Alkylation of 4-hydroxyproline ester derivatives. Diastereoselectivity guided by the anomeric effect and π-interaction

Konformative Fluoreffekte in der Organokatalyse: eine neuartige Strategie zum molekularen Design

“…[55] Eine interessante aus dieser Untersuchung hervorgehende Beobachtung ist die mit geringerer Ausbeute verlaufende und weniger selektive Aldolisierung von 1,5-Cyclononadion bei Verwendung von cis-4-F-Pro (50 % Umsatz, e.r. 79 [51,53] Der Einfluss des 4-Fluorsubstituenten auf die Enantioselektivität fügt der gegenwärtigen Diskussion über den Mechanismus prolinkatalysierter Aldolreaktionen einen weiteren Grad an Komplexität zu. [56] 2.1.4.…”

“…Anzumerken ist, dass die Stereoselektivität analoger Reaktionen mit substituierten 4-Hydroxyderivaten von der Substitution des Stickstoffatoms und vom Alkylierungsreagens abhängt. [53] 2.1.3. Katalytische asymmetrische transannulare Aldolisierungen Nur wenige Jahre später verwendeten Chandler und List trans-4-Fluorprolin in organokatalytischen transannularen Aldolisierungen von 1,4-Cyclooctadionen unter milden Bedingungen.…”

Stereoselective Synthesis of Quaternary Proline Analogues

“…In this context, the stereochemical outcomes of alkylation of enolates 29 derived from (2S,4R)-N-Boc-and -N-benzoyl-4-tert-butyldiphenylsilyloxyproline methyl esters have been particularly studied. [39,40] Kawahara et al found that the diastereoselectivity of the alkylation reaction is dependent on the alkylating reagent and the N-protecting group. [40] Whereas retention of configuration was observed upon alkylation of the N-Boc derivative with allylic and homoallylic halides, the use of benzylic halides led preferentially to products with inversion of configuration.…”

“…[39,40] Kawahara et al found that the diastereoselectivity of the alkylation reaction is dependent on the alkylating reagent and the N-protecting group. [40] Whereas retention of configuration was observed upon alkylation of the N-Boc derivative with allylic and homoallylic halides, the use of benzylic halides led preferentially to products with inversion of configuration. In the case of the N-benzoyl derivatives the alkylation occurred with inversion of configuration when benzylic or allylic halides were used, and in the case of benzylic halides high diastereoselectivities were observed (Scheme 7).…”

Ring Expansion: Synthesis of the Velbanamine Piperidine Core

“…After deprotection of the hydroxy groups at C-4 in 3 and 3', using TBAF (MS 4 Å, THF, r.t.), the hydroxyesters 10 and 10' were isolated in 87% yield and separated by chromatography. Compound 10' was treated with LiOH and transformed to the hydroxy acid 11 (89%) which, after treatment with diphenylphosphorylazide (DPPA) in the presence of Et 3 N in DMF, 10 led to lactone 12 11 in 44% yield (Scheme 3).…”