Anna Gamba

Low molecular weight solutes in desiccated and ABA-treated calli and leaves of Craterostigma plantagineum

Abstract Craterostigma plantagineum, a resurrection plant, displays extreme desiccation tolerance; dehydrated leaves and ABA-treated calli resume a normal metabolic state upon rehydration. Samples of calli and leaves of C. plantagineum were analysed for the major classes of compounds such as sugars, fatty acids, amino acids and derivatives, and phytosterols. Calli solutes are characterized by large amounts of glutamine and tyramine, triterpene acids and colneleic acid, an inhibitor of lipoxygenase. Among the sugars extracted from calli, sucrose, together with its hydrolytic products, predominates, while maslinic, ursolic and oleanolic acids are the most important triterpenes. In leaves the most common triterpene alcohols were campesterol, stigmasterol and β-sitosterol. Almost 50% of the weight of the lyophilized material consisted of 2-octulose, a sugar present as a minor component in a limited spectrum of plant species. Acquisition of desiccation tolerance in ABA-treated and desiccated calli was accompanied by an increase of fatty acids, accumulation of colneleic acid and by the disappearance of glucose and fructose. The most relevant biochemical effect of desiccation on leaves was the conversion of 2-octulose into sucrose.

Steric effects vs secondary orbital interactions in nitrone cycloadditions. Steric effects in cycloreversions of isoxazolidines

Abstract 3,4-Dihydroisoquinoline-N-oxide 1 reacted readily with both acyclic (Z)- and (E)-disubstituted alkenes bearing electron-attracting substituents (methoxycarbonyl, cyano, phenylsulphonyl and benzoyl groups) and with cyclic derivatives (e.g. maleimides) to give mixtures of the two possible diastereoisomers. Similar amounts of endo - and exo -adducts were formed in the reactions of (Z)-cyano, methoxycarbonyl and benzoyl derivatives whereas exo -addition clearly won over its endo -counterpart in the case of (Z)-(phenylsulphonyl) and cyclic derivatives. High exo -selectivity was also observed in the sluggish reactions of 1 with electron-rich alkenes [(Z)-stilbene, vinylene carbonate, acenaphthylene etc.] Our results, which revise previous literature data, clearly show that an “ endo -rule” does not hold for the reactions of 1 with (Z)-1,2-disubstituted alkenes. We conclude that in these reactions repulsive steric interactions either counteract efficientlt or clearly win over stabilizing secondary orbital overlaps in controlling endo/exo -selectivity. These reactions were found reversible under mild conditions so that relative formation rates of related pairs of (Z)- and (E)-dipolarophiles in cycloreversion processes of isoxazolidines could be determined; as a rule (E)-alkenes are extruded faster then (Z)-isomers. These results provide unambiguous experimehntal evidence that increase in steric compression between the substituents in (Z)-alkenes on there way toward transition state, is not a major factor in dertermining their lower reactivity with respect to (E)-isomers in 1,3-dipolar cycloadditions.

Heterodiene syntheses—V : 1,2- Versus 1,4-cycloaddition reactions of enamines to N-substituted 3-oxindolideneacetophenones

Abstract The reaction of some enammes with N-substituted 3-oxindolideneacetophenones has been investigated. When the oxindole N-substituent is an acyl group, 1,4-cycloaddition occurs leading to 2,3-dihydropyrano[2,3-b]indole derivatives: when the N-substituent is an alkyl group, 1,2-cycloaddition takes place and spirocyclobutanoxindole derivatives are formed. Some intermediate cases are also presented. A possible rationalisation of the electronic effect N-substituent on the reaction pathway is proposed.Abstract The reaction of some enammes with N-substituted 3-oxindolideneacetophenones has been investigated. When the oxindole N-substituent is an acyl group, 1,4-cycloaddition occurs leading to 2,3-dihydropyrano[2,3- b ]indole derivatives: when the N-substituent is an alkyl group, 1,2-cycloaddition takes place and spirocyclobutanoxindole derivatives are formed. Some intermediate cases are also presented. A possible rationalisation of the electronic effect N-substituent on the reaction pathway is proposed.

Stereospecificity of 1,.3-dipolar cycloadditions of cyclic nitrones to (E) and (Z)-β-nitrostyrenes

Abstract 3,4-Dihydroisoquinoline-N-oxide (1) reacted readily with (E)-s-nitrostyrene in a regiospecific reaction to give a mixture of 4-nitro-5-phenylisoxazolidines 3a and 4a resulting from endo and exo (with respect to the nitro group) transition states, respectively. This cycloaddition was found reversible under mild conditions. A careful study disclosed ⩾ 99.89% stereoselectivity thus narrowly circumscribing the possibility of stereochemical leakage over the cycloaddition and cycloreversion processes. Moreover, our experimental data showed that eventual loss of stereochemistry should be ascribed to base catalyzed isomerizations in the adducts and/or in the educts. The reaction of 1 with (Z)-s-nitrostyrene ( exo specific and regiospecific) turned out to be faster than that of the (E)-isomer. This is the first example of higher reactivity of a cis than a trans -alkene in 1,3-dipolar cycloadditions. The endo-trans adduct 3a cycloreverted faster than the exo-trans isomer 4a which in turn underwent fragmentation more readily than the exo-cis 6a. The cycloreversion rate was slightly enhanced by increased solvent polarity. This study was extended to the reaction of 5,5-dimethylpyrroline-N-oxide with both the cited dipolarophiles.

