Pyrans from 1,5-dicarbonyl compounds

Pyrans 342 were prepared from furan derivatives 241, readily accessible by the reaction of glucose with /J-dicarbonyl compounds.328 The reaction was accomplished under acidic conditions and probably proceeds according to Eq. (16). 

The reversal of the well-known transformation of sugars into pyrans has been detailed as a method for assembling simple monosaccharides from simple furans (71T1973). A compound of the 2-furylcarbinol type was converted by the Br2/MeOH procedure into a mixture of the cis and trans isomers of the corresponding 2,5-dimethoxy-2,5-dihydrofuran derivative (129). Mild acid hydrolysis of (129) resulted in cleavage of the acetal bonds with formation of the dicarbonyl compound (130) which underwent immediate cyclization to 2,3-dideoxy-DL-alk-2-enopyranos-4-ulose (131 Scheme 29). 

Enamine (235) obtained from cyclic ketones and the acetal of /V-methyl-2-pyrrolidone gave a fused 2-pyrone [83IJC(B)1083]. 2//-Chromenes were obtained from of 3,5-dichlorosalicylaldehyde and enamines (94RRC183) (Scheme 42). The pyran ring is formed by a reaction of aminals of conjugated w-dimethylaminoaldehydes with cyclic /1-dicarbonyl compounds (94IZV285) (Scheme 43). 

Further examples of pyran-2-ones obtained from 1,3-dicarbonyl compounds and HC(OEt)3 include several hetero-fused analogues (95H(41)1299) and cyclodehydration of a y-keto acid yields a pyranopyrrole (95JCS(P1)1131). 

The majority of syntheses of 4/7-pyrans derive from the ring closure of 1,5-dicarbonyl compounds or their equivalents, which is discussed in detail in the previous volumes <1984CHEC, 1996CHEC-II> and is also the subject of a recent review <2000CHE1007>. In this manner, 2,6-bis(silyl)-47/-pyrans 79 are synthesized by the cyclodehydration of their corresponding l,5-bis(silyl)pentane-l,5-diones in good yield (Equation 43) <2000S843>. 

The prerequisite 1,5-dicarbonyl compounds and their equivalents can be formed in situ by a Michael addition of activated methylene groups onto O Ji-unsaturated systems <1996CHEC-II>. In this manner, 5-alkylidene-2-thioxo-dihydropyrimidincM,6(l //,5//)-dione 84 reacts with ethyl 3-oxobutanoate under microwave irradiation to from the intermediate 1,5-dicarbonyl compound 85, which spontaneously cyclize to afford the corresponding 4//-pyrans in high yield (Scheme 28) <2003SC3747>. 

Other 4,5,6-trisubstituted pyran-2-ones are readily accessible from the Re-catalysed reaction of 3-substituted propynoates with 1,3-dicarbonyl compounds. The intermediate enolic adduct can sometimes be isolated (Scheme 24) <07T11803>. 

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... 

The tetrahydropyranol ring system was also formed in a domino Michael-ketabzation reaction. The reaction between cyclohexanone and nitro alkenols in the presence of prohne-based organocatalysts was used to successfully prepare fused cyclohexyl pyran derivatives [111]. Rodrigues and Coquerel reported an NHC-catalyzed Michael-based spirocyclization from 1,3-dicarbonyl compounds bearing... 

Nanoparticulate ZnO was used as an efficient catalyst for the synthesis of cou-marins (84) by the reaction of o-hydroxy benzaldehydes (82) and 1,3-dicarbonyl compounds (83) via Knoevenagel condensation under microwaves and thermal conditions (Scheme 9.24) in moderate to excellent yields (Kumar et al. 2011). This protocol differs from the previous methods for the synthesis of coumarins (84) in terms of simplicity and effectiveness. The application of ZnO/MgO in ionic liquid [bmim] [BF4] was carried out successfully for the synthesis of 4//-pyrans (85) and coumarins (88) at ambient temperature via Knoevenagel condensation reaction of aldehydes (8) or 2-hydroxybenzaldehyde derivatives (86) with active methylene compounds (16, 43, 87) (Schemes 9.25 and 9.26) (Valizadeha and Azimib 2011). The method has several advantages in terms of mild reaction conditions, reusability of the catalyst, high yields of the products, and short reaction times. In comparison with methods mentioned in the literature for the synthesis of 4F(-pyrans (85) and coumarins (88), this protocol has better yield and eco-friendly advantages. 

Additionally, unsubstituted and 6-substimted 2-(perfluoroalkyl)-4/f-pyran-4-ones 4 have been prepared using alkyl enolates derived from p-dicarbonyl compounds. The reaction of acetylacetone enol ether with ethyl perflnoroalkanoates in the presence of i-BuOK, followed by p-TsOH catalyzed cychzation in benzene afforded pyrones 4a,b in 57-75 % yields. Similarly, the parent compounds 4c,d were obtained from the for-mylacetone derivative in 40-64 % yields [4]. Analogue 4e was accessible in low yield from the corresponding triketone [5] (Scheme 2). 

Dicyanomethylene)-l,3-indanedione 254 reacted with Ph3p=C=C=X (291), to yield the corresponding pyran derivatives 292, where X is either O or Different products were obtained by reactions of 254 with 1,3-dicarbonyls. Depending on residues and R, spiro compounds 293 (obtained from 254 and dimedone) and294 or propellanes 295 were isolated. ... 

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