Alkynyl propargyl acetates

The Lewis acid-promoted alkynylation of acetals with alkynylsilanes is frequently used for the synthesis of propargyl ethers [531]. Recent studies have disclosed that the alkynylation of 2-oxy-THP derivatives and 4-oxy-3,4-dihydro-2H-py-rans proceeds with high diastereoselectivity (Scheme 10.202) [532, 533]. 

In connection with the finding that a combination of a Pd catalyst and ammonium formates is effective for hydrogenolysis of allyl esters to olefins, and of propargyl carbonates to allenes, the reactions with allyl ester of alkynyl acid and propargyl acetate under the reaction conditions shown in Scheme 4 were investigated. The results indicate that in both cases reduction of the triple bond takes preference over hydrogenolysis as sununa-rized in Scheme 5. 

Cross coupling of EtsAl with aryl phosphates or alkynyl bromides proceeds with a Ni catalyst to provide alkylated arenes or aUrynes (eqs 5 and 6). While Pd or Cu complexes are used as the catalyst for the coupling of EtsAl with carboxylic acid chlorides or thioesters (eq 7), Iron(HI) Chloride is used for reactions of propargyl acetates to give substituted allenes (eq 8)."... 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-l, 2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5], The reaction can be explained by the transmetallation of the (7-allenylpailadium methoxide 4 with copper acetylides to form the allenyKalk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCb) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7]. 

The alkynylation of estrone methyl ether with the lithium, sodium and potassium derivatives of propargyl alcohol, 3-butyn-l-ol, and propargyl aldehyde diethyl acetal in pyridine and dioxane has been studied by Miller. Every combination of alkali metal and alkyne tried, but one, gives the 17a-alkylated products (65a), (65c) and (65d). The exception is alkynylation with the potassium derivative of propargyl aldehyde diethyl acetal in pyridine at room temperature, which produces a mixture of epimeric 17-(3, 3 -diethoxy-T-propynyl) derivatives. The rate of alkynylation of estrone methyl ether depends on the structure of the alkyne and proceeds in the order propar-gylaldehyde diethyl acetal > 3-butyn-l-ol > propargyl alcohol. The reactivity of the alkali metal salts is in the order potassium > sodium > lithium. 

A number of additional methods involve the addition of alkynylsilanes to electrophiles with concomitant 1,3-isomerization to afford allenylsilanes geminally substituted with the electrophile moiety. The first of these methods employed a trimethyl-silyl-substituted propargylic silane as the alkynylsilane and various acetals as the electrophile precursors (Table 9.29) [53], The allenylsilanes are formed without contamination by alkynyl isomers. 

Carbonyl compounds have been alkynylated to give the corresponding propargyl alcohols, using TMS-alkynes and a base such as acetate or phenoxide ion.193... 

Various metal acetylides are used for smooth coupling with propargylic halides and acetates. 2,3-Alkadien-5-yn-l -ols are obtained by the reaction of 2-(l -alkynyl)oxiranes [28,29], As a synthetic application, the unstable 2,3-octadiene-5,7-diyn-l-ol (136), a fungus metabolite, has been synthesized by the coupling of 4-trimethylsilylbutadiy-nylzinc chloride (134) with 2-ethynyloxirane (135) followed by desilylation [31]. 

Alkynyl ketones can be prepared using the former useful methodology. Starting from propargyl aldehyde diethyl acetals and benzotriazole under toluene reflux, l-(benzotriazol-1-yl) propargyl ethyl ethers were prepared in 83-84% yield. The deprotonation is very... 

Analogous to the allylation with allylsilanes and -stannanes, the transformations, vinylallylation, propargylation, allenylation, alkenylation, alkynylation, and arylation, are viable by the use of an appropriate reagent in the presence of a titanium Lewis acid these are surveyed in the review articles cited both in the Introduction and in this section. The stereochemistry of the reaction of a (vinylallyl)silane in the presence of TiCU has been reported [234]. Equation (113) shows that the major reaction of this silane and isobutyraldehyde occurred mainly in the anti sense with a ratio of anti to syn attack of 90 10 at the terminus remote from the silyl group. Essentially the same stereochemical outcome was observed for the same reaction with the corresponding trimethylsilyl derivative. The intramolecular reaction with an acetal, however, proceeded less selectively the anti syn ratio was 60 40 (Eq. 114) [234]. 

This chapter focuses the attention on the reactions of nonstabilized carbanionic compounds such as alkyl, vinyl, aryl, alkynyl metals, etc., and the chemistry of the stabilized system, i.e. allylic, propargylic or oxaallylic carbanions is presented in Volume 2 of this series. Electrophiles with C=X bonite which are discussed include aldehydes, ketones, epoxides, aziridines, acetals, orthoesters and imines, all of which turn into highly reactive electrophiles in the presence of Lewis acids. 

In addition to their reactions with trlmethylsilyl enol ethers, (propargyl1um)Co2(C0)g complexes react with a variety of other mild carbon nucleophiles including activated aromatic compounds, g-dicarbonyl compounds, other enol derivatives (enol acetates and ketones directly), allylsilanes, and alkyl- and alkynyl-aluminum reagents. These reactions provide a flexible means to introduce the synthetically versatile propargyl function. Key features of propargylations using these complexes are 1) ready... 

Optically pure propargylic alcohols have been obtained by reductive cleavages of a,S-alkynyl acetals with organoaluminium reagents followed by oxidative removal of the chiral auxiliary (Scheme 18), and the same research group has achieved direct, highly efficient... 

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