Voici les éléments 1 - 10 sur 17
- PublicationAccès libreAn Unexpected Tandem Reaction between N-Butadienyl-N-alkylketeneN,0-Trimethylsilylacetals of Propionamide and Activated Dienophiles like N-Phenylmaleimide or Acryloyl ChlorideStarting from the N-butadienyl-N-alkylpropionamides 1a-1c the corresponding N,0-trimethylsilylacetals could be obtained using the mixture ofLDA and trimethylsilyl chloride in THF. The unexpected reaction sequence Diels-Alder reaction/acylation between the N-butadienyl-N-alkylketene N,0-trimethylsilylacetal ofpropionamide (2a-2b) and N-phenylmaleimide produced tricyclic products rac-5a -rac-5b and bicyclic products rac-6a - rac-6b with high diastereoselectivity. The reaction of the N,0-trimethylsilylacetals 2a and 2c with acryloyl chloride in a similar sequence gave the bicyclic products rac-8a and mc-8c. The stepwise synthesis of bicyclic systems of this general structure could only be successfully executed in 26 yield treating the Diels-Alder product rac-10 with LDA.
- PublicationAccès libreThe Synthesis of a Pyrazol Analogon of Porphobilinogen with the Help of the Mukaiyama Aldol ReactionThe synthesis of a pyrazol analogon of porphobilinogen is described. The Mukaiyama crossed aldol reaction is the key step of our approach. The retrosynthetic analysis follows the mechanism for the biosynthesis of porphobilinogen initially proposed by Shemin.
- PublicationAccès libreRac-(2R*,3R*)-S-Ethyl-4-Chloro-3-Hydroxy-2-Phenylbuthanethioate and Rac-(2R*,3R*)-S-Ethyl-2-Phenyl-3-(tosyloxy)buthanethioate: Dichotomy of the Stereoselectivity of the Mukaiyama ReactionThe title compounds, rac-(2R*,3R*)-S-ethyl-4-chloro-3-hydroxy-2-phenylbuthanethioate (I) and rac-(2R*,3R*)-S-ethyl-2-phenyl-3-(tosyloxy)buthanethioate (III), are both composed of a S-ethyl 2-phenylbutanethioate moiety but have different geometries. Compound I is substituted in the 3 and 4 positions by a hydroxyl group and a chlorine atom, respectively. In compound III the hydroxyl group in the 3 position of rac-(2R*,3R*)-S-ethyl-3-hydroxy-2-phenylbuthanethioate (II), has been tosylated in order to obtain suitable crystals for X-ray analysis. In compound I the phenyl substituent and the hydroxyl group have a syn arrangement, whereas in the tosylate derivative of II, i.e., compound III, they have an anti arrangement. In the crystal structure of I centrosymmetric hydrogen bonded dimers are formed via O–H•••O hydrogen bonds, involving the hydroxyl group and the carbonyl O-atom. In the crystal structure of III symmetry related molecules are connect via a weak C–H•••O intermolecular interaction, involving a tosylate O-atom and a phenyl H-atom, so forming zigzag chains propagating in the c direction. The compounds were prepared by the Mukaiyama crossed aldol reaction between the silyl enol ether of S-ethyl 2-phenylethanethioate and simple aldehydes, like 2-chloroacetaldehyde (for I) and acetaldehyde (for II). The syn/anti stereo descriptors clearly indicate that the stereoselectivity of the Mukaiyama aldol reaction has switched from a syn selective process for the reaction using 2-chloroacetaldehyde to an anti selective process for the reaction with acetaldehyde. In both compounds the relative stereochemistry at the newly created chiral centers, positions 2 and 3, is R/R.
- PublicationAccès libreDiastereoselective synthesis of (1S,2S,3R,6S) 3-Chloro-3-methyl-6-isopropenyl-1,2-cyclohexanediol via Prins reaction induced by zinc and trimethylsilyl chlorideWhile attempting to synthesize a potential inhibitor of the biosynthetic step of formation of PBG, a novel catalytic system inducing the Prins reaction was discovered. Treatment of epoxy geranial with Zn and trimethylsilyl chloride gave (1S,2S,3R,6S) 3-Chloro-3-methyl-6-isopropenyl-1,2-cyclohexanediol in high diastereoselectivity.
- PublicationAccès libreSynthesis, Structure, and Complexation Properties of Partially and Completely Reduced meso-Octamethylporphyrinogens (Calixpyrroles)
- PublicationAccès libreApplication of the Novel Tandem Process Diels-Alder Reaction/Ireland-Claisen Rearrangement to the Synthesis of rac-Juvabione and rac-EpijuvabioneThe novel tandem process Diels-Alder reaction/Ireland-Claisen rearrangement shows a high diastereoselectivity for the Ireland-Claisen rearrangement starting from the endo-product of the Diels-Alder reaction. Based on this mechanistic knowledge, the novel tandem process could be applied to the synthesis of rac-juvabione.
- PublicationAccès librerac-Ethyl 2-bromo-2-[(3R,5R)-3-bromo-5-methyltetrahydrofuran-2-ylidene]acetateIn the title compound, C9H12Br2O3, a (tetrahydrofuran-2-ylidene)acetate, the double bond has the Z form. In the tetrahydrofuran group, the relative configuration of the Br atom in the 3-position and the methyl group in the 5-position is anti. The compound crystallizes with two independent molecules per asymmetric unit and, in the crystal structure, the individual molecules are linked to their symmetry-equivalent molecules by C***HO hydrogen bonds, so forming centrosymmetric hydrogen-bonded dimers.
- PublicationAccès libre4,4-Bis(1H-pyrrol-2-yl)pentanolThe title achiral compound, C13H18N2O, crystallized in the chiral monoclinic space group P21. The pyrrole rings are inclined to one another by 62.30 (11)°, and the propanol chain is in an extended conformation. In the crystal, the two pyrrole NH groups are involved in intermolecular N-H ··· O hydrogen bonds, leading to the formation of a helical arrangement propagating along the b axis. An interesting feature of the crystal structure is the absence of any conventional hydrogen bonds involving the hydroxy H atom. There is, however, a weak intermolecular O-H ··· π interaction involving one of the pyrrole rings.
- PublicationAccès libreThe Novel Sequence Diels-Alder Reaction/Ireland-Claisen Rearrangement Applied to Acyclic Dienophiles: New Insights into the Selectivity of the Ireland-Claisen RearrangementThe new dienes 4a-d, 7 and 11 reacted in good yields with acyclic dienophiles like methyl acrylate and diethyl fumarate in the tandem process Diels-Alder reaction/Ireland-Claisen rearangement. Analysis of the relative configuration of products 5, 6, 8-10 and 12 indicated that preference for the chair or boat transition state of the Ireland-Claisen rearrangement is stronlgy influenced by the relative configuration of the substituents of the cyclohexene ring.
- PublicationAccès librerac-(R)-2-[(2R,5R)-5-Methyltetrahydrofuran-2-yl]propanoic acidIn the crystal structure of the title compound, C8H14O3, the 2,5-tetrahydrofuran ring junction is cis. The relative configuration of position 2 in the propanoic acid group was found to be the same as that in positions 2 and 5 in the tetrahydrofuran ring. In the crystal structure, symmetry-related molecules are linked by O***HO hydrogen bonds to form centrosymmetric dimers.