahydrofolate and its role in Ci interconversions and transfers. A. Chemical structures of THF and its Ci-substituted derivatives. B. Major Ci unit interconversions and transfers involving THF derivatives. Sources of Ci units are shown in boxes and their metabolic fates in gray.

The analogy of these dimerization processes to thermal Diels-Alder type reactions which sometimes also yield cyclobutane structures is worth noting and may be taken as one of the arguments for a diradical structure of the transition in the latter process. Also, it may be pointed out that the photoexcited state involved is presumably the same one involved in the well-known photochemical trans-cis interconversion of such olefins. 

Reducing R to 0.7 results in the trans octahedral structure existing as an even deeper minimum (Figure 25). Conversely, increasing R to 1.5 stabilizes the cis structure and, in addition, results in a much flatter potential energy surface (Figure 26). It should be noted that a cis to trans interconversion, or a cis to trans to cis interconversion, is possible, but not a direct cis to cis interconversion. 

Higher selectivities are generally obtained in the presence of a Lewis acid as the reactive conformation is essentially locked into place. The reaction in the absence of a Lewis acid, in contrast, is prone to rotamer control problems as the. v-trans to s-cis interconversion can result in lower selectivities depending on the size of substituents and reaction temperatures. Larger substituents and lower reaction temperatures counteract the s-trans to s-cis rotation. 

Not a minimum but a transition state for the degenerate cis-cis interconversion C1SD(Q)/TZP value " G3(MP2) value... 

Thrl67 bond in the cis Prol67Thr TEM-1 /J-lactamase variant is also characterized by a rate constant between 1 x 10-3 s-1 and 4 x 10 3 s-1 for the trans to cis interconversion [26]. Therefore the trans to cis isomerization can be rate-limiting in protein folding under native-like conditions, as was shown for a proline-free variant of the a-amylase inhibitor tendamistat [27]. This seems to be proven in the discovery of a novel protein dass, the secondary amide peptide cis/trans isomerases (APIases), which can accelerate interconversion of these peptide bonds conformers [28],... 

Recendy, Sneddon and Brooks (39), on the basis of their CHARMM simulations of conformational dynamics of Pro peptides in aqueous solution, have postulated involvement in electron transfer across the -(Pro) -bridge of P — a transitions at the i i angle, as the latter occur more rapidly and bring the donor-acceptor distance to a shorter range than the trans cis interconversion... 

Liu, Z. R, Flashimoto, K., and Fujishuna, A. A Novel Electrochemical Quantification Method for trans/cis Interconversion of Azo Compounds in a Solid Monolayer Him, Chem. Lett. 2177 (1990). 

Because of the interest in stereochemistry of 1,3-biradicals, Chang pyrolyzed optically active rran5-l,2-dimethylspiropentane and. yy -4,4-dideuterio-c/5-l,2-dimethylspiropentane and determined the rate constants for the thermally induced interconversions shown in Scheme 6.34. With the cis isomer, double inversion (cis to cis interconversion) is preferred by a factor of 1.8 over the two single inversion (cis to trans conversion) pathways, but there is little preference for double inversion over single inversion with the trans isomer. Similar observations were made with the 1,2,4-trimethylspiropentanes. ... 

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