Carbohydrates, stereoselective alkene

The chiral pool refers to readily available optically active natural products, some of which are commercially used in quantities of 10 -10 tonnes per year [5], Among them, the most inexpensive compounds are a-amino acids, like monosodium L-glutamate, or carbohydrates, like dextrose or sorbitol. The success of the second method depends on the availability of particular catalysts. One rather special example, how efficient stereoselective synthesis can work, is the syn-selective aldol reaction followed by a stereoselective alkene hydroboration and ketone reduction (Figure 1.8) which were used by Paterson et aJ. [26] to synthesize intermediates for the antibiotic oleandomycin. According to Paterson et al., four new stereocenters are formed in only two synthetic steps [9]. For the purpose of separating racemic mixtures... 

Reactions of alkenes such as 120 with a-chloronitrosoalkanes of type 119 proceed under very mild conditions and result in the formation of nitrones 121 that can be easily hydrolyzed into hydroxylamines 122 (equation 86) . Chiral carbohydrate-derived a-chloronitrosoalkenes 123 possess enhanced reactivity and produce good stereoselectivity in reaction with prochiral alkenes such as 124 (equation 87) . ... 

Intramolecular oxime-alkene cycloaddition has been proved to proceed with complete stereoselectivity in carbohydrate derived hydroxylamine 171 (equation 112). 

Several diastereoselective HDA (hetero-Diels-Alder) reactions of a,/3-unsaturated carbonyl compounds and electron-rich alkenes have been exploited to gain carbohydrate derivatives with good diastereomeric excess. In HDA reaction, up to three chiral centers are formed with high stereoselectivity at each chiral carbon [78]. 

Reactions of ketenes, e.g. dihaloketenes, with electron-rich alkenes bearing the chiral auxiliary, for example enolethers, carbohydrate-derived anomeric enolethers or ketene acetals, lead to cyclobutanones with moderate stereoselectivities. 

Cycloadditions of conjugated dienes to alkenes (dienophiles) provide rapid access to substituted cyclohexenes. In view of the widespread occurrence of carbocyclic compounds in nature, and the recent demands for the synthesis of enantiomerically pure compounds, it is not surprising that carbohydrates were examined as substrates for Diels-Alder reactions. Both carbohydrate-derived dienophiles and dienes have been described in the literature. The products of reactions involving these types of compounds, referred to as annulated sugars, are most often formed stereoselectively. The hi levels of both diastereofacial selectivity and endo/exo selectivity sometimes result in the formation of a single cycloadduct from among several possibilities. 

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