In Diels-Alder reactions

Review Problem 7 a-p-Unsaturated lactones are nsefhl intermediates in syntliesis as they take part in Diels-Alder reactions to buhd larger molecules with more complex functionahty. How would you make tliis one ... 

Palladium catalyzed cycloisomerizations of 6-cn-l-ynes lead most readily to five-membered rings. Palladium binds exclusively to terminal C = C triple bonds in the presence of internal ones and induces cyclizations with high chemoselectivity. Synthetically useful bis-exocyclic 1,3-dienes have been obtained in high yields, which can, for example, be applied in Diels-Alder reactions (B.M. Trost, 1989). 

In Diels-Alder reactions a nitroolefin may function as an electron-deficient ene com-onent or a 1,2-dihydropyridine derivative may be used as a diene component. Both types of iactants often yield cyclic amine precursors in highly stereoselective manner (R.K. Hill, 1962 i. BOchi, 1965, 1966A). 

The compound shown is quite unreactive in Diels-Alder reactions Make a space filling model of it in the conformation required for the Diels-Alder reaction to see why... 

Butenediol is a weak dienophile in Diels-Alder reactions. Adducts have been described with anthracene (108) and with hexachlorocyclopentadiene... 

Diels-Alder Reactions. Acrolein may participate in Diels-Alder reactions as the dieneophile or as the diene (84—89). 

Acrolein a.s Dienophile. The participation of acrolein as the dienophile in Diels-Alder reactions is, in general, an exothermic process. Dienes such as cyclopentadiene and l-dieth5laniino-l,3-butadiene react rapidly with acrolein at room temperature. 

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

Cycloaddition Reactions. Methacrylates have been widely used as dienophiles in Diels-Alder reactions (22—24). 

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

The following compounds have been obtained from thiete 1,1-dioxide Substituted cycloheptatrienes, benzyl o-toluenethiosulfinate, pyrazoles, - naphthothiete 1,1-dioxides, and 3-subst1tuted thietane 1,1-dioxides.It is a dienophile in Diels-Alder reactions and undergoes cycloadditions with enamines, dienamines, and ynamines. Thiete 1,1-dioxide is a source of the novel intermediate, vinylsulfene (CH2=CHCH=SQ2). which undergoes cyclo-additions to strained olefinic double bonds, reacts with phenol to give allyl sulfonate derivatives or cyclizes unimolecularly to give an unsaturated sultene. - Platinum and iron complexes of thiete 1,1-dioxide have been reported. 

Fig. 11.12. Frontier orbital interactions in Diels-Alder reactions.

The use of trifluoromethyl-substituted a-pyrones in Diels-Alder reactions with all types of dienophiles provides an interesting route to trifluoromethyl-benzenes [I/Sj (equation 98)... 

Silver trifluoroacetate is a suitable catalyst for various cationic rearrangements involving multiple carbon-carbon bonds [49 5(1] In the presence of silver trifluoroacetate, 2 propynyl acetates rearrange to the butadienyl acetates to give dienes that are useful in Diels-Alder reactions [49] (equation 22)... 

The Boekelheide reaction has found utility in other synthetic methodology. An approach to 2,3-pyridynes made use of this chemistry in the preparation of the key intermediate 30. Treatment of 28 with acetic anhydride produced the desired pyridone 29. Lithiation was followed by trapping with trimethylsilyl chloride and exposure to triflic anhydride gave the pyridyne precursor 30. Fluoride initiated the cascade of reactions that resulted in the formation of 2,3-pyridyne 31 that could be trapped with appropriate dienes in Diels-Alder reactions. 

It was found that the introduction of a sulfonyl substituent considerably enhances the furanone reactivity in Diels-Alder reaction. Thus, (55)-5-(reacted with cyclopentadi-ene at room temperature in benzene with complete conversion to the adduct 212. Also, the reaction of 211 with 2,3-dimethyl-1,3-butadiene was readily performed in refluxing benzene to give the adduct213 in 98% yield (Scheme 57) (91TL7751). 

The 5-methylene-2(5//)-furanone 216 was found to be a good dienophile in Diels-Alder reactions with acyclic dienes (R = H, 2-Me, 2,3-di-Me, 1-Me, 1,3-di-Me). The reaction took place specifically at the cxo-cyclic double bond to give the corresponding spiro adducts 217 in good yields (Scheme 59) (90JOC3060). 

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