Formyl radical equivalent

Tin-free photolytic conditions for addition of formyl radical equivalents to 3a have been reported by Alonso [53]. Photolysis of 1,3-dioxolane solutions of 3a in the presence of 1 equiv of benzophenone led to formation of the l,3-dioxalan-2-yl radical from solvent, followed by intermolecular radical addition in 87% yield (entry 6). A one-pot variant without isolation of 3a increased the yield and selectivity (entry 7). These conditions gave good yields across a range of chiral /V-acylhydrazones (not shown), but synthetically useful levels were restricted to aliphatic aldehyde precursors. 

Scheme 10-56 A (dichloromethyl)dimethylsilyl ether can be used as a formyl radical equivalent.

As the product 39 still contains the xanthate group, alternative radical transformations can be performed, which also include its removal with tris(trimethylsilyl)silane. Further manipulations of the dithiane ring comprise desulfurization or hydrolysis, thus confirming the use of such xanthates as the synthetic equivalent of a methyl and formyl radical. Two examples of the use of this chemistry for the extension of alkene-containing sugars were successfully examined, as illustrated in Scheme 27. 

By contrast, quinoline gives a mixture of C2 and C4 addition products (Scheme 16), while the use of 1,3-dioxolane as a formyl equivalent leads to mixtures derived from hydrogen atom abstraction at C2 and C4 of the saturated heterocycle (Scheme 17). Interestingly, these reactions only work well when iron(ll) is used catalytically. At higher concentrations, the iron(IIl) salts produced oxidise trioxanyl radical 33 to the corresponding cation, thereby killing the reaction. 

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