Indole-fused pyran-4-ones

Indole-fused pyran-4-ones are prepared in one-pot two-step synthesis. In the first there is the acylation of 1-substituted 3-acetyl-lH-indol-2-ols with acid chlorides using triethylamine as base, in dichloromethane at room temperature. The second consists in the 4ff-pyran-4-one ring formation firom the in situ-obtained 3-acetyl-2-acyloxy-lF/-indole derivatives (13S1235).A highly diastereo- and enantioselective organocatalytic formal hDA reaction of enones with isatins occurs in the presence of amine 34, acid 35, and thiourea 36 to produce spirooxindole tetrahydro-4ff-pyran-4-ones (Scheme 59) (13CEJ6213). 

A set of Diels-Alder reactions of fused pyran-2-ones with ethyl vinyl ether (an appropriate synthetic equivalent of acetylene) gave fused carbocyclic systems. DABCO was used as a catalyst for the elimination of ethanol under microwave irradiation (Juranovic et al., 2012). The Diels-Alder cycloaddition reaction in 3-nitro-l-(p-toluenesulfonyl)indole with dienes under microwave irradiation in solvent-free conditions gave carbazole derivatives after elimination of the nitro group and in situ aromatization (Victoria et al., 2009). 

Snieckus has investigated the directed remote hthiation of indoles at C2 in the context of preparing fused heterocycles [23, 224], The synthesis of indolo[2,3-c] benzo[e]pyran-6-one 110 is illustrative (Scheme 22) [224], Treatment of 108 with LDA led to the formation of 110 via cyclization of lithio intermediate 109. Bisagni used a similar sequence to prepare benzo[/]indolo-6,ll-quinones [331]. 

Cyclocondensation of 3-methyl-l-(5-substituted-3-phenyl-lH-indol-2-ylcarbonyl)-5-(4fi)-pyrazolones with arylidene derivatives of malononitrile using a catalytic amount of triethylamine in refluxing ethanol affords heterocycle [ ]-fused 6-amino-4-aryl-5-cyano-4H-pyrans (14JHC303). Other fused 6-amino-4-aryl derivatives arise from the three-component reaction of benzaldehydes, active methylene compounds (malononitrile/ ethyl cyanoacetate) and 1,3-dicarbonyl compounds catalyzed by 1,8-diaz-abicyclo[5.4.0]undec-7-ene (DBU) in refluxing water (14JHC618). Similar pyrazole[f)]-fused derivatives are obtained from the reaction of benzaldehydes, malonitrile, and 3-substituted-2-pyrazolin-5-ones using sodium... 

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