Nitrones deoxygenation

Other experiments suggest that nitrone deoxygenation proceeds through intermediate formation of nitrone RC, rather than the product of its isomerization, a oxaziridine (441). 

Nitrones can be reduced to hydroxylamines (route A), or deoxygenated to the corresponding imines (route B) (Scheme 2.98). 

Apparently, the reaction involves cycloaddition of nitronate at the C=S double bond of thioketene. This approach can be useful for deoxygenation of labile nitronates. 

The reactions of ammonia or primary amines with five-membered cyclic nitronates containing the EWG -group at the C-5 atom involve deoxygenation of the nitronate fragment, aromatization of the ring, and amidation of the ester... 

Deoxygenation of Cyclic Nitronates Since cyclic nitronates are readily available and are rather stable compounds (see Sections 3.2 and 3.3), simple approaches for their deoxygenation with retention of the configuration of stereocenters can be used for the synthesis of cyclic esters of functionalized oximes. 

The main procedure for deoxygenation of similar nitronates is based on their reaction with trialkyl phosphites (Scheme 3.114). These reactions readily proceed with live- (45, 55, 320, 321) and six- membered (143) cyclic nitronates. 

It should be noted that the configurations of the substituents in the starting cyclic nitronates are retained upon deoxygenation as well. 

Trost and coworkers (71) used tin dichloride for deoxygenation of annelated five-membered cyclic nitronates (140a,b) with retention of stereocenters (Scheme 3.115). 

As mentioned above (303) (Scheme 3.90, Eq. 1), bis-trifluoromethyl thioketene has the deoxygenating ability toward alkyl nitronates, which is also based on cycloaddition to the C=S bond. 

Attack on Oxygen. Nitrones have been deoxygenated to the parent imine with trimethyl phosphite under vigorous conditions.45 Virtually quantitative yields of the enol-phosphates (54) and (55) were obtained from the reaction of tris(trimethylsilyl) phosphite with a-diketones and /j-benzoquinone. 4 6... 

Nitrones and amine A-oxides are deoxygenated by a stoichiometric amount of benzyltriethylammonium tetrathiomolybdate to yield imines and amines, respectively (>80%) [7]. Nitro groups and sulphoxides are not reduced under these reaction conditions. 

Deoxygenation. Hexachlorodisilane is useful for deoxygenation of nitrones and various N-oxides in good yields under exceptionally mild conditions (25° or below).1 It is recommended for deoxygenation of 2,3-disubstitutcd quinoxaline 1,4-dioxides under mild conditions yet at reasonable rates.2... 

The completely reduced oxadiazole ring (26) was recently reported116 as an air oxidation product of a nitrimine. Presumably it was formed by a 1,3-dipolar addition of the intermediate nitrimine nitrone (25) to the nitrimine. The nitrimine results from the deoxygenation of a gem-nitrosonitro compound by triethyl phosphite [Eq. (31)]. 

Treatment of the conanine (372) with bromine-aqueous sodium carbonate yields192 (100%) the conenium salt (375) which is readily converted into (376) (80%). Compound (376) is also formed starting from (373). When C-20 is substituted by two methyl groups as in (374) the product is (377). The conanine (373) is photo-oxygenated,193 in the presence of methylene blue, to (376). The nitrone (378) is quantitatively deoxygenated by triphenyl phosphite with trimethyl phosphite the reaction is more complex.194... 

Deoxygenation of a nitrone. Some years ago Todd and co-workers noted that 1-pyrroline-1-oxides can be obtained by reductive cyclization of a y-nitro ketone. This reaction was used recently in a synthesis of chlonns related to vitamin B,. Thus the nitro ketone 1 on reduction with zinc and acetic acid gives the pyrroline N-oxide 2 in high yield. The N-oxide is stable to triphenylphosphiue or trimethyl phosphite, but is reduced to the imine 3 in high yield by TiCl,. 

Deoxygenation of Nitrones, Nitrile Oxides and Tertiary Amine Oxides... 

Methods of deoxygenation of nitrones (28), nitrile oxides (29), heteroaromatic N-oxides (30) and tertiary amine oxides (31) are described in this section. There are some reagents, such as trialkyl phosphites, which can deoxygenate compounds of all these types as well as those in the preceding section, whereas others are more limited in scope. Oae and coworkers have outlined three distinct mechanistic types of deoxygenation process, which are illustrated in Scheme 17. Clearly, a mechanism of type C will not apply to tertiary amine oxides (31) on the other hand, these compounds are more easily deoxygenated than heteroaromatic N-oxides, such as (30), by some reagents because the aromatic N-oxides are inherently more stable. 

The reaction of 619 with C-nitroso compounds gives azo- or azoxy compounds with A -nitroso compounds one obtains phosphorus heterocycles (e.g., 589) or elimination of HNO, and deoxygenated products are formed with nitrones or A -oxides. ° p-Nitroso-A A -dimethylaniline yields the unusual, purple 631. Sulfoxides are deoxygenated by 619, although some disulfide is formed. ... 

Reduction of 2,2,3,3,5,6-hexamethyl-2,3-dihydropyrazine 1,4-dioxide (78) with sodium borohydride gave l,4-dihydroxy-2,2,3,3,5,6-hexamethylpiperazine (555) and attempted deoxygenation of (78) by zinc and hydrochloric acid gave the bishydroxylamine (79) (555). Compound (78) did not form benzylidene derivatives with benzaldehyde (555) and it was only slowly attacked by selenium dioxide one equivalent of selenium dioxide required 5 days boiling under reflux before the nitrone was completely reacted, and the product was thought to be 6-formyl-... 

---