Stereoselectivity using DAST

Another useful modification of this reagent is the reaction of CF3CCI3 with zinc and DMF in the presence of AICI3 [60, 63] (equation 53). The alcohol product can be treated subsequently with DAST, thionyl chloride, or phosphorus chlorides to afford the allyl substitution product regio- and stereoselectively [66] (equation 54). 

Halogen atoms can be stereoselectively introduced by ring-opening of y-azir-idinyl-a,P-enoates (Scheme 2.39). Treatment of 149 with diethylaminosulfur tri-fluoride (DAST) results in stereospecific ring-opening to yield fluorinated derivative 150 [59]. A related stereoselective conversion of y-aziridinyl-a,P-enoates 151 into allyl halides 152 by use of lithium halide in the presence of Amberlyst 15 was also reported recently [60]. 

The special precautions required for reactions with toxic gases make the use of sulfur tetrafluoride inconvenient. However, the introduction of diethylaminosulfur trifluoride (DAST) removes these handling problems since this reagent is a liquid and is safe below 40 3C 38,39. Anomeric hydroxy groups are replaced stereoselectively by diethylaminosulfur trifluoride in high yield40. 

Notes and discussion. This procedure is used for the stereoselective formation of a, 2-trans configured fluoride. The use of the orthoester precursor allows differentiation of the 0-2, which is a strategic manipulation that is often useful in oligosaccharide synthesis. The product also has participatory acetyl protection of 0-2, which may be used to direct the stereochemistry of its glycosylation reaction. The direct treatment with DAST has the advantage that only a single anomer is formed, compared to the anomeric mixture which results from the two step process of hydrolysis to the hemiacetal followed by DAST treatment. 

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