Model of synthon reactions

The notion of reaction distance can be similarly interpreted in the framework of graph-theory model of synthons. Let Gjj be the SR-graph of isomerization S(A) — S (A) it can be written as follows [15,16]... 

PLE catalyzes the hydrolysis of a wide range of meso-diesters (Table 2). This reaction is interesting from both theoretical and practical standpoints. Indeed, the analysis of a large range of kinetic data provided sufficient information to create a detailed active site model of PLE (31). From a practical standpoint, selective hydrolysis of y j (9-cyclo-I,2-dicarboxylates leads to chiral synthons that are valuable intermediates for the synthesis of a variety of natural products. 

A synthon reaction is understood as a process when a starting synthon 5(A) is changed to a final synthon 5 (A). It is seen that 5(A) and 5 (A) are members of the same FIS(A), i.e. they have to be isomeric. The synthon reaction is modelled by matrix equation... 

The algorithm for the computation of the reaction distance is one of the most important algorithm of the synthon model of organic chemistry. The Principle of Minimal Reaction Distance is a reaction distance analog of the Ugi s Principle of Minimal Chemical Distance and is used as the main heuristic to reduce the number of transformations employed, and synthons generated. The efficiency of the algorithm for the computation of the reaction distance is crucial for unable implementation of the whole synthon model. 

When comparing the model of the mechanism of the Peterson reaction with the scheme of the Wittig reaction generated by PEGAS, we also can find in its output synthons which are analogous to synthons generated for the Peterson reaction (Scheme 20). 

The theory formulated in this section makes possible efficient connection of the synthon model with the reaction mechanisms. This is of great importance, in particular, since the reaction mechanisms are potentially very attractive for a physicalization of our synthon model. Moreover, the suggested theory might also be of value for the computer generation of exhaustive lists of admissible reaction mechanisms. Of course, this latter point will probably need the introduction of further heuristics to substantially reduce the factorial (or perhaps exponential) explosion of chemically unfavorable, but otherwise admissible reaction mechanisms. 

Although this is the standard approach, in our model we propose to allow the chemical reactions to occur not only on the preselected reaction centre but also to affect the atoms in its immediate neigh- bourhood. In practical applications, it is also very convenient to determine not only the atoms of the reaction centre but also those we want to exclude from the chemical processes (e.g. the skeleton-forming atoms). The condition of conservation of all or part of the skeleton, or of a functional group, is very important, e.g., in the synthesis design of biologically active compounds and the generation of reaction networks. Therefore, we introduce the concept of the set of all SPS of the synthon S(A) with respect to the synthon S(X) reduced by the syn-thon S(X). 

The model outlined above and its simple extension contain a theoretical possibility of how to direct the production of SPS towards the required product synthon. This very serious problem is partially overcome by making use of the concept of reaction distance only those new SPS are taken into account that have a smaller distance than their predecessor with respect to the product synthon. Recently, our activity in this field has been concentrated on an application of tools of logical programming for implementation of some principal rules of chemical reactivity. 

Reactions of simple silanediols and disilanols with titanium orthoesters, titanium halides and titanium amides proceed to give cyclic titanasiloxanes [30]. On the other hand, the silanetriols with three functional OH groups would prove appropriate synthons for constructing three-dimensional titanasiloxanes which would in turn serve as model compounds for catalytically useful Ti-doped zeolites [32]. The synthesis of cubic titanasiloxanes has been achieved in two ways. 

In model studies related to the synthesis of compactin (97) and mevinolin (98), the upper-half lactone moiety was constructed from 550 starting with an oxirane ring-opening reaction by vinyl Grignard reagent (Scheme 81) [136]. lodocarbonation of 560 followed by hydrolysis and ketalization affords isomerically pure acetonide 562, the compactin lactone synthon. 

Trimethylsilylhomopropargylmagnesium chloride was used as a masked 2-butanone synthon, on reaction with 95. Hydration with concomitant desilylation, followed by oxidation of the resultant hydroxy ketone and base cyclization yielded the cyclopentenol (118) after reduction. Despite success on the model system, cyclization of the cyclopentenol (118) proceeded only in low yield, to give the olefins 119 and 120. Oxidative ring enlargement " of the major isomer 119 formed the target enone 121, completing the alternate formal total synthesis of alnusenone. Both 119 and 120 provided 122 on demethoxylation. 

Labelle M, Guindon Y (1989) Diastereoselective synthesis of 2, 3-disubstituted tetrahydrofuran synthons via the iodoetherification reaction. A transition state model based rationaU-zation of the allylic asymmetric induction. J Am Chem Soc 111 2204—2210... 

General reviews include the direct aldol/" aldoi and related processes,the Zimmerman-Traxler TS model used to explain the stereochemistry of the aldoi condensation,catalysis of direct asymmetric aldols by prolinamides versus prolinef/zioamides, " " the catalytic asymmetric aldoi reaction in aqueous media (considering both organometallic and organocatalytic approaches), " the use of BINAP oxide in enantioselective direct aldols,and the use of metal enolates as synthons. " ... 

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