Title compound 1 was synthesized by a published route which had to be modified (seven steps from readily obtainable starting materials). Characterization of 1 was achieved by spectroscopic means (FAB-MS, ' H-NMR, including 2D-COSY). Furthermore, commercially available reference material purchased for comparison, was unequivocally established to be 10, i.e. incompletely deprotected 1.
Introduction. -Immunoadjuvant activity of mycobacterial cells was first described by
Freund in 1956 [ 11. N-Acetyl-D-muramoyl-L-alanyl-D-isoglutamine (so-called muramyl dipeptide, MDP) as part of a larger cell-wall species was later found to be the minimal adjuvant-active structure [2] [3] and was first synthesized by Merser et al. [4]. Subsequently, numerous MDP analogs were prepared and tested for activity (cf. reviews [5-71 and ref. cit. therein; for some recent papers, see [8-1 I]).
In the course of our work, we needed reasonable amounts of N2-(N-acetyl-6-0stearoyl-D-muramoy1)-L-alanyl-D-isoglutamine (l), a lipophilic analog of MDP. At first sight this seemed to be a rather easy task, because the synthesis had already been published [ 121, and reference material was commercially available. One step, however, did not yield the desired intermediate as claimed in the published synthesis. After appropriate modification of this step and termination of the synthesis, we obtained a product with the same TLC behaviour as the reference. Neither of the two, however, showed further analytical data consistent with the structure of l! We, therefore, decided to investigate the problem in more depth, especially with regard to 'H-NMR characterization of 1 and some of its precursors which, to the best of our knowledge, had not yet been published (for papers dealing with spectroscopy or showing detailed spectroscopic data of MDP and analogs (other than l), see [14] [15] (MS) and [I61 [I71 (NMR)).
Results and Discussion. -Synthesis of 1 was performed according to Kusumoto et al. [12] [13] with some modifications (cf. Scheme I), starting with the appropriate aminoacid derivatives 2 and 3 (Igl = isoglutamine = 4-amino-4-carbamoylbutanoic acid). It led via 4 and 5, and using N-acetyl-l-O-benzyl-4,6-0-benzylidene-a-~-muramic acid ( = 2acetamido-1-0 -benzyl-4,6-0 -benzylidene-3-0 -[(R)-1 -carboxyethyl]-2-deoxy-a -D-glucopyranose; 6), to the fully protected MDP 7 in three steps of classical peptide chemistry. Subsequent cleavage of the benzylidene group with AcOH/H,O yielded the key intermediate 8. Next, Kusurnoto's [12] acylation procedure (reaction of 8 with a five-fold