Cycloaddition reactions with enones

Cyclopropyl ketones 32 and cyclopropyl imines 33 can also undergo [3+2] cycloaddition reactions with enones 34 in presence of NHC-Ni complexes to afford the corresponding cyclopentane compounds 35 (Scheme 5.9) [11]. The catalytic system is prepared in situ from the use of [Ni(COD),], SIPr HCl salt and KOBu, the reaction also required the use of Ti(O Bu) as an additive to improve yields and increase reactions rates. In most of the cases, th products 35 were obtained in good to excellent diastereoselectivities. 

The stannaphosphenes 21-48 and 21-49 are formed by lit.hia.tion of the FSn-PH unit to give FSn-PLi, followed by the elimination of LiF. Methanol adds across the Sn=P double bond in the same way as with the Sn=N structure, and 21-49 shows cycloaddition reactions with enones and orthoquinones.83"86 No X-ray diffraction study of a stanna-phosphene has yet been reported... 

The Hantzch-Panek condensation of ethylpyruvate with acrylamide followed by treatment with trifluoroaceticanhydride (TFAA) provided 2-vinyloxazole-4-ethylcarboxylate 37 in good yield [57]. The Stille coupling of 2-chlorooxazole-4-ethylcarboxylate with vinyltrimetbylstannane also provided 2-vinyloxazole 37 [26]. The reactivity of 37 towards cycloaddition reactions with enone and coupling reactions with aryl halides were explored [57]. Heck reaction of vinyloxazole with p-iodoanisole gave the cinnamyl oxazole 97 in 71% yield [57]. 

Allenylsilanes also undergo formal [3 + 2] cycloaddition reactions with enones, aldehydes, iminium species generated in situy and acid chlorides to give a variety of five-membered products (Scheme 3-111). The reaction is considered to proceed through stepwise mechanisms. The [3 + 2] cycloaddition reaction can be performed in an enatioselective manner by using the chiral scandium catalyst. 

The absolute configuration of products obtained in the highly stereoselective cycloaddition reactions with inverse electron-demand catalyzed by the t-Bu-BOX-Cu(II) complex can also be accounted for by a square-planar geometry at the cop-per(II) center. A square-planar intermediate is supported by the X-ray structure of the hydrolyzed enone bound to the chiral BOX-copper(II) catalyst, shown as 29b in Scheme 4.24. 

Scheme 6.8. Photochemical Cycloaddition Reactions of Enones with Alkenes and Alkynes... 

A chiral enone derived from carbohydrates, 131, undergoes a stereoselective [3+2] cycloaddition reaction with azomethine ylides to afford pyranopyrrolidines in good yields. The cycloaddition reaction is highly regio- and stereoselective (Equation 64) <2005SL587>. 

While simple unactivated cyclopropanes have yet to be used for [3 + 2] cycloaddition, Tsuji and coworkers have developed a palladium-catalyzed cycloaddition reaction using electron-deficient vinylcy-clopropanes. Thus, vinylcyclopropane (43) undergoes smooth cyclization with methyl acrylate in the presence of a palladium catalyst to give vinylcyclopentane (44) as a mixture of diasteroisomers (equation 35). The cycloaddition probably proceeds through the zwitterionic ( ir-allyl)palladium intermediate (45) and its stepwise reaction with the acrylate (equation 36). Enones such as cyclopentenone and methyl vinyl ketone will also react. Reaction of the same vinylcyclopropane with phenyl isocyanate produces vi-nyllactam (46) (equation 37).Some cycloaddition reactions with (cyclopropyl)Fp complexes have also been reported. However, the substrates are limited to SO2 and TCNE and the yields have not been disclosed (equation 38). ... 

