Consecutive inversions

The fully saturated pyrazolidines have been utilized as models for the study of the nitrogen inversion of hydrazines. For instance, (75), a 2,3-diazabicyclo[2.2.1]heptene derivative, presents a consecutive inversion process at two nitrogen atoms with an activation barrier... 

Another synthetic approach for streptonigrin is based on two consecutive inverse electron demand cycloadditions. In the first one the A-B-C ring system was formed from a 1,2,4,5-tetrazine and the C-D part was constructed from a 1,2,4-triazine (83JOC621 85JA5745). Another novel total syntheses has also been reported (82JAS36). 

The mechanism of enzymic hydrolysis is still a controversial subject (Sinnott 1990 Legler 1990). There is one point of agreement in that retention is the result of two consecutive inversions, one at the time of the attachment to the enzyme, and the other when the glycosyl is transferred to the acceptor. 

Ethyl lactate can also be converted to (R)-( + )-ethyl 2-chloropropionate (85) by treating it neat with thionyl chloride and a catalytic amount of DMF. By a series of manipulations, shown in Scheme 13, 85 is converted in 7 steps to (— )-Lofexidine (89) [30], a stereoselective a2-adrenoceptor agonist used in the treatment of hypertension. This synthesis takes advantage of two consecutive inversion reactions (2 85 —> 86) to produce the desired stereochemistry... 

A short and convergent formal synthesis of streptonigrin was disclosed by Roger s group in a planned manner. The articulate implementation of the two consecutive inverse electron demand Diels-Alder reactions form the basis of their strategy. The substrate 34 initially obtained from 6-methoxyquinoline,... 

Nucleophilic substitution on configurationally labile halides has been involved in compounds with a bromo or iodo atom in the a-position with respect to a carboxylic acid derivative, in which the Sn2 reaction is governed by a chiral auxiliary placed in the carboxylic moiety. Racemisation takes place by consecutive inversions at the labile centre induced by additives such as polar solvents, bases or halide salts (Scheme 1.1). 

Perkins CJ, Kahya E, Roque CT, Roche PE, Newman GC. Fluid-attenuated inversion recovery and diffusion- and perfusion-weighted MRI abnormalities in 117 consecutive patients with stroke symptoms. Stroke. 2001 32 2774-2781. 

As explained above, these experiments have to be performed under continuous proton decoupling. The usual method is inversion-recovery29 (Figure 6) which would require, between consecutive experiments or consecutive scans, a waiting time of 5T unless one has recourse to a variant dubbed "Fast Inversion-Recovery".30... 

As a final remark it must be mentioned that theoretical and experimental works have been dedicated to investigating the effect of the finite size of the chains [65]. In fact, as grows exponentially, at low temperatures it can become comparable with the distance between two consecutive defects (e.g. impurities and vacancies) which are always present in real systems and hardly separated by more than 103 -104 elementary units. In case of Z < , the nucleation of the DW is energetically favoured if occurring at the boundaries, because the energy cost is halved. However the probability to have a boundary spin is inversely proportional to L thus the pre-exponential factor becomes linearly dependent on L, as experimentally found in doped SCMs. As doping occurs at random positions on the chain, a distribution of lengths is observed in a real system. However, as the relaxation time is only linearly dependent on L, a relatively narrow distribution is expected. 

PA = 226 kcal moP ), the predominant formation (6.4 to 1) of the (7 ,5 )-di-2-butyl ether over the (R,R) and (5, 5 )-forms is attributed to a simple backside displacement in the proton-bound adduct of the starting 2-butanol enantiomer with inversion of configuration of the reaction site and loss of a molecule of water. When tri-n-propylamine is replaced by the less basic NH3 (PA = 196 kcal moF ), fast neutralization of the proton-bound dimers of the starting 2-butanol is prevented and, therefore, they can grow, producing aggregates that resemble solution microenvironments in which SnI pathways may be accessible as well. In them or in their primary substituted derivatives, consecutive nucleophilic displacements may take place. As a consequence, the stereospecificity of the process is lost and the [(R,S)-di-2-butyl ether]/[(7 ,7 )- and (5, 5 )-di-2-butyl ethers] ratio falls down to 1.2. In this case. 

Fig. 1. Top Scheme of an inversion recovery experiment 5rielding the longitudinal relaxation time (inversion is achieved by mean of the (re) radiofrequency (rf) pulse, schematized by a filled vertical rectangle). Free induction decays (fid represented by a damped sine function) resulting from the (x/2) read pulse are subjected to a Fourier transform and lead to a series of spectra corresponding to the different t values (evolution period). Spectra are generally displayed with a shift between two consecutive values of t. The analysis of the amplitude evaluation of each peak from — Mq to Mq provides an accurate evaluation of T. Bottom the example concerns carbon-13 Tl of irans-crotonaldehyde with the following values (from left to right) 20.5 s, 19.8 s, 23.3 s, and 19.3 s.

An alternative route to implement local MC moves is provided by the literature on (inverse) kinematics, such as on control systems for robotic arms composed of flexible joints [27,87]. Here, the problem is transformed to either a set of linear equations [27] or finding the roots of a high-order polynomial [87] at comparable computational expense. One of the benefits of such an approach is the ability to introduce arbitrary stiff segments into the loop, that is, the degrees of freedom used for chain closure do not have to be consecutive. Conversely,... 

Light has both wave-like and particle-like properties. As a wave, it is a combination of oscillating electric and magnetic fields perpendicular to each other and to the direction of propagation (Fig. 3.10). The distance between consecutive peaks is the wavelength, A, and the number of complete cycles passing a fixed point in 1 s is the frequency, v. They are inversely proportional through the relationship... 

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