This experiment (ONSC-EXP110) turned out to be very puzzling. On the day that we tried to make solutions we could not get as much material to go in as expected from the paper, even by sonicating at 40C. The next day some of these samples went into solution and then we could not get precipitation to occur for some samples even at 0C.
However, an examination of the NMRs of the samples with lowest concentration revealed the problem. It turns out that they were about 50% converted to the methanol hemiacetal. The NMR clearly shows the coupling between the benzylic and hydroxyl protons at 5.6 and 6.8 ppm, each integrating for 1H each compared with 2H for each of the new peaks in the aromatic region.



It turns out that the hemiacetal was present in all our previous measurements for 4-nitrobenzaldehyde in methanol but we didn't spot it. Furthermore we found significant hemiacetal formation for 2-chloro-5-nitrobenzaldehyde, 2,6-dichlorobenzadehyde and 4-chlorobenzaldehyde but only trace on none for others like veratraldehyde (ONSC-EXP033 has several of these NMRs). So it appears that if the aromatic ring bears sufficiently electron withdrawing groups hemiacetal formation is facilitated without the presence of a catalyst. This makes sense from a mechanistic standpoint.
Since the hemiacetal forms so easily in these cases I wonder why there was no mention made of it in the Maccarone paper. They assayed concentration using GLC - perhaps the hemiacetal decomposed quickly under those conditions? (PNBA is para-nitrobenzadlehyde):

I think that for the Ugi reaction the implication of this is that alcoholic solvents should not be used for aldehydes bearing electron withdrawing groups.
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