Intramolekulare cycloadditionen von N-alkyl-C-phenoxymethyl-nitronen an ortho-staendige C=C-doppelbindungen

Unveiling the intramolecular [3 + 2] cycloaddition reactions ofC,N-disubstituted nitrones from the molecular electron density theory perspective

“…These interesting reactions were first proposed by Le Bel et al [7][8][9][10] for the synthesis of fused bicyclic isoxazolidines 9, in which nitrone 8 was generated by condensation of N-methylhydroxylamine 6 with the unsaturated aldehyde 7 (Scheme 1). Studies on IM32CA reactions were subsequently followed by the pioneering contribution of Opplozer's [11][12][13] group to the synthesis of polycyclic isoxazolidines 11, in which the nitrone and alkene fragments are separated in reagent 10 by the chain incorporating a benzene ring and a heteroatom (Scheme 1). Subsequently, numerous IM32CA reactions 14 of nitrones were employed to synthesize bicyclic and polycyclic isoxazolidines.…”

Intermolecular Diels-Alder Reactions

“…The cw ring fusion of rings Β and C of gibberellic acid is set up in this reaction and in fact the stereochemistry of all the chiral centres in the final product follows from that in (5). The newly-formed six-membered ring in (5) eventually becomes the five-membered ring Β of gibbereUic acid by cleavage of the double bond and aldol recychsation. An intramolecular Diels-Alder reaction was employed at a later stage of the synthesis.…”

“…The intermolecular reaction was early in the synthesis. Reaction of the quinone (4) with íraAi^-2,4-pentadienol gave the single crystalline adduct (5) in 91 per cent yield by way of the endo transition state. The cw ring fusion of rings Β and C of gibberellic acid is set up in this reaction and in fact the stereochemistry of all the chiral centres in the final product follows from that in (5).…”

“…Reaction of the quinone (4) with íraAi^-2,4-pentadienol gave the single crystalline adduct (5) in 91 per cent yield by way of the endo transition state. The cw ring fusion of rings Β and C of gibberellic acid is set up in this reaction and in fact the stereochemistry of all the chiral centres in the final product follows from that in (5). The newly-formed six-membered ring in (5) eventually becomes the five-membered ring Β of gibbereUic acid by cleavage of the double bond and aldol recychsation.…”

“…An intramolecular Diels-Alder reaction was employed at a later stage of the synthesis. After extensive manipulation (5) was converted into the tricyclic intermediate (6) and thence into the ester (7). When (7) was heated in benzene at 160'C the pure crystalline lactone (8) was obtained in 55 per cent yield after crystal lisation, formed by intramolecular addition to the α-face of the diene by way of an endo transition state; no other stereoisomer was detected (intra molecular reactions do not always favour the endo transition state so markedly).…”