Allenyne Alder-ene reactions

A cobalt-mediated formal Alder ene reaction of an allenyne takes place to give a mixture of adducts and ( 4-cycloliexadiene)cobalt complexes (Scheme 16.77) [88], The reaction may proceed via coordination and ensuing jt-allyl complex formation. 

The mechanism proposed for this allenic Alder-ene reaction is shown in Scheme 8.3. The rhodium] I) catalyst coordinates with the allenyne V forming intermediate VI, which undergoes an oxidative addition to form the metaUocycle VII. The metaUocycle then undergoes a /9-hydride elimination producing triene intermediate VIII, which... 

Allenynes, reductive cyclization, 10, 516 Allocolchicine, via C-H functionalizations, 10, 162 Allocyathin, via Alder-ene reactions, 10, 592 (+)-Allopumiliotoxin 267A, via cyclization-hydrosilylation,... 

The ether-tethered allenyne 248 undergoes a rhodium(l)-catalyzed intramolecular allenic Alder ene reaction to afford the ( )-3,6-dihydropyran 249 as the major product (Equation 111) <2002JA15186>. Likewise, ether tethered enynes can undergo rhodium(i)-catalyzed cycloisomerizations to afford 3,6-dihydropyrans <2005JA10180>. 

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. 

Rh-catalyzed Alder-ene reactions of allenynes with an amide-linker 481 and 483 afforded 8- and s-lactams 482 and 484, respectively (Scheme 2-61). Higher temperature was required for the reaction to proceed efficiently, which could be attributed to the presence of the seeondary amide, preferring trans conformation. Yields were found to be lower when the substrates bearing a terminal acetylyne moiety (R = H) were employed. 

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... 

More recently, the Brummond laboratory reported a series of Rh-catalyzed allenic Alder-ene reactions affording 5- and e-lactams 89 from amide-tethered alleneynes 88 (Scheme 46) (94). Higher temperature was required for the reaction to proceed efficiently. This observation could be attributed to the secondary amide, because the reaction of protected amides proceeded at room temperature to afford cyclized product in less than 1 h. The reaction is somewhat sensitive to terminal alkyne in that product yields for amide-tethered allenyne substrates with terminal alkyne were lower. 

Lierena, D., Aubert, C. and Malakcria, M. (1996) First examples of cobalt-mediated formal Alder ene reaction of allenynes. Tetrahedron Letters, 37(39), 7027-7030. 

Brummond et al. disclosed that acyclic allenynes could be efficiently converted into tetracyclic compounds via consecutive rhodium-catalyzed Alder-ene and double Diels-Alder cycloaddition reactions [47]. The former reaction transforms alkynyl allenes such as 45 into triene-ynes (46) using rhodium biscarbonyl chloride dimer. 

Scheme 12.8 One-pot Alder-ene-Diels-/ lder-Diels-Alder reactions initiated by Rh-catalyzed cycloisomerization of an allenyne.

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