Fragmentation reactions reductive

Fragmentation reactions sometimes accompany Clemmenson reduction. See Bailey, K.E. Davis, B.R. Aust. J. Chem., 1995, 48, 1827. Also see Rosnati, V. Tetrahedron Lett., 1992, 33, 4791. 

It should be noted that the Grob fragmentation reaction and the reductive cyclization (homoallylation) discussed in this section involve the same oxanickellacyclopentane 66 as a common intermediate (Scheme 17). The reversibility of these C - C bond cleavage reaction and C - C bond formation reaction is also supported by the isolation and characterization (by X-ray analysis) of an oxanickellacyclopentane-like 66 (without a tether), which is prepared from a stoichiometric amount of Ni(cod)2, a diene, an aldehyde, and a monodentate phosphine ligand [41]. 

A new formal synthesis of ( )-zizaene (136) via the fragmentation reaction is a typical example 45,46). When the enol acetate (132) was irradiated, the isolable photoadduct (133) was formed together with other isomers. Reduction and mesylation of which afforded the mesylate (134). Then, fragmentation reaction would yield an epimeric mixture of the alkenone (755) in the presence of NaOH in aqueous dioxane at 60 °C, thus formulated a total synthesis of (+)-zizaene (136) 45-46). 

Radical chemistry has seen tremendous progress in the past two decades and can now be considered as an eminent sub discipline in synthetic organic chemistry [1-6]. Diastereoselective radical chemistry is well established and many examples of enantioselective radical reactions have appeared in the recent literature. For reviews on diastereoselective radical chemistry see [7-11] for reviews on enantioselective radical chemistry see [12-16] and for reviews on conjugate additions, see [17,18]. This review will detail different ways to introduce asymmetry during a radical reaction. These transformations can be broadly classified into atom transfer reactions, reductive alkylations, fragmentations, addition and trapping experiments, and electron transfer reactions. 

Baeyer-Villiger reaction, 9, 3 43, 3 Bamford-Stevens reaction, 23, 3 Barbier Reaction, 58, 2 Bart reaction, 2, 10 Barton fragmentation reaction, 48, 2 Bechamp reaction, 2, 10 Beckmann rearrangement, 11, 1 35, 1 Benzils, reduction of, 4, 5 Benzoin condensation, 4, 5 Benzoquinones ... 

KOH-ethylene glycol. This latter reaction apparently involves an unusual base-catalysed Beckmann fragmentation and reduction of the resultant aldehyde by KOH-ethylene glycol. 

The hydroperoxide (two isomers) is formed in high yield under these conditions (the nature of the reduction equivalent that is required for this process is as yet not known), and subsequent treatment with a base gives rise to glycolic acid and the base propenal [Grob fragmentation reaction (292) yields near 90%]. 

Fragmentation Reactions Triggered by Carbonyl Reduction with Sml2... 

Other than p-hydride elimination, another important pathway by which palladium(II) intermedi ates can lead to neutral organic fragments is reductive elimination. This forms the basis of the mech anism for cross-coupling reactions between an organometallic reagent and an organic halide o triflate. 

This modification resulted in a yield improvement for the pentacyclization process from 47 % to 66 %. Treatment of the amino ether 192 with diisobutylaluminum hydride in refluxing toluene accomplished Eschenmoser-Grob fragmentation and reduction of the initially formed immonium ion, to give the unsaturated amino alcohol 193 in 86% yield. It was gratifying to find that 193 was the only product formed in this reaction. In the tetrahydropyran derivative, reduction of 192 to 193 is accompanied by about 15 % simple elimination. Displacement of the tosyl group in 196 gives sulfide 197, which is oxidized to sulfone 198. This material is metallated and coupled with enantiomerically pure aldehyde to secure the codaphniphylline skelton [74]. 

Reductive Grob-like fragmentation. Reaction of the mesyloxy alcohol 1 with LiAlHi in refluxing DME gives 2 in almost quantitative yield. The corresponding keto mesylate is also converted into 2 in high yield under the same conditions. This fragmentation was used for a total synthesis of racemic -y-elemene 3. 

Although a previous chapter in this volume provides a broader perspective on the reactivity of radical cations, in this section we will examine intramolecular electron-transfer reactions coupled with or followed by cleavage of a bond in odd electron species such as radical cations, radical zwitterions and radical anions. In particular, this paragraph will be divided in oxidative and reductive bond-cleavage processes. Because this field is however too large to be covered extensively here, the discussion will be limited to selected examples—for oxidative cleavages, side-chain fragmentation reactions of alkylaromatic radical cations and decarboxylation reactions of radical zwitterions derived from benzoic and arylalkanoic acids, and for reductive... 

This chapter discusses free-radical substitution reactions. Free-radical additions to unsaturated compounds and rearrangements are discussed in Chapters 15 and 18, respectively. Fragmentation reactions are covered, in part, in Chapter 17. In addition, many of the oxidation-reduction reactions considered in Chapter 19 involve free-radical mechanisms. Several important types of free-radical reactions do not usually lead to reasonable yields of pure products and are not generally treated in this book. Among these are polymerizations and high-temperature pyrolyses. 

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