Allenic oximes

As a result of Ag (I) catalyzed cyclization of allenic oximes (124) and (125), stable five- (126) and six-membered (127) cyclic nitrones were obtained (Scheme 2.46) (298a). Recently, a novel method of pyrroline-type nitrone formation via P-allenyl-oxime cyclization has been described (298b). 

The analogous transformation of 125, also realized by flash vacuum pyrolysis, gave rise to allenic oximes 126 [165], which are not directly accessible by the classical route starting from allenyl ketones and hydroxylamine (see Section 7.3.2) [122], Because compounds 125 are prepared from allenyl ketones and furan by [4 + 2]-cycloaddition followed by treatment with hydroxylamine, the retro-Diels-Alder reaction 125 —> 126 is in principle the removal of a protecting group (see also Scheme 7.46). 

Silver ion catalysis has also been used with allenic oximes to generate cyclic nitrones, which are trapped by 1,2-dipolarophiles (equation 142).270 This reaction was used in a synthesis of pyrrolizidine alkaloids.270 5... 

Treatment of the oxime 63 with methyl lithium at -20° gave 2,3-dimethyl-1-hydroxy pyrrole (65) in 50% yield, along with 20% of the allene oxime (64), which is the product isolated from this type of reaction when the cyclopropene contains additional alkyl groups (80TL2893). 

This transformation has been less widely studied. We chose two publications from Gallagher s team to show a few examples.69,70 Allenic oximes 40 (Scheme 5.19) were treated with AgBF4 to produce cyclic nitrones 41. Since these nitrones were not sufficiently stable, they were trapped with 1,3-dipolarophiles to give bicyclic or... 

The preparation of vinyl nitrones by catalytic cyclization of allenic oximes has been demonstrated. Oxime 75, prepared from 5,6-heptadienal, was cyclized to nitrone 76 in the presence of silver(l) tetrafluoroborate. This was trapped by, for example, styrene to give 77. Such a compound again opens up a route to 2,6-disubstituted piperidines (85TL6249). 

The allenic oxime (135) undergoes a silver-catalyzed cyclization to form the nitrone (136) (Equation (10)) which may be trapped as the dipolar adduct (137) with vV-methylmaleimide <91JCS(Pl)659>. Azepinonitrones have also been prepared by the spontaneous intramolecular cyclization of alky-nylhydroxylamines, prepared by the reduction of the corresponding oximes <92TL742l, 92TL7425>. 

Protection of the allenic group, by formation of its Diels-Alder adduct with furan, enables otherwise incompatible chemical modification of the a-allenic substituents to be performed. Thermal regeneration of the allenic moiety at 450—510 C completes the synthesis of novel substituted allenes, such as the hitherto unknown a-allenic oximes. 

Methanesulfonates. The most common use of methanesulfonyl chloride is for the synthesis of sulfonate esters from alcohols. This can be readily accomplished by treatment of an alcohol with mesyl chloride in the presence of a base (usually Triethy-lamine or Pyridine). The methanesulfonates formed are functional equivalents of halides. As such they are frequently employed as intermediates for reactions such as displacements, eliminations, reductions, and rearrangements. Selective mesylation of a vicinal diol is a common method of preparation of epoxides." Alkynyl mesylates can be used for the synthesis of trimethylsilyl allenes. Oxime mesylates undergo a Beckmann rearrangement upon treatment with a Lewis acid. Aromatic mesylates have been used as substrates for nucleophilic aromatic substitution. Mesylates are more reactive than tosylates toward nucleophilic substitution, but less reactive toward solvolysis. 

In an attempted synthesis of papaverine based on Rugheimer s preparation of 6 7-dimethoxyisoquinoline from veratrylaminoacetal (McO)2—CjHg—NH—CH2(OEt)2, Allen and Buck reduced deoxy-veratroin oxime, (MeO)2—CgHg—C(NOH)—CHj—C6H3(OMe)2, to a 3-di-(3 4-dimethoxy)-phenylethylamine,... 

The Doering-Moore-Skattebol method including a cyclopropylidene-allene rearrangement is often used for the synthesis of allenes. However, the reaction conditions applied are often not compatible with acceptor substituents. One of the rare exceptions is the transformation 76 —> 77 (Scheme 7.11) [122]. The oximes 77 are not accessible by the classical route starting from allenyl ketone and hydroxylamine (see Section 7.3.2). 

