Conjugated enynes synthesis

A hydroboration-protonolysis procedure for the conversion of conjugated enynes to dienes is far superior to partial hydrogenation over Lindlar s catalyst, in terms of stereoselectivity and yields280. Ratovelomanana and Linstrumelle reported the synthesis of methyl a-eleostearate (equation 163) and methyl punicate by employing this strategy280. 

Several examples are known of the transition metal-catalyzed synthesis of 1,2,3-buta-trienes, which possess one more cumulated C=C double bond than allenes. Most of the reported examples of the butatriene synthesis involve dimerization of terminal alkynes and conjugated enynes are typical side products of the reactions. 

In 1993, Hayashi and co-workers reported a catalytic asymmetric synthesis of alle-nylboranes 256 by palladium-catalyzed hydroboration of conjugated enynes 253 (Scheme 4.66) [105]. Reaction of but-l-en-3-ynes 253 with catecholborane 254 in the presence of a catalyst, prepared from Pd2(dba)3 CHC13 (1 mol%) and a chiral mono-dentate phosphine ligand (S)-MeO-MOP 255 (1 mol%), gave an allenylborane 256. The ee of 256 was determined by the reaction with benzaldehyde affording the corresponding optically active homopropargyl alcohols 257 with up to 61% ee (syn anti= 1 1—3 1). 

It has been shown that the Claisen rearrangement of lithium enolates of amino acid enynol esters allows the synthesis of very sensitive y, 5-unsaturated amino acids with conjugated enyne side chains.The chelate-enolate Claisen rearrangement has also been applied to the synthesis of unsaturated polyhydroxylated amino acids, polyhydroxylated piperidines, and unsaturated peptides. ... 

The reagent employed in the first reaction, with structure 44. is from a method developed by Gtrcx for the synthesis of conjugated enynes.1 Addition of TBA1 leads to quantitative formation of target structure 1... 

Enynes were prepared in good yield from alkynyl iodonium salts and alkenylcopper reagents, stereospecifically. This approach was suitable for the synthesis of conjugated enynes, using a trisubstituted alkene with complete retention of its geometry [47], 1,3-Diynes were similarly obtained by coupling alkynyl iodonium... 

Saito S, Yamamoto Y (2002) Palladium-Catalyzed Benzannulation Reactions of Conjugated Enynes and Diynes. In Negishi E, de Meijere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 1635... 

Doucet H, Hierso JC (2007) Palladium-based catalytic systems for the synthesis of conjugated enynes by sonogashira reactions and related alkynylations. Angew Chem Int Ed 46 834-871... 

Conjugated enynes are of importance for themselves, as well as for the synthesis of conjugated dienes. The cross-coupling reaction of 1-alkenyl(disiamyl)boranes (3c) with 1-bromo-l-alkynes (Scheme 2-34) provides conjugated enynes in high yields [45]. 

Cross-coupling reactions between 1-alkynyl halides and 1-alkenylboranes, which are readily available via hydroboration of alkynes, can also be catalyzed by Pd-phosphine complexes. Stereoselective synthesis of conjugated ( )-enynes 143 was achieved by the coupling of the alkynyl bromide with an alkenylborane [Eq. (48)] [69]. 

This intramolecular carbolithiation of silylated conjugated enynes was then successfully applied to the construction of small carbocycles such as cyclopropanes and cyclobutanes [35]. Due to the stabilization of the propargylic organolithium after carbocyclization, the reversibility of the intramolecular carbometallation of strained carbocycles was impeded (Scheme 7-37), e.g., for the cyclobutane synthesis. 

Htibner, H. Haubmann, C. Utz, W. Gmeiner, P. (2000) Conjugated enynes as nonaromatic catechol bioisosteres synthesis, binding experiments, and computational studies of novel dopamine receptor agonists recognizing preferentially the D3 subtype. J. Med. Chem. 43, 756-762. 

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