Stereochemistry Group VIII

Butterfly Cluster Complexes of the Group VIII Transition Metals Sargeson, Alan M., see Hendry, Philip Sanon, G., see Fleischauer, P. D. Sawyer, Donald T., see Sobkowiak, Andrzej Sawyer, Jeffery F., and Gillespie, Ronald J., The Stereochemistry of SB (HI) 35 437... 

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

Group VIII and Other Transition Metals The structural chemistry of cobalt The stereochemistry of cobalt (ll)-d ... 

THE INFLUENCE OF STERIC AND ELECTRONIC EFFECTS UPON THE STEREOCHEMISTRY OF GROUP VIII METAL COMPLEXES. 

Formation of mixtures of products in these reactions can be attributed largely to the properties of the acetate group. The reactions of a number of cycloalkenes with thallium(III) salts have been investigated in some detail and the results obtained have served both to elucidate the stereochemistry of oxythallation and to underline the important role assumed by the anion of the metal salt in these oxidations. The most unambiguous evidence as to the stereochemistry of oxythallation comes from studies by Winstein on the oxythallation of norbornene (VII) and norbornadiene (VIII) with thal-lium(III) acetate in chloroform, in which the adducts (IX) and (X) could be precipitated from the reaction mixture by addition of pentane 128) (Scheme 11). Both by chemical means and by analogy with the oxymercuration... 

The heteroyohimbine alkaloids and their oxindole counterparts form a large group of compounds that provide an ideal exercise in conformational analysis the oxindole bases, which contain in C-7 an additional asymmetric center, have been particularly thoroughly studied. The oxindole alkaloids that occur in Mitragyna and related genera have previously been discussed in Volumes VIII and X of this series and by 1967 the complete conformations of most of these alkaloids had been elucidated. The known facts concerning the stereochemistry of these alkaloids at that time have been summarized by Shamma et al. (46, 47), and the stereochemistry of the uncarines-A-F has also been... 

The Weinreb group has recently examined the reaction of chiral N-sulfinyl dienophile 23, prepared from (+)-camphor, with 1,3-cyclohex-adiene (Scheme 1-VIII). Whereas the uncatalyzed cycloadcUtion afforded a mixture of diastereomeric adducts, the reaction promoted by TiCU gave a single adduct 24 having the 35,6/ configuration. Stereochemistry at sulfur in this compound could not be determined. As in the phenylmenthol series, one can reasonably consider two reacting dienophile conformations 23A and 23B (Scheme 1-VIU). If conformer 23A is attacked by the diene in an endo manner from the most exposed face, the observed adduct 24 will be formed. Similarly, if conformer 23B reacts with cyclohexadiene via an exo transition state, 24 will result. 

By the end of 1962 the stereochemistry shown in XII had been deduced for tetrahydroalstonine and the evidence on which this conclusion was based was briefly outlined in Volume VIII. This evidence included the chemical correlation of tetrahydroalstonine with transformation products of corynantheine and corynantheidine, which firmly established the presence of a cis D/E ring junction. This was consistent with the weakly basic character of tetrahydroalstonine and with its low rate of reaction with methyl iodide. The methyl group attached to C-19 was placed cis with respect to the hydrogen atoms at C-15 and C-20 on the basis of the NMR spectral data. 

More recently Galivan et al. [283] described a synthesis of y-fluoroMTX (VIII.99) involving condensation of di-t-butyl A -[4-(A -methylamino)ben-zoyl]-y-fluoro-L-glutamate with 2,4-diamino-6-bromomethylpteridine, followed by hydrolysis of the ester groups with trifluoroacetic acid. The overall yield was 45 %, and two products with erythro and threo stereochemistry were shown to be present in equal amounts by F-NMR and ion-exchange HPLC. The proton NMR spectrum of the mixture, taken in DjO-DCl solution, showed the /l-CHj protons as a multiplet at b 2.94 and the y-CHF proton as a markedly deshielded multiplet at b 5.42. 

The NMR spectra of enmein-type diterpenoids in which 6a-H is part of a hemiacetal contain only a singlet assignable to this proton because of its ca. 90° dihedral angle with 5-H [e.g. isodocarpin (64), carpalasionin (77) in Table VIII, and enmein (62), isodotricin (65) etc. in Table II]. In spirolactone-type diterpenoids having a hemiacetal function 6-H is a doublet [e. g. trichorabdal E (96) in Table VIII and trichorabdals F (97) and G (98) in Table II]. In the enmein-type diterpenoids of this group, C-6 is R except for rabdosin A (73), while the spirolactone-type diterpenoids occur as a mixture of 6R- and 6S-isomers 81). Shikodonin has been reported to have structure (88) on the basis of X-ray analyses of its 6-O-methyl and 6-0-ethyl derivatives, but our experimental results are inconsistent with this proposal, and thus the stereochemistries of C-5 and C-6 in (88) seem questionable. 

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