ZUSAMMENFASSUNG:Mit Kaliumcarbonat als Aktivator werden aus Hochamylosestarke (HAS) rnit iiberschiissigem Acetanhydrid bei Reaktionszeiten von 1-2 h in hohen Ausbeuten Produkte mit hohen Durchschnittssubstitutionsgraden (DS) erhalten.In N-Methyl-2-pyrrolidon konnen HAS mit stochiometrischen Mengen Acetanhydrid und rnit nichtfliichtigen starken organischen Basen wie 4-Dimethylaminopyridin oder 1 ,4-Diazabicyclo[2.2.2]octan als Katalysatoren in Reaktionszeiten von weniger als 1 h und in hohen Ausbeuten zu Acetaten mit DS < 2 umgesetzt werden.
SUMMARY:Acetylation of high-amylose starch to products of high average degree of substitution (DS) with yield of > 90% proceeds in 1 -2 h using potassium carbonate as activating agent. Acetylstarches of DS < 2 are available by acetylation with stoichiometric amounts of acetic anhydride in N-methyl-2-pyrrolidone applying organic catalysts as 4-dimethylaminopyridine and 1,4-diazabicyclo[2.2.2]octane, respectively. Reaction times of 1 h and less are sufficient to receive the desired acetates in high yield.
2-Alkyl-2-(prop-2-ynyl)cyclopentane-1,3-diones
2, conveniently prepared from
2-alkylcyclopentane-1,3-diones 1 and prop-2-ynyl bromide, afford the triketones
3 by Hg2+-catalyzed hydration of
the
acetylenic triple bond. Treatment of these triketones with aqueous
sodium hydroxide gives rise to
the 2,3,5-trisubstituted cyclopent-2-enones 5, which are
accompanied by the isomeric 2,3,4-trisubstituted cyclopent-2-enones 7 as byproducts. The
formation of these isomers can be avoided,
when the 2,2-disubstituted cyclopentane-1,3-diones 2 are
first converted by ring cleavage into the
5-alkyl-4-oxooct-7-ynoic acids 4 and then by subsequent
hydration into the 5-alkyl-4,7-dioxoalkanoic
acids 6. An intramolecular aldolization of the latter
forms exclusively the cyclopentenones 5. A
mechanism explaining the simultaneous formation of 5 and
7 from 3 is based on the formation
of
the 2,3-diacylcyclopropanolates 11 and 16 by
intramolecular aldolization and subsequent ring
opening to the 2-acetylcyclohexane-1,4-diones 13 and
18. A further ring opening to the 4,7-dioxoalkanoates 15 and 20 followed by
intramolecular aldol condensation then gives rise to the
isomeric trisubstituted cyclopent-2-enones 5 and
7.
In contrast to earlier reported unsuccessful experiments the Michael addition of prop-2-enal to 2-methylcyclopentane-1,3-dione (1) affords 3-( l-methyl-2,5-dioxocyclopentyl)propanal(3) in 90% yield, if the reaction is carried out in water without a basic catalyst. This aldehyde is converted by an intramolecular aldol reaction to (*)-endo-2-and (f)-exo-2-hydroxy-5-methylbicyclo[3.2.1]octane-6,8-dione (rac-4 and rac-6), both affording ( f )-5-methylcycloheptane-l,4-dione (rac-8) after oxidation to ( f )-5-methylbicyclo(3.2.1]octane-2,6,8-trione (rac-2), hydrolytic ring-opening and decarboxylation. This reaction sequence allows the transformation of the 2-alkylated cyclopentane-1,3-dione 1 into the 5-alkylated cycloheptane-1,4-dione rac-8 in a good overall yield. The pathway of the ringopening process has been determined by an investigation of the reaction intermediates.
Syntheses and Reactions of 2,2-Disubstituted Cyclopentane-1,3diones. Part 8. Conversion of 2-Alkyl-2-(2-oxopropyl)cyclopentane-1,3-diones into 2,3,5-and 2,3,4-Trisubstituted Cyclopent-2-enones by Intramolecular Aldolizations to 2,3-Diacylcyclopropanolates Followed by Remarkable Skeletal Rearrangements.-The title conversion is investigated in dependence of the substituent R in compound (I). The structures of compound (IIIb) and (IVb) are determined by X-ray analysis of their corresponding dicyclohexylammonium salts. Via an independant approach pure (VII) is obtained, which is also a hint for the discussed proposed mechanism. -(SCHICK, H.; ROATSCH, B.; SCHRAMM, S.; GILSING, H.-D.; RAMM, M.; GRUENDEMANN, E.; J.
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