Carbohydrates and Other Polyhydroxylated Compounds

A number of different aldehydes have been utilized as acceptors, which in combination with GO, HA, DHA, and HB donors yielded a plethora of structurally 

In the complementary direction, the poor tolerance of TalB toward HA could be improved by the mutation S176A (i.e., the double mutant the equivalent A129 in FSA, which generates a more hydrophobic environment and facihtates the accommodation of the ethyl moiety of HA [32h]. Indeed, in a competition aldol addition reaction of equal concentrations of both DHA and HA to 3-hydroxypropionaldehyde, showed practically identical conversion 

As pointed out before, the donor quality influences the acceptor tolerance. For instance, FSA mutant was found to furnish 5-O-benzyl-D-xylulose in 60% conversion while FSA wild type gave only a 35% under optimized conditions (Table 16.1, entry 17, other examples in entries 3, 4, 22, 38, 42, and 45). n-Threose (Table 16.1, entry 47) was an excellent acceptor when HA was the donor, whereas no product was detected either with DHA and GO. In this case, a nice cascade reaction was accompHshed consisting of, first, the homoaldol addition of GO, followed by 

Conversions and isolated yields obtained with FSA and TalB catalysts. Isolated yields, nr no reaction, empty no data found. 

and R228, a mutant was found to show enhanced tolerance 

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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. 

Reactions of polyhydroxyl compounds such as carbohydrates with DAST lead to replacement of one or two hydroxyl groups by fluorine, more fluorine atoms are not introduced even when a large excess of the reagent is used [132, 139, 147] Although diethylaminosulfur tnfluonde (DAST) is the most popular, other dialkylaminosulfuranes, such as diisopropylamino- [95] pyrrolidino [95 109 /27], dimethylamino- [148], piperidino- [148] and particularly morpholinosulfur trifluonde [148,149, ISO], are also used as fluonnating agents to convert alcohols into fluorides... 

The stereoselective synthesis of carbohydrates from acyclic precursors is a research topic that has attracted considerable attention over the past decadeT Efforts in this area are easily justified and have maximum impact particularly when directed toward rare sugars or other polyhydroxylated molecules that are not conveniently accessed via classical "chiron" approaches.2 An underlying theme of such efforts, of course, is the development of practical synthetic methodology that will find broad application in the enantio- and diastereoselective synthesis of natural products, their analogues, and other compounds of biological interest. 

Selective biotransformations of polyhydroxylated compounds, such as carbohydrates, have been used since some time ago for the activation or protection of some of their hydroxyl groups, allowing different chemical transformations to be carried out without the need for using tedious protection and deprotection steps. Monoesters of some monosaccharides and disaccharides have been obtained using lipases or proteases in polar organic solvents [3]. In addition, the utility of biotransformations in other natural products such as nucleosides [4] or steroids [5] has also been reviewed. 

This catalytic epoxidation method has been applied to the synthesis of a variety of natural products, particularly polyhydroxylated compounds, including carbohydrates (54) and macrolides. In addition, this reaction has been used for commercial synthesis of disparlure, a gypsy moth pheromone [J. T. Baker Co. (55) and the Shanghai Institute for Organic Chemistry (56)], and more importantly, glycidol, a versatile intermediate for synthesis of /3-blockers and other functionalized chiral molecules (Arco Co.) (Scheme 23) (57). 

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