Whereas the reactions of allenephosphonates 171 (R2 = OEt) with primary aliphatic and aromatic amines 172 and the reactions of the phosphane oxides 171 (R2 = Ph) with aliphatic amines 172 afford the conjugated addition products 173 always in good yields, the addition of anilines to 171 (R2 = Ph) leads to an equilibrium of the products 173 and 174 [231]. However, treatment of both phosphane oxides and phos-phonates of type 171 with hydroxylamines 172 (R3 = OR4) yields only the oximes 174 [232, 233]. The analogous reaction of the allenes 171 with diphenylphosphinoylhy-drazine furnishes hydrazones of type 174 [R3 = NHP(0)Ph2] [234],... 

Particularly good yields of the cydoadduct 329 are obtained if R1 = R2 = H is valid for the allenyl ketone 328 [165]. The Diels-Alder products 329 can undergo many chemical transformations, for example to the oximes 330, which yield the modified allenes 331 after a subsequent flash vacuum pyrolysis. The oximes 331 generated by retro-Diels-Alder reaction are not available from ketones 328 and hydroxylamine hydrochloride directly [122] (see also Scheme 7.19). 

The molecular geometries of oximes in crystals are quite similar to those in the gas phase. The central carbons are planar. The C=N double bonds are primarily between 1.27 and 1.29 A in length (Figure 12) and the C—N—O angles are mostly 110°-114° (Figure 13). These histograms, which combine aldoximes and ketoximes, are based on data taken from Bertolasi and coworkers . For comparison, Allen and coworkers give the mean C=N distance in a collection of 67 crystalline oximes as 1.281 A, with a median of 1.280 A and standard deviation 0.013 A. ... 

Benzofuranyl)pyrroles, 2-(2-thienyl)pyrroles , 2,2 -dipyrroles, 3-(2-pyr-rolyl)indoles , 2-(2-benzimidazolyl)pyrroles and2-(2-, 3- and4-pyridyl)pyrroles were prepared using this method. Reaction of alkynes (for example, propyne) or allene with ketoximes in a superbase system (MOH/DMSO) leads to 2,5-di- or 2,3,5-trisubstituted pyrroles Pyrroles and dipyrroles were synthesized also from corresponding dioximes and acetylene in a KOH/DMSO system It has also been shown that 1,2-dichloroeth-ane can serve as a source of acetylene in pyrrole synthesis. Oxime 52 in the system acetylene/RbOH/DMSO at 70 °C afforded a mixture of three pyrroles 53-55 in low yields (equation 23). The formation of product 53 occurred through recyclization of pyrrolopy-ridine intermediate. ... 

Complexes involving oxime ligands display a variety of reactivity modes that lead to unusual types of chemical compounds. As far as the oxime chemistry of platinum is concerned, these complexes are involved in facile deprotonation of the OH group with formation of oximato complexes, reduction of Pt(IV) species, Pt(II)-assisted reactions with coordinated allene," alkylation by ketones, oxime-ligand-supported stabilization of Pt(III)—Pt(III) compounds, oxidative conversion into rare nitrosoalkane platinum(II) species, and coupling with organocyanamides. ... 

The corresponding nitrogen-containing heterocycles are accessible by silver-catalyzed cyclization of allenic amines, amides, or oximes.334 Thus, Dieter and Yu335 have reported the efficient transformation of various a-aminoallenes 397 or 399 into mono- or bicyclic 3-pyrrolines 398 or 400 in the presence of AgN03 (Scheme 116). Unfortunately, a rather high catalyst loading was required. 

Compounds related conceptually to allenes, in which one double bond is replaced by a suitably substituted ring, e.g, 4-chlorocyclohexanone oxime, are also chiral, and again do not possess a stereogenic centre. 

Methanol and propan-2-ol add non-stereospecifically to cholest-4- and -5-ene as do ethylene and tetrafluoroethylene to 3 -acetoxypregna-5,16-dien-20-one although only 16a,17a-addition products were isolated from allene, acetylene, and dichloroethylene the derived 20-anti-oxime isomerises to the syn-isomer on irradiation in THF and cyclises internally in benzene solution. Irradiation of the trans-acid (561) in methanol resulted only in a cis-trans equilibration, although the trans-acid (562) produced the unsaturated lactone (563). 

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