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Isoxazolidines, reaction with hydroxylamine

A number of other syntheses were discussed by Takeuchi and Furusaki and the most common involved reaction of hydroxylamine with selected a,/3-unsaturated ketones to give isoxazolidine-3- or -5-ols, which exist in equilibrium with an open-chain counterpart (77AHC(21)207). A similar equilibrium was observed in the reaction of a,/3-unsaturated ketones with N-hydroxyurea. The geometric orientation of the ring substituents was studied as a dynamic process (Scheme 158) (75TL2337). [Pg.111]

R = H, Scheme 27). On the other hand, reaction of 255a with N-methylhydrox-ylamine hydrochloride produces a mixture of two regioisomers 257 and 258 (R = Me). When the E-l(10)-unsaturated 5-oxo-5,10-secosteroid 255b was treated with hydroxylamine hydrochloride (R = H) or AT-methylhydroxylamine hydrochloride (R = Me), isoxazolidine 259 was formed regio- and stereoselec-tively in high yield via intramolecular 1,3-dipolar cycloaddition of the nitrone intermediate 256 (R = H or Me). [Pg.38]

Reaction of porphyrins with nitrones has also been studied and the results obtained showed that this is a versatile approach leading to the synthesis of isoxazolidine fused-chlorins (Scheme 26). For instance, chlorin 74 was successfully prepared from the reaction of the jV-methylnitrone, generated in situ from JV-methyl hydroxylamine and paraformaldehyde, with porphyrin Id . It is important to note that bis-addition also took place, yielding exclusively bacteriochlorin type derivatives 76 and 77 (Figure 6). This result contrasts with those obtained in 1,3-DC reactions with azomethinic ylides where isobacteriochlorins were obtained preferentially. [Pg.63]

Starting material which, upon oxidation with PSP, gave aldehydes. These were in turn condensed with primary hydroxylamines, promoted by polymer-bound acetate, to produce nitrones. The nitrones assembled using either method then underwent 1,3-dipolar cyclo-addition reactions with various alkenes to give the corresponding isoxazolidines (Scheme 2.46 and 2.47). [Pg.91]

As part of an extensive study of the 1,3-dipolar cycloadditions of cyclic nitrones, Ali et al. (392-397) found that the reaction of the 1,4-oxazine 349 with various dipolarophiles afforded the expected isoxazolidinyloxazine adducts (Scheme 1.78) (398). In line with earlier results (399,400), oxidation of styrene-derived adduct 350 with m-CPBA facilitated N—O cleavage and further oxidation as above to afford a mixture of three compounds, an inseparable mixture of ketonitrone 351 and bicyclic hydroxylamine 352, along with aldonitrone 353 with a solvent-dependent ratio (401). These workers have prepared the analogous nitrones based on the 1,3-oxazine ring by oxidative cleavage of isoxazolidines to afford the hydroxylamine followed by a second oxidation with benzoquinone or Hg(ll) oxide (402-404). These dipoles, along with a more recently reported pyrazine nitrone (405), were aU used in successful cycloaddition reactions with alkenes. Elsewhere, the synthesis and cycloaddition reactions of related pyrazine-3-one nitrone 354 (406,407) or a benzoxazine-3-one dipolarophile 355 (408) have been reported. These workers have also reported the use of isoxazoles with an exocychc alkene in the preparation of spiro[isoxazolidine-5,4 -isoxazolines] (409). [Pg.61]

The nucleophilic addition of lithiated allyl phenyl sulfone to nitrones at 0 °C afforded 4-(phenylsulfonyl)isoxazol-idines as major products. The process probably involves the isomerization of the allylsulfonyl moiety of the initially formed hydroxylamine anion to vinylsulfone which then undergoes intramolecular Michael addition. For example, the chiral nitrone 536 afforded isoxazolidine 537 with high diastereoselectivity (Equation 88) <2005T3335>. When the same reaction was carried out in the presence of hexamethylphosphoramide (HMPA) at —80°C, the anti-a-sulfonyl homoallyl hydroxylamine was obtained. [Pg.454]

