Terminal alkynes rings, formation

The symmetrical dienyne 58a was converted to a fused bicyclo [4.3.0] ring in 95% yield [17] (Eq. 27). With substrate 58c containing an unsymmetrical diene tether, two different products, 59c and 59c, were obtained in a ratio of 1 to 1 (Eq. 28). The reaction course in the formation of the different bicyclic rings is shown in Scheme 8. This dienyne metathesis is also catalyzed by tungsten or molybdenum complex 62 or 63 (Fig. 1), and a dienyne bearing terminal alkyne 58b could be cyclized to give 59b in 97% yield. 

With the bulky metallo-organic Pd(II) catalyst 98, on the other hand, selective formation of 99 was possible here functional groups are tolerated that would react with an Ag(I) catalyst (for example, terminal alkynes, alkyl chlorides, alkyl bromides and alkyl iodides) [59]. With l,n-diallenyl diketones (100), easily accessible by a bidirectional synthesis, up to 52-membered macrocycles (101) could be prepared in an end-group differentiating intramolecular reaction (Scheme 15.26) [60], For ring sizes lager than 12 only the E-diastereomer is formed overall yields of the macrocydes varied between 17 and 38%. Only with tethers shorter than 11 carbon atoms could the Z-diastereomer of the products be observed, a stereoisomer unknown from the intermolecular dimerization reactions of 96. 

Alternative furan ring fusion involves the reactions of phenyliodonium ylides of cyclic seven-membered jS-diketones with alkynes. These processes lead under mild conditions to cyclization products 152. The high regioselectivity can be explained by the formation of dipolar intermediate 151 favored by the predominant enolization of the carbonyl adjacent to phenyl ring. Terminal alkynes react in the similar fashion, although, in this case, mixtures of regioisomers have been reported due to steric hindrance in the intermediate enol (Scheme 30 (1993JOC4885)). 

More recently McDonald et al. reported a rare example of 7-membered oxacyde formation based on the same concept [21]. Thus, polyoxygenated terminal alkynes 60 containing the dioxolane structure in the tether gave 7-endo cyclized glycals 61 in good yield. It is necessary to have the dioxolane ring in the tether for efficient reaction, probably favoring the suitable conformation for cyclization (Scheme 5.21). 

On the other hand, the reaction of 1,2,3-selenadiazole 166 with 1 equiv of Pt(PPh3)4 in toluene at 140 °C (3 h) led to the formation of new selenoplatinum complex 52 in 35% yield (Equation 13) <2005TL1001>. This reaction may involve the insertion of di(triphenylphosphino)platinum into the selenadiazole ring, followed by 1,3-dipolar addition of an intermediate formed in situ by thermal elimination of dinitrogen with the elimination of triphenylphosphine, similar to the reaction with [Pd2(dba)3] and trialkylphosphine described above. The structure of complex 52 was established by X-ray analysis. Complex 52 is a selective catalyst for the hydrosilylation of terminal alkynes. 

Woerpel and Clark identified silver phosphate as the optimal catalyst to promote di-ferf-butylsilylene transfer from cyclohexene silacyclopropane to a variety of substituted alkynes (Scheme 7.25).95 While this silver salt exhibited attenuated reactivity as compared to silver triflate or silver trifluoroacetate, it exhibited greater functional group tolerance. Both di- and monosubstituted silacyclopropenes were easily accessed. Terminal alkynes are traditionally difficult substrates for silylene transfer and typically insert a second molecule of the starting acetylene.61,90 93 Consequently, the discovery of silver-mediated silylene transfer represents a significant advance as it enables further manipulation of monosubstituted silacyclopropenes. For enyne substrates, silylene transfer the alkynyl group was solely observed. The chemoselectivity of the formation of 99f was attributed to ring strain as theoretical calculations suggest that silacyclopropenes are less strained than silacyclopropanes.96 97... 

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. 

The formaldiminium ion formed from the reaction of 4-hexynylamine (90 R = R = Me) with paraformaldehyde and camphorsulfonic acid is reported not to cyclize when heated for 1 h at 1(X) C in the weakly nucleophilic solvent acetonitrile. However, when nucleophilic salts are added the 3-alkylidene-piperidines (91) are formed in good yields (Scheme 32). Attempted cyclizations of (90) in the presence of weaker nucleophiles such as benzenethiol or methanol were less effective, the former yielding <15% of the expected alkylidenepiperidine product, while the latter provided no products of cyclization. If the strong nucleophile iodide is employed, even a weakly nucleophilic terminal alkyne can be successfully cyclized. In all of these cyclizations of 4-alkynylamines only formation of a six-membered ring product was observed. The (2)-stereochemistry of the alkylidene side chain evolves from antarafacial addition of the internal iminium cation and the external nucleophile to the alkyne. 

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