Diels-Alder reaction, with conjugated ketones

Myrcene with its conjugated diene system readily undergoes Diels-Alder reactions with a number of dienophiles. For example, reaction with 3-meth.5i-3-pentene-2-one with a catalytic amount of AlCl gives an intermediate monocyclic ketone, which when cyclized with 85% phosphoric acid produces the bicycHc ketone known as Iso E Super [54464-57-2] (49). The product is useful in providing sandalwood-like and cedarwood-like fragrance ingredients (91). 

The combination of thionation by Lawesson s reagent [98] of oxoenamino-ketones 96 with normal electron-demand Diels-Alder reaction of conjugated aldehydes allows a variety of thiopyrans 97 to be synthesized by a regio-selective and chemoselective one-pot methodology [99] (Equation 2.28). Thionation occurred at the more electrophilic ketonic carbonyl group. O O... 

The cross-conjugated thione (20 X = S), prepared from the corresponding ketone (20 X = O) with phosphorus pentasulfide, participates in a diene-transmissive Diels-Alder reaction with diethyl azodicarboxylate <90BCJ284>. The 3,4-dihydro-2//-l,2,3-thiadiazine (21) so formed is isolable (see Section 6.13.7.2.2) and on prolonged heating with diethyl azodicarboxylate undergoes further cycloaddition to yield the pyridazino[3,4-e]-l,2,3-thiadiazine derivative (23) in 74% yield (Scheme... 

Fig. (11) The Wieland-Miescher Ketone (1) is converted to keto glycol (125) and then to acetonide conjugated ketone (128) this on sujection to diels-alder reaction with dimethoxybenzocyclobutene affords the adduct (129) which is converted to the triol (132) by the standard organic reactions. The oxidation and cyclization of (132) yield the fiiran-diketone system (133). Its conversion to (+)-halenaquinone (134) is accomplished by the oxidative cleavage, halenaquinol (135) is obtained by reduction of hal enaquinone (134).

Annelatlott. The reagent does not react with saturated ketones, but does undergo conjugate addition to a,/3-unsaturated ketones and esters at —78° in THF to give, after quenching with chlorotrimethylsilane, cyclohexene derivatives. An example is the reaction of ethyl acrylate (equation I). This reaction is an alternative to Diels-Alder reactions with alkoxybutadienes, which require much higher temperatures. 

Finally, the conjugated ketone (III) undergoes Diels—Alder reaction with 2-trimethylsilyloxypenta-1,3-diene to give an adduct (IV), which is hydrolyzed to remove a trimethylsilyloxy group to give V. 

For a long time experiments failed to produce normal carbonylation products starting from dienes. Under the usual conditions the conjugated dienes underwent a Diels-Alder reaction with subsequent hydrocarboxylation of the prior formed isolated alicyclic dienes. Thus, e. g. butadiene first reacted to give vinylcyclohexene which then yielded a mixture of dicarboxylic adds [493]. In other cases cyclic ketones were obtained from dienes and carbon monoxide (see section on ring closure reaction with carbon monoxide). 

Notice that in these compounds the ketone is not conjugated to any of the alkenes and so does not influence the reaction. If we increase the reactivity of the dienophile by putting an ester group in conjugation with it, most of the compound does the Diels-Alder reaction before it does the [1,5]H shift. 

The classical version of the Diels-Alder reaction uses a 1,3-diene as the 4it component and an alkene or alkyne containing electron-withdrawing groups (conjugated aldehydes, ketones, acids and their derivatives, nitroalkenes, etc.) as the 2n reactant. With appropriately chosen dienes and/or dienophiles, a wide array of purely carbocyclic or heterocyclic compounds can be easily synthesized. A representative set of simple examples is given in Scheme 2.118. 

In the presence of a catalytic amount of Sc(OTf)3, conjugated linoleic acid ethyl ester could be used as a diene in Diels-Alder reactions to react with quinines, a,P-unsaturated aldehydes, and ketones in good yields [26]. Sc(OTf)3 catalyst could be recovered by extraction of organic layers with water. After removal of water, the recovered Sc(OTf)3 could be reused in the Diels-Alder reaction without appreciable loss of catalytic activity. 

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