Glycosyl 2-phenylseleno

The reaction of cyclohexene with AT-(phenylseleno)phthalimide in the presence of (S,S)-hydrobenzoin in methylene chloride afforded two diastereomeric oxyselenides (38 and the (li , 2R) diastereomer) in a 1 1 ratio. Compound 38 was separated and converted into the olefin 39 via selenoxide elimination. The second PhSeOTf promoted oxyselenenylation reaction gave only the cis fused bicyclic dioxane 40. Oxidation and subsequent elimination provided the olefin 41. This is the key intermediate for the syntheses of the cyclitols 42 and 43, that were obtained from a series of classical reactions as indicated in the Scheme. Oxyselenenylation reactions have also been employed to promote glycosylation reactions [43]. 

The final SnCl2-mediated glycosidation of the 2-phenylseleno glycosyl fluoride 267 and the alcohol 311 gave the oligosaccharide 312 with complete stereocontrol, which was subjected to Sinay s orthoester formation to afford everninomicin 13,384-1 (314) after successful deprotections, including reductive debenzylation and TBAF-induced desilylation. 

Scheme 10. Orthoester formation via 1,2-phenylseleno migration followed by glycosylation and ring closure after syn elimination. PG - protecting group. Note substituents on carbohydrate rings have been deleted for clarity.

The Michael addition of thiols to enal (24) proceeded with high stereoselectivity to give D-arabino configurated 2-deoxy-3-thioethers (25) in excellent yields. The use of sialic acid derived glycosyl donors with participating 3-phenylthio> and 3-phenylseleno-groups is referred to in Chapters 3 and/or 4. 

Two papers have reported on die addition of diphenyl diselenide to tri-O-acetyl-D-ghical and -D-gaiactal which gives rise to l-phenylseleno 2-azido-2-deoxy glycosides as rqiorted in Ch ter 10. See ref. 119 for a comment on phenyl seloioglycosides as glycosylating agents. 

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