sorry but a reaction of a tertiary amine with aldehyde will not form an hemi-aminal.  At most a reversible salt may be formed. Tertitary amines may be oxidized forming for example nitrosammonium.

@Dr. Lemos, Thanks a lot for your answer. Actually, the product (the quaternary salt) is a natural product and I guess is stable since the isolation chemsit could characetrize it completely. I am at the end of the total synthesis and am looking for reducing one step if possible. My last option would be doing N- demethylation which would react with the aldehyde insitu and after that quaternization of the nitrogen with MeI. Is this option of mine logical? Please suggest me if you have any.

Thanks again.

Hemi-aminal are usually unstable and go further to the imine/iminium or back to the amine and aldehyde, but since the compound has been characterized (and assuming that there is not mistake in the structure elucidation), the additional rigidity should compensate for the hemi-aminal/ammonium high enthalpy.

I understand that you have the corresponding tertiary amine-aldehyde starting material, right? I suggest you try a range of Lewis and Bronsted acids and see which one gives the desired product without degradation of the starting material. Try metallic and borate Lewis acid, and strong to weak Bronsted acids. Try also several mildly acidic aqueous solutions (my guess for this is to form the gem-diol, which would react further with the tertiary amine, and appropriate counter-ion to stabilize the ammonium product).

As for demethylation, the secondary amine will react with the aldehyde to form the iminium, and from there good luck to get the hemi-aminal.

Anyway, all of this is hypotheses, do a proper research to back them up and good luck with this reaction!

@Dr. Lepage, Yes, the product is a six-member ring with the hemiaminal quaternary ion. So, that should compensate for some of the higher enthalpy, I guess. So far, I have tried heating it in different solvents (DCE, ACN, MeOH etc) but nothing happens. Tried with a Lewis acid, no progress. Will try with Bronsted acid, if no progress then will go forward with the de-methylation.

Thank you so much for your input. I really appreciate.