Formation via fragmentation

Single alkene diastereomers are accessible through a Wittig-Homer reaction only if it is performed in two steps (Figure 11.10). A 1 1 mixture of the phosphorylated lithium alkoxides syn- and anti-D is still formed but if the mixture is protonated at this point, the resulting phosphorylated alcohol diastereomers C can usually be separated without difficulty. The suitable diastereomer will be deprotonated with potassium-ferf-butoxide in the second step and then be converted into the stereouniform trans- or cis-alkene E via stereospecific oxaphosphetane formation and fragmentation. 

C. Cyclopropyl Group Formation via Addition of a One-carbon Fragment from... 

However, the lowest energy triplet state of cyclic enones is a 3jt,jt state, not a 3n,Jt state,761 and it is now believed that the regioselectivity in adduct formation rather reflects differences in the efficiencies of cyclization to products and the efficiencies of regeneration of the starting material via fragmentation of the singlet 1,4-biradicals (Scheme 6.63). For example, [2 + 2] photocycloaddition of cyclopentanone... 

From these results a stepwise mechanism has been deduced for the [1,3] rearrangement via fragmentation-recombination, with an intramolecular Michael addition of a cyclohexenyl anion to a conjugated cyclopentenyl moiety in the second stage1182. The exclusive formation of the regiomer 16 in both anionic oxy-Cope processes of 15a and 15b is probably due to nonbonded strain in the alternative intermediate 30 b. 

In another example, Postel and co-workers used functionalized carbohydrates to produce P-substituted polyoxygenated furans. Treatment of functionalized benzylidene precursors with TMS-N3 in the presence of dibutyltin oxide gave the corresponding furan in 10% yield. The authors used a computational approach to rationalize furan formation via a series of cascade fragmentation reactions obtained during 1,5-C-H insertion from alkylidenecarbenes. 

Photochemistry offers a convenient access to the formation of aryl—alkyl bonds under mild reaction conditions. This can be accomplished either via fragmentation of an aromatic derivative to produce a trappable aryl radical or aryl cation as intermediate, or via the activation of an aliphatic component that then reacts with the arene derivative. 

Alkenes (olefins) Rather high intensity (loss of 7i-electron), especially in the case of cyclic olefins Loss of units of general formula C H2n i formation of fragments of the composition C H2 (via McLafferty rearrangement) retro Diels-Alder fragmentation... 

The purine ring, on the other hand, is synthesized from metabolic fragments from five different sources. The carbon atoms at positions 2 and 8 are derived from formate via tetrahydrofolate (page 165). Folic acid antagonists interfere with nucleic acid biosynthesis and are sometimes used in the treatment of cancer. C-6 is derived from carbon dioxide with biotin (page 166) acting as a carrier. 

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