Exploration of Diels-Alder Reaction of 2-Vinylazoles as 1-Azadienes

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Nitrogen containing a six-membered ring system (piperidine ring) is a common partial structure of biologically active compounds. One of the most direct approaches to the system is the nitorogen containing hetero Diels-Alder reaction. 1 However, Diels-Alder reaction of 1-aza-1, 3-butadienes, simple alpha, beta-unsaturated imines, themselves have been difficult due to their low reactivities as dienes, side reactions and instabilities arising from the imine moieties. We found benzylidenecyanomethyl-1, 3-benzothiazoles and -1, 3-benzoxazoles (1) as 1-aza-1, 3-butadienes. The dienes (1) featuring the stabilized imine moieties by constituting heteroaromatic rings, react with both electron-deficient and electron-rich dienophiles. It was found that substrates having the diene systems and dienophiles moiety in the molecules smoothly caused stereoselective intramolecular Diels-Alder reaction to give polycyclic compounds via exo-transition state. Ethyl (E)-3-(1, 3-benzothiazol-2-yl)-3-cyanopropenoate (2) as 1-aza-1, 3-butadiene (2), bearing electronwithdrawing ester group at diene -4-position, reacts with electron -rich dienophiles under extremely mild conditions to give corresponding cycloadducts with regio-and endo-selectivities. Treatment of 2 with electron-donating allyl alcohols causes tandem transesterification and intramolecular cycloaddition to afford cis-fused polycyclic systems in a single step.