Synthetic strategy examples

Another synthetic strategy is based on self-assembly driven by molecular recognition between complementary TT-donors and 7T-acceptors. Examples include the synthesis of catenanes and rotaxanes that can act as controUable molecular shuttles (6,236). The TT-donors in the shuttles are located in the dumb-beU shaped component of the rotaxane and the 7T-acceptors in the macrocycHc component, or vice versa. The shuttles may be switched by chemical, electrochemical, or photochemical means. 

In several important cases, new synthetic strategies have been developed into new production schemes. An outstanding example of this is the production of an entire family of terpene derivatives from a-pinene (29), the major component of most turpentines, via linalool (3) (12). Many of these materials had been produced from P-pinene, a lesser component of turpentine, via pyrolysis to myrcene and further chemical processing. The newer method offers greater manufacturing dexibiUty and better economics, and is environmentally friendly in that catalytic air oxidation is used to introduce functionality. 

On the basis of the examples addressed thus far, it is clear that radical reactions can accomplish manifold transformations in organic synthesis. One of the outstanding achievements of synthetic radical chemistry is the development of synthetic strategies based on controlled, tandem radical cyclizations. The efficiency of such strategies is exemplified in the substantial and elegant synthetic work of D. P. Curran and his group.54 The remainder of this chapter will address the concise total syntheses of ( )-hirsutene [( )-1]55 and ( )-A9(12)-capnellene [( )-2]56 by the Curran group. 

In developing synthetic strategies for macrocycles related to corrins, one initial choice has to be made, i.e. whether to incorporate the direct link between rings A and D or to generate it in the final cyclization step. Examples of both approaches have been reported, but syntheses in which the direct link is formed in the final cyclization are more numerous. The total syntheses of vitamin B12 by Woodward/Eschenmoser and Eschenmoser are prominent examples for the realization of both strategies. 

Nature has created a diverse array of plant and animal toxins that act at mammalian muscle and ganglionic nAChRs or invertebrate nAChRs because the critical physiological functions of these receptors make them prime targets for defensive or predatory strategies. More recently, the perceived validity of neuronal nAChR as therapeutic targets has prompted the generation of new synthetic ligands. Examples are listed in Table 1. 

To rapidly construct complex structures, a recent synthetic strategy uses the Diels-Alder cycloaddition in sequence with another Diels-Alder reaction or with other reactions without isolating the intermediates (domino, tandem, cascade, consecutive, etc., reactions) [4-6]. Scheme 1.2 illustrates some examples. 

Other examples of this synthetic strategy are known for example, a recent zirconium polymer by Illingsworth and Burke (8), who joined amine side groups of a zirconium bis(quadridentate Schiff-base) with an acid dianhydride to give amide linkages. Once again, caution is necesary, as Jones and Power (2) learned when they attempted to link metal bisO-diketonates) with sulfur halides that is, they obtained insoluble metal sulfides because the p-diketone complexes which they used were fairly labile and the insolubility drove the reactions to completion in the wrong direction. 

Numerous synthetic strategies for "preceramics" based on polysilanes are known, but the synthesis of Si-Si chains with different substituents on the silicon skeleton is difficult. In the following the advantages of the triflate method for the synthesis of special substituted oligosilanes are shown by some examples. 

A final example is the synthesis of H-shaped copolymer of (PS PEG (PS)2 by ATRP, i.e. [209]. The synthetic strategy involves the synthesis of 2,2-bis(methylene a-bromopropionate) propionyl chloride (1), the preparation of 2,2-bis(methylene a-bromopropionate) propionyl-terminated poly(ethylene glycol) (BMBP-PEG-BMBP) (2), and then ATRP of styrene at 110 °C with BMBP-PEG-BMBP/CuBr/2,2/-bipyridine as the initiating system. The structure (3) was configured by using NMR and SEC measurements (Scheme 116). 

Considering the entire synthesis illustrated in the previous section, clearly the construction of such a complicated molecule with all the desired stereogenic centers is highly tedious and demanding work. Therefore, an entirely different conceptual method based on double asymmetric induction was finally developed as a less complex synthetic strategy. A good example is the synthesis of 6-deoxyerythronolide B (28), which bears the same 10 chiral centers as eryth-ronolide A (compound la of the previous section). 

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