Organic chemistry fragmentation scheme

Organic chemists have been aware of reversible addition-fragmentation involving xanthate esters in organic chemistry for some time. It is the basis of the Barton-McCombie process for deoxygenation of alcohols (Scheme 9.37).402 404... 

Ortho-Effect. The ortho-effect is one of the most widely known structural phenomena in organic chemistry. It is widely used in organic chemistry for synthetic purposes. The mass spectra of the majority of ort/jo-substituted aromatic compounds possess significant differences in comparison with the spectra of their meta- and para-isomers. A classic example of the ortho-effect in mass spectrometry involves fragmentation of alkylsalicylates. The intense peaks of [M - ROH]+ ions dominate in the El spectra of these compounds. These peaks are absent in the spectra of their meta- and para-isomers. The reaction leading to these ions may be represented by Scheme 5.12. 

Isocyanides, formal divalent carbon functionalities, are ideal candidates for the development of MCRs. Their reaction with carbonyls and imines, through an a-addition process, generates a zwitterionic intermediate, which is then trapped by a nucleophile. The resulting double a-addition adduct is unstable and rapidly undergoes the Mumm rearrangement to afford the final product (Scheme 12.32). The venerable three-component Passerini reaction is the first MCR based on this type of reaction process [116]. It addresses the formation of a-acyloxycarboxamides, which constitute a class of very versatile synthons in organic chemistry. In the present context, this reaction was utilized by Schmidt and collaborators for the elaboration of intermediate 234 [117], a key fragment for the synthesis of the prolyl endopeptidase inhibitor Eurystatin A 231 (Scheme 12.33) [118]. 

Interestingly, a cascade alkoxyl radical fragmentation-peroxidation-hydrogen abstraction reaction occurs in some cases when a hemiacetal is treated with DIB/I2 under oxygen pressure. This reaction may have interesting applications in synthetic organic chemistry. We have used it in a one-step synthesis of A and A rings of the tetranortriterpene limonene and related compounds (Eq. 19, Scheme 6) [48]. 

Formally, vinylidene complexes 8.142 can be considered as complexes of vinylidene carbenes with transition-metal fragments. Free, uncomplexed vinylidene carbenes 8.140 are postulated in organic chemistry as intermediates on the way to terminal and some internal alkynes (the Fritsch-Buttenberg-Wiechell rearrangement) (Scheme 8.39). They can, in some cases, be trapped by nucleophiles. It can be postulated that the equilibrium between the alkyne and the vinylidene carbene, usually entirely in favour of the alkyne, would be less one sided if complexation to a transition metal were involved. 

The fragmentation of simple jS-lactones to carbon-carbon double bonds is a classic reaction in organic chemistry although little used in organic synthesis. Two examples appeared during 1976 illustrating the potential of this fragmentation reaction as shown in Scheme... 

The easiest way to see how particular fragments are isolobal is illustrated in Scheme 21.1. The archetypical molecule of organic chemistry is methane, 21.2. Breaking one C—H bond in a homolytic sense generates the methyl radical, 21.3. It... 

The intensity of activity in the field of thiazole and benzothiazole research reflects the importance of these heterocycles in organic and biological chemistry. There has therefore been a large volume of synthesis literature produced since 1995. The following section has been ordered according to the fragments generated by retrosynthetic analysis, which are shown in Scheme 53. 

In general, olefin metathesis is an organic reaction that allows redistribution of fragments of alkenes by the scission and regeneration of C=C double bonds (Scheme 2.11). The metallic catalysts needed for this reaction have evolved rapidly over the past few decades. The impact of this topic in organic synthesis was demonstrated by the 2005 Nobel Prize in Chemistry awarded to Yves Chauvin, Robert H. Gmbbs, and Richard R. Schrock for elucidation of the reaction mechanism and discovery of highly efficient and selective metathesis catalysts. 

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