A combinatorial library of 26,500 spiro[pyrrolidine-2,3′-oxindoles] was prepared in a single-compound format by a facile intermolecular 1,3-dipolar cycloaddition. An azomethine ylide, generated by the decarboxylative condensation of an isatin 1 with an α-amino acid 2, was trapped by a trans-chalcone3 to afford heterocycles of the general structure 4. The regio- and stereochemistry of a representative product was determined by single crystal X-ray structure.

The considerable gain in efficiency achievable by introducing the principle of convergence into automated parallel synthesis is illustrated with a seven-step synthesis of a sixteen-hundred-compound array of potential serine protease inhibitors, conducted on a 50 mu mole scale (ca. 20 mg. of each compound).

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The application of a photochemical aromatic annulation strategy in highly efficient total syntheses of several diterpenoid quinones isolated from the traditional Chinese medicine Dan Shen is reported. The pivotal step in each synthesis involves the assembly of a key tricyclic intermediate via the application of a recently developed ''second-generation'' photochemical aromatic annulation method for the construction of highly substituted aromatic systems. In the total synthesis of neocrypto-tanshinone,;the synthesis of the requisite diazo ketone annulation substrate 7 was achieved using palladium-mediated coupling reactions and an intramolecular Friedel-Crafts cyclization to form key carbon-carbon bonds. The pivotal aromatic annulation reaction was then accomplished by irradiating a solution of the diazo ketone 7 and the readily available siloxyalkyne 6 in benzene at room temperature. The desired tricyclic phenol 16 was produced in 58-65% yield and was then converted to (+)-neocryptotanshinone (1) by treatment with tetra-n-butylammonium fluoride in the presence of oxygen. Cyclization to generate (-)-cryptotanshinone (2) was accomplished in high yield by brief exposure of 1 to an ethanolic solution of concentrated sulfuric acid, and dehydrogenation of 2 with DDQ furnished tanshinone IIA (3). As a further demonstration of the utility of the photochemical aromatic annulation strategy in the construction of angularly-fused diterpenes, the total synthesis of(+/-)-royleanone (4) was also investigated. Irradiation of a solution of the diazo ketone 18 and siloxyalkyne 25 produced the tricyclic intermediate 26, which was converted in two steps to royleanone by desilylation and oxidation.

This paper describes the first total syntheses of the diterpene quinones aegyptinones A and B via an extremely direct route which should easily accommodate the production of gram quantities of each compound. The key step in the synthetic strategy involves the application of a recently developed ''second-generation'' photochemical aromatic annulation method for the construction of highly substituted aromatic systems. The synthesis of one of the annulation components, the diazo ketone 5, was achieved using a Diels-Alder-based benzannulation strategy employing cyanoallene and the dienamine 13. Cyanoallene proved to be uniquely effective for this cycloaddition, which either failed or proceeded in poor yield using several substituted acetylenes as dienophiles. The pivotal aromatic annulation reaction was accomplished by irradiating a solution of the diazo ketone 5 and the readily available siloxyalkyne 4 in a Pyrex vessel with a 450-W medium-pressure Hanovia lamp at room temperature for 17-20 h. The desired tricyclic phenol 3 was produced in 58-70% yield and was then converted to aegyptinone B (2) by treatment with tetra-n-butylammonium fluoride in the presence of oxygen. Finally, cyclization to generate aegyptinone A was accomplished in high yield by brief exposure of 2 to an ethanolic solution of concentrated sulfuric acid at room temperature. Overall, this strategy provides efficient routes (six and seven steps, respectively) to aegyptinones A and B which should facilitate the systematic investigation of the pharmacological activity of these novel diterpenes.

A photochemical aromatic annulation strategy provides efficient synthetic routes to the diterpenoid quinones (+)-danshexinkun A, danshexinkun B, danshexinkun C, (−)-dihydrotanshinone I, and tanshinone I.