Configuration reaction

The first asymmetric synthesis of (-)-monomorine I, an enantiomer of the natural alkaloid, by Husson and co-workers starts with the chiral 2-cyano-6-oxazolopiperidine synthon (385) prepared from (-)-phenylglycinol (384), glu-taraldehyde (383), and KCN (443). Alkylation of 385 with an iodo ketal led to the formation of a single product (386). The cyano acetal (386) was treated with silver tetrafluoroborate and then zinc borohydride to afford a 3 2 mixture of C-6 epimeric oxazolidine (387) having the (2S) configuration. Reaction of 387 with... 

Two-configuration reactions. Since for most organic reactions one or more covalent bonds are broken while one or more new bonds are formed a reaction profile may be schematically generated as indicated in Fig. 8. For the case in which at least one reactant is a closed shell species, two configuration curves, reaction barrier. The reactant configuration, [Pg.117]

Figures 12a and 12b illustrate the ESR spectra at —180°C and 25°C, respectively, of Nb(7r-allyl)4 after reaction with Manosil VN3 previously dried at 200°C. Figure 12b is complex and shows a mixture of at least two species present in the system. The major peaks are present in the system. The major peaks are also present in the frozen solution spectrum (Figure 10a) indicating that they arise from a rigidly held surface species. This species arises from the reaction of Nb(7r-allyl)4 with two adjacent hydroxyl groups and is therefore the double oxide bridge configuration (Reaction 3). Upon further addition of Nb(7r-allyl)4 to the silica, the pseudo isotropic spectrum (Figure lib) becomes more evident. This suggests that Nb(7r-allyl)4 reacts preferentially with the paired hydroxyl groups, an observation in accord with the literature (63).

Figure 13.4 Effect of the permeance (ranging from 0.01 to 10 times n, used as a reference value ofthe permeance) on the MR performance. Two configurations (reaction in the annular space or in the core of the tube) are considered. The curves indicated with n and 10 jt (tube) and 0.1 n (tube) are the reaction paths when the catalyst is packed inside the tube and thus...

In absence of a catalyst, simple olefins are essentially fixed in their bonding configurations reaction paths to interconversions through molecular collisions, fusions, and disassociations are apparently closed because of orbital symmetry restrictions, as proposed by Hoffman and Woodward 8°). Mango 8 has postulated that in the presence of certain transition metal catalysts, these orbital symmetry restraints are lifted, allowing bonds to flow freely and molecular systems to interchange. Thus, the conservation of molecular orbital symmetry is a key function of the catalyst. 

FIG. 6.—Nucleophilic attack on the carbonyl group of a tranr-oriented 2-acetate can occur either via formation of a cyclic orthoester, which by undergoing a second attack affords a product having the same configuration as the starting material (reactions 1 and 2), or by nucleophilic attack on the anomeric carbon to afford a product having the inverted configuration (reaction S). 

Chloroacetoxylation of 6-benzyloxycyclohepta-1,3-diene was highly diastereoselective and produced only the diastereoisomer 65 shown. Transformation of the chloroacetate 65 to 66 was realized by a Pd(0)-catalyzed substitution of the chloride by a sulfonamide, which occurs with retention of configuration. Reaction of the allylic chloride with the sulfonamide... 

Intermediates (40) react at -110 C with TMS-Cl to give compounds (42), which are stable, readily purified products. The H NMR of (42) is in accord with the assigned stereochemistry and shows that the oxidation of (40) had indeed proceeded with retention of configuration. Reaction of (42) with HF/MeCN gives ( )-alkenes in excellent yields (Scheme 6) with none of the many by-products seen in the original reaction. The ( ) (Z) ratios range from 100 0 to 95 5 and the reaction tolerates NO2, Cl, OMe and alkyl groups in the aromatic aldehyde. ... 

Reactant Catalyst Ligand configuration Reaction time (day) Conversion (%) ee (%)... 

Simple olefins in the absence of a catalyst are essentially fixed in their bonding configurations. Reaction paths to interconversion through molecular collision, fusion, and disassociation [Eq. (13)] are closed because of orbital symmetry restrictions. 

The synthetic utility of the chloral ene adducts has been extensively explored. Reaction with NaOR in ROH gives the a-alkoxy ester with inversion of configuration. Reaction of the corresponding tosyl-ate with sodium ethoxide in ethanol affords ethyl alka-2,4-dienoates (Scheme 9). 

These observations contrast with those of Shoppee and co-workers in a study of the epimeric 3-chloro-A -compounds which were believed to solvolyse via a single allylic carbocation. A re-examination of the solvolysis of the chloro-compounds appeared to confirm the intermediacy of two carbocations since the product ratios were very similar to those from the trifluoroacetates. Treatment of the trifluoroacetates with NaNs-HMPA resulted in substitution at C-3 with inversion of configuration. Reaction of the 17j8-trifluoroacetoxy-17a-vinyl compounds (7) and (8) under similar conditions gave the JB -17(20)-dehydro-21-azido-compounds (9). These reactions are not regarded as pure 5n2 processes since the compounds (7) and (8) may rearrange to compounds (10) in HMPA. Solvolysis of compounds (7) and (8) in MeOH-NaOAc gave product distributions... 

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