Ansamycins naphthalenoid

As described in the introduction, macrocyclic lactams can be classified as ansamycins and other compounds, and the ansamycins consist of an ansa-chain and a chromophore-containing moiety. The ansamycins are typically divided into two groups, i.e., the naphthalenoid ansamycins and the benzenoid ansamycins, according to their chromophores. 

Among the naphthalenoid and benzenoid ansamycin antibiotics, some ansamycins possess a 1,4-naphthoquinone or a 1,4-quinone unit as a chromophore, and others possess a 1,4-hydroxynaphthalene or a 1,4-hydroquinone units (or their derivatives) as a chromophore. Since these two types of chromphore are in most cases reversible, it is not appropriate to classify by the difference of the oxidation stage of the chromophore. In this review, benzenoid and naphthalenoid ansamycins are further divided according to the difference of the length of their ansa chains. Thus, naphthalenoid ansamycins are divided into 3 groups, i.e., naphthalenoid ansamycins with C17 ansa chains, naphthalenoid ansamycins with C23 ansa chains, and naphthalenoid ansamycins with C9 ansa chains. The benzenoid ansamycins are divided into 2 groups, i.e., benzenoid ansamycins with C15 ansa chains and benzenoid ansamycins with Ci7 ansa chains. The relationships between, and the antibiotics within these groups are indicated in Scheme 2. 

As described above, the naphthalenoid ansamycins are divided into three groups according to the size of the macrocyclic ring, i.e., some of them possess Cl7 ansa chains, and other naphthalenoid ansamycins possess C23 and C9 ansa chains, respectively. 

Naphthalenoid Ansamycin Antibiotics with Cj Ansa Chains... 

Naphthalenoid ansamycins with C i ansa chains can be further classified into three groups, namely, the rifamycin group, the proto-streptovaricin group and the streptovarlcin group, based on the difference of their chromophores. Awamycin [34,35] is an example of the naphthalenoid ansamycin antibiotics and belongs to the protostrepto-varicin group with a sulphur atom in the molecule. 

Rifamycin W (20), a naphthalenoid ansamycin antibiotic with a Cjy ansa chain was isolated from a mutant strain of Nocardia mediterranei. Unlike... 

Awamycin (26) was isolated from the cultured broth of Streptomyces sp. 80-217 [34,35] as an antitumor antibiotic. Awamycin (26) is one of the naphthalenoid ansamycin antibiotics containing a sulphur atom in the molecule at the C-3 position of the chromophore, and its structure is closely related to rifamycin W (20). 

Other examples of naphthalenoid ansamycin antibiotics with a sulphur atom are CP-50833 (28), obtained from the culture broth of Streptomyces nigellus subsp. africanus ATCC31496 [80], and the semisynthetic antibiotics derived from rifamycin S (2), i.e., 3-thiomethylrifamycin S and 3-thiomethylrifamycin SV (4) which were obtained by repeatedly treating rifamycin S with MeSH [81]. The double bond at the C4-C5 position of 28... 

Naphthalenoid ansamycins with C23 ansa chains, the naphthomycins, diastrovaricins, naphthoquinomycins, naphthomycinol and actamycin have been isolated and characterized. 

Naphthalenoid ansamycins with C9 ansa chains consist of two compounds, rubradirin and protorubradirin. These antibiotics possess a C9 ansa chain moiety and a C-nitroso-sugar or a C-nitro-sugar in the structure, respectively. As will be described below, rubradirin appears to be a photooxidation product of protorubradirin. 

A description of the biosynthesis of the ansamycins can be divided into two parts, namely, the biosynthesis of the chromophoric units and the biosynthesis of the ansa moieties. It was established that the chromophoric moiety of both the benzenoid and the naphthalenoid ansamycins consists of a AW-C7N unit as the biosynthetic precursor. On the other hand, the ansa chains of these ansamycins are derived from polyketide units composed of such as acetic (C2 unit) and propionic (C3 unit) moieties through the polyketide biosynthesis pathway. The biosynthesis of the ansamycins has been reviewed [20,22]. 

The biosynthetic precursor of the chromophoric moiety was studied vigorously by several research groups. As a result it was established that during the biosynthesis of the ansamycins, a W-C7N unit (Fig. 4) was incorporated into the chromophores of both the benzenoid ansamycins, such as geldanamycin (85) [177, 178] and ansamitocin P-3 (81) [179], and also the naphthalenoid ansamycins, such as the rifamycins [22,180-182] and actamycin (69) [183]. 

It was also found that when [r- C]-AHBA (118) was administered to a streptovaricin C (44) producing culture, C-21 (the quinone methide carbonyl at 188.3 ppm) of 44 was specifically labelled (Fig. 5) [93], and the existence of 118 itself in the fermentation broth was reported [186,187]. It was also reported that the biosynthesis of the naphthalenoid ansamycin antibiotic actamycin (69) was markedly increased by the... 

Thus, naphthalenoid ansamycin antibiotics with a Cjy ansa moiety, naphthalenoid ansamycin antibiotics with a C23 ansa moiety, followed by the naphthalenoid ansamycin antibiotics with a C9 ansa moiety will be discussed. 

Naphthalenoid Ansamycin Antibiotics with a C 17 Ansa Moiety... 

