Spiroketal glycosides

Spiroketal glycosides are widely distributed in nature [56] displaying a broad spectmm of biological activities. Typical examples are the papulacandines, tricyclic spiroketal glycosides... 

The studies on homologation of glycals towards synthesis of the ulosonic acids have been extended recently by the use of an unique sequence of glycosylation. Thus, alkylidene[l,5]-C-H insertion forming the spiroketal glycosides led to DAH ammonium salt in few steps shown in Scheme 41 [121]. 

B. B C40HJ6O24S, Mr 952.93, mp. 208-210°C. Sulfur-containing spiroketal glycosides with significant hypocholesteremic activity. They are isolated from the wood of the Taiwanese shrub Breynia officinalis (Euphorbiaceae). 

Tricyclic spiroketal glycosides from cultures of Pa-pularia sphaerosperma. The P, have antibiotic activities. Similar to the lipopeptide antibiotic echinocandin B, they inhibit the biosynthesis of /S- glucans in yeasts. 

Martin et al. (289) utilized the chiral bicyclic lactone 88 for a total synthesis of (+)-phyllanthocin (Scheme 6.62). Cycloaddition of 88 to acetonitrile oxide, generated in situ from hydroximoyl chloride (89), furnished cycloadduct 90 in 45% yield together with other regio- and stereoisomers. After several steps, methyl glycoside (91) could be obtained. From this, reduction-hydrolysis gave the aldol that was subjected to acid-catalyzed spiroacetalization to produce spiroketal (92), and eventually (+)-phyllanthocin (93) after two additional steps (289). The... 

Gomez, A M, Valverde, S, Fraser-Reid, B, A route to unsaturated spiroketals from phenylthiohex-2-enopyranosides via sequential alkylation, allylic rearrangement, and intramolecular glycosidation,... 

Thus, the glycosidation of 190 followed by oxidation led to the corresponding 2-oxopropyl glycosides 191, which on treatment with [(trimethylsilyl)diazomethane]lithium at low temperature underwent insertion reaction to provide spiroketal derivative 192. Subsequent ozonolysis of 192 liberated an unstable ketoaldehyde, which was immediately oxidized to the carboxylic acid, isolated as its methyl ester 193. Removal of the 2-oxopropyl group was accomplished using Sml2, to afford DAH derivative 194. 

Spiro- and 1,2-fused Bicyclic Systems. Spiroketals involving C-glycosidic structures can more correctly be considered to be chain-extended sugars (Chapter 2, Section 2.3). Sharpless asymmetric dihydroxylation of 5-aryl-2-vinylfuran... 

A different approach to spiroketals of this series is illustrated in Scheme 20. The anthracene derivative (148) was made from the glycal by way of the 1-lithio-derivative and reduced to the 2-deoxy-p-C-glycoside. ... 

The glycosidic spiroketal (16) was formed in good overall yield by cyclisation of the strategically functionalised hemiketal (15), obtained from perbemylated gluconolactone (14) as shown in Scheme 2. Although formally the cyclisation is not reductive, the presence... 

Methylenation of carboxylic acid derivatives A variety of carboxylic acid derivatives can be transformed into heteroatom-substituted olefins (Table 4.9). The C-glycoside congeners are easily prepared by the direct methylenation of aldono-lactones with 30 (entry 2) [67]. Since the generation of titanocene-methylidene 4 from 30 only produces methane as a by-product, 30 can be employed for the olefination of acid-sensitive substrates such as silyl esters and acylsilanes (entries 4 and 6). The tolerance toward acid-sensitive functional groups also allowed the application of dimethyltitanocene 30 in the conversion of a lactone bearing an acetal moiety into the corresponding cyclic vinyl ether, which was further transformed into the spiroketal ketone upon oxidation and rearrangement (Scheme 4.24) [69]. 

Non-tethered intramolecular glycosidations which produce spiroketals [1] or natural products [2, 3] have been reported these will not be discussed here. 

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