Zaragozic acids synthase

Medicinal chemistry has frequently drawn inspiration and important new leads from the examination of natural products, and this was proven to be the case once more. In 1992, researchers at Merck and Glaxo announced, almost simultaneously, the independent discovery of the same new class of natural products from two different fungi. As a consequence, the same family of natural products has two names - the zaragozic acids (Merck)4 or the squalestatins (Glaxo).5 A typical member of the family, zaragozic acid A (squa-lestatin SI) (1) was shown to have a tremendous affinity for squalene synthase (K, = 79 pM for rat microsomal squalene synthase) and could even lower serum cholesterol levels in vivo in a population of marmosets.6... 

Zaragozic acid or the squalastatins were discovered as fungal metabolites and have been identified as potent inhibitors of squalene synthase. These compounds have presented themselves as potential cholesterol lowering agents. 

Zaragozic Acids The zaragozic acids (squalestatins) are a class of competitive inhibitors of mouse, rat, and HepG2 squalene synthases, which also display antifungal activity [104]. The characteristic structural feature of this family of natural products is its highly oxygenated core. 

Bergstrom JD, Dufresne C, Bills GF, Nallin-Omstead M and Byrne K (1995) Discovery, biosynthesis and mechanism of action of the zaragozic acids potent inhibitors of squalene synthase. Annu Rev Microbiol 49, 607-639. 

C Dufresne, KE Wilson, D Zink, J Smith, JD Bergstrom, M Kurtz, D Rew, M Nallin, R Jenkins. The isolation and structure elucidation of zaragozic acid C, a novel potent squalene synthase inhibitor. Tetrahedron 48 10221-10226, 1992. 

The squalestatins (also known as zaragozic acids) are a family of potent squalene synthase inhibitors with potential use as anticholesterol compounds. 

Extensive studies on the structure-activity relationships (SAR) in a series of compounds derived from squalestatins 1 (zaragozic acid A) (25) and 3 (27) have been performed, focusing on the role of the carboxylic acids at C-3, C-4 and C-5 positions of [15 -(la,3a,4p,5a,6a,7P)]-4,6,7-trihydroxy-2,8-dioxabicyclo-[3.2.1 ]-octane-3,4,5-tricarboxylic acid [58-61]. Consequently, the modification of the carboxylic acid at C-5 in squalestatin 1 analogues was not tolerated, but the carboxylic acids at C-3 and C-4 were not absolutely required for the retention of squalene synthase inhibitory activity [58J. 

Hydroxylated analogue (30) of zaragozic acid A was produced from zaragozic acid A by microbial transformation using Streptomyces cyanus ATCC 55214, which inhibited the human squalene synthase in a dose-dependent manner with an IC50 of 0.091 nM in HepG2 cells [62]. 

Zaragozic acids D (31) and D2 (32) were further isolated from cultures of the keratinophilic fungus Amauroascus niger as potent inhibitors of rat liver squalene synthase with IC50 values of 6 and 2 nM, respectively. Structures of zaragozic acids D and D2 are related to those of the previously described zaragozic acids A, B and C, which contain 4,6,7-trihydroxy-2,8-dioxabicyclo-[3.2.1]octane-3,4,5-tricarboxylic acid core unit and hydrophobic alkyl and acyl side chains [63]. 

On the other hand, HMG-CoA reductase inhibitors have also been shown to decrease the biosynthesis of other biologically important isoprenoid compounds derived from mevalonate. Thus, there has been continued interest in developing hypolipidemic agents that inhibit the enzymes involved specifically in the later stages of cholesterol biosynthesis. Zaragozic acid derivatives were isolated from several microbial species and they exhibited dose-dependent inhibition of cholesterol biosynthesis as potent squalene synthase inhibitors. Viridiofungins, bisabosquals, macrolactins, CJ-15,183, CJ-13,981 and CJ-13,982 were also isolated as mammalian squalene synthase inhibitors of microbial origin. 

Squalene synthase is an enzyme catalyzing the formation of squalene from farnesyl diphosphate which is a committed step in the cholesterol biosynthetic pathway. Therefore, squalene synthase is considered a better target than HMG-CoA reductase because farnesyl pyrophosphate, a downstream product of HMG-CoA reductase, is needed for prenylation of proteins and for the biosyntheses of ubiquinone and dolichol (Fig. 2). Before squalestatins and zaragozic acids were discovered, a number of squalene synthase inhibitors were synthesized that showed respectable inhibitory potencies in vitro, but none were successful in animal testing [41]. It was the discovery of squalestatins and zaragozic acids that renewed interest in this biological target, and at picomolar potencies they were the most active inhibitors of squalene synthase. 

