Alkyne cleavage

A plausible mechanism for the ring expansion (Scheme 65) proceeds via an intermediate zwitterion A, resulting from Michael addition of the tertiary nitrogen to the alkyne. Cleavage of the C(l)-N bond occurs via formation of the six-membered transition state B in which a molecule of alcohol facilitates the Sn reaction. 

Alkene and Alkyne Cleavage Systems Not Covered Here but Included in Chapter 1... 

As with alkenes we consider first those oxidations which do not cleave the acetylenic bond giving a-diketones, or oxidation of alkynyl amines and ethers to a-keto amides and esters, and then consider oxidative alkyne cleavage to acids. 

For acyclic alkenes, the facile rearrangement process obviates most possibilities of stereoselective transformation. The one exception is that of stereospecific isotopic labelling, for which hydrozirconation is a potentially powerful technique compounds of type Bu CHDCHDX (36), useful for NMR-based mechanistic studies, are conveniently obtained as in Scheme 9. This requires that four separate processes — hydrozirconation of alkyne, cleavage of alkeiiylzirconium, hydrozirconation of alkene and cleavage of alkylzirconium — all be completely stereospecific. 

Oxidative cleavage of alkenes using sodium periodate proceeds effectively in a monophasic solution of acetic acid, water, and THE with very low osmium content or osmium-free. The orders of reactivity of alkenes are as follows monosubstituted trisubstituted >1,2 disub-stituted > 1,1-disubstituted > tetrasubstituted > alkynes. Cleavage with polymer-supported OSO4 catalyst combined with NaI04 allows the reuse of the catalyst. 

Heterocycles which provide the NOC or CNO component synthon Isoxazoles can be prepared by the thermal or photolytic cleavage of a number of heterocycles, such as 1,3,5-dioxazolidone, furazans, furoxans and 1,3,2,4-dioxathiazole 2-oxides, in the presence of a reactive alkene or alkyne. 

Acidic cleavage of the oxirane ring in (210) results in formation of the alkynic aldehyde (211) (68TL4905). 

AgN02, MeOH, H2O, 24°, cool to 0°, add KCN, then HCl, 96% yield." The reduced electron density of the propargylic alkyne directs the electrophilic silver to the other alkyne and activates it for cleavage. 

The von Richter cinnoline process was further extended to solid-phase synthesis. The route began from benzylaminomethyl polystyrene and the required diverse o-haloaryl resins represented by 21 were prepared from substituted o-haloanilines. A Pd-mediated cross-coupling reaction with 21 and the alkynes provided the alkynylaryl derivatives represented by alkyne 22. The von Richter cyclization reaction with hydrobromic or hydrochloric acid in acetone/HaO and cleavage from the resin occurred in the same step to furnish the cinnoline derivatives 23 in 47-95% yield and 60-90% purity (no yield reported for each entry). 

Thomsoni. Ov Click Organic Interactiveto use a web-based palette to predict products for the oxidative cleavage of alkynes. 

Alkynes, like alkenes, can be cleaved by reaction with powerful oxidizing agents such as ozone or KMnC, although the reaction is of little value and we mention it only for completeness. A triple bond is generally less reactive than a double bond and yields of cleavage products are sometimes low. The products obtained from cleavage of an internal alkyne are carboxylic acids from a terminal alkyne, CO2 is formed as one product. 

With the exocyclic alkylidene at C-13 properly in place, the elaboration of the l,5-diyn-3-ene moiety can now be addressed. Cleavage of both acetate and trimethylsilyl functions in 86 with basic methanol, followed by triethylsilylation of the newly formed tertiary hydroxyl group, efficiently affords alkyne 25 (86 % overall yield). This substance was regarded as a viable candidate for a Pd-catalyzed coupling reaction.12 Indeed, treatment of 25 with (Z)-chloroenyne 26 in the presence of a catalytic amount of Pd(PPh3)4 and Cu1 results in the formation of enediyne 24 in 91 % yield. 

Intramolecular and intermolecular 1,3-dipolar cycloadditions of aziridine-2-car-boxylic esters with alkenes and alkynes have been investigated [131, 132]. Upon heating, aziridine-2-carboxylates undergo C-2-C-3 bond cleavage to form azome-... 

The metathesis of linear alkynes has also been reported, e.g. the metathesis of propyne, 1-pentyne, 2-pentyne, and 2-hexyne (31-33). This reaction can be visualized as the cleavage and formation of carbon-carbon triple bonds ... 

Terminal alkynes can be converted by a process of hydrosilylation followed by oxidative cleavage into carboxylic acids (10). An alternative basic" cleavage yields the corresponding aldehydes. 

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