Diastereoselective conjugate addition/cyclization/bromination: Access to four stereocenters in a single step

A stereoselective tandem conjugate addition reaction with a chiral amine-derived nucleophile is reported in which the enolate intermediate is quenched with 1,2-dibromotetrachloroethane as a mild brominating reagent. X-ray analysis of a subsequent derivative was used to prove the configuration at each of the four newly formed stereocenters. The resulting α-bromoester underwent selective transesterification catalyzed by mild base to allow selective manipulation of the two ester groups of the product.     

An intramolecular para-phenolic allylation free radical cyclization strategy for the synthesis of alkaloids and terpenes with spiro[4.5]decane architectures

A Tsuji-Trost variant of the Winstein-Masamune reaction has been investigated for the synthesis of the AC spirocyclic ring system 9 bearing a quaternary carbon found in the fawcettimine type Lycopodium alkaloids magellanine 1 and lycojaponicumin B 2 and cyclopiane diterpenes such as conidiogenone 3. Annulation of the B ring for the synthesis of tricyclic ABC cores was demonstrated utilizing a 5-exo-trig free radical cyclization of a primary carbon radical onto a cyclohexadienone generated with tri-n-butylgermanium hydride (9 → 11).     

Novel bromination reagents · hexabromocyclopentadiene: bromination of activated saturated sites

Hexabromocyclopentadiene (HBC) readily brominates α-keto and benzylic sites, apparently by bromonium ion transfer.     

The bromination of triethylamine

The products of bromination of triethylamine have been reinvestigated and shown to be N.N-diethyl di- and tribromoacetamides. The rotational barrier for the dibromoamide was measured, E_a = 40 kJ mol^?1 and found to be considerably lower than that of N,N-diethylacetamide.     

Diastereoselective hydroalkoxycarbonylation of terpenes and vinyl-estrone

Hydroalkoxycarbonylation of several monoterpenes (limonene, carvone, dihydrocarvone, pulegone) has been carried out with chiral and achiral palladium–phosphine catalysts. Despite high chemo- and regioselectivities toward the chiral linear products, diastereoselectivity is rather low and cannot be influenced significantly by the selection of the chiral ligand. This observation is in contrast with the high diastereoselectivity of the hydroalkoxycarbonylation of vinyl-estrone used as a model of a vinyl-aromatic skeleton.     

Site-selective bromination of vancomycin

We report the site-selective bromination of vancomycin to produce, with substantial efficiency, previously unknown monobromovancomycins, a dibromovancomycin, and a tribromovancomycin. We document the inherent reactivity of native vancomycin toward N-bromophthalimide. We then demonstrate significant rate acceleration and perturbation of the inherent product distribution in the presence of a rationally designed peptide-based promoter. Alternative site selectivity is observed as a function of solvent and replacement of the peptide with guanidine.     

Raman spectra of some terpenes

Summary Investigation of Raman spectra indicates increase in the interaction between the double bonds as we pass from sylvestrene and isosylvestrene to sylveterpinolene and indicates that the character of the interaction in the last compound is more complex than in 1,3-butadiene and other similar conjugated systems.     

Low and high temperature bromination of exocyclic dienes: high temperature bromination. Part 16

The electrophilic addition of bromine to an exocyclic diene, 5,6-dimethylenebicyclo[2.2.1]hept-2-ene, in CCl₄ at 0°C led to the formation of non-rearranged (73%) and rearranged products (27%). However, high temperature bromination of the exocyclic diene at 77°C suppressed the formation of the rearranged products. Similarly, bromination of 9,10-dimethylenetricyclo[6.2.1.0^2,7]undeca-2,4,6-triene at −10°C gave only the exo-1,2-addition product. Bromination at +5°C resulted in the formation of a mixture consisting of exo-1,2- and 1,4-addition products in a ratio of (1:4). High temperature bromination at 77°C resulted in the formation of the endo-1,2-addition product. Furthermore, it has been shown that the 1,4-addition product converts smoothly to the 1,2-addition product. The formation mechanism of the products is discussed and supported by calculations.     

