Esters, alkynic reaction with allylic alcohols

The Pauson Khand reaction is compatible with a wide variety of functionalities, such as ethers, alcohols, tertiary amines, thioethers, ketones, ketals, esters, tertiary amides, carbamates, and benzene, furan, and thiophen rings. Disubstituted alkynes, alkenes with bulky allylic substituents, and trisubstituted alkenes frequently afford reduced yields of products. Because of the reduced ability of sterically hindered alkenes to coordinate and undergo insertion, insertion of one or more molecules of alkyne occurs instead. 

The alkoxytitanium propene compound Ti(T] -propene)(OTr)2 (46) [153], which is believed to be generated from Ti(0 Pr)4 and two equivalents of /-PrMgCl, reacts with internal alkynes to give titanium-alkyne compounds Ti(ri-alkyne) (0 Pr)2 (47) in quantitative yield (Scheme 6.9) [154,155]. 46 reacts with carboxylic esters to produce cyclo-propan- 1 -ols in modest yields [ 156,157]. Oxidative addition of allyllic halides or allyllic alcohols to 46 proceeds readily to form allyl titanium compounds 48, whose reaction with aldehyde provides a stereoselective synthesis of homoallylic alcohols [153]. 

Reaction of Ta-alkyne complexes with R R2C=0. The Ta complexes formed from TaCls/Zn with unsymmetrical alkynes react with carbonyl compounds to form two rcgioisomcric allylic alcohols with a ratio depending on the substituents on the alkync (both steric and electronic effects) as well as the size of the substituents in the carbonyl group. The complexes from acetylenic esters react with carbonyl compounds mainly at the position or to the ester group, whereas complexes from acetylenic amides react mainly at the position fi to the amide. 

From the fact that in the reaction catalyzed by 2 also internal alkynes 24 can react to give ketones 25 (Scheme 8), the authors conclude that in this case no vinylidene complex acts as an intermediate. Apparently, its formation is favored by the presence of phosphane ligands, and in their absence other reaction paths are followed. The regioselectivity of the reaction as well as its yield can be increased if a mixture of water and DMF is used as solvent. The reaction functions also with alkynes whose triple bond is conjugated to an ester group. The chemoselectivity of the reaction was demonstrated by the coupling of a steroid side chain with an allyl alcohol one a,y0-unsa-turated carbonyl functionality present in the steroid part remained unaltered. 

Alkynes are useful partners in cationic cyclization reactions. Initial reaction of the allylic alcohol moiety in 91 with formic acid gave allyl cation 92. Subsequent attack by the alkyne moiety across the molecule generated vinyl cation 93. This cation trapped formate anion to generate a formate enol ester (C=C—OCHO). Hydrolysis liberated the final ketone product 94, which Lansbury converted to damsinic acid. ... 

Reaction of Ta-alkyne com from TaCIs/Zn with unsymmctncal regioisomeric allylic alcohols with (both steric and electronic effects 11 group. The complexes from acetyl at the position a to the ester gri u mainly at the position to the an... 

The synthesis of 252 began with Brown s asymmetric crotylation to aldehyde 261. The resulting homoallyl alcohol was converted benzyl ester 262, which was reduced to give lactol acetate 263. Axial allylation to 263 formed 2,6-trans-tetrahydropyran 264, which was subjected to ozonolysis to give an aldehyde. Addition of alkenylzinc, prepared by hydrozircona-tion of an alkyne 265, to the aldehyde mediated by chiral ligand 266 yielded allyl alcohol 267 with a 5.1 1 diastereoselectivity [110]. The stereochemistry of the major isomer was found, unexpectedly, to be the S-form at Cl7, which rendered the macrolactonization to adopt the Mitsunobu reaction. The iodide 252, prepared from 267 in three steps, reacted with... 

This reaction was employed to synthesize the perfumery compound, rosefuran 8.180 (Scheme 8.48) by the ruthenium-catalysed combination of alkyne 8.177 with an allylic alcohol. Dihydroxylation of the product 8.178 was followed by acid-catalysed dehydration. Hydrolysis of the ester and thermal elimination of water gave the natural product 8.180. Another synthesis of rosefuran can be found in Scheme 2.15. 

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