Diynes intramolecular cycloaddition reactions

The photochemical reactivity of metastable diacetylene with styrene has been studied. The triplet state of the diyne is involved and several reaction products are formed. Two of the major ones are the eneyne (57) and the ethynylstyrene (58). Irradiation of the tetrayne derivatives (59) in diethyl ether affords a mixture of the trans- and cis-derivatives. Cations can be produced by the photolysis of 4-methoxyphenylalkynes in 2,2,2-trifluorethanol." 1-0-Acetylaminophenyl-2-(pentamethyldisilanyl)ethyne is photochemically active on irradiation in benzene. An intramolecular cycloaddition takes place via a l-sila-l,2-propadiene intermediate. " Park and Baek" have reported the photo-... 

T. Hoye et al. were understandably surprised when a seemingly trivial oxidation of 7 took an unexpected course (Scheme 7.28). Instead of ketone 8 the tricyclic product 9 was obtained in 53% yield. The course of the events could be rationalized readily and pointed to the unprecedented cyclization of ketone 8 to give the benzyne derivative 10 in a [4-1-2] cycloaddition between a diyne and an yne moiety. In fact, this intramolecular cycloaddition turned out to be a viable route to generate arynes bearing electron-withdrawing substituents, a substitution pattern that is not always amenable to the standard methods for generating arynes. Thus, this surprising reaction broadens the scope of benzyne chemistry. 

In 2006, Liu and coworkers reported a gold-catalyzed intramolecular cycloaddition of diynes with tethered arenes 118 to synthesize l,3-dihydroindeno[2,l-c]pyran or 2,3-dihydro-l//-indeno[2,l-c]pyridinederivatives 120 (Scheme 12.52) [56]. On the basis of deuterium labeling, they believed that the first reaction step is the intramolecular arylation of one alkyne to form vinylgold(I) intermediate 119,... 

In the intermolecular reaction of tetraynes, where two 1,6-diyne moieties were directly connected, with monoalkynes, CHIRAPHOS (2,3-bis(diphenylphosphino) butane) was the choice of chiral ligand, and axial chirality was enantiomericaUy generated between the formed benzene rings (Scheme 11.17). Hexaynes with a 1,3-diyne moiety also underwent an intramolecular [2-i-2-i-2] cycloaddition, and the Ir-xylylBINAP (2,2 -bis[di(3,5-xylyl)phosphino]-l,l -binaphthyl) catalyst induced an excellent enantiomeric excess (ee) (Scheme 11.18) [24]. 

The phyllocladane skeleton 131 was constructed efficiently by stereoselective formations of six carbon-carbon bonds and four rings via a one-pot sequence of cyclizations the ene type, [2+2+2], and [4+2] cycloadditions. In this synthesis, the Conia ene reaction of 127 takes place under mild conditions to generate 128, and the cyclotrimerization of its diyne with 118 gives 129. These two reactions are catalysed by CpCo(CO)2. Finally, ring-opening to give 130 and intramolecular Diels-Alder reaction in the presence of DPPE produced the phyllocladane skeleton 131 in a total yield of 42% [55]. 

The diyne 66 undergoes an intramolecular dehydro Diels-Alder reaction in toluene under reflux to afford naphtho[2,3-c]chromene derivatives in reasonable yield (Scheme 20). The reaction is presumed to proceed via a [4+2]-cycloaddition of the alkyne to the aryl alkyne group to form the cyclic allene intermediate 67, which then isomerizes to the aromatic product <2003SL1524>. 

In previous works this group had observed a competition between the PKR and a [2 + 2 + 2] cyclization in the second reaction step of three triple bonds. Thus, when reacting linear triynes 174 under catalytic, high CO pressure, cobalt mediated PKR conditions, they obtained mixtures of products 175 coming from two [2 + 2 + 1] cycloadditions, and 176 from a [2 + 2 + 1]/ [2 + 2 + 2] tandem reaction. When the triple bonds were ether linked, the latter was the favored reaction, while with substrates lacking oxygen atoms, the iterative PKRs was the major pathway (Scheme 51) [166]. When the reaction was performed intramolecularly between a diyne and an alkyne, the only reaction products were the result of a [2 + 2 + 1 ]/[2 + 2 + 2] tandem cycloaddition [167,168]. 