Various photochemical (2 + 2)-cycloadditions of heteroaromatic compounds have been reported in which an enone moiety is incorporated either into the olefinic reagent or into the heteroaromatic compound. Both furan and thiophene have been found to give cycloaddition reactions with maleic anhydride derivatives in the presence of a sensitizer.202 213 The cycloadducts (185 and 186) were formed in high yield, but in the case of 2,5-dimethylthiophene, cyclobutane formation was the minor pathway, as oxetane formation predominated.210 Cyclic enones, such as 2-cyclopenten-l-one and 2-cyclohexen-l-one reacted with furan to afford mixtures of (2 + 2)-cycloadducts (187a, R = H) and (188),... 

A study of the intramolecular (2+2)-addition in the enones (77) has been reported. The outcome of the addition is both temperature and substituent dependent. Thus irradiation of the enone (77a) at 0°C in acetonitrile with benzophenone as the sensitizer yields a single adduct (78) in 90% yield after only 35 min. irradiation. The photocycloaddition of the enone (77b) at lower temperatures yields a mixture of (78) and (79) in a ratio of 1.8 1. The other derivatives (77c,d) afford only one product identified as (79). The products obtained from the cycioaddition can be cleaved to spiroacetals. The synthetic potential of the intramolecular (2+2)-cycloaddition reaction of enones continues to be exploited. In a recent example example the cycioaddition of the enone (80) affords the product (81) which is then subjected to ring-opening and further transformation to provide a path to the natural product (+)-ligudentatol (82). " ... 

Several reviews have been devoted to the subject of, among other cycloaddition reactions, (2 + 2)-cycloaddition reactions to enones. Within these articles the additions of alkynes has been dealt with. Specifically, a short review has discussed the cycloaddition reactions of enones with alkynes. ... 

Scheme 52) [91], The (3-indolylacrylate 240 was thus subjected to a Diels-Alder reaction with enone 239 (prepared via a Diels-Alder cycloaddition of the diene 237 and cyclopentenone 238), thereby providing the endo product 241 in 72% yield and... 

Reactions with D Cenophiles. Azulenes do not undergo cycloaddition reactions with dienophiles but instead they react by conjugate addition to the enone, reaction involving the 1-position of the azulene. Thus maleic anhydride gives an azulenylsuccinic anhydride [73-75] ... 

The two-step formal [3-1-2] cycloaddition reactions of enones or enals with allenylethers afford cyclopentenone derivatives. For example, reaction of the masked enalallene 342 at room temperature in the presence of a gold catalyst affords the bicycUc aldehyde 343 in 90 % yield... 

Photochemical Cycloadditions. Isobutene has been widely used in intermolecular [2 + 2] photocycloaddition reactions with enones. The weakly polarized isobutene is often used to study the regioselectivity of the photocycloaddition. Cyclohexenones (eq 9), cyclopentenones (eq 10), and functionalized enones (eq 11 ) undergo cycloaddition with isobutene. Patemo-Biichi... 

Whereas tropones usually act as dienes in cycloaddition reactions (Section 5.4), tricarbonyl (tropone) iron 59 displays a reactivity that is almost identical to that of a normal enone. High pressure cycloadditions of 59 with 1-oxygen substituted dienes 60 gave the desired cycloadducts 61 in good to excellent yields (Equation 5.9). The subsequent decomplexation of the cycloadducts has been accomplished by treatment with CAN [20]. 

Scheme 5.9 NHC-Ni catalysed [3-I-2] cycloaddition reaction of cyclopropyl ketones or cyclopropyl imines with enones...

Microwave heating has also been employed for performing retro-Diels-Alder cycloaddition reactions, as exemplified in Scheme 6.94. In the context of preparing optically pure cross-conjugated cydopentadienones as precursors to arachidonic acid derivatives, Evans, Eddolls, and coworkers performed microwave-mediated Lewis acid-catalyzed retro-Diels-Alder reactions of suitable exo-cyclic enone building blocks [193, 194], The microwave-mediated transformations were performed in dichloromethane at 60-100 °C with 0.5 equivalents of methylaluminum dichloride as catalyst and 5 equivalents of maleic anhydride as cyclopentadiene trap. In most cases, the reaction was stopped after 30 min since continued irradiation eroded the product yields. The use of short bursts of microwave irradiation minimized doublebond isomerization. 

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... 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

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