Intermediate allenes

It has been shown (87KGS787) that the dehydrobromination of (2-bromopropen-3-yl)pyrazole 23 by KOH in triethyleneglycol (150°C, 0.5 h) gives the intermediate allene which is transformed into 3,5-dimethyl-l-phenyl-4-prop-l-ynyl-l/7-pyrazole (24) (Scheme 33). 

Acid-catalyzed hydrolysis of S-allenylsulfinylamines 425, easily accessible from propargyl alcohols (cf. Scheme 7.8), provides the alkynes 427 (Scheme 7.56) [108, 109]. This transformation is postulated to proceed via the intermediate allenic sulfinic acid 426. However, in some cases with R1 = R2 = alkyl, more complicated products are formed instead of simple alkynes 427 [372]. 

Only a few isolated allene oxides have been synthesized from allenes and characterized. Most often peracids are used but the oxidative and acidic conditions usually result in a complex mixture of products. To overcome this problem, dimethyldioxirane (DMDO) can be used, which rapidly oxidizes allenes to spirodiepoxides. Several synthetically useful methods have been developed via in situ reaction of the intermediate allene oxide or spirodioxide with different nucleophiles. 

An enantioselective synthesis of the Ziegler intermediate 107 of forskolin (108) has been achieved using an intramolecular allenic Diels-Alder reaction (Scheme 19.20) [24], Treatment of propargyl ether 104 with potassium tert-butoxide in tert-butanol affords 106, presumably through the intermediate allene 105. Compound 106 was obtained as a single stereoisomer. 

It is likely that the intermediate allene is protonated by the solvent when reaction takes place in an alcohol. In an aprotic solvent an allene generated from a salt lacking 1,4-substituents could well be protonated by transfer of a proton from a nitrogen atom but, if both nitrogen atoms bore alkyl substituents, such a process is ruled out. In accord with this, treatment of salt (18b) with triphenylphosphine in dry ethyl benzoate as solvent provided the expected protodebrominated product, but similar treatment of (18c) provided only polymeric material, presumably resulting from alternative reactions of the intermediate species [81 AG 193, 81AG(E)I90]. 

Simple allenes (209) react with dimethyldioxirane (200) to give the corresponding spiro-dioxides 210 in instances where diastereoisomeric spiro-dioxides are possible, there is usually an acceptable stereochemical preference for epoxidation to occur anti to the alkyl substituents324,325. Allenic alcohol 211 yields the highly functionalized tetrahydro-furan 212 and tetrahydropyran derivatives by intramolecular nucleophilic addition of the hydroxy group to an intermediate allene diepoxide324. 

The products from this reaction are not invariably homogeneous since the first formed alkyne may isomerise, probably by way of an intermediate allene. With alcoholic potassium hydroxide at 170-180 °C, for example, terminal alkynes tend to rearrange to internal alkynes. The rearrangement process may be minimised by using sodamide in liquid ammonia as the reagent. 

Beltrame et al. (1964) suggested this route for the reaction of 1,1-di-phenyl-2-halopropene (190) with EtO- ion which gives the substitution product (191) and a cyclobutane (192) which is a dimer of the alleged intermediate allene (193). The reaction is characterized by an element effect (fcBr/ ci = 2-54-2-79 at 80-125°), and by rate coefficients which are... 

The same cyclopropanone gives a cycloadduct with furans—this must surely be a reaction of the oxyallyl cation and we can conclude that the three isomeric reactive intermediates (allene oxide, cyclopropanone, and oxyallyl cation) are all in equilibrium and give whichever product is appropriate for the conditions. 

Other 1,3-acyl migrations of propargylic carboxylates catalyzed by gold give intermediate allenes, which can afford heterocyclic compounds by intramolecular attack of the appropriate nucleophiles. The intermediate allenes formed by 1,3-acyl migrations can also react intramolecularly with alkynes to form naphthalenes, although this reaction proceeds more efficiently with Ag(I) catalysts, and is discussed in the aUcyne-carbon nucleophile section. 

Several thiopropargylimines 32 underwent ring-closure to the pyrroles 33 upon treatment with Cul in hot A V-dimethylacetamide (DMA). This process involves a propargyl-allenyl isomerization followed by a 1,2-migration of the thio-group in the intermediate allenes <03AG(E)98>. 

Since epoxidation at the vinyl double bond is unproductive, it is desirable to direct reaction on the al-lene moiety. This can be accomplished by taking advantage of the hydroxy-directed epoxidation of allylic alcohols using the t-butyl hydroperoxide/vanadium(V) system.The directing effects of both allylic and homoallylic type hydroxy groups have been examined at both positions of the vinylallene unit. " At the 1-position (64), primary, secondary and tertiary allylic tdcohols are effective, while only primary homoallylic alcohols have bran examined (equation 35). Presumably the directing effect of the hydroxy groups favors formation of the intermediate allene oxide (65). A sample of the compounds prepared by this route is shown in Scheme 32. ... 

A drawback of these bases is that in the preparation of terminal aliphatic acetylenes they may cause, at the high temperatures used (100-200 °C), partial prototropic rearrangement to the 2-alkynes via the intermediate allenes, as illustrated in equation (13) . Hence, these dehydrohalogenation reagents are preferably used to... 

CCMcaOH gave an apparent dimerization of the intermediate allen-ylidene [RuCp(PPh3)2 Tj -C=C=CMe2 ], affording [Ru2Cp2(PPhj)4 (m-C,oH,2)P+ (26) 21b). 

The method was also used for epoxidation of the alkenylidenecyclopropane (1) the predominant product (64% yield) was the ketone (3). The minor product (27% yield) was identified as (4). Both are considered to be formed from an intermediate allene epoxide (2).3 1... 

Intermediate allenes and internal acetylenes are known to be isomerized by this base to 1-alkynes (6, 476). The reaction with vinyl sulfides is particularly useful for preparation of 1-alkynes. 

Freedman and Huber showed that the cyclization of 253 with NaH/DMSO did not afford 3-methyl-l,4-benzoxazepin-5(4//)-one 255, as previously reported. Instead, they isolated 2-(2-hydroxyphenyl)-5-methyloxazole 256 and confirmed the structure by independent synthesis from 2-hydroxy-A -(2-propynyl)benzamide 257 (Scheme 1.69). The authors considered an intermediate allene but were not able to unequivocally establish the mechanism for conversion of 253 to 256. 

Other methods were also used for the synthesis of trifluoromethylfurans. Thus, flash vacuum thermolysis of silyl enol ether of 1,3-diketone 148 at 800 °C afforded furan 69 (70 %) via intermediate allenic ketone 149 [110], Another synthesis of trifluoromethylated furan from 1,3-diketone comprised reaction of diazomethane with 2-(trifluoroacetyl)dimedone 150. This approach produced dihydrofuran 152 in a mixture with methylated product 151. Compound 151 underwent aromatiza-tion into 153 under heating withp-toluenesulfonic acid [111]. 

Also, double-furan-transfer reactions occur in the sequential cycloaddition of 399 to give 401 via the intermediate allene 400, ... 

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