Terminal alkynes protection

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

Carboxylic acids can be protected as oxazolines [96, 105-107, 186, 191] or as ester functions. Alkynic esters such as silyl esters [153, 211], tert-butyl esters [216], and even benzyl esters [153, 211] have been successfully hydrozirconated when the reactive site was a terminal alkyne or vinyl group (Scheme 8-27). 

Scheme 203 provides a methodology for the conversion of aryl bromides onto 4-aryl-l,2,3-triazoles. In the given example, palladium-copper catalyzed substitution of the bromine atom in indole 1226 by trimethylsilylacetylene provides intermediate 1227. Hydrolysis of the trimethylsilyl protecting group releases terminal alkyne 1228, isolated... 

Molecular scaffoldings with tetraethynylethenes (TEEs, 3,4-diethynylhex-3-ene-l,5-diynes) and trans-1,2-diethynylethenes [DEEs, (E)-hex-3-en-l,5-diynes] are at a particularly advanced stage.114,37 38 441 A collection of dose to one hundred partially protected and functionalized derivatives have been prepared in the meantime, providing starting materials for the perethynylated dehydroannulenes and expanded radialenes shown in Figure 6.136 441 TEEs and DEEs, as well as dimeric derivatives substituted at the terminal alkynes with donor (D, p-(dimethyl-... 

Suitably protected amino acids (112) (cysteine, serine, and lysine) have been added via the side-chain heteroatom (S, O, and N, respectively) to conjugated alkynones, alkynoic ester and alkynoic amide (113). The expected heterosubstituted vinyl product (114) was formed in each case, mainly as the ii-isomer. In an accompanying paper, this type of addition was applied to the derivatives of fluorescein, 7-hydroxycoumarin, Sudan 1, and dansyl chloride with linker arms containing a conjugated terminal alkyne. 

Blechert et al. succeeded in intermolecular CM of terminal alkyne and terminal alkene. A reaction carried out in CH2CI2 at RT in the presence of 5-7mol% Ic gives a mixture of ( )- and (Z)-isomers (Table 2). Because of the nonselective stereochemical course, a silyl-protected ally alcohol is employed and the resulting metathesis product is deprotected and oxidized to afford the desired diene having an -configuration (Equation (13)). 

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

The introduction of CHj requires that the terminal alkyne C first become a carbanion and then be methylated. Such a carbanion, acting like the R group of RMgX, would react with the C==0 group of another molecule before it could be methylated. To prevent this, C==0 is protected by acetal formation before the carbanion is formed. The acetal is stable under the basic conditions of the methylation reactions. The aldehyde is later unmasked by acid-catalyzed hydrolysis. 

Terminal alkynes are acidic, and the end hydrogen can be removed as a proton by strong bases (e.g. organolithiums, Grignard reagents and NaNH2) to form metal acetylides and alkynides. They are strong nucleophiles and bases, and are protonated in the presence of water and acids. Therefore, metal acetylides and alkynides must be protected from water and acids. 

Eastmond, R. Johnson, T. R. Walton, D. R. M. Silylation as a Protective Method for Terminal Alkynes in Oxidative Couplings, Tetrahedron 1972,28, 4601. 

Monosubstitution of acetylene itself to prepare terminal alkynes is not easy. Therefore, trimethylsilylacetylene (134) is used as a protected acetylene. After the coupling, the silyl group is removed by the treatment with fluoride anion. The hexasubstitution of hexabromobenzene (135) with 134 afforded hexaethynylbenzene (136) after desilylation in total yield of 28%. The product was converted to tris(benzocyclobutadieno)benzene (137) using a Co catalyst (see Section 7.2.1). Hexabutadiynylbenzene was prepared similarly [60], As another method, terminal alkynes 139 are prepared in excellent yields by the coupling of commercially available ethynyl Grignard (138) or ethynylzinc bromide with halides, without protection and deprotection [61]. 

TMS group is used for protection of terminal alkynes. However, alkynylsilanes themselves can be used for the coupling with aryl and alkenyl triflates using Pd-CuCl as a catalyst [74], Thus the internal alkyne 160 is prepared by stepwise reactions of two different triflates 157 and 159 with trimethylsilylacetylene (134) via 158. 

Van der Eycken s group developed a silver(I)-mediated synthesis of substituted furo[2,3-6]pyrazines.53 Starting from -methoxybenzyl-protected 3,5-dichloropyr-azine-2(l//)-ones 26 (Scheme 5.13), after a regioselective microwave-assisted Sonogashira reaction with various terminal alkynes, the cycloisomerization reaction could occur using AgOTf (2 mol%) with trifluoroacetic acid (TFA, 5 equiv) to yield... 

Let us now give some thought to Myers retrosynthetic analysis of 4 (Scheme 8.1). After a partial clearance of reactive epoxide functionality from within 4, Myers disconnected bond a in compound 5 to yield the partially protected diyne 7 and ketone 6 as possible building blocks. Note how one of the terminal alkyne units in 7 has been retrosynthetically protected with a r-butyldimethylsilyl group to permit regiospecific alkynyl... 

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