Cycloisomerization, alkynyl ketones

Intramolecular 2 + 2 + 2-cycloisomerizations of cyclic triynes and enediynes have been reported with RhCl(CO)(PPh3)2.126 The transition metal-catalysed rearrangement of alk-5-ynals to /-alkynyl ketones and cyclopent-l-enyl ketones was developed using [Rh(P(OPh)3)2]BF4 or Cu(OTf)2 as a catalyst and the effect of substituents on the partition to products was elaborated (Scheme 84).127... 

Gevorgyan et al. demonstrated that somewhat unstable and not simply accessible allenones could be substituted with far more easily available conjugated alkynyl ketones [83,134,135] in furan syntheses. Thus, it was shown that cycloisomerization of alkynyl ketones 46 proceeded smoothly in the presence of Cu(I)-catalyst providing furans 47 in high yields (Scheme 8.20) [116]. This protocol allowed a highly efficient... 

A very mild and fadle Au( 111)-catalyzed cycloisomerization of skipped alkynyl ketones 79 into 2,5-disubstituted fiirans 80 was first reported by Hashmi (Scheme 8.32) [110, 111]. Interestingly, this catalyst was shown to be completely inefficient for an analogous transformation of conjugated substrates [111]. In addition, an interesting cascade transformation of the propargyl ketone 81, occurring... 

In a similar way, iminoalkynes containing aryl, alkenyl, and alkyl substituents undergoes Cul-catalyzed cyclization in excellent yields and short reaction times to give isoquinolines, pyridines, and pyrroles via a cycloisomerization (eq 18)4 Copper chloride is preferred, however, for the conversion of cyclic alkynyl imines to pyrrole-containing heterocycles. Copper iodide catalyzes the formation of furans from alkynyl ketones. ... 

Cycloisomerization of alkynylated ketones in a 5-exo-dig manner is an atom-economical approach to furans. In 1999, Picquet et al. reported the PdCla-catalyzed cycloisomerization of an alk-4-yn-l-one derivative 116a to the corresponding furan... 

Gold-catalyzed cycloisomerizations of alkynyl ketones/aldehydes provide convenient access to 2-benzopyrylium salts, which are important intermediates for natural product syntheses. For example, the benzaldehyde derivative 151 was converted directly to the 2-benzopyrylium salt 152 in the presence of a catalytic amount of Au(OAc)3 in (CH2C1)2/TFA (Scheme 19.37) [62], IBX-mediated oxidation of 152 in the presence of n-Bi NI (as a phase-transfer catalyst) cleanly afforded the azaphilone 153 in 84% yield after reductive workup, which is a precursor of ( )-S-15183a (154). Similarly, AgNOs-promoted cyclization of the alkyne 155 furnished direct construction of the tricyclic 8/7-pyrano[2,3,4-rfe]chromen-8-one core of cassiarin A (158) (Scheme 19.38) [63]. 

It has been shown by Marshall and Sehon that in the presence of AgNOs absorbed on silica gel, /3-alkynyl allylic alcohols, which are more accessible than allenyl ketones, also undergo cycloisomerization to furans (Equation 19) <1995JOC5966>. Similarly, 3-trifluoromethylfurans can be prepared utilizing a Pd(ii) catalyst <2000JOC2003>. 

Substituted furans are important structural imits, which are often seen in natural products or pharmaceuticals. Hashmi and his colleagues have found that 2-substituted furans are formed from aUenyl ketones by gold-catalyzed cycloisomerization [ 197]. Halofurans and highly substituted furans can be also prepared according to this strategy [198, 199]. Gold(III) porphyrin complexes have been developed as recyclable catalysts for the cycloisomerization [200]. 2-(l-Alkynyl)-2-alken-l-ones [201], l-(l-alkynyl)-cyclopropyl ketones [202], and (Z)-2-alken-4-yn-l-ols [203] are also suitable substrates for highly substituted furans. 

Analogous 1,2-alkyl migration of an intermediary metal carbene complex is proposed in catalytic cycloisomerization reactions of o-alkynylbenzaldehyde acetal [36], alkynyl azides [37], 1,2,4-trienes [38], allenyl ketones [39], and o-alkynylanilides [40]. 

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