Anomalous Zemplén deacylation reactions of α- and β-d-mannopyranoside derivatives

A Versatile Synthesis of Pentacosafuranoside Subunit Reminiscent of Mycobacterial Arabinogalactan Employing One Strategic Glycosidation Protocol

“…Without much difficulty, orthoacetate 13 was transformed into di‐ O ‐silyl ether 17 , which was treated with allyl alcohol and AuBr 3 to obtain allyl arabinoside 18 with acetate at the C‐2 position. In continuation, acetate was saponified easily under Zemplén conditions and protected again with levulinic acid under DIC/DMAP conditions to obtain levulinoate 19 . The next step in the sequence is to convert the allyl glycoside into hemiacetal 23 ; however, under normal conditions, the Pd‐catalysed allyl deprotection resulted in the formation of compounds 20 – 22 .…”

“…Without much difficulty,o rthoacetate 13 was transformed into di-O-silyl ether 17,w hichw as treated with allyl alcohol and AuBr 3 to obtain allyl arabinoside 18 with acetate at the C-2 position. In continuation,a cetate was saponified easily under ZemplØnc onditions [34] and protected again with levulinic acid under DIC/DMAP conditions to obtain levulinoate 19.T he next step in the sequence is to convertt he allyl glycoside into hemiacetal 23;h owever,u nder normalc onditions, the Pd-catalysed allyl deprotection resulted in the formation of compounds 20-22.T he formation of compound 20 can be envisaged from the migration of levulinoyl group from the C-2 positiont ot he more reactive hemiacetal positionw here the formationo fb icyclic compound 21 was more intriguing. [35] Departure of the allyl glycoside canr esult in the formation of an oxocarbenium ion intermediate that could be trapped by the ketone intramolecularly to give bicy-clic 21 or be trapped by the methanol to afford compound 22.…”

Synthesis of novel N-acyl-β-d-glucopyranosylamines and ureas as potential lead cytostatic agents

“…Subsequent deacetylation of analogues 6a-f under Zemplén conditions (Agoston et al, 2001) at 0°C for 20 min afforded the fully deprotected derivatives 7a,b,d,e,f after flash chromatography, in good yields (72-82 %). Deprotection of acyl urea 6c either with basic or acidic transesterification conditions (sodium methoxide, methanolic ammonia, methanolic hydrogen chloride) was unsuccessful, due to faster cleavage of the N-acyl moiety than removal of the O-protecting groups.…”

The synthesis of a series of modified mannotrisaccharides as probes of the enzymes involved in the early stages of mammalian complex N-glycan formation

“…It is worth noting that this deprotection was slow to proceed to completion, a phenomenon previously reported for the Zemplen deprotection of structurally similar acylated branched mannotrisaccharides. 26 The regioisomeric trisaccharide 9 was synthesized from the common, selectively protected mannoside 15. gave disaccharide 27 ready for addition of the final sugar moiety.…”