Boron enolates in stereoselective syntheses of 2-enitols from O-benzyl aldoses by reaction with borohydride in 2-propanol. Configuration of E and Z enols from proton relaxation rates

Abstract

Isomerically pure E and Z forms of 2,4,6-tri-0-benzyl-2-dehydro-3-deoxy-D-eryt/iro-hex-2-enitol (4 and 3, respectively) were synthesized from 2,3,4,6-tetra-O-benzyl-D-glucopyranose by reactions employing sodium borohydride and 2-propanol under differing conditions. A cyclic boron enolate is proposed as a transition state to account for the formation of the Z olefin through regio-and stereoselective elimination, followed by hydride reduction. The elimination step was effected in the absence of boron to ensure generation of the E isomer. The relative configurations of 3 and 4 were determined by measuring the effects on spin-lattice relaxation rates of replacing specific protons in the vicinity of the double bonds with deuterium. Also described are notable differences between lithium and sodium counterions in their influence on stereo-and regioselectivity and the synthesis of 2-enitols corresponding to 3 and 4 from 2,3,5-tri-O-benzyl-D-arabinofuranose.The reaction of 2,3,4,6-tetra-O-benzyl-D-glucopyranose (1) with sodium borohydride in 2-propanol has been shown1 to give not only the normal reduction product, alditol 2, but also a 50% yield of an elimination product, 3, R= H 4, R= H 5, R= Ac 6, R = Ac namely, 2,4,6-tri-0-benzyl-2-dehydro-3-deoxy-D-erythrohex-2-enitol (3). As there were two reactions in compe-

Well‐Defined Glycopolymers via RAFT Polymerization: Stabilization of Gold Nanoparticles

“…This is important in order to investigate the formation of the glycopolymer layer onto the gold NPs and the final properties of the resulting hybrid NP S , as compared with naked NPs, analyzing exclusively the influence of the glycopolymer layer rather than the shape or size of the particles. In addition, due to the partial decomposition of NaBH 4 in water at neutral pH, the further reduction of the glycopolymer dithiobenzoate end groups into thiols after the formation of the gold NPs may proceeds in mild conditions, limiting the possible degradation of the unprotected glucopyranose residues in strong reducing conditions …”

Synthesis of cycloprop[c]indol-5-ones from 4-diazo-3-[n-(2-propenyl)amido]cyclohexadien-1-ones. Exploration of copper(I) and copper(II) complexes as catalysts

“…Nitroindole 9 (5 g, 11.8 mmol), CaCl2 (1.20 g, 10.8 mmol) and acid-washed zinc (30 g, 0.46 g-atom) were suspended in 250 mL of 95% ethanol. The mixture was refluxed for 2 h, cooled to room temperature, filtered, and evaporated to yield the aminoindole 10a: NMR (CDC13) 2.18 (d, J =1.2 Hz, 3 H), 4.93 (s, 2 ), 6.09 (d, J = 2.5 Hz, 1 H), 6.34 (d, J = 2.5 Hz, 1 H), 7.20-7.58 (m, 11 H); EIMS m/z 392,251,91 (100), 77. The amine was suspended in 100 mL of CH2C12, pyridine (1.02 g, 13.0 mmol) was added, and the mixture was cooled to 0 °C.…”

“…NMR (CDC13) 2.25 (s, 3 ), 2.80 (s, 3 H), 4.99 (s, 2 H), 6.40 (br s, 1 ), 6.61 (d, J = 1.5 Hz, 1 H), 6.88 (12b). N-Allylamide 10c (5.1 g, 10.8 mmol) and ammonium nitrate (861 mg, 10.8 mmol) were suspended in 20 mL of CH2C12.…”

“…NMR (CDClg) 1.43 (2t, J = 7 Hz, 6 ), 1.90 (s, 3 ), 2.12 (s, 3 H), 3.74 (dd, J = 15,6 Hz, 1 H), 4.35 (q, J = 6 Hz, 2 ), 4.40 (q, J = 7 Hz, 2 H), 4.68 (dd, J = 15, 7 Hz, 1 ), 5.05-5.15 (m, 2 H), 5.77-5.92 (m, 1 H), 6.94 (s, 1 H), 7.55 (s, 1 H).…”

“…4 The alkene selectivity of copper(I) triflate has been compared with other carbene catalysts. 6 The special features of copper(I) triflate are attributed to its greater electrophilicity and stronger alkene coordination, as compared with Cu(I) in the presence of better donor ligands. Within the general mechanistic framework of metal ion catalyzed cyclopropanations,11 the reaction would be formulated as an oxidative addition of the diazo compound to the Cu(I), with formation of a copper-carbene complex.…”