3-Formylchromone Diels-Alder reaction

Diels-Alder reactions have featured heavily during the period of review. Ar-Vinyl-2-oxazolidinone has been reported as a dienophile for the first time, including the preparation of various tetrahydro-277,77/-pyrano[4,3-. ]pyrans <2002SL952>. The heterodiene cycloaddition reaction of 3-formylchromone with a series of ketene acetals formed from C2-symmetric l,2-diarylethane-l,2-diols is completely diastereoselective (Scheme 40) <1995J(P1)2293>. 

The reactivity of 47/-benzopyran-4-ones in Diels-Alder reactions is well documented <1987T3075>, and recently high asymmetric induction has been achieved in the reaction of 3-alkoxycarbonyl-substituted chromones with chiral auxiliaries and Danishefsky s diene <1991JOC2058>. It should be noted that 3-formylchromones can react as heterodienes in the stereoselective inverse electron Diels-Alder reaction with enol ethers <1994T11755> to provide a route to pyrano[4,3-A][l]benzopyrans a heterocyclic nucleus which occurs naturally in the fungal metabolite fulvic acid <1984CC1565>. The thermal Diels-Alder reaction of 477-pyran-4-one 405 in the presence of an excess of Danishefsky s diene 404 provided cycloadduct 406 (Equation 32) <1996H(43)745>. 

It was achieved by reacting the tryptamine 38 with the formylchromone 36 to form the corresponding imine 52, followed by an aza-Diels-Alder reaction with acetylene dicarboxylates 37 catalyzed by zinc chloride in dimethyl sulfoxide (DMSO) under dry conditions (molecular sieves). This reaction provided a direct access to the intermediate 51 of the 12-step domino process to yield the halogenated indoloquinolizines 39 in significantly higher yields (53-91%) (Scheme 13.11). 

There are only few examples of asymmetric inverse hetero-Diels Alder reactions using a,P-unsaturated aldehydes. As a recent example, a cinchona alkaloid was successfully used to catalyse the oxa-Diels Alder reaction of heterodienes such as 3-formylchromones with acetylene dicarboxylates, providing the corresponding tricyclic benzopyrones in moderate to high yields (54-87%) and enantioselectivities (46-84% ee), as shown in Scheme 6.12. ... 

Scheme 6.12 Oxa-Diels-Alder reactions of 3-formylchromones with acetylene dicarboxylates.

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