cally:~'51 The E-isomer (with K=1.9, in CDC13) is favored.The keto ester 2c furnishes the Z/E mixture of the butatrienediester 5c. In contrast to 5b, 5c is configurationally stable at room temperature. Consequently, the isomers could be separated by conventional column chromatography. E-5c and 2-5c were obtained in 33% and 9% yield, respectively, in each case as pale yellow, crystalline and stable compounds (m.p. 87-88 "C and 71-73 "C, respectively). All the butatriene derivatives 5 prepared were characterized by accurate analytical and spectroscopic data.The esters 5 can be converted into the carboxylic acids 6-8. Since they have a pronounced tendency to undergo nucleophilic addition,l6I protic conditions had to be avoided. The hydrolysis was carried out with [ 18lcrown-6 as phase transfer catalyst of the THFIKOH system. 5a thus furnished the acid 6 in 83% yield as an orange-yellow crystalline substance. Eand 2-5c yielded the isomerically pure dicarboxylic acids Eand 2-8, respectively, in 65% yield as bright-yellow crystals. In contrast, E-5b forms a Z/E mixture of the dicarboxylic acids 7, from which isomerically pure, orange-colored E-7 precipitates in 7 1% yield upon crystallization. 2-7, on the other hand, could-due to sparing solubility-only be obtained by a very tedious chromatographic separation.Upon determination of the melting points, still in the crystalline state, 2-7 and Z-8 rearranged into the E-isomers.The dicarboxylic acids Eand 2-7, like their esters 5b, have different 'H-NMR spectra;['61 the equilibrium constant K , with a value of 2.9 (in [DJDMSO) in favor of the E isomer, is significantly higher than in the case of 5b.
Owing to their bond system, the cumulenes represent a separate entity in the class of unsaturated compounds. Whereas numerous derivatives of the allenes have been prepared and investigated,[" in the butatriene series only very few derivatives with functional groups have been reported,[*I among them no butatrienecarboxylic acids. Since we were interested, inter alia, in the solid state chemistry of such butatrienes and their complexes with amidesJ3] we decided to search for a general entry to this class of compounds.The conventional methods of butatriene synthesisi4] such as dehydrohalogenations (of 2-halogenobutadienecarboxylic acid derivatives) with bases,[51 dehalogenations (of 2,3-dihalogenodienecarboxylic acid derivatived5I) with Zn or other reducing agents, and 1,4-elimination reactions (on 1,4-dihydroxy-2-butynecarboxylic acid derivativesF6]) with SnCI,, P21, etc., failed. Finally, a successful strategy proved to be that based on the concept of constructing the butatriene system stepwise from homologous allene derivatives['] (Scheme 1).As starting material we used the 1-phenylallenyl ester 1, which was formed in 75-79% yield by Wittig reaction of (cr-methoxycarbonylbenzylidene)triphenylphosphaneis1 with acetyl chloride using the method of Lung and Hansen.[91 Reaction of 1 with equimolar amounts['01 of lithium hexamethyldisilylamide at -78 "C afforded the mesomerically stabilized anion, which reacted with benzophenone 2a and the keto esters 2b and 2c to give the allene alcohols 3a-c.["] The isomeric alkynols 4["l are not formed, obviously for steric reasons. The p-toluenesulfinyl residue proved to be a suitable leaving group for the subsequent 1,2-elimination. In the preparation of the sulfinyl ester of 3a, elimination and formation of the desired butatrienecarboxylate 5a already occurs at T < -20°C. Thep-toluenesulfinyl esters could therefore not be isolated and characterized. The extreme easiness of this eliminati~n"~] enabled a one-pot variant of the reaction sequence 1 -+ 5, thus circumventing the high losses in yield during purification of the alcohols 3 :['I1 a solution of p-toluenesulfinyl chloride [**I This work was supported by the Deutsche Forschungsgemeinschaft, the in T H F was added dropwise at -78°C to the reaction mixtare containing the lithium alkoxides of 3 which had previously been treated with an equimolar amount of triethylamine; finally, the mixture was allowed to warm to room temperature. Purification by column chromatography furnished intense yellow crystals (m.p. 94-95°C) of 5a in 31% yield (referred to 1 and with maximally 50% conversion of 2a). Slow crystallization at -20°C yielded a second, orangecolored modification of 5a (m.p. 112-1 13 "C). Both crystal forms are storable in the dark, but are photolabile.In an analogous way the keto ester 2b affords a Z/E mixture of the butatriene diester 5b. Treatment of the reaction mixture with ethanol leads to crystallization of isomerically pure E-5b (no cumulene band in the IR spectrum; strong butatriene band at 2040 c m -' in the Raman spe...
Mit dem Ketoester 2c resultiert das Z/E-Gemisch des Butatriendicarbonsaureesters 5c. Im Gegensatz zu 5b ist 5c bei Raumtemperatur konfigurationsstabil. Daher konnten die lsomere durch konventionelle Saulenchromatographie getrennt werden. E-5c wurde in 33%, Z-5c in 9% Ausbeute als jeweils blaBgelbe, kristalline und stabile Verbindungen erhalten (Fp= 87-88 bzw. 7 1-73 "C). Alle Butatrien-Derivate 5 sind durch analytische und spektroskopische Daten charakterisiert.Die Ester 5 liefien sich in die Carbonsauren 6-8 iiberfiihren. Da sie eine ausgepragte Neigung zur nucleophilen Addition habed"], mufiten protische Bedingungen vermieden werden. Die Hydrolyse gelang mit [18]Krone-6 als Phasentransfer-Katalysator des Systems THF/KOH. 5a lieferte so in 83% Ausbeute die Siure 6 als orangegelbe, kristalline Substanz. E-und 2-5c ergaben die isomerenreinen Dicarbonsauren E-bzw. 2-8 in 65% Ausbeute als hellgelbe Kristalle. E-5b bildete dagegen eine Z/E-Mischung der Dicarbonsauren 7, aus der isomerenreines orangefarbenes E-7 bei der Kristallisation in 71% Ausbeute anfiel.2-7 war wiederum nur durch eine -aufgrund der geringen Loslichkeiten -sehr muhsame chromatographische Trennung isomerenrein zu erhalten.Bei der Bestimmung der Schmelzpunkte lagern sich Z-7 und Z-8 noch im kristallinen Zustand in die E-lsomere um.Die Dicarbonsauren E-und 2-7 haben wie ihre Ester 5b unterschiedliche 'H-NMR-Spektren['61; die Gleichgewichtskonstante K ist mit einem Wert (in [DJDMSO) von 2.9 zugunsten des E-Isomers deutlich hoher als bei 5b.
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