Radical Cyclization Followed by the Fragmentation of Carbonyl Compounds: Effect of an α‑Benzoyl Group

To study a recently developed radical cyclization reaction followed by a fragmentation process in more detail, a series of α-benzoyl carbonyl compounds were prepared, including precursors with aldehyde and ketone moieties. Initiated by tributyltin hydride and AIBN, radical cyclization followed by fragmentation proceeded to give the desired carbonyl translocation products in 4-benzoyl-5-pentanal, 4-benzoyl-5-pentanone, 5-benzoyl-6-hexanal, and 5-benzoyl-6-hexanone radical systems. In comparison with early reports on radical cyclization reactions of α-oxy carbonyl compounds, neither a geminal dialkyl effect nor a conformationally rigid system was required to give the desired carbonyl translocation products. This effect clearly proves that a benzoyl group serves as a protecting group in these radical processes, which not only enhances cyclization efficiency but also increases the rate of the fragmentation step, eventually producing the desired carbonyl translocation products. These observations provide an alternative point of view in the field of radical cyclization reactions. Since the fragmentation step could be enhanced by appropriately positioning an α-benzoyl group, these four radical processes could be used to convert the naturally occurring d-sugars, such as d-pentoses and d-hexoses, into rare deoxy-l-sugars. Furthermore, a cascade radical process involving radical cyclization and fragmentation followed by addition to the allyltin reagent was developed and is reported herein