A nitrone-based cycloaddition approach to the synthesis of the glycosyl system of nogalomycin, menogaril, and their congeners

Abstract

of 0.08 M potassium hydride/THF, 0.30 mmol of 12, and 0.30 mmol of 18-crown-6 ether was degassed and sealed in a glass tube. The tube was heated at 80 °C for 48 h. The reaction mixture was diluted with water and extracted with ether. The ether extract was washed with water, dried, and concentrated, leaving 85 mg (98% recovery including 10% unreacted 12) of a white solid, mp 80-95 °C. Spectral Characterizations of Elimination Products. The 'H NMR spectra of elimination product mixtures (23 + 24) show pairs of partially resolved doublets for the bridgehead protons (H, in 23 and 24) in the region & 4.9-5.2. The vinyl protons lie within the aromatic regions (approximate b 7-8). The methylene protons absorb as varying types of multiplets for products from 12 (5 2.8-3.9), four sets of triplets for products from 20 (5 2.9,3.1, 4.2,4.4), and two singlets for products from 22 (b 4.8). In the case of vinyl chlorides from 22 the area integrations for the two singlets could be used to confirm the product isomeric distribution from analyses of the bridgehead proton absorptions.

α-N-Methyl/Phenyl Furan Derivatives as Dipolarophiles for Synthesis of Spiro Isoxazolidine Derivatives with α-Chloro and Simple Nitrones

“…The 3-H and CHCl protons are also syn as evidenced from their coupling constant values (J 3,CHCl ~ 9.40 Hz). 17,18 Almost similar coupling constant values are obtained for H-3 and H-4 protons in the case of other reported cycloadducts (5)(6)(7)(8). Cycloaddition of Z nitrone via exotransition state geometry results in syn spiro isoxazolidine derivatives.…”

“…The stereochemistry of the isoxazolidine derivatives (3)(4)(5)(6)(7)(8) were rationalised by considering the multiplicity of the proton signals at 3-H, 4-H, CHCl (in the case of α-chloro nitrones only) asymmetric centres along with their coupling constant values. 17,18 In the 1 H NMR spectrum of cycloadducts 3-4, 3-H resonates around δ H 2.50-3.50 ppm while 4-H around δ H 3.00-5.85 ppm and the coupling constant is J 3,4 ~ 9.16 Hz implying a cis relationship between H-3 and H-4. The CHCl proton also resonates upfield around δ H 2.20-2.60 ppm.…”

Microwave-assisted synthesis of novel spiro isoxazolidine derivatives

“…We have also successfully tested these reactions in large scale (taking 6 mmols of nitrone and dipolarophiles) following conventional methodology as far as the feasibility and process chemistry of these reactions are concerned (respective yields are mentioned in Experimental section). The stereochemistry of the spiro isoxazolidine derivatives 3a Á 3d was rationalized by considering multiplicity of the proton signals at 3-H, 4-H, CHCl asymmetric centers along with their coupling constant values [21,22]. Prominent double doublet signals for H-3 protons were obtained in all the novel spiro cycloadducts while H-4 protons appeared as doublet.…”

“…In all the spiro cycloadducts, the methylene protons of furan ring, butanol substituent, and cyclohexyl groups are merged and appeared as multiplets. In the 1 H NMR spectrum of spiro cycloadducts [21,22]. Cycloaddition of Z nitrone via exo-transition state geometry results syn spiro isoxazolidine derivatives.…”

Highly Stereoselective Synthesis of New Aziridines via Baldwin Rearrangement and Their Potential Biological Activities

“…The rearrangement gave predominantly one diastereomer, in 83/17 to >95/5 diastereomeric ratio (dr). The cis relative stereochemistry of the major compound has been assigned from the comparison of the coupling constant ( J ) value (7.9 Hz) . The 4.2‐Hz J constant value of the minor compound indicates a trans relationship between the C substituents.…”