Neuartiger Ringschluß von Hex‐5‐enpyranosiden zu carbocyclischen Verbindungen

Abstract

Unter Erhaltung der Konfiguration am anomeren C‐Atom läuft eine neuartige Umlagerung bei Kohlenhydraten ab. So wird beim leicht zugänglichen Vinylacetal 1 unter Wirkung von Triisobutylaluminium ein Sauerstoff‐ durch ein Kohlenstoffatom im Ring ausgetauscht und damit ein neuer Syntheseweg zum hochfunktionalisierten Cyclohexan 2 eröffnet.

Synthesis of gem‐Difluorocarba‐D‐glucose: A Step Further in Sugar Mimesis

“…The 5a‐ gem ‐difluorocarba‐α‐ D ‐glucopyranose 1 27 was then easily obtained by controlled reduction of the triple bond, reductive ozonolysis, and debenzylation (Scheme ). As previously observed,15 the rearrangement occurs with retention of configuration. The reduction of the putative transient ketone is stereoselective and is classically explained by the attack of the hydride ion, which is attached to the isobutyl group, on the less‐hindered face of the molecule to provide the axial hydroxyl group (Scheme ).…”

“…We recently reported a novel sugar‐to‐carbocycle rearrangement promoted by triisobutylaluminum (TIBAL)15 or Cl 3 TiO i Pr 16. The proposed key step in this transformation is the opening of the ring to give a carbocationic intermediate.…”

“…The observed stereoselectivity of this reduction process is in sharp contrast to the case in which the fluorine atoms are absent from the molecule. Indeed, when the unsaturated sugar 15 32 was submitted to the same reaction conditions, only the axial alcohol 16 15 was obtained in 75 % yield (Scheme ). This result supports the directing role of fluorine in the reduction of the ketone.…”

Triisobutylaluminium and Diisobutylaluminium Hydride as Molecular Scalpels: The Regioselective Stripping of Perbenzylated Sugars and Cyclodextrins

“…A few years ago, we serendipitously observed mono‐de‐ O ‐benzylation as a side reaction during our exploration of the scope9 of the reductive rearrangement of 6‐deoxyhex‐5‐enopyranosides with triisobutylaluminium (TIBAL) 10. By taking advantage of this unplanned finding, we promptly discovered that use of TIBAL led to highly regioselective mono‐de‐ O ‐benzylation of perbenzylated mono‐ and disaccharide derivatives 11.…”

Synthesis of Methoxy‐Substituted Exocyclic (E)‐ and (Z)‐Unsaturated Methyl Pyranosides and a Study of Their Reactivity towards Lewis Acids

“…[1] We have recently developed triisobutylaluminium-(TIBAL-) [2] and TiCl 3 OiPr [3] -mediated carbocyclisations which, in contrast with the classical Ferrier-II reaction, [4] retain the anomeric substituent. This methodology has been successfully applied to the synthesis of carbasugar analogues of -idose, [5] S-, Se-, and C-arylglycosides, [6] and di- [7] and trisaccharides.…”