Oximes a-chloro

Treatment of a mixture of a chloro oxime 177 (R = H, Ph, 4-BrCgH4 or 2-furyl) and a diene (cyclopentadiene or 2,3-dimethylbuta-l,3-diene) with solid sodium carbonate results in the formation of a dihydrooxazine, the intermediate nitrosoalkene 178 having reacted as a heterodiene (equation 98)94. In contrast, l,l-dichloro-2-nitrosoethene and cyclopentadiene yield the epoxyepimine 179 (equation 99)94. 

Dipolar cycloadditions have been used successfully in assembling a number of these ring systems. Nitrile oxides, generated from a-chloro oximes and base, add to a wide range of aliphatic and aromatic aldehydes and ketones, but not to esters (Scheme 42), yielding... 

A review including the synthesis of 3-azetidinones has been published <03T7631>. Azacyclobutadiene (azete) is a highly reactive and unstable molecule. A base-induced novel condensation reaction of a-chloro oxime derivatives to furnish alkynyl oximes through an azacyclobutadiene intermediate has appeared <03AG(E)5613>. 

The addition of nitrosyl chloride to alkenes is a well-known reaction1 3, nevertheless the mechanistic and stereochemical aspects are still not properly understood. The course of the reaction depends on the nature of the alkene and the experimental conditions. After the initial addition step, which gives a blue or sometimes green 1 1 adduct, three competitive pathways may be followed 1) dimerization of the nitroso group to give a white crystalline compound 2) oxidation of the nitroso group to a nitro group (see Section 7.2.1.8) 3) isomerization to a-chloro oximes. 

Disubstituted piperazin-2-ones (83) were first synthesized by the reaction of a-chloro oximes (81) with esters of amino acids, followed by reductive cyclization... 

A different, interesting method of preparing a-chloro oximes was discovered recently 321 nitroolefins, dissolved in ethereal hydrogen chloride, are reduced with ice-salt cooling by ethereal stannous chloride containing HC1 ... 

Nitrile oxides are conveniently generated in situ by dehydration of primary nitro compounds (with phenylisocyanate or ethyl chloroformate or di-tert-butyl dicarbonate) or from a-chloro-oximes (by treatment with a base). The nitrile oxide reacts with an alkene to form an isoxazoUne or with an alkyne to give a heteroaromatic isoxazole (3.131). Nitrile oxides are prone to undergo dimerization, although this can be minimized by maintaining alow concentration of the dipole in the presence of the dipolarophile. 

For example, elimination of halogens from imines to give nitrilium ions is strongly facilitated by an antiperiplanar n o LG interaction (Figure 7.35). The Z-isomer of a-chloro oximes undergoes elimination -450 times faster than the F-isomer. ... 

The asymmetric a-sulfenylation of ketones is a particularly challenging reaction, as demonstrated by the poor success reported in the stereoselective variants via classical enolate/azaenolate reaction with an electrophilic sulfur reagent [71]. An umpolung approach has been devised by Coltart and co-workers [72] to effect the first asymmetric a-sulfenylation of ketones with arene thiols. Nitroso alkene derivatives, in i/tM-generated under basic conditions from a-chloro oximes, reacted with arene thiols in the presence of cinchona thiourea 27, which promoted the conjugate addition of thiophenol (Scheme 14.25). The chiral nonracemic a-sulfenylated oximes were directly hydrolyzed by IBX to ketones in high yield and good enantioselectivity. 

Et) to 3/-chiral acyclic nitrosoalkenes, such as PhCH(Me)CH=CHNO, generated in situ from the corresponding a-chloro oxime derivatives, for example, PhCH(Me)CH(Cl)CH=N-0-t-butyldimethylsilyl (NOTES), and potassium bis(trimethylsilyl)amide (KHDMS) and BU4NF, have been found to afford exclusively the anti-products.In a similar way, addition of 5-nucleophiles 0 R SH to nitroalkenes R CH=CR NO, generated in situ from cr-halo oximes R CH(X)C(R )=NOH on treatment with NaHC03, can be catalysed by chiral thioureas. The resulting a-sulfenylated oximes R CH(SR )C(R )=NOH were obtained with <82% eeP (J)... 

The synthesis of ketones from j9-keto-sulphoxides shows the comparability of these compounds with j8-dicarbonyl compounds. " Other synthetic uses include the formation of a-chloro-oximes RC(Cl)=NOH on nitrosation... 

