Allenyne cross-conjugated trienes

Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

Brummond and Shibata independently reported the Rh(i)-catalyzed cycloisomerization of allenynes to cross-conjugated trienes. The rhodium conditions were shown to have broad functional group tolerance. Brummond et al 9 observed rate and selectivity enhancements when they switched to an iridium catalyst (Equation (77)). The rate acceleration observed in the Alder-ene cyclization of aminoester containing allenyne 121 (Equation (78)) was attributed to the Thorpe-Ingold effect.80... 

In 1996, Malacria et al. [139] reported on cobalt-mediated reactions of the related allenynes. Heating the allenyne 222 in the presence of cpCo(CO)2 accompanied by a photochemical activation of this organometallic compound delivered the cross-conjugated trienes 223 (Scheme 15.71). The second triple bond present in the substrate did not participate in the reaction, underlining the higer reactivity of the allene unit. 

Wilkinson s catalyst also allows the intramolecular cycloisomerization of allenynes 243 to interesting cross-conjugated trienes 244 (Scheme 15.76) [146], Similar compounds are observed as side-products in Pauson-Khand reactions of allenynes [147]. 

Ti-mediated cyclization of an allenynes having a leaving group provides a five-membered ring with cross-conjugated trienes which might be produced by the elimination of an alkoxy group from a titanacycle (Scheme 16.73) [79]. 

The ether-tethered allenyne 4 is converted into a cross-conjugated triene, the 5,6-dihydro-2tf-pyran 5, in a Rh-catalysed allenic Alder ene reaction <02JA15186> and allylic halides undergo a Pd-catalysed coupling reaction with 3,4-allen-l-ols to give 5,6-dihydro-2//-pyrans (Scheme 2) <02JOC6104>. 

In the presence of different transition metal catalysts, allenyne 359 can be diversified into different carbocycles and heterocycles depending on reaction conditions (Scheme 2-23). In the presence of Mo(CO)6, allenyne 359 gave bicyclo[3.3.0] product 360. By contrast, in the Rh-catalyzed reaction, the terminal olefin of359 reacted to give the bicyclo[4.3.0] product 361. In the absence of carbon monoxide, allenyne 359 underwent an Alder-ene reaction to form the cross-conjugated triene 362. 

Cycloisomerization of Aiienenes and Alleneynes. Allenes are a special class of compounds that possess cumulated double-bond systems. For a long period of time, the development of the chemistry of allenes was impaired under the false notion that such cumulated double-bond systems are highly unstable (90-92). Unlike the chemistry of alkene and alkyne that have been comprehensively developed, allenes only started to emerge in recent decades as versatile precursors in organic synthesis. The Brummond Laboratory was the first to illustrate that cross-conjugated trienes 83 could be prepared by Rh-catalyzed allenic Alder-ene reaction of allenyne 82 (Scheme 42) (93). 

Upon the discovery of the allenic Alder-ene reaction, Brummond and coworkers have expanded this synthetic protocol to biologically relevant substrates. Allenynes 86 with amino acids as part of the tether were subjected to the Alder-ene reaction and afforded cross-conjugated trienes 87 in high yields. The Alder-ene reaction of these AT-alkynyl allenic amino acids was substantially faster (<10 min) than the previous substrate series. The authors attributed this rate... 

Five years ago, Houk, Toste, and coworkers [31] suggested a fascinating new reaction mechanism. The cycloisomerization of 1,5-allenynes 23 by the [(Ph3PAu)30]Bp4 catalyst delivers cross-conjugated trienes 24 (Scheme 4.8). 

Brummond s group [23] and independentlythe group of Shibata [24] discovered that rhodium(I) complexes are excellent catalysts in the formal Alder-ene cycloisomerization of allenynes to give cross-conjugated trienes under mild conditions... 

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