Fenestranes are compounds of theoretical interest in which the central carbon atom undergoes severe planarization distortion. Reactions sequences involving double intramolecular Pauson-Khand reactions of ene-diynes, or intramolecular Pauson-Khand of dienynes followed by photochemical [2 + 2] cycloaddition, successfully lead to [5.5.5.5]- or [4.5.5.5]fenestrane, respectively [42], For instance, compound 37 was obtained from ene-diyne 36 in moderate yield as a single all-czs stereoisomer [43] (Scheme 18). 

A slightly modified synthesis of dihydrobenzo[c]furan was achieved, in which the key reaction involved the exposure of a dibromide to dry alumina and one equivalent of water in toluene instead of in hexanes <07TL3039>. Another synthesis of dihydrobenzo[c]furans was reported by Hashmi and Teles <07SL1747> and an example is depicted below. Base-promoted cycloaddition of 1-aryl- or l-aryl-7-substituted-4-oxahepta-1,6-diynes in DMSO were found to involve an anionic intramolecular Diels-Alder reaction, and also resulted in the formation of dihydrobenzo[c]furans <07JA4939>. 

Without question, the metal-promoted cycloaddition of three alkynes to produce benzenes is the most extensively studied organometallic cycloaddition in intramolecular versions. Early work indicated the utility of Ni° systems e.g. Ni(CO)2(PPh3)2), Ziegler catalysts and rhodacyclopentadienes in the partially intramolecular cocycloaddition of a,b>-diynes with additional alkynes. Ziegler catalysts were noteworthy in giving rise to products containing the benzocyclobutene moiety from reactions of 1,5-hex-adiyne, while the Rh systems showed considerable utility in the preparation of anthraquinone derivatives from appropriate diyne precursors (Scheme 29). 

Polyynes have served as starting materials for the synthesis of a wide variety of heterocyclic ring systems. The reactions used involve addition to triple bonds, and any of the common mechanistic pathways may be followed, i.e. nucleophilic, electrophilic or free radical attack as well as concerted cycloadditions. Although the evidence does not permit unequivocal classification of many of the reactions into one of these categories, the ones considered here are those which most likely involve nucleophilic attack at some stage. In a formal sense the reactions amount to successive additions of a divalent nucleophile to two triple bonds the first involves intermolecular and the second intramolecular attack, as illustrated in equation (19) for the addition of HoS to a diyne. 

Intramolecular Reaction of 1,6-Dienes, -Enynes, and -Diynes Cycloisomerization, Tandem Addition-Cyclization, and Cycloaddition... 

Enantioselective intermolecular [2-F2-F2] cycloadditions of 1,6-diynes with a,fS-unsaturated carbonyl compounds are valuable methods for the synthesis of chiral cyclohexadienes in a single step [36]. The intramolecular Diels-Alder reaction (IMDA) is known to be a powerful strategy for the construction of complex multicyclic skeletons [37]. Tanaka reported an efficient enantioselective domino intermolecular [2-F2-F2]/IMDA cycioaddition reaction occurring between 1,6-diyne 106 and amide-linked 1,5-diene 107 bearing two sterically and/or electronically... 

During the studies on the scope and limitations of the CO-SiCaT reaction, it was found that the reaction of 1-substituted dodec-ll-ene-l,6-diynes 216 catalyzed by [Rh(cod)Cl]2 in the absence of a hydrosilane also gave carbonylative-fused tricyclic products 217 in good to excellent yields. This process turns out to be the first example of an intramolecular [2-1-2-1-2-1-1] cycloaddition (Scheme 96). 

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