Target molecules structures

Alterations in target molecule structure Use of partial structure... 

Formation of two bonds between a four-atom fragment and a group IV element atom provides a route to various germanium, tin and lead heterocycles, especially to 1,3-diheterametallacyclo-pentanes and -cyclopentenes. Different types of reactants and reactions are used depending on target molecule structures. 

Factors influencing the rates of H abstraction by alkoxyl radicals are H abstract-ability on target molecules > structure of the alkoxyl radical > solvent system (298). Hydrogen availability and solvent have critical effects in lipid oxidation ... 

Greer, J., Erickson, J.W., Baldwin, J.J., Varney, M.D., 1994. Application of the three-dimensional stmctures of protein target molecules — structure-based drug design. J. Med. Chem. 37, 1035-1054. 

MW fraction of reagents that absolutely do not end up in the final target molecule structure. Hypsicity index, HI... 

As an example, let s analyse the synthesis of y-lactones (e.g. TM 334) and see how we may choose one of a number of strategies depending on the structure of the target molecule. We ll consider in turn each of the three C-C bond disconnections. The one with the most appeal is probably b complete the analysis for this approach. 

We have considered the following structural features affecting the choice of antitheticaf steps starting from simple target molecules ... 

An important concern to chemists is synthesis the challenge of preparing a particular compound m an economical way with confidence that the method chosen will lead to the desired structure In this section we will introduce the topic of synthesis emphasiz mg the need for systematic planning to decide what is the best sequence of steps to con vert a specified sfarfmg mafenal fo a desired producf (fhe target molecule)... 

The reactions described so far can be carried out sequentially to prepare compounds of prescribed structure from some given starting material The best way to approach a synthesis is to reason backward from the desired target molecule and to always use reactions that you are sure will work The 11 exercises that make up Problem 6 32 at the end of this chapter provide some opportunities for practice... 

Structural symmetry, either in a target molecule or in a subunit derived from it by antithetic dissection, can usually be exploited to reduce the length or complexity of a synthesis. 

Relationships between stereocenters vary between two extremes. On the one hand, stereocenters may interact strongly in a spatial sense if they are directly joined, proximate to one another, or part of a compact rigid-ring structure. On the other hand, two stereocenters which are remote from one another and/or flexibly connected may be so independent that one cannot be used to provide substrate spatial control for the other. Nonetheless, this latter type of stereorelationship may still be clearable if the target molecule can be disconnected to divide the two stereocenters between two precursors or if an appropriate enantioselective transform is available. 

It is not surprising that multistep synthesis of challenging and complex target molecules is an engine for the discovery of new synthetic principles and novel methodology which may have very broad application. Just as each component of structural complexity can signal a strategy for synthesis, each obstacle to the realization of a chemical synthesis presents an opportunity for scientific discovery. 

Antithetic Analysis. (Synonymous with Retrosynthetic Analysis) A problem-solving technique for transforming the structure of a synthetic target molecule to a sequence of progressively simpler structures along a pathway which ultimately leads to simple or commercially available starting materials for a chemical synthesis. 

Ex-Target Tree. (EXTGT Tree) A branching tree structure formed by retrosynthetic analysis of a target molecule (treetop). Such trees grow out from a target and consist of nodes which correspond to the structures of intermediates along a pathway of synthesis. 

Supra Retron. A structural subunit in a target molecule which consists not only of a complete retron for a particular transform but also of additional structural elements which signal the applicability and effectiveness of that transform. A supra retron contains ancillary keying groups in addition to those of the basic or minimal retron. 

Part Three is intended to balance the coverage of Parts One and Two and to serve as a convenient guide to the now enormous literature of multistep synthesis. Information on more than five hundred interesting multistep syntheses of biologically derived molecules is included. It is hoped that the structural range and variety of target molecules presented in Part Three will appeal to many chemists. 

With these words, E. J. Corey defines for us the concept of retro-synthetic analysis for which he received the Nobel Prize in chemistry in 1990. Nowadays, it has become routine to think about a target molecule in terms of its retrosynthetic analysis. Furthermore, it is hard to imagine how chemists developed synthetic strategies prior to the formulation of these concepts in the 1960s, without thinking, at least subconsciously, in these terms about complex organic structures. 

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