F. (1993) The zaragozic acids structure elucidation of a new class of squalene synthase inhibitors. Tet Lett. 34, 399-402. 

An example of this approach is given by a group of metabolites the squalestatins (also known as zaragozic acids), isolated from the fungus, Phoma sp. (see Fig. 1). Extracts of this organism were found to have squalene synthase inhibitory activity, and extraction resulted in the isolation of the major squalestatins of which there were approximately four (2,3). As these compounds appeared to hold potential as leads for the development of cholesterol-... 

Using assay conditions where both substrates were used at km levels chaetomellic, actinoplanic, oreganic and zaragozic acids and alkyl citrates (for example, viridiofungins) were discovered from different microbial extracts. These compounds were potent selective inhibitors of FPTase activity and did not inhibit GGPTase or squalene synthase. All five classes of compounds were competitive with respect to FPP and were reversible inhibitors of FPTase activity. Three additional reports have appeared during the intervening period that described the isolation of CP 225917, manumycin, and RPR 113228 as FPP competitive inhibitors. These inhibitors, their in vitro activity and their corporate sources are summarized in Table 1. 

Potent squalene synthase inhibitors were simultaneously discovered and named squalestatins and zaragozic acids by Glaxo-Wellcome and Merck and Co., respectively. These series of compounds contain an alkyl bicyclic core with three free carboxyl groups and an acyl side chain and are representative of the class (Figure 17). 

Pfizer reported the discovery of a new class of structurally complex alkyl carboxylic acids, as exemplified by CP-225917 and CP-263114 (Figure 20), that are weak and non-selective inhibitors of squalene synthase and FPTase. These compounds were isolated by acid base liquid-liquid partition, Sephadex LH20 chromatography and reverse phase HPLC from ethyl acetate extracts of an unidentified fungus that also produced zaragozic acid A [97]. 

Discovery, Biosynthesis, and mehanism of action of the zaragozic acids Potent inhibitors of squalene synthase Bergstrom, J. D. Dufresne, C. Bills, G. F. Nallin-Omstead, M. Byrne, K., Eds. Annual Reviews Inc. Palo Alto, CA, 1995 Vol. 49. 

Zaragozic acids (squalestatins). Highly active, competitive inhibitors of squalene synthase (SQS) and thus of cholesterol biosynthesis. The different names are a result of the independent, simultaneous discovery of this class of natural products in various ascomycete strains (e. g., Phoma sp., Setosphaeria khartoumensis) by three research groups. All Z. possess a 4,6,7-tri-hydroxy-2,8-dioxabicyclo[3.2.1]octane-3,4,5-tricarbox-ylic acid skeleton carrying acyl groups on the C-6 oxygen atom as lipophilic side chains and alkyl groups on the C-1 carbon atom. 

A recent survey collects over 50 targets, in whose total synthesis the Evans aldol protocols served as key steps [123]. As a small selection is shown in Scheme 4.55 the squalene synthase inhibitor zaragozic acid D [124] and the... 

The squalestatin (or zaragozic acid) class of squalene synthase inhibitors presents an interesting challenge to the synthetic chemist and studies directed towards this goal have been reported. 1,6-Anhydro-D-galactose has formed the basis of the synthesis of 107 and the more advanced intermediate 108 (sugar numbering is shown). ... 

In comparison with ketone-based SAMP/RAMP hydrazones, the use of aldehyde-derived SAMP/RAMP hydrazones in natural product syntheses is much more prevalent. For instance, Nicolaou et al. synthesized the side chain of zaragozic acid A 49 using two successive SAMP hydrazone alkylations (Scheme 1.1) The zaragozic acid class of natural products inhibits squalene synthase, the enzyme responsible for the biosynthesis of cholesterol. Inhibition of this enzyme has implications in the treatment of coronary artery disease. The synthesis of the side chain of zaragozic acid began with the formation of 42, which... 

Evans et al. applied the Mukaiyama aldol reaction to the total synthesis of the squalene synthase inhibitor zaragozic acid C (Scheme 8.23). ° Di-f-butyl tartrate 135 was protected as acetal 137, which was converted into silyl enol ether 138. The partner aldehyde 141 was synthesized by the Evans aldol reaction (139 —> 140). The Mukaiyama aldol reaction with silyl enol ether 138 and aldehyde 141 in the presence of (i-PrO)TiCl3 gave adduct 142 as a single isomer. These transformations gave the desired stereochemistry at the C3 to C7 positions. 

---