RESUMEN
The SmI(2)-mediated radical coupling reactions of ß-hydroxylated pyrrolidine/piperidine aza-hemiacetals 8 and 9 and N,S-acetals 6 and 33 with α,ß-unsaturated compounds are described. This method allows a rapid access to ß-hydroxylated pyrrolidines, piperidines, pyrrolizidinones, and indolizidinones. Starting from N,S-acetal 33 and via a common intermediate 27, the alkaloids hyacinthacine A(2) (2), uniflorine A (3, 6-epi-casuarine), and the unnatural epimer 7-epi-casuarine (37) have been synthesized in four and five steps with overall yields of 34%, 16%, and 13%, respectively. The radical mechanism of the coupling reactions has been confirmed by controlled experiments, which also allowed deducing the anionic mechanism in the coupling between N,S-acetal 6 and carbonyl compounds. This demonstrates that the mechanisms of these SmI(2)-mediated reactions are switchable from Barbier-type anionic to radical by cooperative action of BF(3)·OEt(2) and t-BuOH.
Asunto(s)
Acetales/química , Alcaloides/síntesis química , Compuestos Aza/química , Indolizinas/síntesis química , Yoduros/química , Pirroles/síntesis química , Alcaloides de Pirrolicidina/síntesis química , Samario/química , Radicales Libres/química , Hidroxilación , Estructura Molecular , EstereoisomerismoRESUMEN
The synergistic action of BF(3).OEt(2) and SmI(2) allowed a series of intermolecular cross-couplings of readily available N-acyl N,O-acetals with alpha,beta-unsaturated compounds to be performed in high yields, which was applied to the stereoselective synthesis of pyrrolizidine alkaloid (+)-xenovenine.
Asunto(s)
Acetales/síntesis química , Acetales/química , Boranos/química , Yoduros/química , Alcaloides de Pirrolicidina/síntesis química , Alcaloides de Pirrolicidina/química , Samario/química , EstereoisomerismoRESUMEN
PURPOSE: To observe the formation of canal aberrations in S-shaped root canals prepared by every file of hand-used ProTaper. METHODS: Fifteen S-shaped simulated resin root canals were selected. Each root canal was prepared by every file of hand-used ProTaper following the manufacturer instruction. The images of canals prepared by S1, S2, F1, F2 and F3 were taken and stored, which were divided into group S1, S2, F1, F2 and F3. One image of canal unprepared was superposed with the images of the same root canal in these five groups respectively to observe the types and number of canal aberrations, which included unprepared area, danger zone, ledge, elbow, zip and perforation. SPSS12.0 software pakage was used for Fisher's exact probabilities in 2x2 table. RESULTS: Unprepared area decreased following preparation by every file of ProTaper, but it still existed when the canal preparation was finished. The incidence of danger zone, elbow and zip in group F1 was 15/15, 11/15, 4/15, respectively, which was significantly higher than that in group S2(2/15,0,0) (P<0.001). Ledge appeared after prepared by F2, and increased sharply in group F3. None perforation was found in all groups. CONCLUSIONS: The incidence of canal aberrations begins to increase after prepared by finishing files of ProTaper.The presence of unprepared area suggests that it is essential to rinse canal abundantly during complicated canal preparation and canal antisepsis after preparation.