In response to a query by James on ChemUnpub about hints on doing the Ugi reaction, Francesco Mazzini responded:
Regarding aromatic aldehydes in Ugi reaction, I think that this paperThis actually looked like a really promising way of accelerating the Ugi reaction using titanium tetrachloride. We discussed it for a while until we realized that the HCl liberated by the reaction of TiCl4 with methanol would knock off the BOC group required in the synthesis of all our diketopiperazines. Doh...
"Titanium Catalysis in the Ugi Reaction of r-Amino Acids with Aromatic Aldehydes"
Thomas Godet, Yannick Bonvin, Guillaume Vincent, DaphneÂ´ Merle, Alain Thozet, and Marco A. Ciufolini
Org Lett 2004,6, 3281-3284
can help you (if have not already read it)
At least we got a solid reference for the noted slower reaction of aromatic aldehydes in the Ugi reaction, which supports what we are finding.