Unsaturated carbohydrates, carbon chain branching

An important example of a carbohydrate ester is the so-called cord factor from Mycobacteria spp. This contains an ester of the disaccharide, trehalose, with two molecules of a complex acid, mycolic acid. The latter is a general term embracing a whole series of fatty acids containing 60-90 carbons. They are hydroxy fatty acids which differ in their degree of unsaturation and chain branching (Section 1.9). In the example given in Fig. 2.6 the mycolic acid is the 60-carbon compound found in Mycobacterium smeg-matis. 

These developments are here surveyed within the same guidelines as were used previously that is, discrete compounds will be dealt with which have one carbon-carbon multiple bond within the main sugar-chain. As before, enols, enediols, enones, dienes, and dienones will not be considered, nor will unsaturated cyclitols or heterocyclic compounds composed partially of acyclic carbohydrates. The first derivatives having carbon substituents attached by a double bond at a branch-point have now been prepared by application of the Wittig reaction to suitably protected glycosulose compounds these compounds are also arbitrarily excluded, as are ethers or acetals bearing unsaturated substituents. 

Some natural products outside the carbohydrate field - particularly those with highly hydroxylated cyclohexane ring components - have been subject to synthetic studies which are dependent on the mercury-based rearrangement reaction the alkaloid (+)-lycoricidine (30) [12] and the cycloheptane-based (+)-calystegine 62(31) which stimulates growth of nitrogen-fixing Rhizobia [29] are examples, and compound 32 which offers novel access to the anthracyclinone components of anthracyclin anti-cancer compounds, has been produced by cycloaddition of a naphthalene-based o-xylylene to a 2,3-unsaturated hex-4-uloside followed by carbocyclization of the product by use of the mercuration procedure [30]. Studies on HMG-CoA reductase inhibitors like compactin have afforded the tetra-carbon-substituted 33 made from a hex-5-enopyranoside with deoxy-branch chains at C-2, C-3 and C-4 [31]. 

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