Ether, allenic

Another type of lithiated allenic ether or sulfide is obtained from 2-alkynyl... 

The stability of the various cumulenic anions depends to a large extent upon the nature of the groups linked to the cumulenic system. Whereas solutions of lithiated allenic ethers and sulfides in diethyl ether or THF can be kept for a limited period at about O C, the lithiated hydrocarbons LiCH=C=CH-R are transformed into the isomeric lithium acetylides at temperatures above about -20 C, probably via HC C-C(Li )R R Lithiated 1,2,4-trienes, LiCH=C=C-C=C-, are... 

A solution of 0.40 mol of butyllithium in about 270 ml of hexane was cooled to -50°C and 250 ml of dry THF or diethyl ether were added, while maintaining the temperature below -20°C. The allenic ether (0.42 mol, freshly distilled) was subsequently added in 16 min at -30°C. After an additional 10 min at this temperature the solution was ready for further conversions. 

Note 1. If the lithiation of the allenic ether is performed with butyllithium in hexane and THF as a co-solvent, subsequent alkylation (in the presence of a small amount of HMPT) is much faster. The separation of the volatile product from the hexane and THF is difficult, however. 

Similar results are probably obtained when the metallation of the allenic ether is carried out with butyllithium in hexane-THF or diethyl ether. 

Note 2. The allenic ether is extremely sensitive towards oxygen. All operations during the work-up must be carried out under nitrogen. After termination of the distillation nitrogen should be admitted to the distillation apparatus. Too strong heating during the distillation involves the risk of an explosion. 

The contaminants were the starting compound (about 8%) and a trace of the allenic ether (CH3)2N-CH2-CH=C=CH-0CHj. The yields varied from 62 to 755 . 

A solution of 0.22 mol of butyllithium in 150 ml of hexane was cooled below -40°C and 140 ml of dry THF were added. Subsequently 0.20 mol of 1-dimethyl amino--4-methoxy-2-butyne (see Chapter V, Exp. 14) were added in 10 min with cooling between -35 and -45°C. After an additional 15 min 100 ml of an aqueous solution of 25 g of ammonium chloride were added with vigorous stirring. After separation of the layers four extractions with diethyl ether were carried out. The solutions were dried over potassium carbonate and then concentrated in a water-pump vacuum. Distillation of the residue gave a mixture of 8-10% of starting compound and 90-92% of the allenic ether, b.p. 50°C/12 mmHg, n 1.4648, in 82% yield (note 1). 

J/ote 1. The same procedure with 2-alkynylethers. Alkyl-CCC-CH2OR, gave lower relative amounts of the corresponding allenic ethers (varying between about 60 40 for R = CH3 to 75 25 for R = teri.-CcHg in favour of the allenic isomer). 

Enol ether additives were used to probe the protonation of 3-cyclopen-tenylidene (127). Treatment of A-nitroso-A-(2-vinylcyclopropyl)urea (124) with sodium methoxide generates 2-vinylcyclopropylidene (126) by way of the labile diazo compound 125 (Scheme 25). For simplicity, products derived directly from 126 (allene, ether, cycloadduct) are not shown in Scheme 25. The Skat-tebpl rearrangement of 126 generates 127 whose protonation leads to the 3-cyclopentenyl cation (128). In the presence of methanol, cyclopentadiene (130) and 3-methoxycyclopentene (132) were obtained.53 With an equimolar mixture of methyl vinyl ether and methanol, cycloaddition of 127 (—> 131)... 

Scheme 4.71 Camphor-derived auxiliary for the asymmetric synthesis of chiral allene ether 280.

Scheme 4.72 Fructose-derived auxiliary forthe synthesis of chiral allene ethers 281.

Although the preparation of the substituted allene ether substrates for the Nazarov reaction is not the topic of this chapter, it is necessary to mention a few aspects of their synthesis. Lithioallene 1 (Eq. 13.13) can be trapped with chlorotri-methylsilane to give 35 [6]. Exposure of 35 to sec- or tert-butyllithium leads to allenyl-lithium 36, which can be trapped with alkyl halides or other electrophiles to give 37. Desilylation of 37 leads to 38. This is somewhat laborious, but it leads to allene 38 uncontaminated by propargyl ether 39. Exposure of 39 to n-butyllithium, followed by quenching with acid, typically produces mixtures of 38 and 39 that are difficult to separate. Fortunately, one need not prepare allenes 38 in order to access the C6-sub-... 

Investigations of base-catalyzed isomerizations of allene derivatives have been recently continued. For instance, the rearrangement of allene ethers 53 under superbasic conditions (KOH-DMSO) is considered as one of the steps in hydration of acetylene derivatives (equation 17)31,32. 

The allenic ether (204) cyclizes to spiro compounds in the presence of potassium r-butoxide and dicyclohexano-18-crown-6. Acid hydrolysis yields the dihydrofuranone (205). The new carbonyl compound l-oxaspiro[4,4]nonan-4-one can be subjected to another spiroannela-tion sequence. The carbonyl group in (206) has two faces available for addition of a nucleophilic species. Only one product is formed, a cyclopentyl[3]helixane (207). One can in principle continue this reiterative reaction (Scheme 53) (B-81MI31200). 

Orientation of azide addition to allenic ethers is nonspecific addition occurs preferentially to the activated double bond in the j ,y-position to the oxygen atom.254... 

On treatment with a Pd-phosphine complex, allene ethers 54 undergo cyclization (Scheme 17) to the benzox-epines 55, which can be transformed to a dibenzoxepine derivative 56 via Diels-Alder reaction and aromatization <1994JOC4730>. 

Some acetoxylations of various substrates are listed in Table 3.1. It should be noted that the experimental conditions may vary widely for example, allenic ethers reacted at -78°C, whereas arylacetonitriles required reflux in acetonitrile and catalysis by benzoyl peroxide. 

Cyclopentannelation (12, 310). This reaction was mistakenly ascribed to a-methoxyallene, which fails to undergo cyclopentannelation under the cited conditions. Actually, the reagent is (methoxymethoxy)allene. A few other allenic ethers can be used, but all must have a departing group capable of forming a stable cation.1... 

Acylation of aryl and alkenyl halides. The zinc salts of enol ethers and allenic ethers couple with aryl and alkenyl halides in the presence of several Pd(0) catalysts Pd[P(QH,),l4 or bis(dibenzylideneacetone)palladium with added phosphine ligands. 

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