the final refinement and difference map. In the final refinement, the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were included in the structure-factor calculations but their parameters were not refined. The final discrepancy indices are R = 0.044 and R, = 0.055 for the remaining 1328 Synthesis starting from ethyl 2,3-di-O-benzyl-~-~galactofuranceide (3), which is readily available from Dgalactose, provided (2R,3S,4S)-2-(4-methoxybenzyl)pyrrolidine-3,4-diol (2, deacetylanisomycin) in eight steps with 18-20% overall yield. The sequence proceeded through the aldehyde 4 generated from 3 by periodate oxidation, conversion of 4 into ethyl 2,3-di-O-benzyl-5-deoxy-5-C-(4-methoxyphenyl)-cu-~-arabinofuranoside (6) by Grignard reaction and subsequent ionic deoxygenation of the resulting alcohols 5, and elaboration of the corresponding sugar oxime 8. The latter was transformed by dehydrative mesylation into the L-arabinononitrile 4-mesylate 9, which was reduced by diborane to the amine and cyclized by internal S N~ reaction to give the dibenzyl ether 10 of 2. Catalytic transfer hydrogenolysis then furnished 2.

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Die N-monosubstituierten Cyanthioformamide 1 und ihre Acylderivate 2 gehen rnit einer Reihe von 1,3-Dienen Diels-Alder-Reaktionen unter Bildung der Addukte 3a-t und 4 s -t ein. Diels-Alder Reactions of CyanothioformamidesN-monosubstituted cyanothioformamides 1 and their acylated derivatives 2 undergo Diels-Alder reactions with a variety of 1,3-dienes to give the adducts 3 a -t and 4 a -t. Die dienophilen Eigenschaften der

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Die Enthalpie-und Entropiedifferenz zwischen meso-und ~~-2,3-Dimethoxy-2,3-diphenylbernsteinsauredinitril (2) wurde aus der Temperaturabhangigkeit des thermisch uber reversible Bildung von a-Cyan-a-methoxybenzyl-Radikalen (3) eingestellten Gleichgewichts bestimmt und durch Kraftfeldrechnungen bestatigt. Die Aktivierungsenthalpien der Thermolyse von meso-und DL-2 bestatigen diese Differenz ebenfalls und stimmen uberein mit der ESR-spektroskopisch bestirnmten Dissoziationsenthalpie von 2 in 3. Die Radikale 3 sind demnach nicht kinetisch stabilisiert. Aus der Diskrepanz zwischen AS * von 2 und der Dissoziationsentropie A S wird auf primare Bildung sandwichartiger Radikalpaarkomplexe im DissoziationsprozeR geschlossen. Aus den Aktivierungsparametern der Thermolyse und den Spannungsenthalpien der Diastereomeren 2 sowie aus der experimentell bestimmten C,e, -H-Bindungsenthalpie von a-Methoxybenzylcyanid (1) ergibt sich kein Hinweis fur eine uber das Additive hinausgehende capto-dative Stabilisierung der Radikale 3. Substituent Effects on the CC-Bond Strength, 5 l) Kinetics and Thermochemistry of the Homolytic Dissociation of meso-and ~,~-2,3-Dimethoxy-2,3-diphenylsuccinonitrile The differences in enthalpy and entropy between meso-and ~~-2,3-dimethoxy-2,3-diphenylsuccinonitrile (2) were deduced from the equilibrium constants of their interconversion at elevated temperatures via reversible dissociation into a-cyano-a-methoxybenzyl radicals (3). They were reproduced by force field calculations and by the difference in activation enthalpy of dissociation of meso-and DL-2. The identity of A H * (2) and A H for the dissociation process 2 + 3 as obtained by ESR spectroscopy proves that the radicals 3 have no kinetic stabilization. From the discrepancy between AS * (2) and the entropy of dissociation AS (2 + 3) it is concluded that sandwich radical pair complexes are intermediates in the dissociation process of 2. From the relationships between AG * of thermolysis and the strain enthalpies H, of 2 and from the experimentally determined Cf,,, -H bond enthalpy of a-methoxybenzyl cyanide (1) it is concluded that no particular capto-dative stability effect exists for the radicals 3.In jiingster Zeit wurde vor allem durch die Arbeiten von Viehe2) auf die in mancher Hinsicht ungewohnliche Reaktivitat sogenannter ,,capto-dativer" Radikale aufmerk-0 Verlag Chemie GmbH, D