The chemical reactivity of cyclobutanones and cyclobutenones is considerably different from that of cyclic ketones with larger rings; this is due to their ring strain of ca. 25 kcal/mol. Detailed knowledge regarding the influence of this ring strain on regio‐, chemo‐ and stereoselective transformations of four‐membered ring ketones is of particular importance. While several reactions, such as the Baeyer–Villiger reaction, the Beckmann and Favorskii rearrangements and cine‐substitution often proceed in a manner specific to four‐membered rings, other reactions such as the facile ring‐opening by nucleophiles, the rearrangement to tropolones, the thermal [2+2]‐cycloreversion, the isomerization to vinylketenes and the photochemical formation of oxacarbenes are rather specific to cyclobutanones and cyclobutenones. The remarkable selectivity and the excellent yields of such transformations, which are favored or caused by ring strain as the inherent driving force, offer the synthetic chemist fascinating possibilities for the development of new strategies for the synthesis of natural products and biologically active compounds.

Abstract

The modern agrochemical industry is searching, more intensively than ever, for new substances to combat pests (weeds, deleterious insects, plant pathogens, etc.). In the complex and costly selection and optimization process, state-of-the-art scientific methods are always needed. The aims of the interdisciplinary optimization are mainly the reduction of the rate of application of the new substance, an increase in the selectivity against the target organism, and the optimal ecological profile. If a promising crop protection compound is a racemate or a diastereoisomeric mixture, the chemist has a unique opportunity to contribute to this optimization process through the synthesis of enantiomerically pure isomers for testing purposes. If the single isomer proves to be biologically superior to the racemate, the development of an economical and ecologically sound process for the production of the single isomer presents an even greater challenge. The average price of a crop protection compound is much lower than that for a pharmaceutical product, and this fact imposes a severe limitation upon the flexibility of the chemist who is concerned with the synthesis and production of a stereochemically pure agrochemical. This forces the crop protection chemist to make full use of both his scientific and creative capabilities. Fortunately, parallel to the development of the above optimization aims of a modern and ecologically sound crop protection research, there has been a continuous and worldwide advance in the area of asymmetric synthesis. Due to the interplay of these two parallel efforts there has been a great accumulation of chemical, biological, and agronomical knowledge in recent years, which should have implications beyond merely the synthesis of enantiomerically pure agrochemicals. reactions of small rings, photochemistry, cycloadditions and rearrangements (reactions without reagents), heterocyclic and metal-catalyzed reactions, and crop protection chemicals (1985-1991). Amongst his honours he prizes the ScientiJic Award of the City of Basel (1982) and an honorary doctorate (Dr. sc. techn. h.c.) from his Alma Mater (1991) the highest. Angew. Chem. Int. Ed. Engl. 30 (1991) ii93-1215 15 7 >99%ee Scheme 4. Syntheses of 7 by catalytic homogeneous hydrogenation of 15. MCPBA = rnetn-chloroperbenzoic acid. A OH 132 H 134 Scheme 40. Azadirachtin 132 and two synthetic substructures with similar antifeedant activity against S. littoralis.

On the Mechanism of the Cope Rearrangement SummaryThe rates of the Cope rearrangement of 2,5-dicyano-3-methyl-hexa-l, 5-diene

During the reaction of allyl ethers, allyl sulfides and allyl selenides with in situ prepared dichloroketene (1), 2 competing pathways are observed. Besides [2+2]‐cycloaddition, an unprecedented [3,3]‐sigmatropic (Claisen) rearrangement via a 1,3‐dipolar intermediate takes place. It leads to O‐, S‐ or Se‐esters of α,α‐dichloro‐γ, δ‐unsaturated acids containing an inverted allylic group. Starting from cyclic n‐membered α‐vinyl‐substituted ethers, lactones with n+4‐membered rings are formed. A very facile synthesis of the naturally occurring macrolides (±)‐phoracantholide I (10) and (±)‐phoracantholide J (11) illustrates the synthetic utility of this new ‘ketene’ Claisen rearrangement.