Darstellung ungesättigter Kohlenhydrate durch Esterpyrolyse, II. Thermische <i>cis</i>‐Eliminierungen aus vollständig acetylierten Aldopyranosen

Microreactor technology and continuous flow processing in general are key features in making organic synthesis both more economical and environmentally friendly. When preformed under a high-temperature/pressure process intensification regime many transformations originally not considered suitable for flow synthesis owing to long reaction times can be converted into high-speed flow chemistry protocols that can operate at production-scale quantities. This Focus Review summarizes the state of the art in high-temperature/pressure microreactor technology and provides a survey of successful applications of this technique from the recent synthetic organic chemistry literature.

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Continuous Flow Organic Synthesis under High‐Temperature/Pressure Conditions

Razzaq

Kappe

2010

Chemistry An Asian Journal

291

151

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Accessing Novel Process Windows in a High‐Temperature/Pressure Capillary Flow Reactor

2009

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High-temperature/pressure organic synthesis can be performed under continuous flow conditions in a stainless steel microtubular flow reactor capable of achieving temperatures of 350°C and 200 bar (X-Cube Flash™). Using these extreme experimental environments, the Claisen rearrangement of allyl phenyl ether together with the ensuing rearrangement chemistry of the resulting 2-allylphenol product was investigated. Reaction optimization was performed by changing the temperatures, pressures and flow rates "on-the-fly". In addition, the high-temperature/pressure flow system allowed the study of these transformations in low boiling point solvents in or near their supercritical state. In general, the chemistry optimized under high-temperature microwave batch conditions could be successfully translated to a scalable flow regime.

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Darstellung ungesättigter Kohlenhydrate durch Esterpyrolyse, III. Thermische cis‐Eliminierungen aus vollständig acetylierten Ketopyranosen

et al. 1982

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Die in Aceton gelOsten Pentaacetate 1 und 6 der a-L-Sorbopyranose und der p-D-Fructopyranose eliminieren im Temperaturbereich 230-280°C bei Verweilzeiten von 0.5 -1 min in einer StrOmungsapparatur regioselektiv die 2-0-Acetylgruppe als Essigslure. Hierbei werden primar die Z-Isomeren 2 und 7 mit exocyclischer Doppelbindung gebildet (1,3,4,5-Tetra-O-acetyl-2,6-anhydrohex-1-enite). Bei hBheren Temperaturen werden auch die thermodynamisch stabileren E-Isomeren 3 und 8 gefunden. Aus den 'H-NMR-Spektren ergibt sich, dafl bei 2 und 7 ein Gleichgewicht der alternativen Sesselkonformationen 'C, und 'C, vorliegt, hingegen die E-Isomeren 3 und 8 ausschliefllich 'C,-Konformationen rnit drei bzw. zwei axialen Substituenten einnehmen. Als Produkt einer nachfolgenden [3,3]-sigmatropen Umlagerung (Allylverschiebung) werden weiterhin die Tetraacetyl-2,6-anhydro-3-desoxy-ul-hex-2-enosen 4 und 9 beobachtet. Ihre Konformation wird durch den ,,Allyl-Effekt" bestimmt. Die katalytische Hydrierung von 4 bzw. 9 fiihrt zu den 2,6-Anhydro-3-desoxy-ul-hexosen 11 und 12 bzw. 13. Entsprechende Reaktion der ungeslttigten Zucker 2 und 3 liefert das 1,5-Anhydro-~-sorbit-Derivat 14. Preparation of Unsaturated Carbohydrates by Ester Pyrolysis, IIII). -Thermal cis Eliminations from Complelely Acelylated KetopyranosesThe pentaacetates 1 and 6 derived from a-L-sorbopyranose and Po-fructopyranose dissolved in acetone regioselectively eliminate at temperatures of 230 -270°C in a flow apparatus within 0.5 -1 min the 2-0-acetyl group as acetic acid. Primarily the Z isomers 2 and 7 with exocyclic double bond (1,3,4,5-tetra-O-acetyI-2,6-anhydro-hex-l -enitoh) are formed. At higher temperatures the thermodynamically more stable E isomers 3 and 8 are also found. 'H NMR spectra show an equilibrium of alternative chair conformations 'C, and 'C, in 2 and 7, but E isomers 3 and 8 exclusively adopt '(2, conformations with three or in case of 7 two axial substituents. From these compounds the tetraacetyl-2,6-anhydro-3-deoxy-al-hex-2-enoses 4 and 9 are formed by a 13,31 sigmatropic rearrangement (ally1 shift). Their conformation is dominated by the allylic effect. Catalytic hydrogenation of 4 or 9 yields 2,6-anhydro-3-deoxy-ul-hexoses 11 and 12 or 13. Unsatured sugars 2 and 3 are reduced to give the 1,5-anhydro-o-sorbitoI derivative 14.

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Darstellung ungesättigter Kohlenhydrate durch Esterpyrolyse, II. Thermische cis‐Eliminierungen aus vollständig acetylierten Aldopyranosen

Cited by 12 publications

References 36 publications

Continuous Flow Organic Synthesis under High‐Temperature/Pressure Conditions

Continuous Flow Organic Synthesis under High‐Temperature/Pressure Conditions

Accessing Novel Process Windows in a High‐Temperature/Pressure Capillary Flow Reactor

Darstellung ungesättigter Kohlenhydrate durch Esterpyrolyse, III. Thermische cis‐Eliminierungen aus vollständig acetylierten Ketopyranosen

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