Oximes regioselectivity

A process for the preparation of pyrimidine A-oxides from carboxamide oximes has been developed (98T4387). Direct oxidation of pyrimidines is inconvenient for the preparation of nonsymmetric substituted pyrimidines because it gives a mixture of oxidation products ( -N- and 3-A-oxides). Methoxybutenone with carboxamide oximes regioselectively forms A-oxides of pyrimidines 281, which indicates... 

Regioselective Beckmann rearrangements were used as key steps in the synthesis of phosphonoalkyl azepinones (Scheme 36) [43b] and in a formal total synthesis of the protein kinase C inhibitor balanol (Scheme 37) the optically active azide 197 derived from cyclohexadiene mono-oxide was converted into ketone 198 in several steps. After preparation of the oxime tosylates 199 (2.3 1 mixture), a Lewis acid mediated regioselective Beckmann rearrangement gave the lactams 200 and 201 in 66% and 9% yield, respectively. Lactam 201 underwent a 3-e im-ination to give additional 200, which served as a key intermediate in a balanol precursor synthesis (Scheme 37) [43 cj. 

Alkyl Co oxime complexes have been used as chain transfer catalysts in free radical polymerizations.866,867 Regioselective hydronitrosation of styrene (with NO in DMF) to PhCMe=NOH is catalyzed by Co(dmg)2(py)Cl in 83% yield.868,869 Catalytic amounts of the trivalent Co(dmg2tn)I2 (192) (X = I) generate alkyl radicals from their corresponding bromides under mild reaction conditions, allowing the selective preparation of either saturated or unsaturated radical cyclization products.870... 

Hydrazones are also useful substrates in the preparation of pyrazoles. Reaction of N-monosubstituted hydrazones with nitroolefins led to a regioselective synthesis of substituted pyrazoles <060L3505>. lf/-3-Ferrocenyl-l-phenylpyrazole-4-carboxaldehyde was achieved by condensation of acetylferrocene with phenylhydrazine followed by intramolecular cyclization of the hydrazone obtained under Vilsmeier-Haack conditions <06SL2581>. A one-pot synthesis of oxime derivatives of l-phenyl-3-arylpyrazole-4-carboxaldehydes has been accomplished by the Vilsmeier-Haack reaction of acetophenone phenylhydrazones <06SC3479>. 

Kondo and Watanabe developed allylations of various types of aldehydes and oximes by using nucleophilic (7r-allyl)ruthenium(ll) complexes of type 154 bearing carbon monoxide ligands (Equation (29)).345 These 73-allyl-ruthenium complexes 154 are ambiphilic reagents and the presence of the carbon monoxide ligands proved to be essential to achieve catalytic allylation reactions. Interestingly, these transformations occur with complete regioselectivity only the more substituted allylic terminus adds to the aldehyde. 

Although oximes are ambident nucleophiles, a neat regioselectivity allows the formation of oxazolinones. The alpha effect, which is especially relevant for the substimtion of R2C=NO anions at a carbonyl carbon, may offer a good explanation for the Bac2/Bai2 sequence ((9-methoxycarbonylation and A-methylation... 

Cycloadditions with monosubstituted oleflns proceed rapidly and regioselectively to yield the 5-substituted dihydroisoxazoles. Thus, addition of styrene to nitrile oxide 164, formed from oxime 163, results in the formation of the 5-phenyl- 165 and 4-phenyldihy-droisoxazoles 166 in a 99 1 ratio (equation 72) ° °. ... 

Fluoride ion catalyzed 1,3-dipolar cycloaddition of bromo nitrile oxide, obtained in situ from dibromoformaldehyde oxime 184, to nonactivated alkynes provides a new approach to the synthesis of neuroactive isoxazoles. However, the regioselectivity of cycloaddition in this case is not high—products 185 and 186 are obtained in a 1 1 to 1 1.4 ratio (equation 80). Cycloaddition reaction of hydroximoyl chlorides and acetylene was snc-cessfully carried out also in the presence of NaHCOs as a base. For instance, a-keto oximes 187 were reacted with acetylene and NaHCOs to give isoxazoles 188 in good yields (equation 81). 

Beckmann rearrangement of the oxime from 3-phosphonoalkylcyclohexenones 401 afforded lactams and regioselective migration of the non-unsaturated substituent was observed (equation 164). 

Regioselective oxidation by brominolysis of dioxastannolane obtained from methyl / -L-ara-binopyranoside made it possible to accomplish (after subsequent oximation and reduction) a facile synthesis of 4-amino-4-deoxy-L-arabinose, an amino-sugar found in lipopolysaccharides of some Salmonella R. mutant strains <83CPB3778>. 

This compound is easily available by the pyrolysis of D-galactose [25] or, on the small scale, by Angyal and Beveridge s method [26]. Regioselective catalytic oxidation led to the corresponding 5-ulose which, via the oxime and subsequent reduction, could be converted into inhibitor (5) in approximately 6% overall... 

One exception is the reaction of acetone oxime with divinyl ketone in the presence of an equimolar amount of zinc(II) bromide (162). Acetone oxime reacts with divinyl ketone on heating in THE at reflux, leading to both conjugate addition and nitrone cycloaddition, producing a 5 1 mixture of regioisomers with 8-oxa-l-azabicyclo[3.2.1]octan-4-one as the major isomer (Scheme 11.42). On the other hand, in the presence of an equimolar amount of zinc(II) bromide, 7-oxa-l-azabicyclo[3.2.1]octan-4-one is the major isomer (97 3) in a total yield of 97%, indicating that the Lewis acid has controlled the regioselectivity of the second step, namely, the cycloaddition. 

The reaction of aliphatic and aromatic ketone oximes 97 with a dialkyl carbonate 98 in the presence of K2CO3 at 180-190 °C yields 3-alkyl-4,5-disubstituted-2(3//)-oxazolones 104 in 22-48% yields. Mechanistically, it is proposed that N-alkylation of the initially formed oxime O-carbonate 99 yields 100, which affords the enamine 101 in the presence of base. A [3,3] sigmatropic rearrangement ensues to produce 102, which then cyclizes to 104. In cases where 97 contains two methylene groups in proximity to the C=N bond, one of which is benzylic, the above reaction sequence is regioselective for the benzylic methylene group (Fig. 5.25 Table 5.6, Fig. 5.26). ... 

As shown in Table I, this reaction sequence has wide generality and is readily applicable to the straightforward synthesis of various naturally occurring alkaloids such as coniine,9 pumiliotoxin C,11 1 and solenopsin A and B.11 Oxime sulfonates of either linear or cyclic structures may be used. Obviously, the regioselectivity of the reaction follows the general rule of... 

Regioselective alkylative opening of a,(1-epoxy ketone oxime [40] and tosylhydra-zones [41] with organocopper reagents is initiated by the generation of unsaturated azo compounds which undergo conjugate addition. 

---