Subject terminal alkynes

Chlorobenzenes activated by coordination of Cr(CO)3 react with terminal alkynes[253). The 1-bromo-1,2-alkadiene 346 reacts with a terminal alkyne to afford the alka-l,2-dien-4-yne 347[254], Enol tritlates are used for the coupling with terminal alkynes. Formation of 348 in the syntheses of ginkgolide[255] and of vitamin D are examples[256] Aryl and alkenyl fluorides are inert. Only bromide or iodide is attacked when the fluoroiodoalkene 349 or fluoroiodoar-ene is subjected to the Pd-catalyzed coupling with alkynes[257-259]. 

Alkynes are interesting radical acceptors for cyclization reactions because the products contain double bonds that can be subjected to further transformations. In the case of terminal alkynes, the desired products can be obtained in high yields as single isomers. With non-terminal alkynes as acceptors, the alkene products are generated as mixtures of ( )- and (Z)-isomers in high yields but with low selectivity [36],... 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

The synthesis of novel azetidine derivatives remains the subject of intensive study. New procedures for the preparation of this class of compounds include, e.g., rearrangement of /3,7-aziridino-a-amino esters <2007OL4399>, copper-catalyzed multicomponent reactions of terminal alkynes, sulfonyl azides, and carbodiimides <20070L1585>, regioselective addition of 1,3-dicarbonyl dianions to iV-sulfonyl aldimines <2007T4779>, elaboration of a-amino acids <2007TL2471>, palladium-catalyzed iV-arylation of azetidines <2007S243> and... 

With the exception of (trimethylsilyl)acetylene (equation26)79, BF3-etherate methodology has not been applied to typical terminal alkynes [RC=CH]. However, when various alkynoic acids are subjected to the action of iodosylbenzene and BF3-etherate, intramolecular participation of the carboxyl group occurs, and lactonic vinyliodonium fluorobo-rates are obtained135. Some examples are presented in equation 176. H NMR studies of these compounds showed no appreciable NOE enhancement between the vinylic and allylic protons 135, consistent with the -configuration about the carbon-carbon double bonds. 

Takahashi et al. also reported a route to muconin. Their synthesis adopted Keinan et al. s strategy to construct the stereochemistries by Sharpless AD and AE upon multiple olefin containing fatty acid (Scheme 10-35). The di-olefin 214 was subject to Sharpless AD conditions and then treated with acid, yielding a THP-containing diol. This diol was further protected as acetonide 215. The reversion of stereochemistry of alcohol 215 was achieved by Dess-Marlin oxidation and Zn(BH4)2 reduction. Williamson etherification of tosylate 216 and epoxide formation afforded tri-ring intermediate 217. Opening with acetylene, 217 was converted into the terminal alkyne 218, which was coupled with vinyl iodide to finally give muconin. 

In order to understand the polymer structures that are obtained in the polymerization of 1,6-heptadiynes, one needs to consider all possible polymerization mechanisms. If 1,6-hep tadiynes are subject to cyclopolymerization using well-defined Schrock catalysts, polymerization can proceed via two mechanisms. One is based on monomer insertion, where the first alkyne group adds to the molybdenum alkylidene forming a disubstituted alkylidene, which then reacts with the second terminal alkyne group to form poly(ene)s consisting of five-membered rings. Analogous to 1-alkyne polymerization, one refers to this type of insertion as a-insertion (Scheme 4). 

Since our direct route to angularly fused triquinanes from cycloaddition of l-(4-pentynyl)cyclopen-tenes is limited to trisubstituted alkenes and simple terminal alkynes, bisnorisocomene, but not iso-comene itself, could be prepared (Scheme 22). However, this limitation is not a factor for most other compounds in this class of natural products, and the steric interactions described earlier worked to our advantage in a diastereocontrolled synthesis of pentalenene (see structure. Scheme 20). The natural product was obtained by subjecting the product of Scheme 14 to the sequence i, Li, NH3, MeOH ii, MeLi, Et20 iii, p-TsOH, benzene, reflux. ... 

The same research group reported on synthesis of [2]rotaxane employing Diels-Alder reaction of terminal alkyne with 1,2,4,5-tetrazine to provide stopper units (Scheme 8.2) [11]. Synthetic concept is identical as in the previous example. Two components in acetonitrile after evaporation gave pseudorotaxane 9, which was subjected to ball milling with ym-tetrazine 10. After 9h, [2]rotaxane 11 was obtained in high yield. Silica gel was added to facilitate transformation in solid state. Under mechanochemical solvent-free conditions, small, but sufficiently bulky pyridazine rings were used for stoppering the pseudorotaxanes. 

C-19 methyl functionality in the tetracyclic ketone 164, the Nt-H group was alkylated with the optically active i .-tosylate 224 (which was in turn obtained from the R propargylic alcohol via a two-step sequence) in aceto-nitrile/K2C03, followed by treatment with tetrabutylammonium fluoride hydrate to obtain the acetylenic ketone 225 in 96% yield. The terminal alkyne in 225 was converted into the iodoolefm functionality by treating with dicyclohexyhodoborane [I-B(Cy)2], followed by protonolysis. The iodo-olefin 226 was obtained in 74% yield. It was subjected to a Pd-catalyzed a-vinylation to obtain the key C-19 methyl-substituted pentacy-clic system 227. This was followed by a Wittig/hydrolysis/epimerization... 

With the palladium-isonitrile catalyst, terminal alkynes with a variety of substituents and internal alkynes were subjected to the silaboration with 2-silyl-l,3,2-dioxaborolane in refluxing toluene (Table 4). The reaction proceeded not only with high yields but also... 

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