The reaction of a-chloro-oximes or the isomeric nitrosochlorides with... 

Sterically hindered derivatives of isoxazole carboxylic acids have yielded a goodly number of antibiotics. Chlorination of the oxime of the appropriately substituted benzaldehydes (15) leads to the intermediates, ( 16. Condensation of the chloro oximes with ethyl acetoacetate in base gives the esters (17) of the desired isoxazole carboxylic acids. Alternately, the esters... 

Only in one case (498), a- chloro-substituted oxime was isolated as the only product of silylation of secondary -functionalized AN. 

Solid-state photochemistry of (—)-2-chloro-2-nitrosocamphane 275 was studied145 by irradiation of the blue-crystal with red light to invert the configuration at C(2) (equation 123). This also causes a photochemically initiated Beckmann rearrangement to form chloroxime 276 to give nitroxide radical 278 (equation 124). The intermediate chloro oxime 276 is proposed to arise from the njr excitation and is believed to be the common intermediate for the photo-epimerization and Beckmann rearrangement. Extended... 

Acetonitrile, phenyl-, 55,91, 94 Acetonitrile-18-crown-6 complex, 57, 31 Acetophenone, 58, 57, 61 ACETOPHENONE, 2-(2-ACETAMIDO-ETHYLE4,5-DIMETHOXY-, 56, 3 Acetophenone, 2-(2-acetamidoethyl)-4,5, 6-trimethoxy-, 56, 7 Acetophenone, 4-chloro, 55, 40 Acetophenone, 4-chloro-, oxime, 55, 39, 40 7-Acetoxy-4,4,6,7-tetramethylbicyclo-[4 2 0] octan-2-one, 57, 113 Acctylacetone, 58, 52, 56 Acetyl chloride, a rert-butyl-o-cyano-, 55, 38... 

Azine approach. Substituted 6,7,8,8a-tetrahydro-5H-[l,2,4]oxadiazolo[4,5-a]-pyrimidines (613) have been prepared by cycloaddition of nitrile oxides or their chloro-oxime equivalents to a tetrahydro pyrimidine (73GEP2155753). 

Nitrosoalkenes.1 Nitrosoalkenes are usually generated by reaction of a-halo oximes with bases, and have been used for [4 + 2] cycloadditions. They are also generated efficiently from a-chloro silyloximes by fluoride ion. This elimination is used to effect an intramolecular cyclization of the nitrosoalkene generated from 1 to give a mixture of the dihydrooxazines 2 and 3. The choice of the metal fluoride is critical for acceptable yields. Highest yields are obtained when the nitrosoalkene is generated slowly by a... 

The use of sulfur dioxide as the solvent and low temperatures allows efficient and clean formation of the a-chloro nitroso compound4 6. The primary adducts, containing nitroso, nitro, or oxime functions, can be reduced to chlorine-free amines or /i-chloro amines 8. Additionally, chlorine readily undergoes substitution by nucleophiles (hydrides, alcohols, amines, etc.), such that halogen-free and /i-functionalized nitroso and nitro compounds, which can further be reduced to amines, can be prepared by this route. Recently, the conversion of nitroso compounds (RNO) to amines (RNHEt) using a triethylborane/borane mixture has been described9 10, although not applied specifically to /i-chloro nitroso compounds. 

Indoles have been prepared from reactions of o-aminophenylketones with reactive , or stable " arsonium ylides. Oxo-stabilized ylides reacted with 2-chloro-oximes to give trans-5-acyl-A -isoxazolines, and isoxazoles have been obtained from reactive arsonium ylides and a-isonitrosoketones, and from triphenylarsonium methylide and nitrile oxides The latter ylide reacts similarly with nitrile imines to give pyrazoles. With triphenylarsonium benzylides and benzoylylides,benzene diazonium salts give 1,3,4,6-substituted 1,4-dihydro-1,2,4,5-tetrazines in a reaction in which initial coupling of the reagents is followed by a dimerisation. ... 

Use is made of the facile hydrogenation of nitro groups to prepare a-hydroxy primary amines from nitromethane-ketone addition products (Eqn. 19.22). The addition of a little acetic acid to the reaction medium can sometimes improve the yield. The p-hydroxyamines are more easily prepared in this way than by the hydrogenation of the corresponding cyanohydrin. a-Chloro nitro compounds are selectively hydrogenated to the oxime over palladium under... 

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