Indirect Syntheses

Indirect syntheses are the antithesis of direct syntheses. They are associated with sacriflcial-type reactions such as those itemized in Chapter 8. Reaction types involved are eliminations or fragmentations, subtractive redox reactions, and to some extent rearrangements. 

Plans that are stacked with these kinds of reactions suggest that wrong starting materials were probably used. As a result indirect syntheses necessarily have an increased number of steps to the final target structure. The following corrections are usually found in indirect syntheses  

Elimination reactions required to change bond type between already connected atoms, for example, C-C bond changes to a C=C bond. 

Epimerization reactions required to change the configuration of a stereogenic center. 

Cis-trans isomerization reactions are required—for example, Inhoffen [4] and Roche G1 [5] plans for beta-carotene require heating in the last steps, respectively Al-Hassan [6] plan for tamoxifen requires photoisomerization in the final step. 

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Both direct and indirect syntheses of chloro-l,2,3-triazoles have been carried out. In the former approach, the reagent employed exerts an important influence on the product yield (Eqs. 2,3). A large number of additional combinations were tried but without synthetically useful results. It has also been shown that cyanogen chloride reacts with trimethylsilyl-diazomethane to give chloro-1,2,3-triazoles (Eq. 4) in fair yield. 

Examples of indirect syntheses taken from the synthesis database are the following. 

Chromones containing suitable substituents at C-2 may be converted into chromone-2-carboxylic acids although such indirect syntheses have not been widely used. Methyl, aldehyde or cyano groups are potential precursors of a carboxyl function. It is also possible to obtain a chromone from a chroma-none. 

There are a few examples of indirect syntheses of 2/f-azirines from unsaturated azides. The photolysis of azidoallenes has provided a route to methylene-2/f-azirines The... 

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