Synthesis of Enynes

Wang s approach for the synthesis of enyne-allenes focused on ene-allenyl iodide 45 (Scheme 14.12) [24]. Palladium-catalyzed Sonogashira reaction of 45 with terminal alkynes 46 (R= Ph or CH2OH) proceeded smoothly under mild reaction conditions in the presence of the cocatalyst cuprous iodide and n-butylamine. The initially formed enyne-allene 47b with substituent R= CH2OH cyclized spontaneously to the corresponding a-methylstyrene derivative 48. 

Wang s synthesis of enyne-allenes by cross-coupling of ene-allenic iodides with alkynes has already been mentioned in Sect. 14.2.1.1 (Scheme 14.12). In a continuation of this work, the same group developed an alternative coupling reaction of allenylzinc chlorides 74 with enyne iodides 75 catalyzed by Pd(PPh3)4, which provided the expected enyne-allenes 76 in high yield and with excellent Z/E selectivity (Scheme 14.17) [38],... 

Scheme 20.5 One-step synthesis of enyne-allenes from enediynyl propargylic alcohols.

Use of alkenyl halides in this reaction has proven to be an effective method for the synthesis of enynes.116 117 The reaction can be carried out directly with the alkyne, using amines for deprotonation. 

In general, this synthesis of enynes has no special advantages over the alternative method which starts with allenylmagnesium bromide [144] ... 

TABLE 12. Synthesis of enyne 183 from magnesium alkylidene carbenoids 134a with lithium acetylides and electrophiles... 

SCHEME 113. Synthesis of enynes using zinc organometallics... 

Cascade Synthesis of Enynes/Pauson-Khand Reaction. 221... 

The synthesis of enynes is not always trivial. Some synthetic reactions have functional group compatibility problems, others give poor yields due to instability of the products. Transition metals can be useful in reactions that give enynes which can suffer subsequent PKRs. 

Scheme 27 Catalytic synthesis of enynes from terminal alkynes can result in the formation of mixtures of products...

Synthesis of Enyne-Allenes, Dienyne-Allenes, and Trienyne-Allenes, and Their Cycloisomerization... 

Synthesis of Enyne-AUenes, Dienyne-AUenes, and Trienyne-AUenes... 

N. Sakai, R. Komatsu, N. Uchida, R. Ikeda, T. Konakahara, Org. Lett. 2010, 12, 1300-1303. A single-step synthesis of enynes Pd-catalyzed arylalkynylation of aryl iodides, internal alkynes, and aUcynylsilanes. 

The synthesis of enynes is of interest in the chemistry of certain natural products. Terminal enynes occur in several natural products such as histrionicotoxin and laurencin, and the internal enyne unit is found along with polyacetylenes and allenes in natural products from Compositae and Umbelliferae. Both internal Z-and. E-enynes are also useful as precursors to stereochemically define d dienes on their partial reduction. A group from Phillips-Duphar has described an efficient synthesis of the functionalized enyne (95), which serves as the C5 synthon for the convergent synthesis of vitamin A. The l,3-dichloro-2-ether (94) is dechlorinated, substituted, and isomerized in one step on reaction with two molar equivalents of sodium acetylide in liquid ammonia, giving (95) with an E Z ratio... 

The CuTcatalyzed synthesis of enynes has also been performed by coupling divinylic chalcogenides with alkynes in the presence of a catalytic amount of nickel. These conditions established a new stereoselective route to Z- and -enynes in good... 

The cross-coupling of aluminum organyls (alanes) with electrophiles has been intensively investigated (and discovered) by the Negishi group. Since aluminum alkenyls are readily available by hydroalumination of alkjmes, this Negishi coupling is very useful for the synthesis of enynes and styrenes (Scheme 5-104). ... 

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