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Substituted 4,5-dihydro-3-methylene-2(3H)-furanones 1 react as dipolarophiles with diphenylnitrilimine to give the corresponding spiro[furan-pyrazo1in)ones 2. From the compound la, 1,3-diphenyl-?-oxa-l,2-diazaspiro[4.4]non-2-en-6one (2a) is formed. In the other cases, a primarily formed cycloadduct undergoes rearrangement during the reaction to the bicyclic compound 5. Thus, addition of diphenylnitrilimine to (E)-lb, (Z)-lb and (E)-lc leads to compound 5b, whereas reactions of (E)-or (Z)-lf and (E)-lg give the corresponding compound 5f and 5g, respectively. The structure of 5 b was determined by X-ray analysis. Labeled methyl formate (H13C02CH3) was used for labeling the methylene carbon atom in 4,5-dihydro-3-(p-tosyloxymethylene)-2(3H)-furanone [(E)-lb] in order to ascertain the structure of the intermediate cycloadduct as well as the pathway of the consecutive rearrangement.

Benzonitrile oxide cycloadds to the CC bond of substituted 4,5‐dihydro‐3‐methylene‐2(3H)‐furanones 1a–1g. Thus, (E)‐1a and (E)‐1b are converted into the corresponding isoxazolinespirodihydrofuranones (4RS,5RS)‐2a and (4RS,5RS)‐2b, and (E)‐1c is converted into a mixture of isomers (4RS,5RS)‐2c and (4RS,5RS)‐3c. The reactions of (E)‐1d–1g yield mixtures of the corresponding diastereomeric isoxazolinespirodihydrofuranones (4RS,5RS,8RS)‐2d and (4RS,5RS,8SR)‐2d, (4RS,5RS)‐3e and (4RS,5SR)‐3e, (4RS,5RS)‐3f and (4RS,5SR)‐3f, (4RS,5RS)‐3g and (4RS,5SR)‐3g. The addition of benzonitrile oxide to (Z)‐1a gives (4RS,5SR)‐2a and 4a. In the cycloaddition with (E)‐1a the p‐tolylsulfonyloxy group in the intermediate (4RS,5RS)‐2a is partially replaced by chlorine to give (4RS,5RS)‐2h. – The structures of (4RS,5RS)‐2h, (4RS,5RS)‐3e, (4RS,5SR)‐3e, and 4a are determined by X‐ray analysis. The 1H‐ and 13C‐NMR spectra of the products are discussed and the reaction course is rationalized on the basis of HOMO‐LUMO energy gaps.

Sodium salt of 3-hydroxymethylenetetrahydro-2H-pyran-2-one (V), obtained by Claisen condensation of δ-valerolactone with ethyl formate, was converted into its sulfonates and carboxylates IV, VII - X, which were obtained either as pure E-isomers or as mixtures of E- and Z-isomers; the mixtures were chromatographically separated. Substitution reaction of α-tosyloxymethylene lactones II, III and IV with aromatic thiols, azide anion, secondary amines and sodium enolates XI, XII and V was studied. The stereochemical outcome of this substitution is discussed from the viewpoint of mechanism of nucleophilic vinylic substitution (SNV).