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-C-glycosyl
alanines. The keyC-glycosyl radical is generated
through photocatalysis by either the new generation copper(I) complex
[(DPEPhos)(bcp)Cu]PF6or [Ru(bpy)3](BF4)2. The reactions proceed smoothly, affording the desired
α-C-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.N,N-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 inC-glycosyl radical formation.