Aminoacetone (AA) is a threonine and glycine catabolite long known to accumulate in cri-du-chat and threoninemia syndromes and, more recently, implicated as a contributing source of methylglyoxal (MG) in diabetes mellitus. Oxidation of AA to MG, NH(4)(+), and H(2)O(2) has been reported to be catalyzed by a copper-dependent semicarbazide sensitive amine oxidase (SSAO) as well as by Cu(II) ions. We here study the mechanism of AA aerobic oxidation, in the presence and absence of iron ions, and coupled to iron release from ferritin. Aminoacetone (1-7 mM) autoxidizes in Chelex-treated phosphate buffer (pH 7.4) to yield stoichiometric amounts of MG and NH(4)(+). Superoxide radical was shown to propagate this reaction as indicated by strong inhibition of oxygen uptake by superoxide dismutase (SOD) (1-50 units/mL; up to 90%) or semicarbazide (0.5-5 mM; up to 80%) and by EPR spin trapping studies with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), which detected the formation of the DMPO-(*)OH adduct as a decomposition product from the DMPO-O(2)(*)(-) adduct. Accordingly, oxygen uptake by AA is accelerated upon addition of xanthine/xanthine oxidase, a well-known enzymatic source of O(2)(*)(-) radicals. Under Fe(II)EDTA catalysis, SOD (<50 units/mL) had little effect on the oxygen uptake curve or on the EPR spectrum of AA/DMPO, which shows intense signals of the DMPO-(*)OH adduct and of a secondary carbon-centered DMPO adduct, attributable to the AA(*) enoyl radical. In the presence of iron, simultaneous (two) electron transfer from both Fe(II) and AA to O(2), leading directly to H(2)O(2) generation followed by the Fenton reaction is thought to take place. Aminoacetone was also found to induce dose-dependent Fe(II) release from horse spleen ferritin, putatively mediated by both O(2)(*)(-) and AA(*) enoyl radicals, and the co-oxidation of added hemoglobin and myoglobin, which may be viewed as the initial step for potential further iron release. It is thus tempting to propose that AA, accumulated in the blood and other tissues of diabetics, besides being metabolized by SSAO, may release iron and undergo spontaneous and iron-catalyzed oxidation with production of reactive H(2)O(2) and O(2)(*)(-), triggering pathological responses. It is noteworthy that noninsulin-dependent diabetes has been frequently associated with iron overload and oxidative stress.

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A adição de aliltrimetilsilano, promovida por TiCl 4 , a íons N-aciliminios cíclicos de 5-e 6-membros derivados do ácido (S)-(+)-mandélico, (1R,2S)-trans-2-fenil-1-cicloexanol e (1R,2S,5R)-8-fenilmentol ocorreu com baixas a moderadas razões diastereoisoméricas (1:1-6:1) e forneceu as respectivas amidas e carbamatos em bons rendimentos. A melhor diastereosseleção facial foi observada com o uso de (1R,2S,5R)-8-fenilmentol como auxiliar quiral. As amidas e carbamatos 2-substituídos foram convertidos nos alcalóides (S)-e (R)-propil pirrolidina e coniina com eficiente recuperação dos auxiliares quirais.The TiCl 4 -promoted addition of allyltrimethylsilane to chiral 5-and 6-membered N-acyliminium ions employing (S)-(+)-mandelic acid, (1R,2S)-trans-2-phenyl-1-cyclohexanol and (1R,2S,5R)-8-phenylmenthol derivatives as chiral auxiliaries occurred with low to moderate diastereoisomeric ratios (1:1-6:1) to afford 2-substituted amides and carbamates in good yields. The best diastereoselection was observed with (1R,2S,5R)-8-phenylmenthol as the chiral auxiliary. The 2-substituted amides and carbamates were converted to the corresponding alkaloids (S)-and (R)-propyl pyrrolidine and coniine with efficient recovery of the chiral auxiliaries.

Keywords: dimethyldioxirane; oxidation of sulfur compounds; cyclic peroxides. REVISÃO INTRODUÇÃODioxiranos 1 (Fig. 1), peróxidos cíclicos de 3 membros 1 , são oxidantes bastante seletivos e suaves frente aos produtos de oxidação. Facilmente preparados a partir de substâncias comercialmente disponíveis esses oxidantes não constituem uma ameaça ao meio-ambiente.Durante muito tempo dioxiranos foram postulados como intermediários em reações de oxidações, com pouca ou nenhuma evidência de sua existência. A dicotomia existente entre dioxiranos e os seus isômeros óxidos carbonílicos 2 ( Fig. 1) dificultou a reunião de evidências não ambíguas da participação de dioxiranos em reações químicas.Cálculos teóricos 5-9 e dados espectroscópicos 10-15 estabeleceram inequivocadamente que dioxiranos 1 e óxidos carbonílicos 2 são espécies isoméricas separadas por uma elevada barreira energética. Bach e colaboradores 6,7 , através de cálculos ab initio, encontraram para a interconversão do dioxirano 1a (R 1 = R 2 = H) no respectivo óxido carbonílico 2a uma barreira energética na faixa de 29,7 a 54,1 kcal.mol -1 . Empregando-se o nível QCISD(+)/ 6-31G*//MP2/6-31G* eles observaram que a energia de ativação para a epoxidação do etileno é de 16,7 kcal.mol -1 para 1a e de 11,9 kcal.mol -1 para 2a. PREPARAÇÃO DE DIOXIRANOSOs métodos mais eficientes e mais práticos para se obter Figura 1Já em 1899, Baeyer e Villiger propuseram que um dioxirano poderia ser o intermediário formado na oxidação da mentona 3 à lactona 4 através do emprego do ácido peroximonossulfúrico (ácido de Caro; Esquema 1) 2 . Posteriormente, contrariando a proposição de Baeyer e Villiger, Doering e Dorfman 3 mostraram, através de estudos com 18 O, que a oxidação de cetonas por peroxiácidos não envolvia a formação de dioxirano como intermediário.

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