Stereochemistry and rearrangement reactions of hydroxylignanolactones

Various conflicting data on the rearrangement and absolute stereochemistry of hydroxylignano-9,7'-lactones are resolved using 18O labeled compounds, also confirmed by an X-ray analysis of a pure lignano-9,7'-lactone enantiomer, obtained for the first time. Under NaH/DMF rearrangement conditions a silyl protected hydroxylignano-9,9'-lactone underwent an unexpected silyl migration.     

Stereochemistry in the palladium-catalyzed rearrangement of some cyclohex-2-enyl acetoacetates

The palladium-catalyzed decarboxylation and rearrangement of $\text{cis}$-5-substituted-cyclohex-2-enyl acetoacetates to give the corresponding methyl ketone proceed mostly with retention of configuration, indicative of an enolate addition to the π-allylic ligand trans to the palladium. The reaction from the trans-isomer is not stereoselective.     

Stereochemistry of 1-hydroxyphosphonate-phosphate rearrangement. Retention of configuration at the phosphorus atom

1-Hydroxyphosphonate 1 in the presence of triethylamine in acetonitrile solution undergoes irreversible rearrangement to phosphate 2 and reversible retro-phospho-aldol (retro-Abramov) reaction. The X-ray structures for 1 and 2 revealed that the phosphonate-phosphate rearrangement occurs with retention of configuration at the phosphorus atom.     

Stereochemistry of the 1,2-Wittig rearrangement: A synthesis of syn-1,3-diol monoethers

The [1,2] Wittig rearrangement of β-alkoxyalkyl allyl ethers has been studied and found to provide syn-1,3-diol derivatives in 14–32% yield and with useful levels of diastereoselection.     

Stereochemistry of the methylenecyclopropane rearrangement—II: Exclusion of the concerted 45° pivot mechanisms

Mechanistic options for the thermal rearrangement of methylenecyclopropane are narrowed further by experiments with the four racemic diastereoisomeric 2-cyano-3-methyl-ethylidenecyclopropanes. The explicit assumption being made that these structural perturbations have neither weakened nor invalidated the mechanistic restrictions imposed by previously published work, it has been concluded that concerted birotational mechanisms involving transition states such as the 45° parallel twisted allylic (Woodward-Hoffmann, Möbius controlled) or the 45° perpendicularly twisted allylic (Berson-Salem subjacent orbital controlled) can be excluded from the presently acceptable set of structurally compatible, chirality-retaining, configuration-inverting mechanisms. A competitive rearrangement of the corresponding carboxylic esters to 1-methyl-2-ethyl-4-methoxyfuran makes these molecules unsuitable for study of the methylenecyclopropane rearrangement. Assignment of stereochemistry to the cyano compounds has been accomplished by application of base-catalyzed equilibration of the cyano center and photochemical iodine-catalyzed equilibration between geometrical isomers.     

Unusual fragmentations under polonovski reaction conditions

N_b-oxides $\text{2a}$ and $\text{2b}$ in the presence of acetic anhydride led to products $\text{5}$, $\text{7}$ or $\text{8}$ with participation of the indole nucleus during the fragmentation reaction.     

Configurationally and conformationally homogeneous cyclic n-aryl sulfimides—III : Stereochemistry of rearrangement

Configurationally and conformationally homogeneous thiane- and trans- and cis-1-thiadecalin-1-N-p-chlorophenyl imides were rearranged to the corresponding 2-(2'-amino-5'-chlorophenyl)-sulfides and the configurations of the products were assigned by ^1H-NMR spectroscopy. If sterically unhindered, rearrangements proceeded with a high degree (95%) of stereospecifity: sulfimides with equatorial S-N-bond gave rearranged products with axial, sulfimides with axial S-N-bond gave rearranged products with equatorial aryl substituent. The proposed concerted mechanism via a cyclic transition state is supported by these results.     

Stereochemistry as a tool in deciphering the processes of a tandem iminium cyclization and Smiles rearrangement

To understand the detailed mechanism of a recently reported tandem iminium cyclization and Smiles rearrangement, the reaction processes of a chiral substrate were investigated by monitoring its stereochemical courses. Under the tandem reaction conditions, chiral aldehyde 1 derived from l-prolinol led to two surprising results. First, the iminium cyclization gave a diastereomeric mixture with the cis-configured product as the predominant one. Second, Smiles rearrangement of both cis- and trans-2 led to the same product 3a directly derived from the trans isomer. The former was rationalized by the postulation of a Cram's chelate transition state leading to the cis product as kinetically favored. The latter was due to the equilibration between the trans/cis pair involving a carbocation intermediate and the steric hindrance, which prevented the cis isomer from undergoing the intramolecular nucleophilic substitution. This hypothesis was further supported by the results of a competition experiment in which the addition of 1 equiv of p-methoxyaniline in the rearrangement step led to a significant amount of anilinyl-exchanged rearrangement product.     

Novel applications of the modified polonovski reaction - VII preparation of isoquinuclidines

The first successful preparations of the biochemically important isoquinuclidine ring system by the modified Polonovski reaction are described.     

Asymmetric oxidation of achiral selenides to optically active selenoxides : Stereochemistry of the allyl selenoxide-selenenate [2,3] sigmatropic rearrangement

Asymmetric oxidation of methyl phenyl selenide ($\text{3}$), under anhydrous conditions, by chiral 2-sulfonyloxaziridines, $\text{1–2}$, gives optically active methyl phenyl selenoxide (8.1–9.3% ee). The stereochemistry of the selenoxide is determined by the configuration of the oxaziridine three membered ring with non-bonded steric interactions responsible for the chiral recognition. Asymmetric oxidation of E-phenyl cinnamyl selenide ($\text{7}$) by $\text{1–2}$ affords optically active l-phenylallyl alcohol ($\text{9}$). A concerted [2,3] sigmatropic rearrangement via an exo transition state is proposed.     

Novel applications of the modified polonovski reaction. A biomimetic synthesis of quinuclidines.

Suitably substituted aminoesters can be transformed to the corresponding α-aminonitriles and in the case of piperidino-acetic ester derivatives, into the biochemically important quinuclidine ring system.     

Stereochemistry of the hygrolidins

The stereochemistry shown in 6 has been determined by computer modelling for hygrolidin, prototypical member of a new class of macrolide antibiotics.     

Stereochemistry of heterocycles. XL. Stereochemistry of the synthesis of 2,3-dialkyloxetanes

It is shown that trans-2,3-dialkyloxetanes are formed from the erythro isomers of 2-alkyl-1,3-butanediols, whereas cis-2,3-dialkyloxetanes are formed from the threo isomers.See [1] for communication XXXIX.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 164–168, February, 1976.     

On the Stereochemistry of Vincetene

Summary.  The absolute configuration of the centre of asymmetry in the side chain of the alkaloid vincetene (1) was determined by ozonolysis and subsequent unequivocal transformations affording the (S)-enantiomer of 1,2-propanediyl dibenzoate (4). Received February 22, 2001. Accepted March 1, 2001