Glycosidation, intermediates

Emetine and other related alkaloids are biosynthesized from dopamine and secologanin of mevalonate origin through a glycosidic intermediate, desacetylipe-coside (Fig. 5.2.14) (6). Alkaloids having biogenetic importance, such as protoemetine, have also been found in the same plant (7). 

The synthesis of eleutherobin, which has antitumor activity and cytotoxic properties similar to those of taxol, has also been reported (Scheme 41) [181, 182], The carvone-derived acceptor was stereoselectively glycosylated with the arabinosyl trichloroacetimidate (a P = 8 1). The resulting glycoside intermediate was treated as for sarcodictyins A and B to yield eleutherobin and eleuthosides A and B. 

Structurally, 0-glycosides are mixed acetals that involve the anomeric position of furanose and pyranose forms of carbohydrates. Recall the sequence of intermediates in acetal formation (Section 17.8) ... 

In this paper, we study the stabihty of the carbonium ion intermediate formed in the cleavage of a glycosidic bond by lysozyme. It is found that the electrostatic stabilization is an important factor in increasing the rate of the reaction step that leads to the formation of the carbonium ion intermediate. Steric factors, such as the strain of the substrate on binding to lysozyme, do not seem to contribute significantly. 

An attractive feature in this reaction is the possibility of direct substitution and formation of unblocked sugar derivatives containing one or more chlorodeoxy function in essentially two steps. Another facet is the formation of methyl 4,6-dichloro-4,6-dideoxy-hexosides from certain methyl glycosides in one step. Such compounds could be valuable intermediates in the synthesis of dideoxy and diamino sugars of biological importance. 

The completion of the synthesis of the polyol glycoside subunit 7 requires construction of the fully substituted stereocenter at C-10 and a stereocontrolled dihydroxylation of the C3-C4 geminally-disub-stituted olefin (see Scheme 10). The action of methyllithium on Af-methoxy-Af-methylamide 50) furnishes a methyl ketone which is subsequently converted into intermediate 10 through oxidative removal of the /j-methoxybenzyl protecting group with DDQ. Intermediate 10 is produced in an overall yield of 83 % from 50) , and is a suitable substrate for an a-chelation-controlled carbonyl addition reaction.18 When intermediate 10 is exposed to three equivalents of... 

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