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The increasing garbage mountain is more and more recognized as an ecological threat. Space for landfills is limited and additional incineration capacities require high capital investments and pose additional envrironmental problems. In the FRG from total annual 14 million t of household waste, 700,000 t are non-degradable plastic materials (polydefines. polystyrene and polyvinylchloride), especially for packaging. Different strategies are being followed to reduce the 5% plastics in household waste: prevention, recycling, chemical valorization, thermic utilization (incineration) and use of degradable polymers. The latter are reviewed in detail: chemical modification of classical polymers into photo-and chemo-degradable materials, physical mixing of 6-20% granular starch with polyethylene or polypropylene, coprocessing of more than 50% disintegrated starch with hydrophilic polymers like polyacrylate or polyvinylalcohol to biodegradable films for agricultural mulch or carrying bags, thermoplastic extruded starch with plasticiser containing more than 90% starch for blister packaging and disposables for fast food and finally biodegradable polyesters like poly-hydroxyhutyric acid (PHB) and polylactic acid, produced by fermentation processes. Die Funktion derStiirke in biologisch abbaubaren thermoplastischen Materialien. Der wachsende Miillberg wird mehr und mehr als okologische Bedrohung angesehen. Platz fur Deponien ist begrenzt, zusatzliche Verbrennungsanlagen erfordern hohe Investitionen und crgeben zusatzliche Umweltprobleme. In der Bundesrepublik Deutschland werden jahrlich 14 Millionen t Haushaltsabfall erzeugt, rnit einem Anteil von 700000 t nicht abbaubarer Kunststoffe (Polyolefine. Polystyrol und Polyvinylchlorid), insbesondere Verpackungsmaterialien. Verschiedene Entsorgungskonzepte werden verfolgt um den 5% -Anted an Kunststoffen im Hausmull zu verringern: Abfallvermeidung, Recycling, chemische Venvertung, thermische Verwertung (Verbrennung) und der Gebrauch abbaubarer Polymerer. Letztere werden eingehend besprochen: chemische Modifizierung klassischer Polymerer zu photolytisch-und chemisch abbaubaren Materialien, physikalisches Mischen von 6-20% granullrer Starke mit Polyethylen oder Polypropylen, Verarbeiten von mehr als 50% desintegrierter Starke mit hydrophilen Polymeren wie Polyacrylat oder Polyvinylalkohol zu biologisch abbaubaren Folien fur die Landwirtschaft oder fur Tragetaschen, thermoplastisch extrudierte Starke mit mehr als 90% Starkc fur Klarsichtverpackungen und Wegwerfartikel im Fast Food-Bereich und schlieRlich biologisch abbaubare Polyester wie Polyhydroxybuttersaure (PHB) und Polymilchsaure, die fermentativ hergestellt werden. starcwstarke 42 (1990) Nr 4. S 123-130 0 VCH Verlagsgcsellschaft mbH, D-6940 Weinhelm, 1990 0038-905619010404-0123~02 5010

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We have investigated several 1,3-dipolar cycloadditions of a chiral nitrone prepared from L-erythrulose. While cycloadditions to carbon-carbon multiple bonds of dipolarophiles such as ethyl acrylate, ethyl propiolate, or dimethyl acetylenedicarboxylate were poorly stereoselective, reaction with acrylonitrile provided predominantly one diastereomeric adduct. Furthermore, the regioselectivity exhibited by the two structurally similar dipolarophiles ethyl acrylate and ethyl propiolate was found to be opposite. The molecular mechanisms of these cycloadditions have thus been investigated by means of density functional theory (DFT) methods with the B3LYP functional and the 6-31G and 6-31+G basis sets. A simplified achiral version of nitrone 1 as the dipole, and methyl propiolate or acrylonitrile as the dipolarophiles, were chosen as computational models. The cycloadditions have been shown to take place through one-step pathways in which the C-C and C-O sigma bonds are formed in a nonsynchronous way. For the reaction with methyl propiolate, DFT calculations predict the experimentally observed meta regioselectivity. For the reaction with acrylonitrile, however, the predicted regioselectivity has been found to depend on the computational level used. The calculations further indicate the exo approach to be energetically favored in the case of the latter dipolarophile, in agreement with experimental findings. The main reason for this is the steric repulsion between the nitrile function and one of the methyl groups on the nitrone that progressively develops in the alternative endo approach.

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