Stereoselective 1,3-dipolar cycloaddition of diazoalkanes to ( E )-5-alkylidene-1,3-dioxan-4-ones 1 and the ( Z) -isomers (Z)-1 afforded spiropyrazolines 3 and 10 , respectively. The optically active 2-benzylidene-3-methyl-β -lactone (8) was synthesized from the corresponding 5-benzylidene-1,3-dioxan-4-one 1f by hydrolysis and recyclization and added diazomethane with opposite stereodifferentiation. Photochemical elimination of nitrogen from spiropyrazolines 3 gave enantiomerically pure spirocyclopropanes 12 that could be further transformed to cyclopropanecarboxylic acids 13 and derivatives 14 .Recently, the cycloaddition of diazomethane to 5-alkylidene-1,3-dioxan-4-ones 1 , derivatives of the naturally occurring polyhydroxybutyrates (PHB), was reported in a short communication. 1 Spiropyrazolines 3 were obtained with complete stereoselectivity, which could then lose N 2 affording spirocyclopropanes 12 or 5-alkylidene-1,3-dioxan-4-ones 15 by irradiation or thermolysis, respectively. In addition to detailed investigations and extension of these reactions to substituted diazoalkanes 2 (R 3 ≠ H) and chemical transformations of the spirocyclopropanes 12 , we report here the possibility of changing the stereochemistry of the diazomethane cycloaddition (α versus β -attack) by the application of a corresponding 3-alkylidene-β -lactone 8 . The addition of diazomethane to ( E )-5-alkylidene-1,3-dioxan-4-ones 1 gave β -products 3 (Scheme 1 and Table 1) with high stereoselectivity. 1 The same holds true for the addition of diazomethane to the ( Z )-5-alkylidene-1,3-dioxan-4-one ( Z ) -1a (R 1 = Me, R 2 = t -Bu) affording the diastereomer 10a (Scheme 2 and Table 1). In the case of the less reactive 5-arylidene-1,3-dioxanones 1 and ( Z ) -1 (R 1 =Ph, 3-MeOC 6 H 4 ) long reaction times were necessary (1 week) and mixtures of the diastereomeric spiropyrazolines 3 and 4 or 10 and 11 were obtained (Schemes 1 and 2), that could be separated by chromatography affording at least the major isomer in analytically pure state (Table 1). The configuration and constitution of the pyrazolines 3 , 4, 10 and 11 were elucidated by NMR spectroscopy (NOE-investigations included), and in some cases by their transformation to cyclopropanes 12 (vide infra). In addition, the structure of the cycloadduct 10b was proved by X-ray crystal analysis ( Figure).In order to check the scope of the dipolar cycloaddition of diazoalkanes to 5-alkylidene-1,3-dioxan-4-ones 1 , diazoethane (R 3 = Me), trimethylsilyldiazomethane (R 3 = TMS) and ethyl diazoacetate (R 3 = CO 2 Et) were included in reactions with the ethylidene compound 1a . Whereas diazoethane gave comparable results to diazomethane, reactions with trimethylsilyldiazomethane and ethyl diazoacetate proved more difficult to realize. Finally, high pressure conditions (10 kbar) or heating in benzene at 80 o C afforded complete cycloadditions (Table 1) to the corresponding 1-pyrazolines 3 or 2-pyrazoline 6a , respectively. The stereoselectivity was somewhat lower in these cases.Since most of the spiropyr...
