Ene intramolecular

Giguere performed tandem ene-intramolecular Diels-Alder reactions between 1,4-cyclohexadiene (29) and dimethyl acetylenedicarboxylate (26b) in sealed tubes in a commercial microwave oven (Scheme 9.6) [47]. 

Photosensitised [2+2] cycloreversions have been reported in the aryl cage compounds (115), yielding the dienes (116). Analogous cycloreversions have also been observed in cis- and trans-bicyclo[5.2.0]non-8-enes.Intramolecular photosensitised pyrimidine dimer splitting in the indole-pyrimidine (117) is remarkably solvent dependent and is thought to proceed via electron transfer from the excited indole moiety. The relative values of the activation parameters in photosensitised pyrimidine dimer splitting have been determined. ... 

Caryophyllane-2,6-a- and -P-oxides have been prepared from (— )-isocaryophyll-ene. Intramolecular epoxide opening has been studied for seven diols prepared by oxidation of the exocyclic double-bond of caryophyllene and isocaryophyllene epoxides. ... 

An array of multi-substituted tetrahydrofurans and tetrahy-dropyrans can be prepared through the indium-catalyzed ene-type cyclizations. "" In one example, the efficient formation of tetrahydropyran rings through the self-tandem carbonyl-ene, intramolecular (2,5) oxonium-ene cyclization of aldehydes and methylenecyclohexane is demonstrated. Methylenecyclohexane and benzaldehyde react initially to afford a homoallylic alcohol and subsequent (2,5) oxonium-ene cyclization involving a further molecule of aldehyde provides the tetrahydropyran in very good 3field and excellent diastereoselectivities, favoring the 2,3 anti, 2,6-syn isomers (eq 17). 

Similarly to alkenes. alkynes also insert. In the reaction of 775 carried out under a CO atmosphere in AcOH, sequential insertions of alkyne, CO. alkene. and CO take place in this order, yielding the keto ester 776[483]. However, the same reaction carried out in THF in the presence of LiCl affords the ketone 777, but not the keto ester[484]. The tricyclic terpenoid hirsutene (779) has been synthesized via the Pd-catalyzed metallo-ene carbonylation reaction of 778 with 85% diastereoselectivity as the key reaction[485], Kainic acid and allo-kainic acid (783) have been synthesized by the intramolecular insertion ol an alkene in 780, followed by carbonylation to give 781 and 782[486],... 

The allenyl moiety (2,3-aikadienyl system) in the carbonylation products is a reactive system and further reactions such as intramolecular Diels-Alder and ene reactions are possible by introducing another double bond at suitable positions of the starting 2-alkynyl carbonates. For example, the propargylic carbonate 33 which has l,8(or 1.9)-diene-3-yne system undergoes tandem carbonylation and intramolecular Diels-Alder reaction to afford the polycyclic compound 34 under mild conditions (60 C, 1 atm). The use of dppp as ligand is important. One of the double bonds of the allenyl ester behaves as part of the dieneflSj. 

The most recent, and probably most elegant, process for the asymmetric synthesis of (+)-estrone appHes a tandem Claisen rearrangement and intramolecular ene-reaction (Eig. 23). StereochemicaHy pure (185) is synthesized from (2R)-l,2-0-isopropyhdene-3-butanone in an overall yield of 86% in four chemical steps. Heating a toluene solution of (185), enol ether (187), and 2,6-dimethylphenol to 180°C in a sealed tube for 60 h produces (190) in 76% yield after purification. Ozonolysis of (190) followed by base-catalyzed epimerization of the C8a-hydrogen to a C8P-hydrogen (again similar to conversion of (175) to (176)) produces (184) in 46% yield from (190). Aldehyde (184) was converted to 9,11-dehydroestrone methyl ether (177) as discussed above. The overall yield of 9,11-dehydroestrone methyl ether (177) was 17% in five steps from 6-methoxy-l-tetralone (186) and (185) (201). 

Hydroxymethylmethyldiazirine (209 unprotonated) formed propionaldehyde as the sole product by thermal nitrogen extrusion 4-hydroxy-l,2-diazaspiro[2.5]oct-l-ene (218) formed a mixture of cyclohexanone (73%), cyclohexenol (21%) and cyclohexene oxide (5%). Thermal decomposition of difluorodiazirine (219) was investigated intensively. In this case there is no intramolecular stabilization possible. On heating for three hours to 165-180 °C hexafluorocyclopropane and tetrafluoroethylene were formed together with perfluorofor-maldazine 64JHC59). 

Another reaction that has potential is the intramolecular ene reaction, which preferentially results in five-membered rings Because fluorinated ketones have a propensity for undergoing intermolecular ene reactions [5S], this reaction has tremendous synthetic possibilities Only one example of an ene reaction of an unsaturated trifluoromethyl ketone has been reported, but it indicates the potential of the methodology [59] (equation 59)... 

The reaction of 4,4-bis(tnfluoromethyl)-I,3-diaza-1,3-butadienes with certain a,P-unsaturated ketones yields pyrimidine derivatives A two-step mechanism, metathesis-electrocyclic ring closure and metathesis-intramolecular ene reaction, is a plausible explanation for the experimental results (pathway 4, equa-bon 25) [259]... 

Stereoselective azepine ring formation via intramolecular ene reaction 97YGK725. 

Desimoni et al. have found that chiral BOX-manganese(II) complexes can catalyze the intramolecular cycloaddition and ene reactions, with the latter as the major product [37]. 

The ene reaction as a reaction principle has been first recognized and systematically investigated by Alder It is a thermal addition reaction of a double bond species 2—the enophile—and an alkene 1—the ene—that has at least one allylic hydrogen. The intramolecular variant is of greater synthetic importance than is the intermolecular reaction. 

For the intramolecular variant, synthetically valuable applications have been developed during the last decade." " Three types of intramolecular ene reactions are formulated—depending on the structure of the starting material ... 

A modern variant is the intramolecular magnesium-ene reaction, e.g. the reaction of the alkene-allylic-Grignard compound 9 to give the five-membered ring product 10. This reaction proceeds regio- and stereoselectively, and is a key step in a synthesis of the sesquiterpenoid 6-protoilludene ... 

The retro-ene reaction also is of synthetic importance. While the application of high pressure facilitates the ene reaction, the retro-ene reaction is favored at higher temperatures. Furthermore small-ring strain can shift the equilibrium towards the side of the dienes. The vinylcyclopropane 11 rearranges by a synchronous process to the open-chain diene 12. Formally this process is the reverse of an intramolecular ene reaction ... 

The reaction conditions for the ene reaction of simple starting materials are, for example, 220 °C for 20 h in an aromatic solvent like trichlorobenzene. Lewis acid-catalyzed intramolecular reactions have been described, e.g. with FeCls in dichloromethane at -78 °C." Yields strongly depend on substrate structure. 

DinitrQ-ll-Qxatricyclo[6 2 1 0 undec-9-ene has been prepared by an intramolecular Diels-Alder reacdon of the furan with a idtroalkene group as shown in Eq 8 26 This tricyclic compound is a versatile synthedc tool for the preparadon of ergot alkaloids... 

Hodgson very recently reported an efficient intramolecular and completely dia-stereoselective cyclopropanation of bisliomoallylic and trisliomoallylic epoxides based on the use of a-lithiated epoxides. In a seminal paper, Crandall and Lin had reported that the reaction between t-BuLi and l,2-epoxyhex-5-ene (100) gave, inter alia, small amounts oftrans-bicyclo[3.1.0]hexan-2-ol (102, 9%) (Eq. a, Scheme 8.28)... 

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