Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Acetylene intramolecular reaction

To avoid problems with the separation of regiomers, dimethyl acetylene dicarboxylate (DMAD) was chosen as a dienophile. The intermolecular Diels-Alder reactions were performed in refluxing dichlorobenzene (bp 132 °C), while the intramolecular reaction of alkyne tethered pyrazinone required a solvent with a higher boiling point (bromobenzene, bp 156 °C). In the case of 3-methoxy or 3-phenyl pyrazinones a mixture of pyridinones and pyridines was obtained, while for the alkyne tethered analogue only the di-hydrofuropyridinone was isolated as the single reaction product. [Pg.296]

Two precedent examples had been reported of the enantioselective [2+2+2] cycloaddition of alkynes. In one case, an enantioposition-selective intermolecular reaction of a triyne with acetylene generated an asymmetric carbon at the benzylic position of a formed benzene ring [19]. In the other case, an intramolecular reaction of a triyne induced helical chirality [20]. Both reactions were developed by chiral Ni catalysts. [Pg.281]

The synthesis of oxotetrahydroindoles by an intramolecular reaction of an azomethine ylide and an acetylene functionality has been reported by MartineUi and co-workers (89). Conversion of the precursor ester 317 to the requisite acid by... [Pg.236]

Acetylenic pyrimidines undergo hetero-Diels-Alder reactions yielding pyrido-fused lactams with recent reports of improved yields using microwave-assisted conditions <2005TL3423>. The use of alkynes as dieneophiles has also been reported in an intramolecular reaction with chloropyrimidine (Scheme 55) <2000SL625>, in this case toward the total synthesis of cerpegin via demethylation. [Pg.255]

A new one-pot tandem C-C coupling intramolecular acetylenic Schmidt reaction using Pd/C-Cu catalysis was developed to afford 3-substituted I (2/7)-isoquinolones 139 <07CC1966>. In this one-pot procedure, 2-iodobenzoyl azide 140 and the appropriate alkyne were reacted in the presence of Pd/C, PPh3, Cul and TEA to provide the products in good to high yields. [Pg.310]

Chelidonine, a representative benzo[c]phenanthridine alkaloid (1,2), was synthesized as the first application of this reaction in natural product synthesis (145). Initial thermal opening of the four-membered ring in 299 led to the formation of a transient -o-quinodimethane (301), which has a dienamide structure in the diene part and is then trapped by the suitably positioned multiple bond in the same molecule. They applied this intramolecular reaction to the acetylenic cyclobutene 300 for the synthesis of ( )-chelidonine (Scheme 110). [Pg.266]

Because the trimerization of acetylene is so enormously exothermic (—143.6 kcal/mol) and is allowed by the Woodward-Hoffmann rules, one might expect a very low activation energy for this transformation. The decrease in entropy might be costly, but should not be prohibitive in intramolecular cases. Experimentally, acetylene undergoes reaction at 400 °C in the gas phase to give a wide variety of products, of which benzene constitutes only a small fraction26. ... [Pg.12]

The reactions of Fischer carbene complexes with alkynes can under certain conditions lead to products that result from the incorporation of two alkynes, the carbene ligand and a carbon monoxide. In inter-molecular reactions, this is most commonly observed for acetylene itself or for sterically unhindered al-kynes. °2 As can be anticipated by the mechanism in Scheme 36, two-alkyne incorporated products of the type (258) are also favored for high alkyne concentration. Synthetically, the two-alkyne reactions are most useful in intramolecular reactions, two of which have been reported and are exemplified by the reactions in Scheme 43. The typical product from the reaction of a Fischer carbene complex with a diyne, such as (308), is a bicyclic phenol of the type (309). ° These products are apparently the result of the assembly of pieces indicated by (311). Under some conditions, dienones of the type (310) and (314) can be isolated, and it is thought they are the immediate precursors of the phenol products via an in situ reduction by a chromium(O) species. This reaction is completely regioselective with diyne (308) and the phenol (309) results from incorporation of the terminal alkyne of (308) before the disubstituted alkyne. Phenols of the type (309) have also been observed from the reaction of diynes with carbyne complexes. ... [Pg.1102]

Internal alkynes induce olefin loss from alkylboranes to form alkenyl boranes. Thus, (C HsjaB reacts rapidly and reversibly with alkynes at 160-200°C to form trial-kenylborane and C2H4. With (r-C4H9)3B, the reaction proceeds under milder conditions. It is not known whether such reactions proceed via B-H intermediates produced by -hydride elimination, or by intramolecular hydride transfer from the alkyl group to a coordinated acetylene. This reaction is useful for reduction of acetylenic hydrocarbons. [Pg.569]

