Without a spectra of the crude and so on it is hard to say exactly, but from a general point of view I think your problem may be the OAc group on the phenol.  This may be quite labile and could participate in acyl transfer reactions.  If the OAc transfers to one of the alcohols on the amine (particularly after iminium formation) then you have a set up which could undergo a huge amount of side-reactions.  Alternatively, perhaps the OAc is transferring directly to the amine, by passing the reversible iminium formation, thus stopping the reaction. 

I would suggest moving to a different protecting group (ether perhaps) and trying again under mildly acidic condition or with a Lewis acid (BF3) to assist.  Failing that, try the reaction on 2-bromobenzaldehyde and install the phenol by cross-coupling.

Best of luck

Luke

Dear Luke,

Thank you for your interest.

I had already tried the same condensation under different reaction conditions, and with different pg (tbdms and oboc) with pretty much the same result.

when making the condensation with the tbdms as pg in dmf rt 12h, i saw a white solid which i think being the byproduct deriving from the transfer of the tbdms to the amine.....Once i tried between diethanolamine and unprotected aldehyde in toluene 120 degrees in presence of MgSo4 or molecular sieves, and still i obtained the same oil (yellowish, insoluble in dcm or acn, and showing sal aldh in tlc)..... which lead me thinking that any side reaction occurring is not ascribable to the presence of an pg....

Or that it's not going to happen in the presence of a transferable protecting group.  What about a methylether that you can remove with BBr3? or a benzyl to be removed by hydrogenolysis

That´s a really good idea....about the oil, would you exclude the chance that in case it is my product, it breaks in TLC due to its high acid lability?

Right yesterday i worked up another batch, this time between acetyl salicylaldehyde and diethanolamine....this time something seems to be precipitating from diethyl ether...but still kind of yellowish oily.

Am afraid tht making a mass analysis would not help, as i got available an lc ms where the compound could still hydrolize in the 0.1 % TFA based eluent employed to run the analysis...Some weeks ago, the oxazolidine was obtained as solid by condensing H-Thr-OH and unprotected sal. aldh.....in that case in TLC i could not see any aldh. left, with an UV visible spot on the base of the plate, which i could not manage to run if not in highly polar eluent where it gives enormous dragging to the top...using AcOH in the eluent, i see again sal. aldh. Funny isnt it?

Thank you Luke for the advises.

In general silica TLC is unreactive. But it may be in a limited cases such as some bromoinated compounds.

I think that you should change the MgSO4, because Mg2+ cation is so strong one, it broke some  rings such as oxiranes.

Thank you Nejib Mekini.

In other cases I tried molecular sieves with the same result...i really do not know what happens :)

For a material that was supposedly in solution in toluene, "insoluble in dichloromethane" is really a remakable observation.  It sugggests a polymer . . . does it re-dissolve toluene?

yes it does, but hot...