Deoxygenation of nitrone

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... 

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. 

Deoxygenation of Nitrones and AT-Oxides. Deoxygenation of nitrones and A-oxides is a valuable transformation in organic... 

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

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. 

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

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)]. 

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,. 

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-... 

Tervalent phosphorus compounds (phosphorous derivatives) are weil-established reagents for the deoxygenation of amine oxides and nitrones, and they provide a clean and convenient method for the preparation of furazans from furoxans. Mukaiyama et al.425 and Grundmann426 used trialkylphosphites, and they have been widely employed since that time, both with and without solvents.80,85,93,95,97,352 The simple dialkyl- and diarylfuroxans usually require more vigorous conditions (e.g., refluxing triethyl phosphite) than do strained derivatives or the benzofuroxans.85,97... 

Stereoselective cycloaddition between functionalized six-membered cyclic nitrone 166 derived from o-lyxose and sugar alkenes was employed for the synthesis of aza-C-disaccharides [72]. Reaction of nitrone 166 and alkene 167 prepared from methyl a-o-mannopyranoside in toluene at reflux led to the isolation of a crystalline cycloadduct 168 in 84% yield. The isoxazohdine 168 was acetylated and cleaved using Mo(CO)6. This was followed by deoxygenation and routine deprotection to give the aza-C-disaccharide 169, isolated as its hydrochloride (44% overall from 168). 

Similarly, secondary nitroalkanes (37) can be reduced to ketoximes (38) by treatment of the corresponding nitronate anions with hexamethyldisilane through a 1,2-elimination process (eq 19). The conversion of nitrones (39) to imines (40) is also performed by using trimethylsilyllithium, which is prepared in situ from hexamethyldisilane and MeLi (eq 20). Under the same conditions, heterocyclic A -oxides (41) can be deoxygenated to the corresponding A -heterocycles (42) in 73-86% yields (eq 21). ... 

The deoxygenation of certain sterically hindered aromatic compounds has been studied, contrasting results being obtained. 2,5-Di-t-butylnitrosobenzene with TEP gives a complex mixture containing 2,5-di-t-butylaniline (127), 2,5,2, 5 -tetra-t-butyl-azobenzene (128), N-(2,5-di-t-butylphenyl)-a-t-butyl-a-2-(5-t-butylpyridyl) nitrone (129), and diethyl N-(2,5-di-t-butylphenyl)-phos-phoramidate (130), while the similar reaction of 2,4,6-tri-t-butylnitrosobenzene yielded mainly 3,3-dimethyl-5,7-di-t-butyl-dihydroindole (131) together with small amounts of 2,4,6-tri-t-butylaniline (132) and 2-(2-methylallyl)-4,6-di-t-butylaniline (133). 

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