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Novel Trichloroacetimidates and their Reactions

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2003

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Ali, Ibrahim Ahmed Ibrahim

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3-86537-025-X
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Göttingen : Cuvillier-Verl.

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Titel in einer weiteren Sprache

Neuartige Trichloracetimidate und ihre Reaktionen
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Zusammenfassung

A successful multi-step synthesis of complex oligosaccharide structures requires an appropriate protecting group strategy. Generally, the presence of three or more hydroxy groups in each sugar residue necessitates the protection of those hxdroxy groups which are not involved in the glycosylation step. In our study, the phthalimidomethyl protecting group has been used for the protection of hydroxy groups. The trichloroacetimidate 2 was prepared by the reaction of N-ydroxymethyl phthalimide (1) with trichloroacetonitrile in dichloromethane as solvent and in the presence of DBU in 87 % yield. The trichloroacetimidate 2 was reacted with primary and secondary hydroxy groups in various types of organic compounds. The deallylation of O-1 in 22 which possesses a phthalimidomethyl group on O-2 and reaction with trichloroacetonitrile in the presence of DBU as a base led to trichloroacetimidate 24. Glycosylation of methanol, n-octanol and 6-O-unprotected glucopyranoside 15 with 24 as glycosyl donor in the presence of TMSOTf as a catalyst afforded glucosides 25-27 in high yields. Thus, it was demonstrated that the Pim group on O-2 controls the anomeric selectivity essentially based on steric hindrance. The required trichloroacetimidates 41 and 42 of the DPM 128 and Fl, respectively, were prepared by the reaction of diphenylmethanol (39) and 9-fluorenol (40), with trichloroacetonitrile in the presence of 1,8-1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as catalyst. Also, trichloroacetimidates 41 and 42 were reacted with different hydroxy groups in alcohols and carbohydrates. It became interesting to study the effect of the DPM and Fl group of glycosyl donors 67 and 68 on the stereoselectivity during the glycosylation reaction as shown for the reactions leading to compounds 69-74. Also, the stereoselectivity in mannosylation reactions was studied. The coupling of the trichloroacetimidate donor 78 with n-octanol, glucose derivatives 15, 84 and 4-OH free glucose derivative 17 as acceptor was carried in dry dichloromethane at room temperature and at -40oC in the presence of TMSOTf as catalyst to afford the desired mannopyranosides 81, 82, 85 and 86. Thus, it became obvious, that compared with the benzyl group the DPM group supports in most cases β-mannopyranoside formation. This part describes the reaction of cyclopropylmethyl and cyclobutyl trichloroacetimidates, respectively, with hydroxy groups of varied nucleophilicities in order to investigate its use as alkylating agent under mildly acidic condition and to throw some light on the mechanism of the trichloroacetimidate procedure in forming glycosyl bonds. The cyclopropylmethyl cation has been found to be formed readily from the trichloroacetimidate 101 in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) as catalyst. Dibenzyl phosphate 120, as weak acid, gave only cyclopropylmethyl derivative 121 without any catalyst and the reaction proceeded without rearrangement. When the acceptor has a slightly more acidic character such as diphenyl phosphate 122 reaction with trichloroacetimidate 101 gave, in addition to 123, rearrangement products 124 and 125. When the acceptor has a slightly more acidic character such as diphenyl phosphate 122 reaction with trichloroacetimidate 101 gave, in addition to 123, rearrangement products 124 and 125. In the case of 4-toluenesulfonic acid, the cyclobutyl 127 and homoallyl derivatives 128 were formed. The required cyclobutyl trichloroacetimidate 102 was prepared in 87% yield by the reaction of cyclobutanol 99 with trichloroacetonitrile in the presence of DBU as catalyst. The cyclobutyl trichloroacetimidate 102 was reacted with acceptors such as benzyl alcohol (5), dinitrobenzyl alcohol (11) and the O-6-unprotected hydroxy group in glucose derivative 15; it gave the same reaction products as cyclopropylmethyl trichloroacetimidate 101 and also in about the same ratio. The double bond rearrangement of many unsaturated compounds can take place on treatment with acids. Thus, rearrangement of allyl compounds carrying a leaving group of the type shown in the following scheme may take place in presence of acids via carbonium ions, which in presence of alcohol may give two products. It has been found that the phthalimidomethyl group (Pim) can be used as an aminomethylating agent for C-nucleophiles.

