Natural products aglycon

Natural Products. 16-Membered macroHdes are divided into leucomycin- and tylosin-related groups, which differ in the substitution pattern of their aglycones. Multi-factor complexes are usually produced and some compounds have been isolated from culture broths of different organisms and then been given different names. 

As impressive as the oligosaccharide domain is, calicheamici-none, the aglycon sector 7 (see Scheme 3) is the most striking substructure of calicheamicin y. The rigid bicyclic framework of 7 accommodates an unusual allylic methyl trisulfide and a novel pattern of unsaturation that had not been encountered in natural products before. 

The oxidative formation of p-benzoquinones from anilides such as 7-108 was used for the synthesis of the core scaffold of the natural products elisabethin A (7-106) and pseudopterosin A aglycone (7-107) (Scheme 7.30). Exposure of anilide 7-108 to DMP [53] led to the formation of the o-imidoquinone 7-109, which underwent an intramolecular Diels-Alder reaction to give 7-110 in 28% yield after hydration. In a competitive pathway, the p-quinone 7-111 is also formed from 7-108, which on heating in toluene again underwent an intramolecular Diels-Alder reaction to give cycloadduct 7-112 in 25% overall yield. Hydrolysis of 7-112 furnished the carbocyclic skeleton 7-113 of elisabethin A (7-106). 

Avermectins are a series of eight related natural products with potent anthelmintic and insecticidal activities produced by Gram-positive, filamentous soil bacteria Streptomyces avermitlis [30]. All of these molecules share a 16-membered macrocyclic PK aglycon with the attachment of a dimeric o-methyl-a-L-oleandrose to C-13 (Figure 13.6). Avermectins differ in three substituents R1 R2 and R3. Rj can be either a hydroxyl or hydrogen, in which case there is a double bond between the C-22 and C-23. R2 can be either methyl or ethyl, and R3 can be either hydrogen or methyl. 

Saponins are glycosylated secondary metabolites that are widely distributed in the Plant Kingdom.3,4 They are a diverse and chemically complex family of compounds that can be divided into three major groups depending on the structure of the aglycone, which may be a steroid, a steroidal alkaloid, or a triterpenoid. These molecules have been proposed to contribute to plant defense.3 6 Saponins are also exploited as drugs and medicines and for a variety of other purposes.4 Despite the considerable commercial interest in this important group of natural products, little is known about their biosynthesis. This is due in part to the complexity of the molecules, and also to the lack of pathway intermediates for biochemical studies. 

Intermolecular cycloadditions or Diels-Alder reactions have proved to be a successful route to several valuable intermediates for natural product syntheses. In creating new chiral centers, most of these reactions apply single asymmetric induction. As mentioned in Chapter 3, in the asymmetric synthesis of the octa-hydronaphthalene fragment, the Roush reaction is used twice. Subsequent intramolecular cyclization leads to the key intermediate, the aglycones, of several natural antitumor antibiotics. On the other hand, the Diels-Alder reaction of a dienophile-bearing chiral auxiliary can also be used intramolecularly to build... 

A C9-C14 cycHzation forms the ring C and leads to the amphilectane skeleton which is foimd for instance in pseudopterosins A-F (10, aglycone) [10] isolated from Pseudopterogorgia elisabethae extracts stemming from a Bahamian collection site (Fig. 5). This class of natural products can be characterized as... 

Saponins are glycosylated natural products with soap-like properties [3]. The aglycones (sapogenins) are either triterpenoid- or steroid-derived. Dicotyledonous... 

Previously in the literature, for simplicity, under indolo[2,3-fl]carbazole natural products, only the indolo[2,3-fl]carbazoles with and without the pyrrolo[3,4-c]ring were considered. The recent past has witnessed an explosive growth of these natural products. Due to this, and the difference in the aglycon framework, in this article we have classified the indolo[2,3-fl]carbazoles with a pyrrolo[3,4-c]ring as indolo[2,3-fl]pyrrolo[3,4-c]carbazole alkaloids, and the indolo[2,3-fl]carbazoles without a pyrrolo[3,4-c]ring as simple indolo[2,3-fl]carbazole alkaloids. 

This checklist of chalcones, dihydrochalcones, and aurones contains compounds of these classes reported in the literature as natural products to the end of 2003. Compounds published before 1992 are cross-referenced to numbered entries in volumes 1 and 2 of the Handbook of Natural Flavonoidf using the abbreviations H1 and H2, respectively. Compounds published from 1992 to 2003 are cross-referenced to Table 16.1-Table 16.15 using numbers in bold type. The compounds are listed according to the system outlined in Section 16.1.1, with the exception that isoprenylated derivatives are included under the heading of aglycones. Bn, benzyl. 

Table 4 lists some representative examples of macrocyclization via the intramolecular SNAr reaction. In addition to the 14- and 17-membered cycloisodityrosines shown in the table, a variety of mono-, bi-, and tricyclic systems from the vancomycin family of natural products including the orienticin C/40 vancomycin J44-47 and teicoplanin[48 aglycons have been prepared by this method.This versatility, coupled with the high yields obtained, makes the intramolecular SNAr reaction currently the most widely applicable method for cycloisodi-tyrosine formation. 

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