The first total syntheses of (+/-)-didehydrostemofoline (1) and (+/-)-isodidehydrostemofoline (3) are reported. The synthesis begins with the Diels-Alder reaction of readily available pyrrole 9 and ethyl (E)-3-nitroacrylate, the latter serving as a regioinverted equivalent of ketene. After hydrogenation to prevent retro-Diels-Alder reaction, the major cycloadduct is transformed to 7-azabicyclo[2.2.1]heptanol 14. Aza-Cope-Mannich reaction of the formaldiminium derivative of 14 delivers 1-azatricyclo[5.3.0.04.10]decane 15, which in 15 additional steps is converted to 1 and 3.

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Stereoselection in tethered Biginelli condensations can be tuned to give either the cis or trans stereoisomer of 1-oxohexahydropyrrolo[1,2-c]pyrimidine and 1-iminohexahydropyrrolo[1,2-c]pyrimidine products (see eq 1). With substrates having urea and N-sulfonylguanidine functionality (Schemes 2 and 4), cis stereoselection (4-7:1) is observed when the condensation is accomplished under Knoevenagel conditions, while trans stereoselection (4-20:1) is observed when the condensation is carried out in the presence of polyphosphate ester (PPE). Under both conditions, steroselectivity is highest in the N-sulfonylguanidine series. With substrates bearing a basic guanidine unit, the trans product is formed exclusively under Knoevenagel conditions (Scheme 3).

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The oxidation of two tethered Biginelli adducts was examined as a potential key step in total syntheses of highly oxidized batzelladine and crambescidin alkaloids. Although angular hydroxyl substitution could not be introduced, dehydrogenation was readily accomplished. This latter conversion is a key step in the first total synthesis of dehydrobatzelladine C. [structure: see text]