Sugars chirality

Other methods that rely on the use of the carbohydrate template to construct fused carbocycles have been successfully investigated as strategies for the formation of carbocycles from sugars. Chirality transfer from the sugar to the new carbocycle is observed for these reactions. These annulation reactions are the subject of the second part of this section. 

A commoner way to make heterocycles by pericyclic reactions is to use 1,3-dipolar cycloadditions. These often occur without catalysis and so are compatible with many other reactions. The starting material 182 for this asymmetric synthesis of swainsonine was derived from a natural sugar (chiral pool strategy, chapter 23). An exceptionally stereoselective Wittig reaction gave the Z-alkene 183 (chapter 15) and the alcohol was converted into the azide 184 with diphenylphos-phoryl azide.24... 

Keywords Anhydro sugars Chiral heterocycles Synthesis... 

In our laboratory we have developed a novel approach to the synthesis of an enantiopure AB segment of the anthracyclinones based on a new type of chiral substrate, enone 131 [131,132]. Di-O-acetyl-L-rhamnal (129), obtained from L-rhamnose, was converted into benzyl glycoside 130 [133] by the Perrier procediue [134], Hydrolysis and oxidation of the C-4 (carbohydrate numbering) hydroxyl group in 130 gave enone 131 in 67% overall yield (Scheme 27). It is worthy of note that uloside 131 is also readily available from a non-sugar chiral pool. Enzymatic resolution of racemic l-(2-furyl)ethanol yielded the (S)-enantiomer 138 in 98% ee [135]. This was transformed in two steps, by known procedures, into ulose 139 (Scheme 28) [136,137] from which uloside 131 was readily obtained. 

The marine natural product Reiswigin A 52 has been synthesized from the known compound 51.Two Claisen rearrangements serve as key transformations for chirality transpositions. The synthesis is notable in utilizing the sugar chiral centres to introduce four other chiral centres, while all the original stereocentres are lost (Schone 14). 

A major trend in organic synthesis, however, is the move towards complex systems. It may happen that one needs to combine a steroid and a sugar molecule, a porphyrin and a carotenoid, a penicillin and a peptide. Also the specialists in a field have developed reactions and concepts that may, with or without modifications, be applied in other fields. If one needs to protect an amino group in a steroid, it is advisable not only to search the steroid literature but also to look into publications on peptide synthesis. In the synthesis of corrin chromophores with chiral centres, special knowledge of steroid, porphyrin, and alkaloid chemistry has been very helpful (R.B. Woodward, 1967 A. Eschenmoser, 1970). 

Another name for glucitol obtained by reduction of d glucose is sorbitol it is used as a sweetener especially in special diets required to be low in sugar Reduction of D fructose yields a mixture of glucitol and mannitol corresponding to the two possi ble configurations at the newly generated chirality center at C 2... 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

A large number of chiral crowns have been prepared by numerous groups. The reader is directed to the tables at the end of this chapter to obtain an overview of these structures. It would not be useful to try to recount the synthetic approaches used in the preparation of all of these compounds we have chosen rather to subdivide this mass of compounds into three principal groups. The groups are (1) Cram s chiral binaphthyl systems (2) chiral crowns based on the tartaric acid unit and (3) crowns incorporating sugar subunits. These are discussed in turn, below. 

As in the previous categories in this section, there are numerous compounds which have been prepared based on a sugar subunit. Examples may be found in Refs. 7,35,42-45, 57, 82-85, 117—121,175,176,193 and 208. Much of the work in these references has been reported by Stoddart and his coworkers, who have pioneered this field. As with the compounds prepared by Cram, the goal was to prepare a chiral receptor for ammonium ions which could be utilized in enzyme model studies. 

Xylose is a common sugar found in many types of wood, including maple and cherry. Because it is much less prone to cause tooth decay than sucrose, xylose has been used in candy and chewing gum. Assign ft or S configurations to the chirality centers in xylose. 

Figure 25.2 Some naturally occurring D sugars. The -OF group at the chirality center farthest from the carbonyl group has the same configuration as

Assign R or S configuration to each chirality center in the following monosaccharides, and tell whether each is a o sugar or an t. sugar ... 

Aldotetroses are four-carbon sugars with two chirality centers and an aldehyde carbonyl group. Thus, there are 22 = 4 possible stereoisomeric aldotetroses, or two d,l pairs of enantiomers named erythrose and threose. 

The addition of (Z)-3-(alkylamino)-2-butenoic acid esters to nitroalkenes derived from open-chain sugars gave a 50 50 mixture of diastereomeric products 32. The reaction of chiral 2-sub-stituted l-(2-nitroethenyl)pyrrolidines with zinc enolates of 3-substituted tetrahydro-2//-pyran-2-oncs in 1,2-dimethoxyethane at — 78 °C afforded the corresponding 3,3-disubstituted products in 82-96% ee via an addition-elimination process33. The stereochemical course of the reaction was determined by chemical correlation of (S)-( )-3-ethyltetrahydro-3-(2-ni-troethenyl)-2//-pyran-2-one with ( + )-quebrachamine. 

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