I don't know if you can get commercial reduced bond dipeptides. We
synthesized hexapeptides with reduced bonds some years ago using
Fmoc-chemistry, and had between 20-40% yeilds at the level of the
reduced bond isostere (A. Tossi, N. Antcheva, D.Romeo and S. Miertus
(1995) Development of pseudopeptide inhibitors of HIV-1 aspartic
protease: analysis and tuning of the subsite specificity., Peptide Res.
8:328-334).
A technical article on syntheses compatible with Boc-chemistry is
present in Meth. Molec. Biol Vol. 35 (1994) Chapter 12 p241 "Solid-Phase
synthesis of peptides containing the CH2NH reduced bond surrogate" M.W.
Pennigton
Hope this is of use,
Bye, Alex