Padwa and co-workers have shown that the amides 55 can be conveniently prepared in high yield by [4 + 2]-cycloaddition of an acetylenic dienophile to the corresponding 2-amino oxazole derivative 54 (Eq. (6)). The reaction was extended to intramolecular reactions (e.g. 56 - 57, Eq. (7)), giving convenient 2-aminofuran precursors for further intramolecular Diels Alder furan (IMDAF) reactions (see Section Il.C.l.e) (99JOC3595, 99TL1645). [Pg.17]

Since these reductions of carbon-heteroatom bonds proceed via carbon radical intermediates, the cyclized products are obtained by their intramolecular reaction with carbon-carbon multiple bonds when an olefin or acetylene moiety is present in their structures (eq (115)) [110]. [Pg.415]

One typical example is given by G. Kaupp with the [2+2] cycloaddition of diphenyl-acetylene to 1,4-dioxene leading to a bicyclic cyclobutene in good yield (cf. iv of this chapter). Another example is presented by T. Miyashi starting from naphthoquinonor-bomadiene. In an intramolecular reaction a quadricyclane derivative is quantitatively formed upon irradiation at X > 400 nm. It is assumed that this process involves the diketo-norbomadiene in its triplet state. Although the spin multiplicity of intermediates have not... [Pg.204]

New Au(III)-pyridine-2-carboxylate complexes were developed to catalyze the intramolecular reaction between furan and acetylene to form phenols <04AE(G)6545>. These pre-catalysts provide higher reaction conversion than AuClj. The Lewis acid catalyzed vinylogous Mukaiyama-Mannich addition of trimethylsilyloxyfuran to aldimines, that generates 6-amino-Y-butenolide intermediates, was applied to the synthesis of piperidines... [Pg.144]

A related transformation of azides has been reported by the group of Toste to afford pyrroles by an acetylenic Schmidt reaction (equation 30).In an intermolecular-related addition, gold triazolates are obtained. The intramolecular hydroamination of trichloroacetimidates derived from propar-gyl and homopropargyl alcohols also proceeds with cationic Au(I) as catalysts. ... [Pg.6582]

It is accepted that this reaction involves the formation of the alkynedicobalt hexacarbonyl complex from an alkyne and Co2(CO)s by the evolving of two CO ligands, followed by the alkene coordination at one of the two enantiotopic Co atoms with concomitant CO insertion, and final reductive elimination of the metal to an a,(3-unsaturated cyclopentenone. In the traditional protocol, the reaction mixture is heated in toluene at 110°C, or tertiary amine A-oxides are added to promote the reaction at ambient temperature. For the purpose of stereochemical control, many Pauson-Khand reactions are designed as intramolecular reactions P " or using cyclic alkenes, such as norbornene. It has been found that the reactivity of cyclic alkenes is in the order of cyclohexene < cyclopentene < norbornene. For the intermolecular Pauson-Khand reaction, alkene is positioned adjacent to the less bulky acetylenic substituent during coordination because of steric hindrance, and a subsequent C-C bond forms between an alkenic... [Pg.2131]

Toste has reported a gold(I)-catalyzed protocol for the intramolecular addition of an alkyl azide to an alkyne with loss of dinitrogen (acetylenic Schmidt reaction) to form pyrrole derivatives [19]. For example, treatment of homopropargyl azide 16 with a catalytic 1 2 mixture of (dppm)Au2Cl2 (dppm = l,2-bis(diphenylphosphino)meth-ane) and AgSbFfi in methylene chloride at 35 °C led to isolation of2,5-dibutylpyrrole in 82% yield (Eq. (11.13)). The method was also effective for the cyclization of a-unsubstituted homopropargyl azides and tolerated both alkyl and aryl substitution of the alkyne. [Pg.441]

Variation 8 Gold(I)-catalyzed Intramolecular Acetylenic Schmidt Reaction (Toste )... [Pg.362]

As in the case of bis-silylation of acetylenes, intramolecular bis-silylation of alkenes is more effectively performed compared to the intermolecular reactions. The isonitrile-palladium catalyst provides 5-exo cyclization products stereospeciflcally from (Z)- and ( )-alkenes tethered to disilanyl groups by ether linkage (Scheme... [Pg.1165]


See other pages where Acetylene intramolecular reaction is mentioned: [Pg.118]    [Pg.118]    [Pg.639]    [Pg.564]    [Pg.1500]    [Pg.1595]    [Pg.564]    [Pg.329]    [Pg.639]    [Pg.564]    [Pg.707]    [Pg.334]    [Pg.308]    [Pg.125]    [Pg.320]    [Pg.368]    [Pg.905]    [Pg.1864]    [Pg.499]    [Pg.215]    [Pg.191]    [Pg.616]    [Pg.522]    [Pg.227]    [Pg.273]    [Pg.16]    [Pg.480]   
See also in sourсe #XX -- [ Pg.363 ]




SEARCH



Acetylene reactions

Intramolecular reactions gold-catalyzed acetylenic reaction

© 2024 chempedia.info