Carbocyclization of the Sugar Backbone

Variants of this reaction used to construct inositols were introduced by Prestwich [301] and Sinay [302]. These new methods have been reviewed elsewhere [303]. Additionally, a chapter of this book deals with methods of carbocyclization to form carbasugars from sugars. 

SCHEME 11.80 Reagents i (1) NBS, BaCOj, (2) MeONa, MeOH and (3) NaH, BnBr ii HgCE iii (1) MsCl, pyridine, (2) LiBH4, CeClaTHjO and (3) HOzCCHzCHfCHslCHfOTBSfCHjCHfOTBSlCHfCHs) CHzOBr, EDCl iv LDA, HMPA, TBSCl, and then heat. 

Dehydration and reduction of the resulting cyclohexenone under Luche s conditions gave the corresponding allylic alcohol. Esterification with an acid representing the C39 to C26 part of rapamycin gave compound 389. Finally, Ireland-Claisen rearrangement gave the branched-chain cyclohexane 390. 

Ferrier carbocyclization can form only cyclohexanones. Nevertheless, chemical manipulations of the latter can provide us with seven-membered rings. This is illustrated by the synthesis of (—)-calystegine B2 by Boyer and Lallemand [305]. These natural polyhydroxylated nortropane compounds are growth stimulators of nitrogen fixing bacteria. 

C2 epimer failed [309]. Inversion of configuration at C2 was thus performed and final deprotection gave enantiomerically pure lycoricidine 402. 

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