Enones 2+2 cycloaddition with allene

Intramolecular [2 + 2]-cycloadditions of allenes and enones occur with fair to good site- and regioselectivities. Thus, cyclohexenone 40 bearing an allenic side-chain at the C3 position was irradiated and a single product 41 was isolated in 95% yield [45],... 

The intramolecular 2 + 2-photo-cycloadditions of optically active allenesilanes (5) with enones and enoates produce silyl-substituted exo-methylenecyclobutanes (6) in high enantiometric excess. Photo-desilation leads to the parent unsaturated exo-methylenecyclobutanes (7) (Scheme 3).19 The cycloaddition of naphthoquinone to allyltrimethylsilane in the presence of Me2 A1C1 yields the expected 2 + 2-cycloadduct that slowly rearranges to the 2 + 3-adduct.20 hi the presence of bases, Cephalosporin triflates (8) undergo 2 + 2- and 4 + 2-cycloaddition with alkenes, alkynes, and dienes via an intermediate six-membered cyclic allene (9) (Scheme 4).21... 

SCS-MP2 and the new perturbative B2-PLYP density functional methods provide accurate reaction barriers and outperform MP2 and B3-LYP methods when applied to the 1,3-dipolar cycloaddition reactions of ethylene and acetylene.39 Phosphepine has been shown to catalyse the asymmetric 3 + 2-cycloaddition of allenes with a variety of enones (e.g. chalcones) to produce highly functionalized cyclopentenes with good enantiomeric excess.40 The AuPPh3SbF6 complex catalysed the intramolecular 3 + 2- cycloaddition of unactivated arenyne- (or enyne)-yne functionalities under ambient conditions.41 A review of the use of Rh(I)-catalysed 3 + 2-cycloadditions of diaryl-and arylalkyl-cyclopropenones and aryl-, heteroaryl-, and dialkyl-substituted alkynes to synthesise cyclopentadienones for use in the synthesis of natural products, polymers, dendrimers, and antigen-presenting scaffolds has been presented.42... 

Wilson, J.E. and Fu, G.C. (2006) Synthesis of functionalized cyclopentenes through catalytic asymmetric [3 + 2] cycloadditions of allenes with enones. Angewandte Chemie, International Edition, 45, 1426-1429. 

High regioselectivity is found in the photochemical addition of allene to the enones (98). With enone (98a) cycloaddition affords (99) while (98b) yields the adduct (100). This compound was used for subsequent reactions in syntheses of taxane-like molecules. 

Nitrogen-containing heterocyclic enone systems also reacted with allene to give (2 + 2)-cycloadducts, as was shown in alkaloid synthesis.178 As the keystep in the synthesis of an annotinine derivative, allene was added to 157 and the adduct 158 was obtained in quantitative yield. Various uracils have been modified by photochemical (2 + 2)-cycloaddition with olefins, e.g., vinylene carbonate,17,18° vinyl ethers, vinyl acetates, and (cetene acetals yielded 159.181 Very recently the photochemical addition of cyanoethylenes to 2-pyridones has been observed to yield mixtures of tetrahydroazocin-2-ones (160) and (2 + 2)-cycloadducts (161).182... 

The nickel-iminophosphine-catalysed 4- -2-cycloaddition of enones with allenes formed highly substituted dihydropyrans. The enantioselective amine-catalysed 4-I-2-cycloaddition of allenoates with oxo-dienes produced polysubstituted dihydropyrans in high yields and with high enantioselectivities. Novel enam-ine/metal Lewis acid bifunctional catalysis has been used in the asymmetric inverse-electron-demand hetero-Diels—Alder reactions of cyclic ketones with Q ,j9-unsaturated a-ketoesters. The 4- -2-cycloaddition of acylketenes (80) with 2-unsubstituted and 2-monosubstituted 3-aryl-2//-azirines (81) produced 1 1 (82) or 2 1 (83) adducts, being derivatives of 5-oxa-l-azabicyclo[4.1.0]hept-3-ene or 5,7-dioxa-l-azabicyclo[4.4.1]undeca-3,8-diene. The formation of the monoadducts proceeds via a stepwise non-pericyclic mechanism (Scheme 25). A-heterocyclic carbene-catalysed 4- -2-cycloaddition of ketenes with 1-azadienes yielded optically active 3,4-dihydropyrimidin-2-ones (93% ee) ... 

