Synthesis of Conjugated Enynes

Another important synthesis of 1,3-rrms-enynes has also been reported (Eq. 98)153). 

Many insect pheromones have conjugated cis, trans-diene system l52, l53). The versatility of these reactions is exemplified in the synthesis of many natural products l53-154, 55 In Eq. 99, the four step synthesis of a natural sex pheromone of the European grape vine moth (Lobesia botrana), (7E, 9Z)-7,9-dodecadien-l-yl acetate, is illustrated 155). 

Recently, Rossi et al. have applied the cross-coupling of 1-alkenylboranes with 1-alkynyl halides for the synthesis of natural products. Thus, (7 , 9Z)-7,9-dodecadien-1-yl acetate (32), the sex pheromone of Lobesia botrana, has been synthesized by the following sequence involving  

Many insect pheromones have conjugated cis, trans-diene system The ver- 

European grape vine moth (Lohesia botrana), IE, 9Z)-7,9-dodecadien-l-yl acetate, is illustrated 

The versatility of such cross-coupling reactions of 1-alkenylboranes has been 

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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... 

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... 

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]. 

The synthesis of conjugated enynes via cross-coupling of A-tosylhydrazones with terminal alkynes catalysed by Pd(OAc)2-tris(2-furanyl)phosphine has been reported (Scheme 133). ... 

Brown and Molander [1] have reported the synthesis of conjugated enynes. The reaction involves the synthesis of alkenylcopper intermediates from alkenylbo-ron derivatives of 9-BBN, which undergo coupling with 1-halo-l-alkynes in a stereodefined manner and afford the conjugated trans enynes (>95% pure) in almost quantitative yields. The cuprous iodide is the reagent of choice as compared to cuprous bromide-dimethylsulfide (Eq. 24.21 Table 24.21) [1]. 

Fig. 27 Synthesis of conjugated enynes by oxidative coupling of tosylhydrazones with terminal alkynes...

A 1,2,3-triazole-promoted iron(III)-catalysed propargyl alcohol dehydration has been developed for the synthesis of conjugated enynes (Scheme 3) The products were obtained in good to excellent yields (up to 95%) with a large substrate scope and excellent Z stereoselectivity. Addition of readily available 1,2,3-triazole to FeCl3 provided a practical and efficient catalyst system, which gave the critical chemoselectivity for the carbon-oxygen bond activation. 

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. 

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). 

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... 

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]. 

Hydroboration of enynes provides 1,3-alkadienylboron derivatives. The coupling of dienylboron compounds with haloalkenes allows a short-step synthesis of conjugated trienes such as the synthesis of leukotriene B4 shown in Scheme 2-31 [88, 89]. 

In this account, an overview of the methods employed for the synthesis of conjugated dienes and polyenes is presented. Dienes and polyenes with isolated double bonds are excluded, as they are accessed through methods usually employed for alkene synthesis. Oligomerizations and polymerization reactions leading to polyenes are also not covered. Synthesis of 1,2-dienes, i.e. allenes, is excluded from the purview as there is a volume in the present series devoted to this functional group. Synthesis of heterodienes, conjugated enol ethers, [n]-annulenes and related compounds are also not covered here. However, enynes, dienynes and enediynes syntheses have been included in a few cases in view of their emerging importance. 

The Suzuki reaction provides a versatile, general method for the stereo- and regiospe-cific synthesis of conjugated dienes, enynes, aryl substituted alkenes, and biaryl compounds via Pd-catalyzed cross-coupling of vinyl halides or aryl halides with... 

Scheme 10 6 Synthesis of conjugated trienes by carbomagnesiation of enyne propargylic alcohols and ethers [71],...

The ability of conjugated enynes to serve as a four-carbon unit in thermal or Lewis acid-mediated cycloaddition reaction with alkynes to form aromatic product was first recognized by Danheiser et al. [1] (Scheme 14.1). Extensive experimental and theoretical mechanistic studies suggest intermediacy of strained cyclic allene in this transformation [2]. Although this enyne-yne cycloaddition reaction represents a powerful method for the synthesis of bicyclic aromatic compounds [1,3], it is limited to intramolecular processes. 

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