Catalytic asymmetric bromination and chlorination of beta-ketoesters

The first general catalytic asymmetric bromination and chlorination of beta-ketoesters has been developed. The reactions proceed for both acyclic and cyclic beta-ketoesters catalyzed by chiral bisoxazolinecopper(II) complexes giving the corresponding optically active alpha-bromo- and alpha-chloro-beta-ketoesters in high yields and moderate to good enantioselectivities. For the optically active chlorinated products the isolated yields are in the range of 88-99 % and the enantiomeric excesses up to 77 % ee, while the optically active brominated adducts are formed in 70-99 % isolated yield and up to 82 % ee. Based on the absolute configuration of the optically active products, the face selectivity for the catalytic enantioselective halogenation is discussed based on a bidentate coordination of the beta-ketoester to the chiral catalyst and a X-ray structure of chiral alpha,gamma-diketoesterenolatebisoxazolinecopper(II) complex.     

Oxidative bromination of aniline and its derivatives

Oxidative bromination of aniline and its derivatives containing various substituents (CH₃, Cl, NO₂, COOH) in ortho, meta, and para positions with a brominating mixture of NaBr (KBr) and 20–22% hydrogen peroxide in 6–8% hydrochloric acid at the molar ratio aniline : NaBr (KBr) : H₂O₂ : HCl = 1 : 3.5 : 3.2 : 4.5 is described.     

Regioselective bromination of 6-hydroxytetrahydroisoquinolines

Regioselective bromination of 6-hydroxytetrahydroisoquinolines using molecular bromine was disclosed. Treatment of 6-hydroxytetrahydroisoquinolines with molecular bromine under different temperatures afforded 5-bromo-6-hydroxytetrahydroisoquinolines as sole products in high isolated yield with excellent regioselectivity.     

Unexpected bromination ring-opening of tetraarylporphyrins

An unexpected, mild, efficient bromination ring-opening method has been developed for convenient synthesis of various novel biladienes or brominated porphyrins by controlling the amounts of NBS used.     

Solid-phase bromination of hindered phenols

The solid-Phase bromination of a series oftert-butyl-substituted phenols withN-bromosuccinimide and dioxane dibromide afforded halogenated cyclohexadienones, quinobromides. Under the extrusion conditions, the latter underwent further transformations, mainly, debromination. A new reaction, dioxane dibromide catalyzed anhydroheterocyclization of 2,2-dihydroxy-3,3,5,5-tetra-tert-butyldiphenyl to 2,4,6,8-tetra-tert-butyldibenzofuran, was discovered and the mechanism of this reaction was proposed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1310–1312, May, 1996.     

The bromination of isatoic anhydride

Bromination of isatoic anhydride in acetic acid gives N-dibromoacetyl-3,5-dibromoanthranilic acid. Three new anthranils are described.     

Selective bromination of dihydroquinopimaric acid

The reaction of dihydroquinopimaric acid methyl ester with bromine was found to be chemo- and stereoselective. Regardless of the solvent (acetic acid, methanol, dioxane), bromination of the title compound with an equimolar amount of bromine occurs as electrophilic addition at the double C¹⁹=C²⁰ bond with formation of 14α-hydroxy- or 14α-methoxy-19R-bromo derivatives. The reaction with excess bromine (3 equiv) leads to the formation of 16S-bromo derivatives. The bromination process is accompanied by formation of epoxy bridge between the C¹⁴ and C²⁰ atoms. X-Ray analysis revealed two polymorphic modifications of (16S,19R)-16,19-dibromo-14β,20-epoxydihydroquinopimaric acid methyl ester.     

Surfactant control of bromination products

The product distribution in the bromination of cyclohexene is almost completely controlled by the addition order of the reactants to the surfactant, the obtained bromohydrin being 99% pure. Water seems to be present all around the micelles head groups.     

Higher terpenes from lower terpenes: A short,simple and versatile route to bisabolane sesquiterpenoids

An efficient procedure for the elaboration of abundantly available monoterpenes to bisabolane sesquiterpenes utilising catalysed ene addition as the key reaction is described.