Konformationsanalyse, III. exo‐Anomerer Effekt und Circulardichroismus von Glycopyranosylaziden

Abstract

Tetra-0-acetyl-pentopyranosen und Penta-0-acetyl-hexopyranosen setzen sich mit Trimethylsilylazid bei Gegenwart von BF3 oder SnC14 leicht zu den entsprechenden Glycosylaziden l a bis 11 um (Tab. I). Es entsteht stets nur das anomere Glycosylazid. in dem die Azidogruppe und 2-OAc ,,trans" zueinander angeordnet sind. In Glycosylaziden sind die Bindungen CI -N, und N,--N, in der Weise polarisiert, daR der Dipol in beiden Flllen zum N, weist. Hieraus wird abgeleitet, daD bei Glycosylaziden wie bei Methylglycosiden ein exo-anomerer Effekt wirksam sein sollte. der die Konformeren 9 und 12 stark bevorzugt. Die Anwendung der Arid-Oktantenregel sagt fur das a-D-Glycosylazid 12 einen negativen, fur das p-D-Glycosylazid 9 einen positiven Cotton-Effekt voraus. Messungen des Circulardichroismus stimmen mit diesen Voraussagen gut uberein, was fur einen exo-anomeren Effekt bei Glycosylaziden spricht. Conformational Analysis, 111 1) em-Anomeric Effect and Circular Didvoism of Glycopyranosyl AzidesTetra-0-acetyl-aldopentopyranoses and penta-0-acetyl-aldohexopyranoses react readily with trimethylsilyl azide in the presence of BF3 or SnC14 to afford the corresponding glycosyl azides l a -11 (table 1). Only the anomeric glycosyl atide having the azido group ,,trans" to the 2-acetoxy group is obtained in each case. The polarization of thc C1 -N, and N, -N, bonds in glycosyl azides is such that in both cases the dipole is directed towards N,. It can be deduced that glycosyl azides, like methyl glycopyranosides, should exhibit an exo-anomeric effect which strongly favors the conformers 9 and 12. The application of the azide-octant rule predicts a negative Cotton effect for the a-D-glycosyl a i d e 12 and a positive effect for the p-o-glycosyl azide 9. Circular dichroism measurements are in good agreement with these predictions, indicating the operation of an exo-anomeric effect in the glycosyl azides.

Total Synthesis of Glycocinnasperimicin D

“…Finally, removal of the Boc group in 29 with trifluoroacetic acid and subsequent cleavage of the benzoyl and two acetyl groups in a mixture of aqueous ammonia and methanol yielded glycocinnasperimicin D (1). To our delight, careful and laborious purification by HPLC provided pure synthetic material 1 (23 % yield, isolated as the hydrochloride), the spectral data ( 1 H NMR, 13 C NMR, HRMS), TLC behavior, and antimicrobial activity of which were in good agreement with those of the natural glycocinnasperimicin D.…”

“…Treatment of 18 with trimethysilyl azide in the presence of tin(vi) chloride furnished the b-glycosyl azide 19 exclusively (86 %). [13] The isonitrile group at the anomeric position was constructed in a three-step sequence by 1) Staudinger reaction of the glycosyl azide 19, 2) formylation of the resulting iminophosphorane with acetic formic anhydride (72 % yield over two steps), and 3) dehydration of the formamide 20 by the modified Appel procedure [14] to afford the isonitrile glycoside 7 (85 % yield).…”

Enantioselective Organocatalysis of Strecker and Mannich Reactions Based on Carbohydrates

“…[5] Compound 5 is sufficiently stable to the conditions required for the SnCl 4 -catalyzed reaction with trimethylsilyl azide [6] to give glycosyl azide 6. Formation of the urea linkage with the l-tert-leucine benzylamide 7 was achieved by a Staudinger-azaWittig-type reaction according to PintØr [7] and yielded N-glycosylurea derivative 8.…”

General methods for the synthesis of glycopyranosyluronic acid azides

“…5 Alternatively, per-O-acetylated sugars can be activated with a Lewis acid in the presence of azidotrimethylsilane to give the 1,2-trans product. 6 However, in the case of mannosyl azides, only the a anomer has been prepared efficiently. In view of our recent studies on the use of glycosyl iodides in organic synthesis, 7 we decided to probe the potential of these donors in the preparation of glycosyl azides.…”