Brønsted acids catalyze the addition of enolizable b-keto esters to a,b-unsaturated aldehydes leading to substituted 2H-pyrans in good yields under mild conditions via a formal [3 + 3] cycloaddition.Keywords: cycloaddition; electrocyclic reactions; organocatalysis; oxygen heterocycles; a,b-unsaturated carbonyl compounds 1,3-Dicarbonyl derivatives constitute important synthetic intermediates as they incorporate electrophilic or nucleophilic functionalities.[1] Rodriguez and colleagues developed anionic methodologies based on the reactivity of stabilized carbanions for the stereoselective synthesis of bicycloA C H T U N G T R E N N U N G [3.2.1]octanols, [2a] arylidenecycloalkanones, [2b] cycloheptanols, [2c] and cyclooctane derivatives [2d] as well as functionalized cyclic enol ethers.[2e] These 1,3-dicarbonyl compounds, in the presence of a,b-unsaturated aldehydes, can also lead to pyrans and oxodecalins via a formal [3 + 3] cycloaddition (Scheme 1).[3] Such derivatives are common motives and are present in numerous natural products.[4] Elegant approaches to these intermediates, and their use in synthesis, were described by several groups using either a,b-unsaturated iminium salts as activated electrophiles, [5] Lewis acids as catalysts, [6] and tandem Stille-oxo-electrocyclization reaction. [7][8][9][10] Electrophilic activation by small-molecule chiral Hbond donors has emerged as an important tool for enantioselective catalysis, with new applications and developments appearing at a rapidly increasing pace.[11] Such organocatalysts do not contain any metals and, therefore, are advantageous from environmental perspectives. Compared to Lewis acid, they are less expensive, stable and moisture insensitive. Among the known carbonyl activators, Brønsted acids have recently demonstrated their potential to serve as active catalysts for a variety of synthetically useful reactions in organic chemistry. [12] We report here our preliminary results on the synthesis of pyran compounds by a cascade reaction involving diketones 2 and conjugated enals in the presence of a catalytic amount of a Brønsted acid under mild conditions. For this work, the phosphoric acids 4 and 5 were prepared following known procedures, [13] and isolated in good yields (69 % and quantitatively, respectively, Scheme 2). [14] 3,3-Dimethyl-1,5-cyclohexanedione 2a has been used as the starting material to synthesize substituted pyrans. A blank reaction carried out at room temperScheme 1. Scheme 2.
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