Intramolekulare En-Reaktionen in der organischen Synthese

Abstract

Thermische Cyclisierungen von Dienen, Eninen, Enonen und ahnlichen ungesattigten Systemen illustrieren das praparative Potential der intramolekularen En-Reaktion. Eine Fiille substituierter, anellierter und iiberbriickter Ringsysteme, darunter auch Naturstoffe, sind so auf regio-und stereoselektive Weise zuganglich. Zahlreiche Beispiele dienen dam, die Moglichkeiten, Grenzen und gemeinsamen GesetzmaBigkeiten dieser Cyclisierungs-und der dazu inversen Ringoffnungs-Reaktion zu umreiBen. Manche dieser Cyclisierungen sind in industrieller GroDenordnung durchfuhrbar.

Cycloisomerization of 1,6‐Dienes Mediated by Lewis Super Acids without Additives: Easy Access to Polysubstituted Six‐Membered Carbocycles

“…In the presence of Lewis acids, the intramolecular ene reaction [15] of 1,6-dienes generally leads to five-memberedring compounds. [16] In a particular example, an alkenylpentenolide was transformed in the presence of pure sulfuric acid into a cyclohexane ring, as a result of a Wagner-Meerwein transposition with the formation of a spirocyclic ring system.…”

Ring Reversal of a Spirocyclic Patchouli Odorant: Molecular Modeling, Synthesis, and Odor of 6‐Hydroxy‐1,1,6‐trimethylspiro[4.5]decan‐7‐one

“…Just like the Katsuki-Sharpless oxidation, [30] the Plietker ketohydroxylation reaction uses catalytic amounts of RuO 4 , generated in situ from RuCl 3 with Oxone ® (2KHSO 5 ·KHSO 4 ·K 2 SO 4 ) as stoichiometric oxidant, but in contrast to the Katsuki-Sharpless conditions, the Plietker protocol does not lead to cleavage of the double bond, although the cyclic ruthenate formed by [3+2] cycloaddition and subsequent oxidation was proposed to be identical. [29c] Yet, instead of an electrocyclic fragmentation as in the case of the Katsuki-Sharpless reaction, in the presence of Oxone ® the nucleophilic addition of the peroxomonosulfate anion (SO 5 2-) leads to the formation of an α-hydroxy ketone.…”

“…Stirring was continued for 30 min at room temp., prior to quenching with cold saturated aq. NH 4 Cl solution (50 mL). After separation of the layers, the aqueous one was extracted with Et 2 O (3ϫ 25 mL).…”

“…or purum) from SAFC that were used without further purification. 1,4-Dibromo-2,2-dimethylbutane (18) was prepared from diethyl 2,2-dimethylsuccinate by reduction with LiAlH 4 and subsequent bromination with phosphorus tribromide according to the procedure of Brown and van Gulick. [31] The odor thresholds were determined by GC olfactometry: Different dilutions of the sample substance were injected into a gas chromatograph in descending order of concentration until the panelist failed to detect the corresponding substance at the sniffing port.…”

“…At first sight, the spirocyclic ring system seems easily accessible by a thermal Alder ene reaction [4] of 2-methyl-3-(4Ј-methylpent-4Ј-enyl)cyclohex-2-en-1-one (3, Figure 2). Yet, both double bonds of dienone 3 bear allylic hydrogen atoms, and consequently both can act as ene components.…”