Efficient total syntheses of natural pterin glycosides: limipterin and tepidopterin

The key, versatile precursors N-2-(N,N-dimethylaminomethylene)-1'-O-(4-methoxybenzyl)-3-[2-(4-nitrophenyl)ethyl]biopterin (29a) and its ciliapterin analog (29b) were prepared, respectively, from D-xylose (in 14 steps) and L-xylose (in 11 steps). Treatment of 29a and 29b with 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-beta-D-glucopyranosyl bromide in the presence of silver triflate and tetramethylurea, followed by removal of the protecting groups, led to the first selective syntheses of limipterin (3) and tepidopterin (5), respectively.