A Facile Synthesis of Fully Protected meso-Diaminopimelic Acid (DAP) and Its Application to the Preparation of Lipophilic N-Acyl iE-DAP

Synthesis of beneficial protected meso-DAP 9 by cross metathesis of the Garner aldehyde-derived vinyl glycine 1b with protected allyl glycine 2 in the presence of Grubbs second-generation catalyst was performed. Preparation of lipophilic N-acyl iE-DAP as potent agonists of NOD 1-mediated immune response from 9 is described.


Introduction
Peptidoglycan (PGN) is an essential component of the cell walls of virtually all bacteria. The function of the PGN is to preserve cell integrity by withstanding the internal osmotic pressure [1,2]. The biosynthesis of PGN is a well-recognized target for antibiotic development [3]. Bacterial cell wall PGN can function as a potent immunostimulator and an adjuvant for antibody production. PGN partial structures are recognized by the intracellular nucleotide-binding oligomerization domain proteins 1 and 2 (NOD1 and NOD2) that mediate host recognition of bacterial molecules [4][5][6]. The recognition core of Nod1 stimulatory molecules is γ-D-glutamyl-meso-diaminopimelic acid (iE-DAP) which is a constituent of most Gram-negative and some Gram-positive bacteria. In addition, synthetic, lipophilic, N-acyl iE-DAP derivatives have been shown to be potent NOD 1 agonists [7]. Thus, DAP scaffold OPEN ACCESS peptides would be expected to function as NOD 1 agonists. In connection with our interest in the synthesis of DAP [8], we report herein on a new synthesis of orthogonally protected meso-DAP and applications to preparing N-acyl iE-DAP from protected iE-DAP ( Figure 1).
A variety of substituted olefins 1a-e as vinyl glycine equivalents were obtained from the Garner aldehyde in high yields according to the corresponding literature reports [27][28][29][30][31]. It is expected that a homocoupling of 1 would scarcely occur due to a bulkiness of N-Boc-oxazolidine. First, a coupling began with the CM of 1a with 2 [32,33]. Treatment of 1a (5 mmol) with 2 (1 mmol) using the Grubbs second-generation catalyst A (5 mol%, Figure 2) in CH 2 Cl 2 under reflux gave a desired product 3 in 56% together with homo-coupling products traces of 4 and 5 (22%) (entry 1 in Table 1).
Next, the use of a combination of catalysts B [34] and C [34] gave lower yields (28% and 33%) of 3, respectively (entries 2 and 3). On the other hand, when toluene was used as the solvent in place of CH 2 Cl 2 , a higher yield (64%) of 3 was obtained (entry 4).
In addition, the use of CM using several substituted olefins 1b-e as vinyl glycine units in toluene was examined and the results are shown in Table 2. The use of 1b (E:Z = 1:10) gave the best yield (76%) of 3. Unfortunately, CM using other derivatives, such as 1c-e, resulted in lower yields (entries 4~6). Furthermore, the CM of 1b with the Hoveyda-Grubbs 2nd generation D [34] and the Blechert E [35] catalysts afforded lower yields (56% and 9%), respectively. Accordingly CM in conjunction with a combination of 1b and A as a catalyst resulted in better yields. Additionally, CM using the pure E isomer and the Z isomer of 1b resulted in nearly the same yields (75%) (entry 7). Furthermore, the CM in Table 2 produced no the homocoupling product 4 as expected.
With 3 in hand, our interest was focused on the synthesis of orthogonally protected meso-DAP. The hydrogenation of 3 in the presence of PtO 2 as a catalyst gave 6, which was transformed by hydrolysis of the amino acetal with with p-TsOH in aqueous MeOH into the alcohol 7 in 81% yield in two steps.
Having the desired 9 in hand, we embarked on the synthesis of N-acyl iE-DAP, which is known to function as a strong agonist for the stimulation of NOD 1 [6]. Deprotection of the tert-butoxycarbonyl group of 9 by treatment with trifluoroacetic acid followed by condensation of the resulting amine 10 [37] with Fmoc-D-Glu-OBn [38,39] using EDC in the presence of HOBT and triethylamine gave the protected iE-DAP 11 in 54% yield in two steps. Next, 11 was treated with diethylamine to afford the deprotected amine 12, which was subsequently acylated with capryloyl chloride and myristoyl chloride to produce the corresponding N-acyl derivatives 13 and 14 in 68% and 89% yields, respectively. Finally, the deprotection of 13 and 14 with Pd(OH) 2 as the catalyst under hydrogen gave N-capryloyl iE-DAP 15 and N-myristoyl iE-DAP 16, respectively, in quantitative yields (Scheme 2).