Verzweigte Zucker, XVIII. Stereoselektivität der 1,4‐Addition von Carbanionen an Pyranosid‐Enone zu verzweigten Zuckern

Abstract

~~Pyranosid-Enone vom Typ 14 reagieren mit 2-Lithio-1,3-dithiolan-2-carbonsaure-ethylester (15) unter 1,4-Addition und Einfuhrung einer C-Kettenverzweigung am C-2. Die Addition erfolgt streng stereoselektiv und liefert nur das Addukt, in dem die Seitenkette am C-2 und die l-OCH3-Gruppe ,,trans" zueinander angeordnet sind. Die Lenkung der Reaktion erfolgt durch die anomere OCH3-Gruppe. Dieses zeigen die vier Beispiele der Pyranosid-Enone 4, 14, 26 und 36. Die eingefuhrte Seitenkette kann in eine Glycoloylverzweigung umgewandelt werden. Das Verfahren ist zur Darstellung von verzweigten Zuckern mit funktionalisierter Seitenkette ohne Hydroxylgruppe am Verzweigungspunkt bestens geeignet. Hex-2-enopyranosid-4ulose 4 reagiert mit Lithiumdimethylcuprat(1) unter 1.4-Addition stereoselektiv, wobei die Ankniipfung der Seitenkette am C-2 entsprechend durch die anomere Gruppe gelenkt wird. Branched-chain Sugars, XVIII')Stereoselective Sjmthesis of Branched-chain Sugars by 1,4-Addition of Carhanions to Pyranoside b o n e s Pyranoside enones of type 14 react with ethyl 2-lithio-l,3-dithiolane-2-carboxylate (15) by way of a 1.4-addition and attachment of a C-side chain to C-2. The addition takes place strictly stereoselectively and provides only the adduct in which the side chain at C-2 and the 1-OCH3 group are arranged in a ,,trans" position. The reaction is influenced by the anomeric OCH3 group. This can be seen from the four examples of the pyranoside enones 4,14,26, and 36; the attached side chain can be converted into a glycoloyl branching. The procedure is extremely suitable for the synthesis of branched-chain sugars having a functionalized side chain but not hydroxyl group at the branching point. Hex-2-enopyranoside-4-ulose 4 reacts stereoselectively with lithium dimethylcuprate(1) under 1,4addition whereby the attachment of the side chain at C-2 is correspondingly influenced by the anomeric group.In einer vorhergehenden Untersuchung 2, hatten wir gefunden, daD das PyranosidEnon 4 Carbanionen sowohl unter 1,2-Addition an die Carbonylgruppe, als auch unter 1 ,CAddition an die konjugierte Doppelbindung addieren kann. Die Umlenkung zur 1 ,CAddition ist durch den Einsatz von Kupferorganylen ') oder unter kupferfreien Bedingungen in vorzuglicher Weise mit dem von Schlessinger4) angegebenen Reagenz 2-Lithio-

Palladium−Cobalt-Mediated Double Annulation Process: A New Strategy to Chiral and Polysubstituted Bis-Cyclopentanoids on Carbohydrate Precursors

“…For formation of the olefinic sugars 6 and 7 , the MsCl/pyridine and TsCl/pyridine methods were employed, but no satisfactory results were obtained. However, if the iodohydrin 2 and the mixture of 4 and 5 were treated with POCl 3 in pyridine at −40 °C, the unsaturated sugars 6 and 7 were isolated in 70% and 55% yield, respectively. After deacetylation of 7 with a mixture of MeOH/H 2 O/Et 3 N (3:2:1), the allyl alcohol 8 was obtained in quantitative yield.…”

“…Synthesis of Unsaturated Sugars. The starting sugars 6 and 9 , needed for the present study, were prepared from easily accessible benzyl 2- O - p -tosyl-3,4-anhydro-β- l -arabinopyranoside ( 1 ) and benzyl 2,3-anhydro-4- O -acetyl-α- d -ribopyranoside ( 3 ), respectively. , The first step proceeds via epoxide ring opening of 1 and 3 . When 1 was refluxed with sodium acetate, sodium iodide and acetic acid in acetone for 4−5 h, the iodohydrin 2 was obtained as a single product adopting the 4 C 1 conformation.…”

“…The starting sugars 6 and 9 , needed for the present study, were prepared from easily accessible benzyl 2- O - p -tosyl-3,4-anhydro-β- l -arabinopyranoside ( 1 ) and benzyl 2,3-anhydro-4- O -acetyl-α- d -ribopyranoside ( 3 ), respectively. , The first step proceeds via epoxide ring opening of 1 and 3 . When 1 was refluxed with sodium acetate, sodium iodide and acetic acid in acetone for 4−5 h, the iodohydrin 2 was obtained as a single product adopting the 4 C 1 conformation. The 1 H NMR spectrum shows two diaxial relationships between H-2/3 and H-3/4 with coupling constants J 2,3 = 9.4 Hz and J 3,4 = 10.5 Hz and one equatorial−axial relationship between H-1/2 with a small coupling of 3.6 Hz.…”

Total Synthesis of Selected Bioactive Natural Products: Illustration of Strategy and Design

“…The substrate 81 for enantiodivergent synthesis would be obtained by Wittig reaction with lactol 82 . The pyranonaphthalene skeleton of 82 might be synthesized by tandem Michael−Dieckmann reaction to connect lactone 83 and enone 84 …”

“…The pyranonaphthalene skeleton of 82 might be synthesized by tandem Michael-Dieckmann reaction to connect lactone 83 48 and enone 84. 49 The chiral enone 84 would be derived from commercially available L-rhamnose 85.…”