Reaction of bis(/J-chloroethyl)-y-chloropropylamine with H202-acetic anhydride gives the isoxazolidinium chloride 47a.153,154 Various isoxazo-lidinium salts of this type have been prepared.153-157 Dehydro-halogenation of W-(y-chloroalkyl)hydroxylamine is an obvious method of preparing isoxazolidines. Thus, a hydroxy-4//-1,3-benzoxazine (48) gives the isoxazolidine 49 with sodium methoxide.158,159... [Pg.233]

Vasella [58] has shown that the hydroxy-oxime 122 (2,3-0-isopropylidene-5-0-trityl-D-ribose oxime) can be, via its tautomeric form a-alkoxy-hydroxylamine 123, converted by reaction with carbonyl compounds (formaldehyde, acetaldehyde, etc.) to N-(alkoxyalkyl) nitrones 124 which then reacts via 1,3-dipolar cycloaddition with methyl methacrylate to give the protected isoxazolidine ribosides 125, 126 epimeric at C5. This reaction proceeds in high yield (97 %) and with high stereoselectivity (84 16) (Fig. 8.35). [Pg.247]

Other types of cyclic compounds can sometimes be generated in the oximeforming reaction if the carbonyl is sufficiently close to the ester/acid moiety. Reaction of 1.107 (prepared by literature methods) with hydroxylamine, for example, gave isoxazolidine 1.108 via reaction of the initially formed oxime with... [Pg.21]

Reactions of Allylation and Propargylation Allylation of prochiral and chiral nitrones (292) with allylmagnesium chloride leads to homoallylic hydroxylamines (416), which via an iodo cyclization step are converted to 5-(iodomethyl)isoxazolidines (417) (Scheme 2.186) (202, 213, 666-668). [Pg.283]

Isoxazolidines display the properties consistent with their cyclic hydroxylamine structure. Thus, isoxazolidines behave as nucleophiles and thus can be alkylated or acylated. Isoxazolidines are also strong bases (pKa 5.05), and undergo reactions such as quatemization, hydrogenolysis, oxidation, thermolysis, photolysis, decomposition by bases, etc. (84CHEC(6)i, 9iHC(49)i). [Pg.432]

It has been demonstrated that N-hydroxytryptophan can be converted to /3-carbolines in two ways (Fig. 41). Pictet-Spengler reaction of 1 with acetals provided the N -hydroxytetrahydro-/8-carbolines (2) (287). A modified Bischler-Napieralski reaction of 1 with trimethylorthoformate gave N -0X0-3,4-dihydro-/3-carbolines (3), the nitrone function of which can undergo 1,3-dipolar cycloaddition with alkenes (288) and nitriles (289), providing isoxazolidine (4) and dehydro-1,2,4-oxadiazoline (5), annulated TBCs, respectively. Nitrone 3 also was obtained by oxidation of the N-hydroxy-j8-carboline 2 with 2,3-dichloro-5,6-dicyano-l, 4-benzoquinone (DDQ). N-Oxygenated TBCs showed no affinity for the benzodiazepine and tryptamine receptors (290). Unfortunately, no toxicity data were recorded for these substituted hydroxylamines. [Pg.170]

The same methodology was employed by Tiecco to effect the cyclizations of 0-allyl oximes 164 indicated in Scheme 25. Treatment of the intermediate iminium salt 165 with sodium borohydride affords M-alkyl isoxazolidines 166 [95]. Alternatively, 165 can be treated with water to produce M-unsubstituted isoxazolidines 167 [96]. This synthesis of M-alkyl isoxazolidines represents a valid alternative to that described in Scheme 23. Moreover, the present cyclization reaction is much more stereoselective than the cyclization of 0-allyl hydro-xylamines. In fact, when the methyl derivative 168 was employed, the two isomeric isoxazolidines 153 and 154 were obtained in a ratio of 95 5. The same two products were, in contrast, formed in a 1 1 ratio when the reaction was carried out starting from the corresponding 0-allyl hydroxylamines. Clearly, the steric requirements for the selenium-induced cyclization of the 0-allyl oximes are much greater than those for the corresponding 0-allyl hydroxylamines. [Pg.35]

