Small molecule compound libraries based on medium rings 1 are virtually unknown. 2 This critical gap in the literature is probably caused by two main factors. First, medium ring natural products are themselves uncommon and therefore less likely to be the object of total synthesis efforts. 3 Second, there is a widely held view that medium rings remain the most challenging and difficult systems to synthesize. 4 As part of a program to identify interesting chemotypes for library development, we examined several medium-ring natural products. One system in particular, namely, octalactin A (1) (Figure 1), 5 continues to attract intense interest because of its fascinating architecture and potent antitumor profile. 6 The rare, saturated eight-membered lactone core appeared to be an attractive feature on which to base a new library template. Our previous experience with the total synthesis of the octalactins7 and of other eight-membered lactone natural products 8 by means of a facile direct lactonization from the saturated seco acid ("zip-up" approach) and via an equally efficient ringclosing metathesis to afford oxocenes from the diene ester ("zip-down" approach), 9 provided the enabling technology for this project and prompted us to proceed with this scaffold.Because it was desirable for us to have the flexibility to introduce additional functionality that could later be easily modified, we settled on a convergent-divergent strategy of constructing lactam rather than lactone libraries via RCM (Scheme 1).In the convergent phase, three simple, commercially available building blocks served as sources of diversity for the scaffold: protected L-amino acids, benzaldehydes, and benzyl amines. Once the desired frameworks were in hand, diversification at the secondary amine R 2 N was carried out in parallel fashion (divergent phase) to afford compound libraries of amines, amides, sulfonamides, carbamates, and ureas. As a guiding principle for the creation of molecules most likely to have druglike properties, all of the entries were screened in silico prior to synthesis to comport with Lipinski's Rules. 10 We are now pleased to report a 163-member demonstration library, the first based on a monocyclic medium-ring platform. The synthesis of the scaffold began with the commercially available t-butyl ester hydrochloride salts of the amino acids glycine, alanine, and phenylalanine (Scheme 2). The salts were allylated by a modified one-pot literature procedure (CH 2 =CHCH 2 Br, NaHCO 3 , LiI), 11 and then protected with the Fmoc group to afford the Nallyl esters 12-14 in 53-81% yield. Attempts at unmasking the carboxylic acid using conventional ester hydrolysis protocols proved unsuccessful and resulted in significant racemization with the two chiral amino acids. However, the use of Et 3 SiH in TFA/CH 2 Cl 2 (1:1) 12 produced the desired acids 15-17.To maximize the number compounds available for biological screening with a minimum of synthetic effort, we decided to employ a single racemic coupling partner. The secondary allylic amin...

Abstract. Pilot-scale libraries of eight-membered medium ring lactams (MRLs) and related tricyclic compounds (either seven-membered lactams, thiolactams or amines) were screened for their ability to inhibit the catalytic activity of human recombinant 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase in vitro. A dozen of the synthetic compounds mimic the inhibition of purified HMG-CoA reductase activity caused by pravastatin, fluvastatin and sodium salts of lovastatin, mevastatin and simvastatin in this cell-free assay, suggesting direct interaction with the ratelimiting enzyme of cholesterol biosynthesis. Moreover, several MRLs inhibit the metabolic activity of L1210 tumor cells in vitro to a greater degree than fluvastatin, lovastatin, mevastatin and simvastatin, whereas pravastatin is inactive. Although the correlation between the concentrationdependent inhibitions of HMG-CoA reductase activity over 10 min in the cell-free assay and L1210 tumor cell proliferation over 4 days in culture is unclear, some bioactive MRLs elicit interesting combinations of statin-like (IC 50 : 7.4-8.0 μM) and anti-tumor (IC 50 : 1.4-2.3 μM) activities. The HMG-CoA reductase-inhibiting activities of pravastatin and an MRL persist in the presence of increasing concentrations of NADPH. But increasing concentrations of HMG-CoA block the HMG-CoA reductase-inhibiting activity of pravastatin without altering that of an MRL, suggesting that MRLs and existing statins may have different mechanisms of enzyme interaction and inhibition. When tested together, suboptimal concentrations of synthetic MRLs and existing statins have additive inhibitory effects on HMG-CoA reductase activity. Preliminary molecular docking studies with MRL-based inhibitors indicate that these ligands fit sterically well into the HMG-CoA reductase statin-binding receptor model and, in contrast to mevastatin, may occupy a narrow channel housing the pyridinium moiety on NADP + .

We have prepared a novel speculative eight-membered lactam demonstration library based on the skeletal structure of the potent antitumor marine natural product octalactin A. The basic scaffold was readily constructed in a convergent fashion via ring-closing metathesis chemistry from the corresponding diene amides. A cursory examination of the biological properties of the library validates the relevance and significance of these structures. KeywordsEight-membered; Natural product; Lactam; Library; Ring-closing metathesis; Medium-ring; Octalactin; Parallel synthesis Libraries of compounds based on natural products have gained in importance in recent years. 1,2 Although the structures contained in these libraries tend to be fairly diverse, one class of compound remains to be fully exploited, namely, the monocyclic medium-ring. 3,4 An interesting natural product from the standpoint of structural architecture and biological profile is the powerful antitumor marine natural product octalactin A, 1 (Fig. 1). 5,6 This fascinating natural product which features a rare saturated eight-membered lactone core continues to receive considerable attention in total synthesis efforts from many laboratories. 7 We have previously constructed this lactone by means of an especially facile and direct lactonization from the corresponding seco acid ('zip-up' approach), 7a,8 and by an equally facile ring-closing metathesis (RCM) route ('zip-down' approach) to afford the unsaturated oxocene. 9 These enabling technologies prompted us to investigate the feasibility of generating a speculative eight-membered ring library inspired by octalactin A.In order to simplify the synthesis, we elected in the present work to construct a simpler lactam scaffold that at a minimum retained the eight-membered core feature of the octalactins. The strategy is depicted in Scheme 1. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptThe unsaturated lactam scaffold 2 would be constructed by means of RCM. The diene amides 3 would result from the condensation of the racemic Fmoc-protected 3-amino-5-hexenoic acid 4 and the sublibrary of various racemic secondary allylic amines 5. The allylic amines in turn would be derived from vinyl Grignard addition to the Schiff bases. By using readily prepared racemic components, a mixture of diastereomers 2 would be obtained resulting in a larger library with a relative minimum of effort and cost. Separation of the diastereomeric scaffolds would precede derivitization of the primary amines, thus generating the desired lactam library.The allylic amine sublibrary was prepared as shown in Scheme 2. Reaction of a series of commercially available benzaldehydes 6a-h and benzyl amines 7a-f in ether at room temperature in the presence of magnesium sulfate 10 gave after filtration the Schiff bases in nearly quantitative yield in all cases.Addition of vinyl Grignard with BF 3 ·OEt 2 at −78 °C for 12 h gave a representative 13-member sublibrary in 67−87% yield (Fig. 2).Synthesis of the racemic Fmoc-protect...