Ungewöhnliche faciale Diastereoselektivität in der Paternò-Büchi-Reaktion eines chiralen Dihydropyrrols – eine kurze Totalsynthese von (+)-Preussin

trans-Selective Aryldifluoroalkylation of Endocyclic Enecarbamates and Enamides by Nickel Catalysis

Efficient methods for the dicarbofuntionalization of the cyclic alkenes 2-pyrroline and 2-azetine are limited. Particularly, the dicarbofunctionalization of endocyclic enecarbamates to achieve fluorinated compounds remains an unsolved issue. Reported here is a nickel-catalyzed trans-selective dicarbofunctionalization of N-Boc-2-pyrroline and N-Boc-2-azetine, a class of endocyclic enecarbamates previously unexplored for transition metal catalyzed dicarbofunctionalization. The reaction can be extended to six- and seven-membered endocyclic enamides. A variety of arylzinc reagents and bromodifluoroacetate, and its derivatives, undergo the reaction, providing straightforward and efficient access to an array of pyrrolidine- and azetidine-containing fluorinated amino acids and oligopeptides, which may have applications in the life sciences.

Photochemical reactions as key steps in natural product synthesis

Photochemical reactions contribute in a significant way to the existing repertoire of carbon-carbon bond-forming reactions by allowing access to exceptional molecular structures that cannot be obtained by conventional means. In this Review, the most important photochemical transformations that have been employed in natural product synthesis are presented. Selected total syntheses are discussed as examples, with particular attention given to the photochemical key step and its stereoselectivity. The structural relationship between the photochemically generated molecule and the natural product is shown, and, where necessary, the consecutive reactions in the synthesis are illustrated and classified.

The [2+2]-photocycloaddition of aromatic aldehydes and ketones to 3,4-dihydro-2-pyridones: regioselectivity, diastereoselectivity, and reductive ring opening of the product oxetanes

3,4-Dihydro-2-pyridones [3,4-Dihydropyridin-2(1H)-ones] 6 were evaluated with respect to their use as alkene components in stereoselective Paternò-Büchi reactions. The parent compound 6a was shown to be a versatile synthetic building block that reacted with various photoexcited aromatic carbonyl compounds (benzaldehyde, benzophenone, acetophenone, methyl phenylglyoxylate, 3-pivaloyloxybenzaldehyde) with high regio- and diastereo-selectivity (51-63% yield). The products can be subjected to hydrogenolysis, opening a new and efficient route for the synthesis of 2-arylmethyl-3-piperidinols. As examples, the oxetanes 7a and 8a were hydrogenolytically cleaved and yielded the products 12 (88%) and 13 (93%). The ability of compound 6a to bind to a chiral lactam host through two hydrogen bonds was used favorably to differentiate the enantiotopic faces of its double bond. In the photocycloaddition to the chiral aldehyde 15, which was conducted at -10 degrees C in toluene, a high facial diastereoselectivity (>90% de. 56% yield) was recorded. The stereoselectivity results from a 1:1 association of dihydropyridone 6a to the aldehyde. The 4-substituted dihydropyridones 6b-6d (R = methyl, isopropyl, phenyl) were found to be less suited for potential use in photochemistry. The yields and facial diastereoselectivities recorded in their photocycloaddition to benzophenone remained low.

Inhibition of cyclin-dependent kinase activity and induction of apoptosis by preussin in human tumor cells

In this paper, we report that (+)-preussin, a pyrrolidinol alkaloid originally identified as an antifungal agent, has growth-inhibitory and cytotoxic effects on human cancer cells. Preussin was found to be a potent inhibitor of cyclin E kinase (CDK2-cyclin E) in vitro (50% inhibitory concentration; approximately 500 nM) and to inhibit cell cycle progression into S phase. In agreement with these findings, the level of the cyclin-dependent kinase inhibitor p27(KIP-1) is increased in response to preussin treatment while the expression of both cyclin A and the transcription factor E2F-1 is down-regulated. Preussin also induces programmed cell death (apoptosis), which requires caspase activation and involves the release of cytochrome c from mitochondria. This induction of apoptosis is not blocked by high levels of Bcl-2, which usually confers resistance to chemotherapeutic agents. Taken together, our data indicate that preussin could be a promising lead compound for the development of a new class of potent antitumor drugs.