Ene Reactions of 1,7-Dienes

Intramolecular ene reaction of 1,7-dienes.1 The ZnBr2-catalyzed reaction of the activated 1,7-diene 1 gives the frans-disubstituted cyclohexane 2 (>98% selectivity.) Under the same conditions, the chiral 1,7-diene 3 [from (R)-citronellal] is converted into two diastereomeric frww-disubstituted cyclohexanes, 4 and 5 in the ratio 96.5 3.5. Slightly higher diastereoselectivity obtains with (C2H5)2A1C1 (96% de), but the thermal ene reaction gives 4 and 5 in the ratio 86 14. 

Intramolecular ene reactions of 1,7-dienes with doubly activated enophiles (123 X, Y = CO2R, CN, COMe) proceed readily either thermally or with Lewis acid catalysis to give (124) and stereoisomers. In an elegant series of studies, Tietze and coworkers have explored the stereochemistry... 

Ene reactions of 1,7-dienes are less common. Both six-membered ring products (such as 691 via path a) and eight-membered ring products (such as 692 via path b) are possible. xjje first ene reaction with a 1,7-diene was probably reported by Hunstman et al.. Six-membered rings are usually preferred to eight-... 

These results point out an important difference between intramolecular ene reactions of dienes and unsaturated aldehydes. Intramolecular ene reactions of 1,6-dienes to give vinylcyclopentanes are much faster than ene reactions of 1,7-dienes to give vinylcyclohexanes due to tihe less negative ASt for formation of five-membered rings. Both reactions are thermodynamically favored even at elevated temperatures, Ene reactions of unsaturated aldehydes are much faster than those of dienes due to the polarity of the carbonyl double bond. However, the ene reactions of aldehydes are less... 

The tandem-Knoevenagel-ene reaction is a powerful tool to synthesize five-and six-membered carbocycles.2 5 The process is exemplified by the diastereoselective synthesis of 4a. Compound 4a has been obtained In both enantiomeric forms and as a racemate according to the procedure described here. The sequence includes the Knoevenagel reaction of citronellal, 1, and dimethyl malonate, 2, followed by the intramolecular ene cyclization of the chiral 1,7-diene 3 to yield the trans 1,2-disubstituted products 4a and 4b. Whereas the thermal cyclization of 3 at 160°C provides 4a and 4 b in a ratio of only 89.7 10.3, the Lewis acid... 

Diastereoselective ene reactions. Thermal cyclization via an intramolecular ene-tvpc reaction of the (7.)-1,6-diene 1 results in a mixture of trans-1 and cis-2 in the ratio 75 25. This reaction is markedly accelerated by addition of (C2H5)2AIC1, with only trans-1 being formed. The (E)-isomer of I is also cyclized thermally to give... 

As mentioned in Scheme 7.3, although thermal ene reaction of the 1,6-enyne 320 gives only 1,4-dienes 324, the 1,3-diene 325 is formed by the Pd-catalysed reaction. No thermal ene reaction is possible with 342, but the smooth Pd-catalysed cyclization affords the 1,3-diene system 343, which has a skeleton analogous to 325 in Scheme 7.3. Notably, the reaction proceeds with migration of vinylic hydrogen. The 1,3-diene system formed by the cyclization is useful for further modification, typically Diels-Alder reaction. The 1,3-diene 343 formed from 342 is converted to sterepolide (344) by Diels-Alder reaction [140]... 

Ene cyclization of a variety of 4-aza-1,7-dienes (46) (R1 = H, R2 = C02Me R1 = R2 = C02Me) affords 3,4-disubstituted piperidines (47) with up to >200 1 diastereo-isomeric ratios for the reactions catalysed by MeAlCl2.55 This high preference for the... 

A ruthenium based catalytic system was developed by Trost and coworkers and used for the intermolecular Alder-ene reaction of unactivated alkynes and alkenes [30]. In initial attempts to develop an intramolecular version it was found that CpRu(COD)Cl catalyzed 1,6-enyne cycloisomerizations only if the olefins were monosubstituted. They recently discovered that if the cationic ruthenium catalyst CpRu(CH3CN)3+PF6 is used the reaction can tolerate 1,2-di- or tri-substituted alkenes and enables the cycloisomerization of 1,6- and 1,7-enynes [31]. The formation of metallacyclopentene and a /3-hydride elimination mechanism was proposed and the cycloisomerization product was formed in favor of the 1,4-diene. A... 

A major problem in the reaction of a,/3-unsaturated carbonyl compounds and alkenes proves to be the competition between hetero Diels-Alder and ene reactions. Intramolecular cycloadditions of 1,6- and 1,7-dienes with ester and cyano groups at the double bond yield the ene product nearly exclusively, but with alkylidene- and benzylidene-ketoesters and 1,3-diketones the Diels-Alder reaction is preferred under thermal conditions, however under Lewis acid catalysis also ene reactions occur [12]. 

Section 1.1.3.3 Formation of 1-vinyl-2-alkylcyclohexanes from 1,7-dienes. Narasaka demonstrated that the chiral titanium alkoxide prepared from TiCl2(OPr )2 and (213) catalyzes the asynunetric intramolecular ene reaction of oxazolidinone (212) in Freon 113 at 0 °C to give 63% of ene adduct (214) in >98% enantiomeric excess and 25% of the hetero Diels-Alder adduct. ... 

Thermal and photochemical decompositions of cyclic diazenes usually proceed via diradicals as intermediates. Good synthetic routes to polycyclic diazenes are available, and consequently many interesting members of this class of compounds, containing three-membered rings, have been prepared and their decompositions studied under a variety of conditions. The thermal conversion of exo-6,7-diazatricyclo[3.2.1.0 ]oct-6-ene to cyclohexa-1,4-diene and nitrogen (Table 18, entry 1) was so much more rapid than the analogous decompositions of model cyclic diazenes that it probably occurred by a synchronous reaction, i.e. a diradical intermediate... 

Possible side-reactions, such as the formation of allyl hydroperoxides from compounds having an allylic hydrogen (a competitive ene reaction Section 6.7.3) or 1,4-endoperoxides from 1,3-dienes, may be suppressed by changing the experimental conditions.1359,1438 For example, the extent of [2 + 2] and [4 + 2] photooxygenation in styrenes, such as the propenylanisole 522 (Scheme 6.253), is controlled by solvent polarity and pH, possibly due to protonation of a perepoxide/zwitterion intermediate.1421,1439 The [4 + 2] product 523 is preferentially produced in non-polar benzene or chloroform, whereas the 1,2-dioxetane 524 is almost exclusively formed in methanol or acidified non-polar solvents. 

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