Zusammenfassung in einer weiteren Sprache

Eine erfolgreiche Mehrstufensynthese von komplexen Oligosacchariden erfordert eine angemessene Schutzgruppenstrategie. Die Gegenwart von drei oder mehr Hydroxylgruppen in jedem Zuckerrest erfordert in der Regel die Schützung derjenigen Hydroxylgruppen, die nicht an der Glycosylierungsreaktion beteiligt sind. In unserer Studie wurde die Phthtalimidogruppe als Schutzgruppe für Hydroxylgruppen eingesetzt. Das Trichloracetimidat 2 wurde durch die Reaktion von N-Hydoxymethylphthalimid mit Trichloracetonitril in Dichlormethan als Lösemittel und in Gegenwart von DBU in 87 % Ausbeute dargestellt. Das Trichloracetimidat 2 wurde mit primären und sekundären Hydroxylgruppen von verschiedenen organischen Verbindungen umgesetzt. Die Entschützung der Allylgruppe an der O-1 Position in Verbindung 22, welche eine Phatalimidomethylgruppe an der O-2 trägt, und die Umstzung mit Trichloracetimidat in Gegenwart von DBU als Base führte zum Trichloracetimidat 24. Die Glycolysierung von Methanol, n-Octanol und 6-O-freiem Glucopyranosid 15 mit 24 als Glycosyldonor in der Gegenwart von TMSOTf las Katalysator lieferte die Glucoside 25-27 in hohen Ausbeuten. Somit wurde gezeigt, dass die Pim-Gruppe an O-2 die anomere Selektivität hauptsächlich über sterische Hinderung beeinflusst. Die benötigten Trichloracetimidate 41 und 42 von DPM und Fl wurden jeweils hergestellt durch die Umsetzung von Diphenylmethanol (39) und 9-Fluorenol (40) mit Trichloracetonitril in Gegenwart von DBU als Katalysator. Trichloracetimidate 41 und 42 wurden auch mit verschiedenen Hydroxylgruppen von Alkoholen und Kohlenhydraten umgesetzt. Es war von Interesse, den Effekt der DPM- und Fl-Gruppe der Glycosyldonoren 67 und 68 auf die Stereochemie während der Glycosylierungsreaktion zu untersuchen, wie gezeigt ist für die Reaktionen, die zu den Verbindungen 69-74 führen. Weiterhin wurde die Stereoselektivität in Mannosylierungsreaktionen untersucht. Die Kupplungsreaktionen des Trichloracetimidatdonors 78 mit n-Octanol, den Glucosederivaten 15 und 84 sowie dem 4-OH-freien Glucosederivat 17 als Akzeptor wurde durchgeführt in trockenem Dichlormethan bei Raumtemperatur und bei -40°C in Gegenwart vom TMSOTf als Katalysator, was die gewünschten Mannopyranoside 81, 82 und 86 lieferte. Somit erwies es sich, dass die DPM-Gruppe im Vergleich zur Benzyl-Gruppe in den meisten Fällen die Entstehung von β-Mannopyranosiden fördert. Ein weiterer Abschnitt beschreibt die Reaktionen von Cyclopropylmethyl- bzw. Cyclobutyl-Trichloracetimidat mit Hydroxylgruppen unterschiedlicher Nucleophilität, um deren Nützlichkeit als Alkylierungsmittel unter leicht sauren Bedingungen zu untersuchen und um etwas Licht auf den Mechanismus der Trichloracetimidat-Methode bei der Bildung von Glycosylbindungen zu werfen. Es wurde herausgefunden, dass das Cyclopropylmethyl-Kation sich aus dem Trichlor-acetimidat 101 in der Gegenwart von TMSOTf als Katalysator leicht bildet. Dibenzylphosphat 120, als eine schwache Säure, ergab nur das Cyclopropylmethyl-Derivat 121, wobei kein Katalysator benötigt wurde und die Reaktion ohne Umlagerungen erfolgte. Wenn der Akzeptor einen etwas saureren Charakter hat, wie z.B. Diphenylphosphat 122, ergab die Reaktion mit Trichloracetimidat 101 zusätzlcih zu der Verbindung 123 noch die Ulagerungsprodukte 124 und 125. Im Fall von 4-Toluolsulfonsäure wurden die Cyclobutyl-Verbindung 127 und das Homoallyl-Derivat 128 gebildet. Das benötigte Cyclobutyl-Trichloracetimidat 102 wurde in 87% Ausbeute hergestellt durch die Reaktion von Cyclobutanol 99 mit Trichloracetonitril in der Gegenwart von DBU als Katalysator. Das Cyclobutyl-Trichloracetimidat 102 wurde umgesetzt mit Akzeptoren wie Benzylalkohol (5), Dinitrobenzylalkohol (11) und der O-6-ungeschützten Hydroxlygruppe im Glucosederivat 15; es ergab die selben Reaktionsprodukte wie das Cyclopropyl-Trichloracetimidat 101 und ebenfalls in Ungefähr dem selben Verhältnis.

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540 Chemie

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ISO 690ALI, Ibrahim Ahmed Ibrahim, 2003. Novel Trichloroacetimidates and their Reactions [Dissertation]. Konstanz: University of Konstanz. Göttingen : Cuvillier-Verl.. ISBN 3-86537-025-X
BibTex
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