Grab fragmentations

IOB alone can oxidize some alcohols, but catalysed oxidations are much more efficient. Thus, in the presence of RuCl2(PPh3)2 primary aliphatic alcohols were oxidized cleanly to aldehydes, at room temperature the use of m-iodosylbenzoic acid instead of IOB considerably increased the yields for example, hexanal was formed from hexanol quantitatively (by GC) [19], Another catalytic system involved the use of simple lanthanide salts such as ytterbium triacetate [20]. Cyclic y-stannyl alcohols, readily available from cyclic vinyl ketones and Bu3SnLi, underwent oxidation accompanied by carbon-carbon bond cleavage (Grab fragmentation), when treated with IOB.BF3 and DCC. The products were unsaturated aldehydes or ketones. 

The reaction by which (6) is transformed to (7) and the iminium ion (8) is, in fact, a manifestation of the Grab fragmentation, and is of considerable interest because extensive modification of the tertiary amine structure is achieved in a single operation. 

L.A. Paquette and co-workers accomplished the first total synthesis of the antileukemic agent jatrophatrione. " This natural product has a [5.9.5] fused tricyclic skeleton with a trans-BIC ring fusion. The key step in their approach was the Grab fragmentation to obtain the tricyclo[5.9.5] skeleton. The tetracyclic 1,3-diol was monomesylated on the less hindered hydroxyl group and then treated with potassium fert-butoxide, triggering the concerted fragmentation to afford the desired tricyclic product in almost quantitative yield. 

The addition of strongly polarized carbon-carbon double bonds (enols) to simple iminium ions, known as the Mannich reaction (equation 1), is a fundamentally important route to amines, especially in biosynthetic processes.12 This reaction type is less synthetically useful if weakly polarized or unpolarized carbon-carbon ir-bonds are used as nucleophiles (equation 2).3 Because of the low electrophilic reactivity of the iminium moiety, the reverse reaction, known as the Grab fragmentation (equation 2), is often the more important process.4 However, if the iminium moiety has a carbonyl substituent on nitrogen, its... 

Irradiation of the 6 4 mixture of enol acetates 205 and 206 produces a 7 3 mixture of the photo adducts 208 and 207. Reduction of the major photoadduct 208 followed by mesylation of the resulting alcohol leads to a mixture of isomers of the mesylate 209 in 70% yield. Treatment of the mesylate with sodium hydroxide effects simultaneous saponification and the Grab fragmentation, with the formation of a 1 2 mixture of a- and P-methyl... 

---