Alkynes Corey-Fuchs reaction

Another common method for the conversion of an aldehyde into an alkyne is the transformation of the aldehyde via a Corey-Fuchs reaction into a geminal vinyl dibromide followed by reaction with nbutyllithium and aqueous work-up.22... 

Steps 1-2 Corey-Fuchs reaction aldehyde to terminal alkyne transformation. 

Steps 1-2 Conversion of a ketone to a terminal alkyne (compare with Corey-Fuchs reaction where an aldehyde is converted to a terminal alkyne). Step 4 Me2CuLi adds to the acetylenic ester in a conjugate manner to provide (Z)-olefin. 

W.J. Kerr and co-workers carried out the total synthesis of (+)-taylorione starting from readily available (+)-2-carene and using a modified Pauson-Khand annulation with ethylene gas as the key step. The key terminal alkyne intermediate was prepared by the Corey-Fuchs reaction. Interestingly, the ketal protecting group was sensitive to the excess of CBr4, so the addition of this reagent had to be monitored carefully to cleanly transform the aldehyde to the desired dibromoolefin. 

Examples of allylation [39a] and vinylation [39b] are shown in Scheme 24. The dichlorovinylation is especially interesting since the products are immediate precursors of alkynes via the Corey-Fuchs reaction. The process is also applicable to aliphatic iodides, exchange of iodine replacing the exchange of xanthate in the mechanistic manifold in Scheme 22 [39b,c[. 

A general one-pot procedure for the synthesis of alkynes from aldehydes (Corey-Fuchs reaction) that involves the synthesis of the triphenylphosphonium dibromomethane reagent has been explored by Michel and coworkers. In general, the base of choice is f-BuOK, but in case of the N-Boc piperidine, use of BuLi in order to avoid unwanted side reactions between the carbamate and the acetylide function provides a cleaner reaction at low temperature (eq 64). 

Sol 3. (d) The first step involves Corey—Fuchs reaction One-carbon homologation of an aldehyde to dibromoolefin, which is then treated with n-BuLi to produce a terminal alkyne. 

Corey-Fuchs reaction is a two step reaction converting an aldehyde to an alkyne by one-carbon homologation of the aldehyde. The Wittig-like reaction of aldehyde 1 and dibromocarbene forms dibromoalkene 2. The treatment of dibromoalkene 2 with two equivalent of -BuLi form a lithium alkynylide, quenched by eletrophiles, such as proton, CO2, aldehydes, ketones, and alkyl halides to form alkyne 3. 

Alkyne 13 was prepared from aldehyde 9 by Corey-Fuchs reaction (10), alkylation (11), 1,2-Wittig rearrangement (12) and silylation reaction. The reaction of alkyne 13 and BuaSnH afforded compound 14 via a 5-exo-dig cyclization and the cleavage of Si-0 bond by MeLi. ... 

The reaction sequence in steps two and three is known as the Corey-Fuchs method to create an alkyne from an aldehyde 10 Reaction of triphenylphosphane with carbontetrabromide gives phenylphosphane-dibromomethylene. This reagent then transforms aldehyde 19 into the corresponding dibromoalkene 20 thereby extending the chain by one carbon. Reaction of the bromo compound with two equivalents of n-butyllithium in THF at -78 °C results in the rapid formation of the acetylenic lithio derivative which forms the terminal acetylene 21 upon aqueous work-up. 

Fig. 11. 29. Aldehyde — alkyne chain elongation via [1 -rearrangement of a vinyl carbenoid (Corey-Fuchs procedure). The aldehyde and phosphonium ylide A generated in situ undergo a Wittig reaction and form the 1,1-dibromoalkene. In the second stage, the dibromoalkene is reacted with two equivalents of n-BuLi and the vinyl carbenoid C is formed. The carbenoid undergoes H migration to form the alkyne B. The alkyne B reacts immediately with the second equivalent of w-BuLi to give the lithium acetylide.

Related reactions Corey-Fuchs alkyne synthesis ... 

The presence of halogen atoms in a molecule is compatible with formation of an ylid and subsequent Wittig reaction. Carbon tetrabromide and triphenylphosphine have been used to give vinyl dibromide products, which are rapidly converted to the corresponding alkyne. Indeed, this is an important synthetic route to alkynes called the Corey-Fuchs procedure. The reaction of 511 with this reagent gave vinyl dibromide 512 in 77% yield as part of Marshall s synthesis of callystatin When 512 was treated with butyllithium... 

---