4 research outputs found
A synthetic approach to palmerolides via Negishi cross coupling. The challenge of the C15-C16 bond formation
The esterification of fragment C1-C8 (2) with fragment C16-C23 (3) to give iodo derivative 4, followed by a Pd-catalysed coupling with a C9-C15 fragment (7 or 8), may provide a common precursor of most palmerolides. Ligands and reaction conditions were exhaustively examined to perform the C15-C16 bond formation via Negishi reaction. With simple models, pre-activated Pd-Xantphos and Pd-DPEphos complexes were the most efficient catalysts at RT. Zincation of the C9-C15 fragment (8) and cross coupling with 4 required 3 equiv of t-BuLi, 10 mol % of Pd-Xantphos and 60 °C
Estudi de reaccions catalitzades per lâoxazolidinona de Seebach i per aurats de tipus Na[AuBrx(OH)y]
[cat] En la present Tesi sâhan abordat dos camps de gran actualitat en la QuĂmica OrgĂ nica actual, lâorganocatĂ lisi i la catĂ lisi promoguda per complexos dâAu(III). En ambdĂłs blocs sâhan orientat els esforços en la recerca de nous mĂštodes sintĂštics o en aprofundir en els ja existents a fi de millorarâlos; sense perdre de vista en cap cas la seva aplicabilitat a casos sintĂštics reals.
En el primer Bloc sâha tractat de solucionar el problema de la lentitud de moltes de les transformacions organocatalĂques. Sâha trobat com la oxazolidinona derivada de la condensaciĂł de la Lâprolina amb el pivalaldehid (oxazolidinona de Seebach, dâara endavant SolPro) permet dur a terme reaccions aldĂČliques entre cetones i aldehids en tan sols dues hores, millorant significativament molts dels temps de reacciĂł descrits en la literatura. Els estudis per RMN de 1H van permetre de determinar que el procĂ©s clau era el bescanvi del pivalaldehid amb la cetona corresponent. TambĂ© es van realitzar una sĂšrie de cĂ lculs teĂČrics a fi dâesbrinar quin o quins podien ser els mecanismes implicats en aquests bescanvis. En el darrer capĂtol del Bloc 1 sâha avaluat lâactivitat catalĂtica de la SolPro en la reacciĂł de Michael entre nitroalcans i cetones alfa,betaâinsaturades, trobantâse que tambĂ© lâaccelera perĂČ no de manera tan espectacular com en el cas de lâaldĂČlica.
En el segon Bloc sâhan estudiat diverses transformacions que involucren, directa o indirectament, el grup nitro. En el capĂtol 2.2 sâhan preparat diverses Nâsulfenilimines quirals amb un procediment desenvolupat recentment en el nostre grup de recerca. En el capĂtol 2.3 sâha posat a punt un mĂštode dâhidrĂČlisi dâNâsulfenilimines realment suau i que evita la racemitzaciĂł dels estereocentres en alfa. DesprĂ©s dâhaver provat diversos Ă cids de Lewis sâha trobat que lâĂșs de quantitats catalĂtiques dâAuBr3 mĂ©s lâaddiciĂł de nitrit dâisoamil, permet dâobtenir cetones quirals a partir de la corresponent Nâsulfenilimina. A mĂ©s, Ă©s possible acoblar el mĂštode de formaciĂł dâNâsulfenilimines amb el dâhidrĂČlisi dâaquestes, de manera que sâha desenvolupat una reacciĂł de Nef formal que transcorre en condicions molt suaus (pH prĂ cticament neutre) i sense racemitzaciĂł dels estereocentres en alfa. En el capĂtol 2.4 sâha estudiat la transformaciĂł dâoximes a carbonils. De nou, lâAuBr3 sâha mostrat com lâĂșnic Ă cid de Lewis que permet la formaciĂł de cetones (o aldehids) a partir dâoximes. Sâha aconseguit que el procĂ©s sigui catalĂtic si sâempra biacetil o isovaleraldehid com additius. Les condicions en les que tĂ© lloc la reacciĂł sĂłn suaus (si bĂ© el pH baixa lleugera o moderadament durant el decurs de la reacciĂł). Sâha fet un estudi pioner per Espectrometria de Masses en Electroesprai en mode negatiu a fi de conĂšixer els detalls mecanĂstics, trobantâse que les espĂšcies de tipus AuBrx(OH)yâ sĂłn les predominants i com la oxima sâhi coordina desplaçant Brâ. El mĂštode sâha aplicat a mĂ©s dâuna vintena de cetoximes i aldoximes obtenintâse resultats excelâąlents.[eng] This Thesis deals about Organocatalysis and goldâmediated catalysis, two fields that have experienced a rapid growth in recent years. In both sections, the efforts have been focused in the development of new synthetic methodologies or in gaining insight into existing procedures in order to improve them.
