Benzylidene radical fragmentation

Crich and Yao have exploited a homolytic substitution at sulfur to trigger a radical cascade that includes a loss of carbon monoxide and a radical fragmentation of a 4,6-0-benzylidene moiety to give esters such as 113 after a final diastereoselective reduction (Scheme 32) [107]. This impressive outcome opens a direct avenue to 6-D-rhamnopyranosides and other 6-deoxy sugars. 

The last few years have seen numerous applications of spin trapping to biological systems, and in these the trapping of hydroxyl radicals has assumed some importance. This work has been confined almost exclusively to nitrone scavengers 4 the fact that the hydroxyl adduct [6] of DMPO is much more persistent than that [7] of the commonly used nitrone, benzylidene-t-butylamine-N-oxide ( phenyl t-butyl nitrone ,3 or PBN) [3], may be due to a fragmentation reaction, with subsequent oxidation of the cr-hydroxybenzyl radical, as shown. 

The relative abundance of these h-fragments is high for benzylidene acetals, owing to the formation of the (stable) benzyl radical. These h-fragments are also formed by components having a 1,3,6-trioxabicyclo-[4.4.0]decane system, such as 4,6-O-benzyIidenehexopyranosides (see Scheme 28). The hi-ion is more abundant than the ha-ion, because of the... 

Benzylidene acetals of diols undergo radical-chain cleavage to give benzoate esters via the formation and subsequent selective fragmentation of a di(a -alkoxyl)henzyl radical. Thus, cychc acetal 24 was quantitatively converted to henzoate 25 under the catalysis of TBST (eq 10). Apphcation to the carhohydrate derived cyclic acetal 26 led to the formation of the 6-deoxy glycoside 27 in high yield (eq 11). ... 

Fragmentation Reactions. The tetrahydrofuranyl radical undergoes fragmentation at 140 °C to give an open-chain acyl radical. The THE radical as well as the rearranged radical are trapped by excess of alkene (eq 15). Benzylidene acetals undergo simi-... 

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