Consonant systems

It must not be forgotten that the retrosynthetic bond-disconnections are only mental processes [27], which may however, coincide with actual processes in the laboratory provided that the reactions under consideration are reversible. Another immediate consequence is that only consonant systems (or molecules) can offer, properly speaking, reasonable bond-disconnection mechanisms, as it is required by "logical bond disconnections". 

The synthesis of dissonant systems is intrinsically more diffficult than the synthesis of consonant systems and because of this chemists have developed, in the past few years, more and more selective and efficient methods for performing it. Table 5.1 gives a summary of the most important methods developed so far, which have been classified into five main strategies ... 

On the other hand, doubly deprotonated nitroalkenes are reagents with a double reactivity inversion (Scheme 5.30) provided they are used to prepare normal 0-, A-derivatives [1]. For instance, the 1-nitrobutadiene dianion 43 reacts with electrophiles to give a mixture of a- and y-isomers, 44a and 44b. Addition of the dianion 43 to 2-cyclohexenone gives only the y-adduct 45 which was transfomed into the 1,7-ketoaldehyde 46 by a Nef-type reaction with TiCl3 [38]. As shown in Scheme 5.30, although the resulting product is a "consonant system" (1,7-C), the... 

In order to fire reliably in a gun or other weapon device, a percussion primer must possess impact sensitivity which is in consonance with the mechanical energy delivered by the firing pin. In all cases, a factor of safety is built into the system in such a manner that the firing pin energy usually exceeds the max energy requirement of the primer by approx 20%... 

Keeping in mind "generalisation 3" concerning the priority of the carbon skeleton over the oxidation state of the functional groups (or even of the carbon chain itself), the synthesis of a consonant molecule (or system) with n bonds between the two functional groups has, in principle, n possible different synthetic pathways. For instance, the 1,3-amino-ketone,... 

As we will see later, in contrast to consonant molecules or systems, the synthesis of dissonant molecules is always a more complex problem, that involves a larger number of steps and intermediates and takes place through essentially irreversible processes, which require more critical and sophisticated experimental conditions. 

Notice that this HP is especially efficient in those cases in which n is equal to 2 and 4, since it means reducing 1,4-D and 1,6-D systems to 4- and 6-membered consonant rings, respectively (see Chapter 6). 

Bifunctional systems In the case of bifunctional systems (or molecules) only two alternatives are possible the bifunctional relationships are either "consonant" or "dissonant" (apart from molecules or systems with functional groups of type A to which we have referred to as "assonant"). In the first case, the synthetic problem will have been solved, in principle, in applying the "heuristic principle" HP-2 that is to say, the molecule will be disconnected according to a retro-Claisen, a retro-aldol or a retro-Mannich condensation, or a retro-Michael addition, proceeding if necessary by a prior adjustment of the heteroatom oxidation level (FGI). 

As in the case of acyclic compounds, the level of difficulty of the synthesis of a cyclic compound depends upon whether the molecule is a consonant or a dissonant system. However, some additional difficulties may be encountered in molecules with medium-sized rings as well as in polycyclic bridged compounds, which are treated in the next Chapter. On the other hand, as we have seen in Heading 4.3, even simple monofunctionalised cyclic molecules may require a FGA operation before bond disconnection of the cyclic network at the ipso-, a- or 13-positions can be effected. 

In contrast, the so-called bis-nor-Wieland-Miescher ketone (2) is a more complex synthetic problem, since the molecule is a multidissonant system with two dissonant bifunctional group relationships (1,4-D and 1,6-D) and two dissonant cyclopentane rings, besides a 1,5-consonant bifunctional group relationship. Its synthesis was only accomplished 30 years after the synthesis of its consonant homologue [5],... 

HP-5 Reconnection of functional groups, such as carbonyl groups which are separated by 2, 3, 4 or 5 carbon atoms, to give 4-, 5-, 6 or 7-membered rings. Of special interest is the reconnection of 1,4-D and 1,6-D systems to the corresponding consonant rings (see below iii-d). 

FINALS, which perform disconnections only if none of the above algorithms could be applied. This usually happens if only one functional group not included in group 2 is present for instance, hydration of an isolated double bond. However, the consonant 1,3-dicarbonyl systems are also included in this group... 

As shown, the disconnection of the consonant 1,3-dicarbonyl system present in 1 would lead to the precursor 2, the acylation of which would present problems of regioselectivity. However, disconnection of the alkyl group at the a-position of the carbonyl groups leads to precursor 3, the alkylation of which would not present any problem of regioselectivity. 

What does seem clear from the new data is that dreaming is driven— strongly—by forebrain systems subserving primary emotions, and that these emotions, are potent shapers of dream plots. The activation of the amygdala, shown by PET studies, is entirely consonant with the phenomenological fact that anxiety is, by far, the leading dream emotion. But anxiety was not a wish for Freud. It was, instead, a symptom caused by... 

So far, only one detailed discussion of boron-11 nuclear magnetic resonance spectra of aminoborane systems has been reported 31>. It was found that the 1 lB chemical shifts of aminoborane systems can be described fairly well in terms of a set of additive substituent contributions. In consonance with earlier work on trisubstituted boron compounds 35> these contributions depend on the mesomeric effects of substituents rather than their electronegativity. 1,3,2-diazaboracycloalkanes can be considered as aminoborane derivatives and in the case of the known heterocycles the exocyclic boron substituent will govern primarily the boron chemical shifts and will do so by mesomeric effects. However, the available data are rather limited and it may be possible that additional factors must be considered. Steric effects appear to be negligible, however, since the heterocycles with either six or seven annular atoms have almost identical shifts (Table 5). 

As for all the systems relegated to Section 2 the attenuation function for structural H2O in the microwave and far-infrared region, as well as that for free H2O, can be understood in terms of collision-broadened, equilibrium systems. While the average values of the relaxation times, distribution parameters, and the features of the far-infrared spectra for these systems clearly differ, the physical mechanisms descriptive of these interactions are consonant. The distribution of free and structural H2O molecules over molecular environments is different, and differs for the latter case with specific systems, as are the rotational dynamics which govern the relaxation responses and the quasi-lattice vibrational dynamics which determine the far-infrared spectrum. Evidence for resonant features in the attenuation function for structural H2O, which have sometimes been invoked (24-26,59) to play a role in the microwave and millimeter-wave region, is tenuous and unconvincing. 

By redirecting the focus from local to global, the concept of chelation is brought into consonance with that of graph theoretical cycles, and thus may be described in the same manner as the system that is used for organic molecules. 

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