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The development of synthetic protocols to small molecules with a complex 3D shape is a relevant problem for modern chemists because such molecules are required by drug discovery. The Diels-Alder reaction is a good synthetic approach to complex three-dimensional structures. In the present paper, a reaction of aroylpyrrolobenzothiazinetriones (oxa-dienes) with electron-rich dienophiles is investigated in order to reach novel alkaloid-like pyrano [4,3-b]pyrroles fused to a 1,4-benzothiazine-2-one moiety. The studied reaction was found to proceed highly regioselectively. Its stereoselectivity was dramatically dependent on the reaction solvent. The experimental results are supplemented with computational studies, which demonstrate that the studied reaction proceeds via a one-step polar mechanism. In addition, an improved synthesis of alkaloid-like pentacyclic 6/6/5/6/5-and 6/6/5/6/6angularly fused pyrano [4,3-b]pyrroles via an acid-catalyzed intramolecular cyclization of Michael adducts to hetero-Diels-Alder cycloadducts was discovered. The synthesized alkaloidlike heterocycles represent an interest to pharmaceutics, since their close analogs show significant antiviral activity.

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