Skeleton polyether

A classification based first on ion specificity, then on stmctural features has been suggested for the polyethers (7). Another method uses the presence of unsaturation or of aromatic groups in the molecular skeleton (8). In this review the compounds are classified based on the number of carbons in the backbone according to the numbering system proposed in reference 9. The carbon backbone or skeleton refers to the longest chain of contiguous carbons between the carboxyl group and the terminal carbon. 

Table 1 Hsts the polyether antibiotics arranged by the number of carbons in the skeleton. Many of these compounds were isolated independendy in separate laboratories and thus have more than one designation. The groups are subdivided depending on the number of spiroketals. Two classes fall outside this scheme the pyrrole ether type containing a heterocycHc ring, and the acyltetronic acid type, that has an acyHdene tetronic acid instead of a carboxyHc acid. These compounds are ionophores and because of their common features are included as polyethers.

The C-30 skeleton group accounts for about 60% of the polyethers for which stmctures have been deterrnined. Most of these contain a sugar moiety, usually 2,3,6-trideoxy-4-0-methyl-D-erythto pyranose [65104-53-2]. The single spiroketal subset is ikustrated by two closely related compounds, semduramicin (4, R = H, R =) and maduramicin alpha (4, R = CH3, R = ). Maduramicin is marketed as an anticoccidial and semduramicin is under development for the same appHcation. 

Bacterial metabolites include novel alkaloids, peptides, and macrocyclic lactams. The most unusual metabolites from dmoflagellate are polyethers, both linear and macrolides. The latter were also found abundantly in brown seaweeds (Chart 7.6.FA/PO). Filamentous fimgi have afforded new isoindole and quinazoline alkaloids, as well as new-skeleton steroids (Chart 7.6.A/P/I). 

The addition of rings into the skeleton of the polyether can also lead to a stiffening of the skeleton and produce selectivity shifts.33... 

The masked tetrahydroxyketone 28 (Fig. 7) which can theoretically give isomers 29 and 30, was found to yield isomer 29 exclusively (26). The structure of 29 was proven by X-ray analysis. Similarly, dibromodihydroxyketone 31 can give either isomer 32 or 33. Upon cyclization, isomer 32 was the product formed (27) and its structure was also established by X-ray (28). The recently reported total synthesis of ionophore A-23187 (29), a polyether antibiotic whih possesses the 1,7-dioxaspiro[5.5]undecane skeleton having a conformation equivalent to 29 and 32 confirms these results. 

Sharpless et al. noted an increase of the specific rotation with increasing generation number in the case of dendrimers having trimesic acid as achiral core unit and a completely chiral polyether dendrimer skeleton based on 1,2-diol branching units (Fig. 4.72) [27]. The observed molar optical rotations corresponded to... 

In another study, success was achieved in clipping a phenanthroline-polyether ring of the usual type on to each of three arms of a much larger cryptand-like skeleton that incorporated a phenantroline group in each arm (Figure 6.10). The prod-... 

The reaction of carbonyl compounds and allylmetal reagents is an important transformation in organic synthesis. Advances in stereoselective carbonyl allylation reactions have been spurred by interest in the synthesis of polypropionate-derived natural products, carbohydrates and other polyhydroxylated compounds. These reactions are ideally suited for the construction of stereochemically rich acyclic skeletons. Additionally, cyclic polyether-containing natural products, among others, have inspired chemists to investigate ring-closing allylation reactions. This review will focus on recent developments in the allylation reaction, with special emphasis on its application towards the synthesis of natural products. 

Since the YTX molecule has a hydrophobic polyether skeleton, some hemolytic activity could be expected. However, the toxin did not induce hemolysis [3], Even an important hemolytic property was described to desulfated YTX by interacting with alfa-helix membrane peptides [20],... 

Along the Florida coast and in the Gulf of Mexico, the dinoflagellate Kare-nia brevis often forms blooms known as red tides that lead to massive flsh kills. The causative toxins, the brevetoxins, possess an unprecedented highly oxygenated structure. The brevetoxins were the first cyclic polyether compounds to be chemically identified, and were shown to possess a characteristic structural feature of a ladder-like skeleton consisting of trans-fused polyether rings. 

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