[Babcock_Hall]

Yes, I dont know the acidity of the phosphonate, maybe its more acidic then the Fmoc-proton. Its a good suggestion to mix the base and phosphonate first.

Some time ago I found a pK_a of 18.6 for (EtO)₂P(O)CH₂CO₂Et, which I believe is in DMSO.  I have not yet found pK_a values for common β-sulfonylphosphonates, but I think that they are likely to be in this vicinity.  The presence of the lithium ion in the reaction will lower the pK_a of the β-sulfonyl phosphonate, but I don't know by how much.  The 1984 Tet. Lett. paper by Blanchette M et al., suggests that the effect is roughly 7 pH units relative to the potassium ion with respect to the carboxylate derivative.

[rolnor]

I think Dibal is much less basic than LAH so maybe there is no problem?
Its not a suggestion for you, but a good way to make aldehydes is to cleave double bonds with OsO4/NaIO4, its very mild and neutral. You can use ozone/dimethyl sulfide also but its more work if you dont have the apparatus to make ozone.
I have used old dinitrophenyl hydrazine and its not critical, if it forms a yellow solution it works.
I think for safety reasons it sold with some moisture in it?

[Babcock_Hall]

A quick update:  We have completed the synthesis of two dipeptides.  We are looking at methods of increasing the yield, especially in the coupling step and in the step in which a Weinreb amide is reduced.  With respect to the former, I think we will move away from NHS esters and try HBTU instead.