In recent years, photochemical reactions have emerged as powerful transformations which significantly expand the repertoire of organic synthesis. However, a certain lack of selectivity can hamper their application and limit their scope. In this context, a major research effort continues to focus on an improved control over stereo- and chemoselectivity that can be achieved in molecular assemblies between photosubstrates and an appropriate host molecule. In this tutorial review, some recent, representative examples of photochemical reactions have been collected whose unique outcome is dictated by the formation of a molecular assembly driven by non-covalent weak interactions. Graphical Abstract

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We report a mild alternative to thermally-driven noble-metal catalyzed decarbonylation protocols for the defunctionalization of benzaldehydes in short reaction times. Our cooperative photocatalytic system involves thioxanthone as an inexpensive HAT-agent...

The carbon skeleton of the Stemona alkaloids stemokerrin and cochinchistemonine was assembled from three building blocks (a piperidine, a furan, and a tetronate). Key steps linking the fragments included a Stille cross-coupling (piperidine/furan) and an aldol type addition of a tetronate. The furan served as a latent 1,4-difunctional compound which was converted to a γ-ketolactone by a type II photooxygenation. Attempts to construct the C12-C13 double bond of stemokerrin by a late stage oxidation or by an elimination remained unsuccessful. The non-natural products dihydrostemokerrin and furostemokerrin were obtained instead.

P re d ic tio n o f Iso to p e E ffe c ts fo r A n tic ip a te d I n te r m e d ia te S tr u c tu r e s in th e C o u rs e o f B a c te ria l D e n itrific a tio n M. A. Morgenstern and R. L. Schowen Department of Chemistry, University of Kansas, Lawrence, Kansas 66045-0046, USA Z. Naturforsch. 44a, 450 458 (1989); received January 10, 1989 This paper is dedicated to Professor Jacob Bigeleisen on the occasion of his 70th birthday Vibrational-analysis methods have been used to estimate the equilibrium 14N/15N isotope effects to be expected for conversion of nitrite anion to thirteen possible intermediate-state and productstate structures [HONO, NO + , NO, NO", FeNO, 0N*N02, 0*NN02, 0 2NN02, ONO*N, 0*N0N, ONNO, *NNO, N*NO] in the reduction of nitrite ion to nitrous oxide denitrifying bacteria. The results, taken in combination with previous experimental isotope-effect and tracer studies of the Pseudomonas stutzeri and related systems, are consistent with a suggestion that a second nitrite anion enters the enzyme-catalytic cycle at the stage of a nitrosyl-ion intermediate but re-emerges after entry of the reducing electrons; the product nitrous oxide is then formed by disproportionation of enzymically generated hyponitrous acid. The calculations are consistent with contributions, under different experimental conditions, of several different transition states to limit ing the rate of the enzymic reaction. These transition states (and the corresponding experimental conditions) are the transition states for N -O fission in the generation of a mononitrogen electrophilic species from nitrite anion (high reductant, high nitrite concentrations), for attack of nitrite on this electrophile (high reductant, low nitrite concentrations) and for electron transfer to a dinitrogen-trioxide-like species (low reductant concentration).