Konformationsanalyse, V. Einfluß des anomeren und inversen anomeren Effektes auf Konformationsgleichgewichte von N‐substituierten N‐Pentopyranosiden

Abstract

Konformationsgleichgewichte von N‐substituierten N‐Pentopyranosiden wurden untersucht. Der anomere Effekt nimmt bei Substitution am C‐1 durch nachstehende Gruppen in folgender Reihe ab: \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm N}\limits^ \ominus - \mathop {\rm P}\limits^ \oplus ({\rm C}_6 {\rm H}_5)_3 $\end{document} > OAc > N3 > NHCOCF3 > NHCOC6H4OCH3‐(p) ≃ NHCO‐ C6H4NO2‐(p) > NH2 ≃ NHAc ≃ \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm N}\limits^ \oplus {\rm PH} $\end{document}(C6H5)3]Cl⊖ > Imidazoliumsalz ≃ Pyridiniumsalz. Bei der NH2‐Gruppe ist kein anomerer und inverser anomerer Effekt nachweisbar. Die Imidazolium‐ und Pyridinium‐Verbindungen 33 und 31 zeigen inversen anomeren Effekt, wodurch in der α‐D‐xylo‐Reihe Inversion zur triaxialen 1C4(D)‐Konformation eintritt. Dem C1N1‐Dipol wird ein überwiegender Einfluß auf den anomeren Effekt zugemessen. Der Substituent mit negativiertem Stickstoff \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm N}\limits^ \ominus - \mathop {\rm P}\limits^ \oplus ({\rm C}_6 {\rm H}_5)_3 $\end{document} weist den größten anomeren Effekt auf, der bei Protonierung des Ylids zur \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm N}\limits^ \oplus {\rm PH} $\end{document}(C6H5)3]Cl⊖‐Gruppe vollständig verschwindet.

Anomeric Effect of the Nitrogen Atom in the Isocyano and Urea Groups

“…The crystal structures of xylopyranosyl isocyanides 8 and 7 offer an appropriate opportunity to analyze the anomeric effect of the nitrogen atom in the isocyano group owing to the fact that both anomers have similar 4 C 1 conformation. The ring oxygen of the α-isocyanide 8 contains an unshared pair of electrons, which is oriented antiperiplanar to a polar C1-N bond.…”

“…Horton reported the result of a comprehensive 1 H NMR spectroscopic investigation of pentopyranose derivatives, [3] which were used to analyze the anomeric effect of N-substituted glycosylamines by Paulsen. [4] The 1 H NMR spectroscopic measurements of the conformational equilibrium for N-substituted N-pentopyranosylamines show that the order of axial preference is as follows: NPPh 3 Ͼ OAcϾ N 3 Ͼ NHCOCF 3 Ͼ NHCOAr Ͼ NH 2 = NHAc. The anomeric effect is largest for more electronegative sp-and sp 2 -hybridized nitrogen groups, whereas substituents such as NH 2 and NHAc show the usual sterically driven preference for the equatorial dispositions.…”

Stereoselective Synthesis of N‐Glycosyl Amino Acids by Traceless Staudinger Ligation of Unprotected Glycosyl Azides

“…[8] Free glucose (equilibrium anomeric mixtures) is also formed by hydrolysis of the same intermediates. The glucosyl amides were purified by flash chromatography on silica gel (4:1 CHCl 3 /MeOH) and fully characterized as their acetylated counterparts.…”

Some recent developments in stereoelectronic theory. Reevaluations of ALPH and the reverse anomeric effect

“…5, Scheme 8; note that all axial/equatorial positions are interconverted in this conformational equilibrium). This was confirmed in several gluco-, xylo-and arabino-pyranosyl systems, 20 and extended to glycosylimidazolium systems (eq. 6, Scheme 8).…”

“…There are indeed exceptions, but the overall evidence is strongly in favour of the RAE -this conclusion itself is widely accepted. 7,[19][20][21] The few exceptions refer mostly to the case of the non-acetylated sugars in aqueous media, but these may be explained on the basis of solvation effects. Other apparent exceptions involve sulphur and phosphorus derivatives, 7 but these again possess altered steric preferencesessentially because of the relatively long C-S and C-P bonds -so their relevance is unclear.…”