Alkynes Corey-Fuchs

The one-step Seyferth procedure offers an alternative to the Corey -Fuchs sequence. After a Horner-Wadsworth-Emmons olefmation of an aldehyde 37 to an unsaturated diazo compound 38. elimination of nitrogen and [1,2]-rearrangement of the resulting vinylcarbene 39 leads here as well to a Cy-extended terminal alkyne 40.19... 

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. 

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. 

Sulfone 47 was prepared in nine steps from known iodide 69 by the sequence shown in Scheme 17.15. After initial conversion of 69 into aldehyde 70 by cyanide displacement and reduction with diisobutylaluminium hydride (DIBAL), a Corey-Fuchs alkynylation25 yielded alkyne 51, the key substrate needed for the proposed carboalumination-epoxide ring-opening step. Carboalumination26 was achieved by treating... 

Fig. 14.32. Aldehyde alkyne chain elongation via [l,2]-rearrangement of a vinyl carbenoid (Corey—Fuchs procedure). The aldehyde and phosphonium ylide A generated in situ undergo a Wittig olefina-tion and form the 1,1-dibro-moalkene (B). In the second stage, the dibromoalkene is reacted with two equivalents of n-BuLi and the vinyl carbenoid D is formed stereoselectively. The carbenoid undergoes H migration to form the alkyne C. The alkyne C reacts immediately with the second equivalent of n-BuLi to give the lithium acetylide and is reconstituted by reprotonation during aqueous workup.

In the laboratory of J.H. van Boom, the synthesis of highly functionalized c/s- and frans-fused polycyclic ethers of various ring sizes via radical cyclization of carbohydrate-derived p-(alkynyloxy)acrylates was developed/ The radical cyclization precursors were prepared iteratively and the terminal alkyne moieties were installed using the Corey-Fuchs procedure. 

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. 

W. Oppolzer et al. utilized the Corey-Fuchs alkyne synthesis for the preparation of a key acyclic enynyl carbonate during the total synthesis of (+)-hirsutene. ... 

The stereoselective synthesis of the C5-C20 subunit of the aplyronine family of polyketide marine macrolides was accomplished by J.A. Marshall and co-workers. The C15-C20 moiety was prepared using the original Seyferth-Gilbert homologation conditions. The diazophosphonate was deprotonated with potassium fert-butoxide at low temperature, and then the solution of the aldehyde was added slowly also at low temperature. Interestingly, the alternative Corey-Fuchs alkyne synthesis was unsuccessful on this substrate, since extensive decomposition was observed. 

Corey-Fuchs alkyne synthesis One-carbon homologation of aldehydes to form the corresponding terminal alkynes. 104... 

Related reactions Corey-Fuchs alkyne synthesis ... 

Intermediate 143 having secured, the synthesis was carried on by deprotection to form the primary alcohol 144 (Scheme 25) which was oxidized to the aldehyde 145 and converted into the envisaged alkyne (corresponding to 139) via a Corey Fuchs chain elongation via dibromide 146. 

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[. 

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... 

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. 

Ene-yne metathesis has been employed in an elegant synthesis of stemoamide 8.415 (Scheme 8.112). The starting lactam 8.407, available itself from glutamic acid, was iV-alkylated, then subjected to deprotection, Swern oxidation and application of the Corey-Fuchs method for installation of an alkyne by Wittig... 

---