Modulable Photocatalyzed Strategies for the Synthesis of α--Glycosyl Alanine Analogues via the Giese Reaction with Dehydroalanine Derivates.

Herein, we present the α-selective Giese reaction between pyranosyl/furanosyl bromides and dehydroalanine analogues, which provides access to a library of highly valuable α--glycosyl alanines. The key -glycosyl radical is generated through photocatalysis by either the new generation copper(I) complex [(DPEPhos)(bcp)Cu]PF or Ru(bpy). The reactions proceed smoothly, affording the desired α--glycosyl alanines in up to 99% yield when diethyl 1,4-dihydro-2,6-dimethyl-3,5-pyridinedicarboxylate [Hantzsch ester (HE)] is used as an additive. ,-Diisopropylethylamine (DIPEA) has been selected as a reductant in both protocols. A mechanistic study by means of transient absorption spectroscopy unveils a halogen-atom transfer (XAT) process in -glycosyl radical formation.