Alkaline -elimination

The amino functional group is not commonly encountered in steroid synthesis except perhaps in steroidal alkaloids. However, certain elimination reactions have been shown to have theoretical and limited preparative importance, largely due to the efforts of McKenna and co-workers. The Hofmann rule for 2 elimination predicts that alkaline elimination of quaternary ammonium salts will occur towards the carbon carrying the most hydrogen atoms cf. the converse Saytzeff orientation, above). In cyclohexyl systems, the requirement for diaxial elimination appears to be important, as in other 2 eliminations, and the Hofmann rule frequently is not obeyed [e.g., (116) (117)]. 

Structural studies by periodate oxidation-alkaline elimination indi-... 

The third adduct (669) was an ether after similar functionalization of the dihydropyran ring to the L-gulo system, 3-0-(benzyl 4-deoxy-/3-L-gulopyranosid-4-yl)- l,2 5,6-di-0- isopropylidene-a-D- glucofuranose (672) was obtained. Cleavage of the ether bond between the sugar units was possible35 by alkaline -elimination. 

C of the acetal and the Lewis acid. Enantiomerically pure cyclic acetals 49 deriving from chiral diols open a route to adducts 50, which can be successively elaborated to chiral /9-hydroxyesters upon oxidation of the alcoholic group followed by alkaline elimination (equation 33). 

Figure 8. Susceptibility to alkaline / -elimination A is from References 47 and 48 B is from Reference 64...

The presence of such enolic ions in alkaline solutions of sugars was shown later by Isbell et al. (67). The mechanism of the alkaline -elimination reaction of substituted serine and threonine glycosidases based on this concept is shown in Figure 9. 

Harbon et al. (72) hydrolyzed the alkaline -elimination product from OSM with 3N HC1 (90 min at 100°C). Pyruvic acid was found in the hydrolyzates to the extent of 83% of the serine lost, and 2-keto-butyric acid accounts for 63% of the threonine which had disappeared. Niederhiser et al. (73) reported quantitative conversions to these keto acids by alkaline treatment of bile mucins and showed that the elimination can be followed by an increase in the absorption at 240 nm. 

Amongst the proteoglycans synthesized by the developing embryos of the Japanese sea-urchin Hemicentrotus pulcherrimus, dermatan sulphate proteoglycan is the major component. This sulphated polysaccharide seems to have an unusual structure in comparison with the normal mammalian dermatan sulphates in that the latter have only polysaccharide-protein linkages which are resistant to alkaline elimination. 

A. Methods of Preparation a. Alkaline Elimination of Halogen or Sulfonic Ester Groups... 

Although the epoxy ring is more readily formed than the hydrofuran ring, the latter is more stable to further anionic attack. Consequently, an epoxy anhydro sugar may be the initial product of alkaline elimination of a halogen or sulfonic ester, but it may in turn be subjected to intramolecular rearrangment to a hydrofuran anhydro sugar (VIII —> IX) 42). 

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