When malononitrile is reacted with allenes in the presence of palladium catalysts, insertion into the C-H bond is observed with formation of a linear adduct Also, phosphine catalysts are used in an asymmetric [3+2] cycloaddition of carbo-ethoxyallene 302 with enones to give the isomeric cycloadducts 303 and 304 in 54-87 % yield (% ee in 303, the major product, 87-89. ... 

Also, a cobalt-catalyzed diastereoselective reductive [3+2] cycloaddition of allenes to enones is observed. Thus phenylallene reacts with methyl vinylketone 340 in the presence of Col2(dppe)/Zn and ZnU, to give the cyclopentane derivative 341 in 88 % yield... 

Phosphine-Catalyzed Reactions Enantioselective [3+2] cycloaddition of allenes with enones leads to the synthesis of cyclopentenes, which can be further transformed to cyclopentanes (Scheme 6.4). The phosphine addition to allenoate generates an allylic carbanion, which undergoes addition to the enone in the a and the p positions followed by the phosphine elimination to afford 3 and 4, respeetively [6]. 

Cycloadditions and cyclization reactions are among the most important synthetic applications of donor-substituted allenes, since they result in the formation of a variety of carbocyclic and heterocyclic compounds. Early investigations of Diels-Alder reactions with alkoxyallenes demonstrated that harsh reaction conditions, e.g. high pressure, high temperature or Lewis acid promotion, are often required to afford the corresponding heterocycles in only poor to moderate yield [12b, 92-94]. Although a,/3-unsaturated carbonyl compounds have not been used extensively as heterodienes, considerable success has been achieved with activated enone 146 (Eq. 8.27) or with the electron-deficient tosylimine 148 (Eq. 8.28). Both dienes reacted under... 

Other unsaturated ketones to which allene 2 undergoes cycloaddition include cyclopent-2-enone, l-phenyl-but-2-enone, l,4-diphenyIbut-2-enone, l-phenyloct-2-enone and acrylonitrile in yields ranging from 60 to 92%. The use of a chiral titanium(IV) catalyst gives chiral methylenecyclobutanes 5 with optical yields of at least 94%.2... 

The mechanistic rationale of the Cl sequence provides reactive intermediates such as enones and elusive allenols (vide supra) that can be exploited even in a domino sense, if a suitable trapping functionality is present. For the synthesis of heterocycles two bimolecular sequences have been elaborated. One is terminated by an intramolecular cyclocondensation with an amino group, which is unprotected and carried through the sequence to trap the evolving enone functionality. The other exploits the generation of a vinyl allene intermediate, which is captured by an intramolecularly tether dienophile in the sense of a (4+2)-cycloaddition. 

Introduction of an alkylthio group on the allene system increased the reactivity of the allene moiety in [2 + 2] cycloaddition reactions. It proved possible to conduct reactions of this allene at much lower temperatures. By adding Lewis acids, the reaction temperature could be decreased even more, as was illustrated by the Lewis acid catalyzed [2-1-2] cycioadditions of l-trimethylsilyl-l-methylthio-l,2-propadiene with a variety of electron-poor alkenes, including cyclic and non-cyclic enones, acrylates, methyl fumarate and acrylonitrile. When a chiral diol 21 based titanium catalyst was employed, the [2-1-2] cycloaddition reactions of /-acryloyl-l,3-oxazolidin-2-ones 17a and 17b with allenyl sulfides 18 yielded methylenecyclobutanes 19 and 20 with high optical purities (equation The highest yields were obtained with electron-poor allenophile 17b. 

The resulting radieal (144)" reaeted with alkenes to give (145). [Ir(dF(CF3)ppy)2(dthhpy)] acted as excellent visihle-light photocatalyst for the intramolecular and intermolecular [2 + 2] cycloaddition of alkenes, styrenes, enones, enolates, enol ethers, haloalkenes, and allenes. ... 

---