Remote centres

The final aldol reaction used in our synthesis of spongistatin 1 was one of the more remarkable reactions of this type our group has witnessed over the years. The aldol union of ketone 64 with ( )-4-chloro-2,4-pentadienal 65 required the creation of the (475) stereochemistry in the resultant alcohol 66. Formally, this would require 1,5-syn induction from the ketone 64, which is opposite to that observed previously for boron aldol reactions with simple [i-alkoxy methyl ketones. However, ketone 64 is densely packed with stereocentres, and predicting the influence of these remote centres on the reaction outcome was not possible with any degree of certainty. It was hoped that should 64 display undesirable 1,5-anti bias, this may be overturned by appropriate choice of Ipc ligands on boron. 

A substituent in the steroid molecule can influence reactions at remote centres in three distinct ways, which may be classified as (i) inductive effects (2) electrostatic field effects and (3) conformational transmission. It is difficult to disentangle these effects completely, for all are likely to be operative to some extent, even though for a partioolar reaction one or another may be dominant. The limited number of detailed investigations so far reported make it unwise to generalise at present. 

The requirement for such a large excess of 224 suggests that a second molecule binds to the OH group thus shielding one face of the ketone from reduction 226. Though asymmetric reduction would now be preferred to this method it is remarkable that such a remote centre (and the OH is five atoms from the carbonyl group) exerts such a powerful effect. 

Instead of sending the two original images for analysis to a remote centre for quality supervision, a smaller package of data is sent, which includes the SNR in a specific ROI, the thumbnail images, a selection of DICOM header tags (for DR systems) or manual input such as anode/filter combination, peak tube voltage and tube current for CR with incomplete DICOM headers, a few line profiles and noise power spectra of six specific ROIs. 

The thermal rearrangements of optically active 1,5-dienes have also been studied, and the products found to have high optical purity. Where mixtures are formed, the major product is as predicted from the above steric considerations. It is thus not only possible to generate chirality at a remote centre, but also to be certain about the absolute stereochemistry at the new asymmetric carbon atom. 

Comparison of the water-induced acceleration of the reaction of 2.4a with the corresponding effect on 2.4g is interesting, since 2.4g contains an ionic group remote from the reaction centre. The question arises whether this group has an influence on the acceleration of the Diels-Alder reaction by water. Comparison of the data in Table 2.1 demonstrates that this is not the case. The acceleration upon going from ethanol to water amounts a factor 105 ( 10) for 2.4a versus 110 ( 11) for 2.4g. Apparently, the introduction of a hydrophilic group remote from the reaction centre has no effect on the aqueous acceleration of the Diels-Alder reaction. 

Studies of the Diels-Alder reaction of the ionic dienophile 2.4g have demonstrated that the acpieous acceleration of the uncatalysed reaction as well as the catalysed reaction is not significantly affected by the presence of the ionic group at a site remote from the reaction centre. 

Remote supervision and automatic load-control of generating units, load dispatclt centres and suh-siatioris to maintain the desired operating parameters, including load sharing. 

So important are lattice imperfections in the reactions of solids that it is considered appropriate to list here the fundamental types which have been recognized (Table 1). More complex structures are capable of resolution into various combinations of these simpler types. More extensive accounts of crystal defects are to be found elsewhere [1,26,27]. The point which is of greatest significance in the present context is that each and every one of these types of defect (Table 1) has been proposed as an important participant in the mechanism of a reaction of one or more solids. In addition, reactions may involve structures identified as combinations of these simplest types, e.g. colour centres. The mobility of lattice imperfections, which notably includes the advancing reaction interface, provides the means whereby ions or molecules, originally at sites remote from crystal imperfections and surfaces, may eventually react. 

The synthesis of (23) illustrates how a six-membered ring may bo used to control even more remote chiral centres. Reverse Michael disconnection leaves enone (24), an oxidation product from allylic alcohol (25). The double bond can come from elimination on bromohydrln (26) and hence from (27). 

By convention, the carbenes displayed in Fig. 1.7 are normal NHCs as they are coordinated to the metal centre throngh the atom. By contrast, abnormal (also named non-classical or unnsnal) are those bound through the atom. Abnormal is also a term used for NHCs having a valence representation requiring additional charges. Remote is a term nsed to describe a carbene which does not have any heteroatom on the a-position to the carbenic carbon (Fig. 1.8) [57]. 

Gourtieu, J. Deuterium NMR stereochemical analysis of threo-erythro isomers bearing remote stereogenic centres in racemic and non-racemic liquid crystalline solvents. Tetrahedron Asymmetry 2000, 11,1911-1918. 

Also in remote areas, such as low-populated islands, without any access to the electricity grid and for using large renewable resources, which are far away from any user centres (so-called stranded resources), hydrogen could be an attractive solution (see also the discussion in Chapter 17). 

Handley fluidised soda glass particles using methyl benzoate, and obtained data on the flow pattern of the solids and the distribution of vertical velocity components of the particles. It was found that a bulk circulation of solids was superimposed on their random movement. Particles normally tended to move upwards in the centre of the bed and downwards at the walls, following a circulation pattern which was less marked in regions remote from the distributor. 

Fat is stored in the body in separate depots, most of which need to be remote from muscles, so as not to interfere with their function. A considerable amount is stored around the waist or hip where it does not affect the centre of gravity of the body, which is important in locomotion. Consequently, the fat fuel requires transport from the site of storage to the tissue of utilisation, via the blood, which requires specific transport mechanisms (see above). 

---