Ethers alkynyl

Note that in these cases the direction of the isomerization is the reverse of that observed with the 2-alkynyl ethers and sulfides. 

Alkynes (see also Alkynyl Halides, Alkynyl Ethers)... 

Acid-Catalyaed Hydration of Alkynyl Ethers and Thioethers. 207... 

Acid-catalyzed hydration of alkynyl ethers and thioethers... 

The earliest evidence for vinyl cations as intermediates comes from thorough studies by Jacobs and co-workers and Drenth and co-workers of the acid catalyzed hydration of alkynyl ethers (3-7) and thioethers (8-10). In particular, the hydration of the following compounds has been investigated ... 

Clearly, a large body of diverse evidence indicates that the acid-catalyzed hydration of alkynyl ethers and thioethers proceeds via a rate-determining protonation through a vinyl cation. However, these vinyl cations are unique in that they have a resonance form where the positive charge resides on the... 

Once again, a large amount of diverse evidence indicates the intermediacy of a vinyl cation in electrophilic additions to arylacetylenes. As in the case of the hydration of alkynyl ethers and thioethers, the vinyl cation formed is especially stable because of resonance interaction and charge delocalization with the adjacent rr center of the aromatic system. 

Alkynylation of halo acetals. In the presence of ZnCl2 (2 equiv.) the organotin acetylide 1 couples with chloromethyl methyl ether or chloromethyl methyl sulfide to form alkynyl ethers or sulfides, respectively (equation I). 

Allenyl ethers are useful key building blocks for the synthesis of a-methylene-y-butyrolactones [129, 130], The synthesis of the antileukemic botryodiplodin was accomplished with the crucial steps briefly presented in Scheme 8.56. Bromoallenyl ethers 225 were easily prepared by base-induced isomerization from the corresponding /3-bromoalkyl alkynyl ether compounds and then subjected to electrophilic bro-mination with NBS. The resulting acetals 226 were converted into 2-alkoxy-3-methy-lenetetrahydrofurans 227 by dehydrohalogenation of the alkenyl bromide unit to an alkyne and subsequent radical cyclization employing tributyltin hydride [130],... 

Cobaloxime(I), electrochemically regenerated from chloro(pyridine)-cobaloxime (III) (232), has been employed as a mediator in the reductive cleavage of the C—Br bond of 2-bromoalkyl 2-alkynyl ethers (253), giving (254) through radical trapping ofthe internal olefin (Scheme 95) [390]. An interesting feature of the radical cyclization (253) (254) is the reaction in methanol, unlike the trialkyltin hydride-promoted radical reactions that need an aprotic nonpolar solvent. An improved procedure for the electroreductive radical cyclization of (253) has been attained by the combined use of cobaloxime(III) (232) and a zinc plate as a sacrificial anode in an undivided cell [391]. The procedure is advantageous in terms of the turnover of the catalyst and the convenience of the operation. 

Scheme 3.22. Stannylcupration of alkynyl ethers [87a] and alkynyl thioethers (TBDMS = t-butyidimethylsilyl) [87bj.

In situ preparation of the vinylidene pentacarbonylchromium complexes was also reported by Fischer et al. [7]. Treatment ofthe acylchromate complexes 18, prepared by acylation of pentacarbonylchromium dianion, with trifiuoroacetic anhydride in the presence of DBU afforded very labile pentacarbonylchromium vinylidene complexes 19, which were, without isolation, reacted with several electron-rich alkynes such as ynamines, alkynyl ethers, and alkynyl thioethers to give the corresponding cyclobutenylidene chromium complexes in reasonable yield [8] (Scheme 5.5). 

Ethyl and higher-alkyl alkynyl ethers eliminate ethene at elevated temperatures [68],... 

Alkynyl ethers, RCH2C CCH2OR and CHjGaCCHCRlOR, are converted into 3,1 -en-ynes by treatment with sodamide in liquid ammonia. The reactions may be visualized as 1,4-eliminations of ethanol and subsequent metailanon of the intermediary cumulenes [143] ... 

Acetylenic precursors employed in the syntheses of sugars may be divided into three groups (a) aldehydes (usually in the form of acetals), (b) alkyl alkynyl ethers, and (c) alkynols or alkynediols. Some of them are commercially available (for example, 2-butyne-l,4-diol), and others are prepared by Grignard-type reactions between 1-alkynylmag-nesium halides or lithium alkynes and suitable aldehydes, ketones, or epoxides. In this way, the synthesis of substrates having the desired number of carbon atoms, as well as the necessary functional groups, can be achieved. The next step consists in partial saturation of the triple bond to afford the desired cis- or trans-alkene. ct.s-Alkene systems... 

Intramolecular alkylation, although it is enticing, has not been developed as a method for cyclohexanone construction. Joseph P.A. Harrity of the University of Sheffield reports (J. Org. Chem. 68 4392, 2003) that TiCl smoothly transforms the enol ether 10, prepared from the corresponding alkynyl phosphonium salt, into the 2-aryl cyclohexanone 11. Alkynyl ethers such as 10 are readily prepared in enantiomerically-enriched form. Would the enantiomeric excess be maintained on cyclization ... 

Thus, strong shieldings are observed for fi carbons of enol ethers and alkynyl ethers, as shown for 1,1 -dimethoxyethene (54.7 ppm) and ethoxyethyne (23.4 ppm) in Table 4.26. In 1-alkoxy-l,3-butadienes, transmission of the electron releasing effect along the conjugated double bonds affects alternate carbons similarly, shielding the carbons in / and <5 position as illustrated for l-ethoxy-2-methyl-l,3-butadiene. 

I-Ethoxy-I -alkyne. R—C=C—OC-Hv. pyroly/es at 120 into ethylene and alkylketene. but the latter is consumed by Ihe original alkynyl ether... 

Palladium-mediated addition of silyl stannane reagents to alkynyl ethers has been employed for the synthesis of aliphatic acyl silanes in very good yields via the intermediate a-alkoxy-/J-stannyl vinyl silanes (enol ethers of acyl silanes)82. In a second palladium-catalysed step, the vinyl stannane moiety could be coupled to suitable halides before hydrolysis to the acyl silanes with trifluoroacetic acid (Scheme 11). 

Ethyl enol ethers of acyl silanes have been prepared by the palladium-mediated addition of silyl stannanes to alkynyl ethers. Hydrolysis using trifluoroacetic acid gave very high yields of acyl silanes (vide supra, Section m.A.4)82. 

C6H5IO-Ru(II) also oxidizes 1-alkynyl ethers and amines to the corresponding oc-keto esters and amides in fair to high yield.3... 

Methylene chromans can be prepared by a palladium-catalyzed intramolecular cyclization of 2-iodophenyl alkynyl ethers 455. Simple modifications to the reaction conditions lead to either (E)- or (Z)-4-methylene chromans (Scheme 103) <2001TL2657>. 2-Iodophenyl alkynyl acetals are also viable substrates for this reaction <2005JOG489>. Addition of tris(2,6-diphenylbenzyl)tin hydride (TDTH) to a triethylborane-mediated intramolecular cyclization of 2-iodophenyl alkynyl ethers ensures complete (A)-selectivity is observed in the resulting 4-methylene chroman <2001AGE411>. 

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