1-Pentene, cycloaddition with

The 1,4-cycloaddition of ketenes to A, A -disubstituted enaminones allows a general synthesis of pyranones. Berchtold and co-workers (61JOC4776 65JOC2642) showed that treatment of ethyl 3-pyrrolidinocrotonate 341 (R = EtO) or 4-pyrrolidino-3-penten-2-one 341 (R = Me) with an excess of ketene resulted in the formation of pyranones 342, Scheme 96. The mode of formation of 342 apparently involves initial acetylation with one mole of ketene at the enaminone fi position followed by cycloaddition with a second mole of ketene. 

CAN-mediated nitration provides a convenient route for the introduction of a nitro group into a variety of substrates. Alkenes on treatment with an excess of sodium nitrite and CAN in chloroform under sonication afford nitroalkenes. When acetonitrile is used as the solvent, nitroacetamidation occurs in a Ritter-type fashion. However, the attempted nitroacetamidation of cyclo-pentene-1 -carboxaldehyde under similar conditions resulted in the formation of an unexpected dinitro-oxime compound. A one-pot synthesis of 3-acetyl- or 3-benzoylisoxazole derivatives by reaction of alkenes (or alkynes) with CAN in acetone or acetophenone has been reported. The proposed mechanism involves a-nitration of the solvent acetone, oxidation to generate the nitrile oxide, and subsequent 1,3-dipolar cycloaddition with alkenes or alkynes. The nitration of aromatic compounds such as carbozole, naphthalene, and coumarins by CAN has also been investigated. As an example, coumarin on treatment with 1 equiv of CAN in acetic acid gives 6-nitrocoumarin in 92% yield. ... 

Azadiene 1, bearing an electron-withdrawing substituent (C02Me), is expected to be electron-deficient and, therefore, more likely to react with electron-rich dieno-philes in an inverse electron demand Diels-Alder reaction. However, for a diene, 1 shows a very unusual reactivity, since it participates in cycloadditions with both electron-rich and electron-deficient dienophiles. As an example, reaction of azadiene 1 with diethyl flimarate yields pyridine 2 whereas in the presence of A-cyclo-penten-l-ylpyrrolidine, tetrahydropyridine 3 is obtained (Scheme 18.1). 

Ketiminium salts have been shown to participate readily in [2-r 2]-cyclo-additions with olefins [35]. Ghosez reported a classic in this area, involving the use of prolinol-derived ketiminium species (Scheme 18.28) [122]. Treatment of a-chloroenamine 172 with ZnCl2 facilitated chloride abstraction under mild conditions to yield an intermediate ketiminium salt 173. The reactive cumulene participated in a diastereoselective cycloaddition with cyclo-pentene to afford the chiral ketone 174 (> 97 % ee), after hydrolytic removal of the auxiliary. These diastereoselective [2 2]-cycloadditions of Ghosez provide a rare example of a chiral auxiliaiy-based approach to chiral cyclobutanones. 

Reactions of fluorinated dipolarophiles. Electron-deficient unsaturated species generally make better dipolarophiles, therefore, fluonnated alkenes become better dipolarophiles when vinylic fluonnes are replaced by perfluoroalkyl groups For example, perfluoro-2-butene is unreactive with diazomethane, but more highly substituted perfluoroalkenes, such as perfluoro-2-methyl-2-pentene, undergo cycloadditions in high yields [5] (equation 2) Note the regiospecificity that IS observed in this reaction... 

Anthronylidene 64 forms no cycloaddition products with cis- or trans-4-methyl-2-pentene, but [1 -f-2]-cycloaddition is observed with stilbene derivatives. This is also attributed to the reacting triplet state I02,i30-i32) ... 

Cycloaddition using the unusually functionalized 2-diazo-1,1,1-trifluoro-3-nitropropane (15) could be achieved with methyl acrylate, methacrylic acid chloride, and esters (Scheme 8.5), but not with the 1,2-disubstituted C=C bonds of p-nitrostyrene, ethyl cinnamate, and 4-methyl-3-penten-2-one (42). In these cycloadditions, 15 is considerably less reactive than 2-diazo-1,1,1-trifluoroethane... 

The ene reaction of an azodicarboxylate ester was first observed in 196218-19 as a process competing with cycloaddition to dienes, but little additional work has been done on the ene reaction with simple alkenes. An elegant study in 1976 provided evidence for a concerted, suprafacial reaction between dimethyl azodicarboxylate and (S)-(Z)-l-deutero-4-methyl-l-phenyl-2-pentene (15) based on the direction and high level of chirality transfer observed20. 

Simple olefins undergo cycloaddition, one way or another, in the metathesis process with remarkable ease e.g., with a tungsten halide catalyst at room temperature, 2-pentene is converted to an equilibrium mixture of 2-butene, 2-pentene and 3-hexene in a few seconds >. One should reasonably expect a similar transformation of simple acetylenes, e.g., 26 - -27. The transformation 26 27 might appear, at first inspection, to be symmetry-allowed. 

The last example consists of the synthesis of lycorane skeletons by an intramolecular cycloaddition of the azomethine ylide generated by the decarboxylation route (Section II,E). Thus, 3,4-(methylenedioxy)phenyIaceto-nitrile is deprotonated with LDA and then alkylated with 5-bromo-l-pentene... 

In contrast to the preferred meta mode of intramolecular photoaddition of 5-phenyl-l-pentenes, where the alkene and benzene groups are separated by three atoms, irradiation of the styrene (64) yields a single stereoisomer of the ortho adduct (65). In (64), not only are the reacting units separated by 4 atoms, but also it is the styrene rather than the benzene which is excited. Comparable photoreactivity is seen for phenanthrene-styrene systems such as (66) which yield 2+2 adducts (67) along with products derived from competing Paterno-Btichi reaction of the ester carbonyl with the alkene side chain. The photochemical cycloaddition also proceeds in an intermolecular fashion between the ester of 9-phenanthrene carboxylic acid and para-methoxy-0-methylstyrene. The mechanism of this reaction is shown to involve addition of the styrene to the singlet excited state of the phenanthrene derivative. °... 

The reaction of (chlorocarbonyl)phenylketene 126 with enaminones takes place effectively in refluxing toluene, furnishing a facile and rapid synthesis of 4-hydroxy-3,4,6-trisubstituted-2(lfJ)-pyridinone 346 in high yield via ring closure of intermediates 344—345 (Scheme 107) (2009JHC96). On the other hand, the cycloaddition of 4-[(2-aminoethyl) amino]-3-penten-2-one 347 with 126 led to the generation of ethyl... 

The photochemical cycloaddition of azirine 35 with cyclopentanone has been found to depend on the experimental conditions. When 35 is irradiated and cyclopentanone is slowly added, the expected s/ /ro-S-oxazoline 53 is the main productHowever, when the cyclopentanone is irradiated first and the irradiation is continued in the presence of azirine 35, the sole product is 3-oxazoline 54. Under the latter conditions, cyclopentanone reacts first by a Norrish type I cleavage and hydrogen transfer to yield 4-pentenal. This aldehyde reacts faster with the nitrile ylide than does the cyclic ketone still present, so that only 54 is formed. Norcamphor and camphor also react with azirine 35 under photolytic conditions via the Norrish type I reaction route to give 3-oxazolines 55 and 56. 

---