Based on their different sources the APN inhibitors APNIs ca

Based on their different sources, the APN inhibitors (APNIs) can be divided into the natural products and synthetic compounds. Bestatin (1, Fig. 1), isolated from the Streptomyces oliuoreticuli by Umezawa et al., is the first reported and only marketed natural dipeptidomimetic inhibitor of APN. After that, many natural products against APN have been documented, including amastatin, actinonin, phebestin, and lapstatin. Our group has a long-standing interest in the development of synthetic APNIs as antitumor agents15, 16, 17, 18, 19, 20, 21, among which the leucine ureido-based APNIs (2, Fig. 1) exhibited promising in vitro and in vivo anti-angiogenisis and anti-metastasis potency.18, 19, 20, 21 Our previous structure-activity relationship studies revealed that structural modifications of the aryl group of 2 were well tolerated.18, 19, 20, 21 1,2,3-triazole, which could be readily obtained by click chemistry, is widely used in various biologically active compounds due to its desirable features in the context of medicinal chemistry.22, 23, 24, 25, 26 For example, triazoles are stable to Difopein and basic hydrolysis, redox conditions and metabolic degradation. This heterocycle also has the potential to form hydrogen bonds as well as π stacking interactions. Herein, the 1,2,3-triazole moiety was introduced as the surrogate for the aryl group in compound 2 to get a novel series of leucine ureido-based APNIs (3, Fig. 1). All target compounds were synthesized efficiently using click chemistry and all the leucine ureido-based compounds exhibited more potent APN inhibitory activities than the positive control bestatin. The superior APN inhibitory activities of several representative compounds were validated using cell-based APN inhibitory assays. The most potent compound 13v exhibited promising in vitro anti-angiogenisis potency and in vivo anti-metastasis potency. Finally, the molecular docking study was used to postulate the binding mode of 13v in the active site of APN.
Chemistry The target compounds were efficiently synthesized following the procedures shown in the Scheme 1, Scheme 2, Scheme 3, Scheme 4. As shown in Scheme 1, compounds 4a–4c reacted with triphosgene to yield isocyanate, which was then immediately reacted with propargylamine to generate the ureido derivatives 5a-5c. The ureido derivatives 5a-5c were converted into key intermediates hydroxamic acids 6a-6c in the presence of NH2OK in dry methanol. As shown in Scheme 2, the aldehydes (7a–7u, 7aa–7ff, 9v–9w) were converted into their alcohol derivatives (8a–8u, 8aa–8ff, 10v–10w) via reduction by sodium borohydride in methanol. Compounds 9v–9w were generated by reaction of 7v–7w with benzyl bromide. The hydroxyl group of compounds 8a–8u, 8aa–8ff and 10v–10w were converted into a mesyl group under basic conditions and then reacted with sodium azide to yield the azide derivatives (12a–12w, 12aa–12ff). The target compounds 13a-13w and 13aa-13ff were prepared by coupling 6a with compounds 12a–12w and 12aa–12ffvia click chemistry, respectively. Compounds 14 and 15 were prepared by coupling 6b with 12a and 6c with 12dvia click chemistry, respectively. Scheme 3 presented the synthesis of compounds 20a–20b. Compounds 16a–16b underwent reduction by sodium borohydride to yield compounds 17a–17b, of which the nitro groups were reduced to amino groups with sodium sulfide. Subsequently, activation of the hydroxyl group with triphenylphosphine in CCl4/DMF and then reaction with sodium azide led to the azide derivatives 19a–19b. Compounds 19a-19b were coupled with 6avia click chemistry to generate compounds 20a–20b. The reactions in Scheme 4 are similar to that of Scheme 2. The aldehydes and ketones (21a–21h) underwent reduction to yield the corresponding alcohols (22a–22h). Activation of the hydroxyl group followed by nucleophilic substitution led to azide derivatives 24a–24h, which were coupled with 6avia click chemistry to yield compounds 25a–25h, respectively.