If the carbons derived from a propionate unit are indicated as P and carbons derived from an acetate unit are indicated as A, the common biosynthetic building units and their sequence in the naphthalenoid ansamycin antibiotics with a Cn ansa moiety (and a part of the chromophore) are presented as P-A-P-P-P-P-P-P-A-P (Fig. 8). 

Funayama et al. suggested the possibility of the application of this model for the determination of the stereochemistry and biosynthetic pathway of the known ansamycin antibiotics [202]. According to this proposal, it is important to consider that, of the C17 ansa chain naphthalenoid ansamycins, the absolute configurations of rifamycins S (2) [16, 194] and B (1) [33, 42], and streptovaricin C (44) [94], were established by X-ray analysis. The rigorous biosynthetic studies of rifamycin S (2) indicated P-A-P-P-P-P-P-P-A-P as the biosynthetic sequence of the ansa chain and a part of the naphthalenoid moiety [197]. 

As examples of other naphthalenoid ansamycin antibiotics, rubradirin (70) and protorubradirin (71) were isolated from the fermentation broth of Streptomyces achromogenes var. rubradiris [109-114,117]. As indicated previously, protorubradirin (71) is the C-nitroso-analogue of 70, and it seems that 71 is the true secondary metabolite produced by S. achromogenes var. rubradiris. Thus, rubradirin (70), reported earlier, is the photo-oxidation product of protorubradirin (71). The ansa moiety of these compounds possesses a C9 chain, instead of a C or a C23 chain. 

The ansa moiety and part of the chromophoric unit of 70 and 71 consist of nine carbons, and the biosynthetic building units of this part, and their sequence are considered to be P-A-P-P-A-P (Fig. 12). Through a comparison of the ansa moiety of this compound with that of the ansa chains of the naphthalenoid ansamycin antibiotics with a Cp ansa chain (P-A-P-P-P-P-P-P-A-P), it seems that the first two to four biosynthetic units (P-A, P-A-P or P-A-P-P), and the last two to four units (A-P, P-A-P or P-P-A-P), are involved in the formation of the antibiotic, whereas the four biosynthetic units (P-P-P-P) in the center of the sequence are missing. 

It appears that the starting three units in the biosynthetic process (P-A-P) are universal to the naphthalenoid ansamycin antibiotics which possess an ansa moiety consisting of the Cp and C23 ansa chains, and also the C9 chains and even an early intermediate such as 72. 

Among the naphthalenoid ansamycin antibiotics, the streptovaricin complex was reported to inhibit focus formation by MSV (MLV murine leukaemia virions) and to inhibit the splenomegaly-induced Rauscher leukaemia by virus selectively [226,227]. On the other hand, awamycin (26) [34,35] was shown to possess activity against HeLa S3 cells and antitumor activity against experimental murine tumors. Antitumor activity tests on streptovaricin C (44) were conducted [226,227]. 

As implied above, there are two kinds of ansamycins, one that possesses a benzene ring and another that possesses a naphthalene ring in the structure these are known as the benzenoid and naphthalenoid ansamycins, respectively. The m-CyN unit is involved in the biosyntheses of the chromo-phores of both these ansamycins, and the polyketide biosynthetic pathway is also concerned in the biosynthesis of the ansa chains of both types of ansamycin and a part of the chromophore of the naphthalenoid ansamycins. 

Regarding the biosynthesis of the ansa chain and part of the chromophore of rifamycin S, it was shown that two molecules of acetic acid and eight molecules of propionic acid were incorporated, as shown in the figure. During the biosynthesis, the methyl moiety (C-34) at the sixth propionic acid residue (C-27, C-28, and C-34) was eliminated, and the bond between C-29 and C-12 at the seventh propionic acid (C-12, C-13, and C-29) was cleaved and an oxygen was inserted [8,9]. The other part of the naphthalene nucleus was shown to be derived from an m-CyN unit. The more detailed biosynthesis of various naphthalenoid ansamycins will be described later (Section 13.4). 

About 70 naphthalenoid ansamycins have been reported until now. In most of these ansamycins, the ansa part is composed of 17 carbons, like the rifa-mycins, and in other instances the ansa moiety is composed of 23 carbons, like the naphthomycins [1]. Models for the biosynthesis and stereochemistry (Celmer s model), as applied to macrolide antibiotics [2], have been developed for these two types of ansamycins [3]. 

It was also shown that the absolute configuration between C-8 and C-16 of rifamycin B [4] and streptovaricin C [5] are the same. In addition, a methyl moiety derived from a propionic acid unit attached to the Cl moiety of the m-CyN unit corresponds to the methyl moiety at the C-20 position of the naphthalenoid chromophore. In all of these alkaloids, an oxygen atom derived from propionic acid remains at the C-19 position. This information can be applied to the elucidation of the chemical structure and stereochemistry of various naphthalenoid ansamycins, for example awamycin, which belongs to naphthalenoid ansamycins [6]. 

In the meantime, the biogenetic units between C-11 and C-20 of the naphthalenoid ansamycins with a Ciy ansa chain, and those between C-17 and C-26 of the naphthalenoid ansamycins with a C23 ansa chain, are the same (P-P-P-A-P). 

Fig. 1. Structures of naturally occurring naphthoquinonoid and naphthalenoid ansamycins...

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