Diaryl-substituted isoxazolidines were synthesized in a one-pot reaction starting from aryl halides and 0-homoallyl hydroxylamines through a diastereoselective cascade reaction catalyzed by Pd(0). The choice of the phosphine ligand was shown to affect the product distribution between the isoxazolidine and the Heck-type product. The best results were obtained when 1 mol% of Pd2(DBA)3/P(o-Tol)3 was used (DBA = dibenzylideneacetone). For example, under these conditions, isoxazolidine 520 was obtained in 79% yield together with a minor amount of the Heck coupling adduct 521 (Equation 83) <2006TL927>. [Pg.449]

Dipolar cycloaddition reactions of nitrones, prepared by the oxidation of suitable hydroxylamines, have been used in a number of alkaloid syntheses. The preparation of tetrahydropyridine /V-oxides by means of the oxidative cleavage of suitable bicyclic isoxazolidines has been studied in connection with projected syntheses of... [Pg.137]

Reaction of various cyclic unsaturated aldehydes with JV-methyl-hydroxylamine can afford complex polycyclic isoxazolidines ,4°-142 e.g., from norbornenylacetaldehyde, adduct 41 is formed.140... [Pg.231]

Intramolecular cycloaddition reactions of nitrones have been used widely in synthesis. The required unsaturated nitrones can be obtained by oxidation of N-alkenyl-hydroxylamines or by condensation of an aldehyde with an A -substituted hydroxylamine. Thus the cis bicyclic isoxazolidine 212 was obtained by reaction of 5-heptenal with AT-methylbydroxylamine, by way of the intermediate ititrone 211 (3.139). [Pg.227]

Dipolar cycloaddition provides a useful and direct process to construct 5-membered heterocycles. Kobayashi et al reported that a three-component assembly of aldehydes, hydroxylamines, and electron-deficient olefins via 1,3-dipolar cycloaddition catalyzed by lanthanide triflate [38]. With 20mol% of Yb(OTf)3, the reaction proceeded smoothly to afford isoxazolidines in good to excellent yield and... [Pg.125]

The dependence of the reaction product on the existence of HfCU was also found in a reaction of nitrone (125) with methyl vinyl ketone (Equation 56) [61]. In this case, the reaction without Hf, isoxazolidine (127) was obtained in a low yield via normal [2 + 3] dipolar addition. On the other hand, the Hf-media ted reaction afforded dihydroisoxazole derivative (126) in high yield. The latter reaction would include transfer of hydroxylamine moiety of nitrone (125) into methyl vinyl ketone, and thus formed alkenylnitrone (129), which was cyclized intramolecularly to produce (130), followed by addition of benzaldehyde and deprotection of N-t-Bu group (Equation 57). [Pg.316]


See other pages where Isoxazolidines, reaction with hydroxylamine is mentioned: [Pg.85]    [Pg.48]    [Pg.60]    [Pg.58]    [Pg.70]    [Pg.356]    [Pg.1005]    [Pg.369]    [Pg.455]    [Pg.364]    [Pg.32]    [Pg.48]    [Pg.42]    [Pg.58]    [Pg.266]    [Pg.317]    [Pg.396]    [Pg.309]    [Pg.330]    [Pg.332]    [Pg.329]    [Pg.214]    [Pg.329]    [Pg.102]   
See also in sourсe #XX -- [ Pg.84 ]




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Hydroxylamine reaction

Hydroxylamines reaction

Isoxazolidine

Isoxazolidines

Isoxazolidines, reaction with

Reaction with hydroxylamine

Reaction with hydroxylamines

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