Heterodiene syntheses—XII : The conformational analysis of cis and trans 2-alkoxy-4-phenyl-2,3-dihydropyran[2,3-c] pyrazoles: Steric interactions and the anomeric effect

Abstract The conformational equilibria of a homogeneous series of cis and trans 2-alkoxy-4-phenyl-2,3-dihydropyran[2,3-c] pyrazoles have been investigated by a graphical method, and the preferences are rationalized in terms of steric interactions between the 4 and 5-substituents and of the anomeric effect. The anomeric interaction depends upon the nature of the alkoxy group and the conformational free-energy differences correlate with the steric parameters E s .

Heterodiene syntheses—XXIV: Changes in the reactivity of 2-oxoindolin-3-ylidene derivatives with ethylvinylether induced by electron-withdrawing groups. The importance of the lumo coefficients

Abstract Ethylvinylether reacts in acetonitrile with 1 - acetyl - 2 - oxoindolin - 3 - ylidene derivatives β,β -disubstituted with electron-withdrawing substituents. When the β-substituents act by inductive effect alone, a regiospecific β-attack 2 occurred giving rise, through a 1,4 - cycloaddition, to 2,3 - dihydropyran[2,3-b]indoles. As a by product Michael adduct can be obtained. When the β-substituent act by conjugative effect also, a regiospecific α-attack 2 occurred giving rise to spiro - dihydropyrane or - cyclobutane - 2 - oxoindolines depending on the nature of the substituents. In a less polar solvent, the latter ones are formed together with the adduct arising from the regioisomeric β-attack. The overall reactivity can be rationalized in terms of LUMO coefficients of the hetcrodiene and of different stabilization offered by the solvent to the various reaction pathways.

Cycloaddition reactions of nitrones to cyclooctatetraene and its derivatives

Abstract The cycloaddition of acyclic nitrones to the Diels-Alder adduct cyclooctatetraene-acetylenedicarboxylate yields three different monoadducts, whose structures are assured through NMR spectroscopy and chemical transformations. Thermolysis of these monoadducts to the new heterocyclic system of 2,3-oxazabicyclo[3.2.0]hepta-6-ene is described. Depending on the nature of the substituents on the N atom and on the stereochemistry of the isoxazolidine C-3, variable amounts of pyrrole aldehydes are also obtained. The reaction of a cyclic nitrone with cyclooctatetraene and some of its diene adducts is also reported.

syn–anti-Isomerism in the cycloaddition of nitrile oxides to cis-3,4-dichlorocyclobutene

1,3-Dipolar cycloaddition reactions of cis-3,4-dichlorocyclobutene with several nitrile oxides yield mixtures of syn- and anti- 4-substituted 6,7-dichloro-2-oxa-3-azabicyclo[3.2.0]hept-3-enes. The structures of the cycloadducts are deduced from n.m.r. and dipole moment data and a possible explanation for the abnormally high proportion of the sterically disfavoured syn-isomer is presented.

Heterodiene syntheses—IX: Polar character of the thermal rearrangement of the adducts from 3-oxindolideneacetophenones and enamines☆

Abstract The cyclic adducts resulting from the reaction between N-substituted 3-oxindolideneacetophenones and enamines undergo thermal rearrangement in CDCl3 at 35°. If the adducts originate from the β-dimethyl substituted enamine, they revert to the starting materials. In contrast the β-monomethyl substituted adducts are transformed into Michael-type enamines. Both classes of adducts react with tetracyanoethylene to give spirocyclohexane oxindoles. All the reported reactions seem to occur through the same intermediate with a zwitterionic character.

Periselectivity in the reactions of cyclopentadienones with 8-aryl-8-azaheptafulvenes

Abstract Periselectivity; Cycloadditions; Dienes; Trienes 8-Aryl-8-azaheptafulvenes 2 reacted smoothly with 2,5-dimethyl-3,4-diphenylcyclopentadienone (1a) to give exo [6+4] adducts, i.e. 4, as the dominant adducts along with minor amounts of endo [4+2] adducts, i.e. 7 and 8, and trace amounts of [8+2] adducts, i.e. 6. Passing from 2,5-dimethyl to 2,5-dimethoxycarbonyl-3,4-diphenylcyclopentadienone (1c) brought about an increase in reaction rate without any relevant change in selectivity. Structures of the exo [6+4] adducts 4 rest firmly on spectroscopic data and chemical behavior. Upon heating the [6+4] adducts 4 underwent a [3,3] aza-Cope rearrangement followed by [1,5-H] sigmatropic shifts to afford the [8+2] adducts 6. Mechanisms of formation of all the adducts are discussed. We conclude that only concerted cycloadditions are at work in the reactions of azaheptafulvenes with cyclopentadienones