Titlebook: Organic Reaction Mechanisms; 40 Solved Cases Mar Gómez Gallego,Miguel A. Sierra Textbook 2004 Springer-Verlag Berlin Heidelberg 2004 Anorga

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Level 1 — Case 5 Rearrangements from Tetrahydropyran Derivativesng in several distinct products that are obtained as diastereomeric mixtures (Scheme 5.1). The formation of these products could be rationalized by considering the initial generation of a carbocationic intermediate (. or .) from which different final products can be obtained. For example, trapping o

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Level 1 — Case 6 Stereospecific Substitution Reactions of Epoxy Sulfidesess of Me.Al yielded after acid quenching, .-hydroxy sulfides ., whereas .-1-(phenylthio)-2,3-epoxyalkanes . under the same conditions, yielded .-hydroxy sulfides .. In both cases the yields of isolated products were greater than 95% (Scheme 6.1).

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Level 1 — Case 8 Addition of Hydroxylamines to α,β-Unsaturated Esterstested with ethyl (.)-cinnamate ., different results were obtained depending on the reagent employed. Thus, the reaction of . with .-methylhydroxylamine yielded the isoxazolidinone ., whereas no reaction took place (the starting material was recovered unaltered) when .-methylhydroxylamine was used (

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Level 1 — Case 14 Cyclization of 2,3-Dibenzylidenesuccinatestural products found in plants. The stereochemistry of the cyclization products depends on the reaction conditions. For example, irradiation of succinate . in EtOH yields almost exclusively .-dihydronaphthalene ., whereas heating of . with trifluoromethanesulfonic acid (CF.SO.H, triflic acid) in CH.

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Level 1 — Case 15 Oxazoline ,-Oxides as Dipoles in [3+2] Cycloadditions proceed in good yields and are highly .- and .-stereoselective. Thus, regioisomers . are obtained preferentially to . (the . ratios are about 95:5), with almost complete endo-selectivity (Scheme 15.1).

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Level 2 — Case 18 Unusual Diels-Alder Reactivity of Acyclic 2-Azadieneson-rich dienophiles in an . Diels-Alder reaction. However, for a diene, . shows a very unusual reactivity, since it participates in cycloadditions with both electron-rich and electron-deficient dienophiles. As an example, reaction of azadiene . with diethyl fumarate yields pyridine . whereas in the

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Level 2 — Case 19 Chelate-Controlled Carbonyl Addition Reactions SnCl. was employed, aldols . were obtained preferentially with selectivities higher than 95:5. Instead, the stereoisomers . were isolated with nearly total stereoselectivity when Me.AlCl (2 eq) was used to promote the reaction (Scheme 19.1a and b)..