In the first section, we attempted to solve the problem of the slow rate of some organocatalytic transformations. Several oxazolidinones were prepared and then were tested in aldol reactions between ketones and aldehydes. The most efficient was Seebachâs oxazolidinone (SolPro). With this catalyst aldol reaction take less than 2 hours. Exchanges between pĂŹvalaldehyde and different ketones were proved by 1H-NMR studies which suggested that these exchanges are essential in the mechanism. Finally, in chapter 1.5, the scope of the catalytic activity of SolPro was evaluated in a Michael reaction between nitroalkanes and alpha, beta-unsaturated ketones.
Transformations in which the nitro group is involved were studied in the second section. In chapter 2.2 several chiral N-sulphenylimines were prepared according a novel procedure developed in our group. In chapter 2.3 a novel method to hydrolyze N-sulphenylimines was developed, which works in very mild conditions (room temperature, almost neutral pH) and avoids the racemization of alpha stereocenters. After an exhaustive screening of several Lewis acids, only AuBr3 catalyzed the reaction at neutral pH and afforded the corresponding ketone without racemization of alpha-stereocenters. Finally, in chapter 2.4, the transformation of oximes into carbonyl compounds was studied. Again, only AuBr3 was able to perform the hydrolysis of oximes into ketones (or aldehydes) in mild conditions (although the pH decreased slightly or moderately during the reaction). A pioneering study by Mass Spectroscopy (Electrospray in negative mode) showed that oximes coordinate to species like AuBrx(OH)yâ and this coordination is crucial in order to obtain the carbonyl compound
Relative tendency of carbonyl compounds to form enamines
Equilibria between carbonyl compounds and their enamines (from O-TBDPS-derived prolinol) have been examined by NMR spectroscopy in DMSO-d 6. By comparing the exchange reactions between pairs (enamine A + carbonyl B â carbonyl A + enamine B), a quite general scale of the tendency of carbonyl groups to form enamines has been established. Aldehydes quickly give enamines that are relatively more stable than those of ketones, but there are exceptions to this expected rule; for example, 1,3-dihydroxyacetone acetals or 3,5-dioxacyclohexanones (2-phenyl-1,3-dioxan-5-one and 2,2-dimethyl-1,3- dioxan-5-one) show a greater tendency to afford enamines than many α-substituted aldehyde
A synthetic approach to palmerolides via Negishi cross coupling. The challenge of the C15-C16 bond formation
The esterification of fragment C1-C8 (2) with fragment C16-C23 (3) to give iodo derivative 4, followed by a Pd-catalysed coupling with a C9-C15 fragment (7 or 8), may provide a common precursor of most palmerolides. Ligands and reaction conditions were exhaustively examined to perform the C15-C16 bond formation via Negishi reaction. With simple models, pre-activated Pd-Xantphos and Pd-DPEphos complexes were the most efficient catalysts at RT. Zincation of the C9-C15 fragment (8) and cross coupling with 4 required 3 equiv of t-BuLi, 10 mol % of Pd-Xantphos and 60 °C