Nitrones reactions with phosphites

Nitrone reactions with lithiated dialkyl phosphite results in the formation of a-(hydroxyamino)phosphonate, which, depending on the character of the R substituent at a-C lead to a-phosphorylated nitrones (429) or fS-phosphorylated nitroxyl radicals (430) after successive oxidations (Scheme 2.200) (686—689). 

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. 

The reaction of the nitrone (23) with alkyl phosphites has been studied. With trimethyl phosphite in refluxing MeOH (23) afforded a mixture of the epimers (32), whereas in AcOH (33) was obtained. It was shown that (33) was not formed from (32) by loss of MeOH. With triethyl phosphite analogous products were formed, but in this case a large amount of the deoxygenated compound (34) was also obtained. With trimethyl phosphite the formation of an immonium structure, reducible to conanine (35), was suggested by the presence in the n.m.r. spectrum of the reaction mixture of a N—Me group. The structures of these compounds were deduced from their spectral data. 

No reactions occur between dialkyl phosphite salts and the A -glycosyl-C-arylnitrones 366 (R = Ph or substituted phenyl) On the other hand, reactions with tris(trimethylsi-lyl) phosphite do take place in the presence of a Lewis acid (ZnCl2, HCIO4) and proceed smoothly. Some unusual results are to be observed, particularly in relation to the nature of the catalyst otherwise, the general course of the present reaction is similar to that found for reactions of the nitrones 366 with dialkyl phosphite anions. Thus, the reaction between... 

The hydroxy-nitrone (8a) gave the isomeric hydroxy-nitrone (9) on treatment with acid. The latter hydroxy-nitrone underwent base-catalysed deuteriation at C-21, presumably via the dihydroxy-enamine tautomer. Treatment of either hydroxy-nitrone with methanolic sulphuric acid gave the unstable methoxy-nitrone (10), which yielded the dimeric compound (11) on attempted crystallization. The reaction of nitrone (8b) with dialkyl phosphite derivatives was reported earlier details of this study have now been published. ... 

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... 

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,. 

Silicon Lewis acids are useful for the C-heteroatom bond-forming reaction of nitrones. For instance, the t-BuMe2SiOTf-promoted reaction of nitrones with diethyl phosphite realizes an efficient synthesis of a-amino phosphonates [146]. 

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