Oxidation with a photolabile carbonyl protecting group

A novel oxidation approach utilizing a robust photolabile carbonyl protecting group reagent (1) as the oxidizing reagent has been developed. Different from existing methods, this approach oxidizes primary alcohols to the photosensitive acetals (e.g., 3), providing another unique approach to the protected aldehydes. Thus, for the first time, oxidation and protection are achieved in one reaction. Secondary alcohols are oxidized to the corresponding ketones. Moreover, the photolabile protecting group (PPG) also oxidizes ethers and esters. The oxidation is presumably via hydride abstraction by the tritylium ion generated from 1 under acidic conditions. However, the mechanisms for primary alcohols and secondary alcohols are slightly different.     

Cation-pi interactions of a thiocarbonyl group and a carbonyl group with a pyridinium nucleus

Attractive interactions between a thiocarbonyl group and a pyridinium nucleus, and between a carbonyl group and a pyridinium nucleus have been proven by (1)H and (13)C NMR studies, UV-vis spectral analyses, and X-ray crystallographic analyses of nicotinic amides 1 and 3, and pyridinium salts 2 and 4. Comparison of the Deltadelta values, which are the differences in the chemical shifts with reference compounds 5 or 6, showed that the absolute Deltadelta values of 2 and 4 are much larger than those of 1 and 3. In the UV-vis spectra, the n-->pi absorption of the C=S group of 2a exhibited a significant blue shift in CHCl(3). X-ray crystallographic analysis of 1-4 clearly showed that the C=S group of 2a and the C=O group of 4 are very close to the pyridinium moiety compared to the case of 1 and 3. In addition, the X-ray crystal packing structure of 2a showed the C=S group is sandwiched between two pyridinium rings. These experimental results strongly suggested the existence of attractive (C=S)...Py(+) and (C=O)...Py(+) interactions in solution and in crystal. The optimized geometries of 1 and 2 calculated at the HF/6-311G level are in good agreement with their X-ray geometries. MP2/6-311G calculations for the model systems of pyridinium salts 2 and 4 predicted that the electrostatic and induction energies are the major source of the attractive interactions. Since the larger contribution of electrostatic and induction interactions are characteristic features of cation-pi interactions, the (C=S)...Py(+) and (C=O)...Py(+) interactions would be classified as a cation-pi interaction.     

Ab initio studies of three-membered ring formation through intramolecular nucleophilic substitution

Three-membered ring (3MR) forming processes of ⁻X(SINGLE BOND)CH₂(SINGLE BOND)CH₂(SINGLE BOND)F and ⁻CH₂(SINGLE BOND)C((SINGLE BOND)Y)(SINGLE BOND)CH₂(SINGLE BOND)F (X(DOUBLE BOND)CH₂, O, or S and Y(DOUBLE BOND)0 or S) through a gas phase neighboring group mechanism (S_Ni) are studied theoretically using the ab initio molecular orbital method with the 6–31+G* basis set. When electron correlation effects are considered, the activation (ΔG^≠) and reaction energies (ΔG⁰) are lowered by ca. 10 kcal mol⁻¹, indicating the importance of the electron correlation effect in these reactions. The contribution of entropy of activation (−TΔS^≠) at 298 K to ΔG^≠ is very small, and the reactions are enthalpy controlled. The ΔG^≠ and ΔG⁰ values for these ring closure processes largely depend on the stabilities of the reactants and the heteroatom acting as a nucleophilic center. The Bell–Evans–Polanyi principle applies well to all these reaction series. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1773–1784, 1997     

13C NMR spectra of 20-ketosteroids with 16α,17α three-membered ring

Conclusions ¹³C chemical shifts have been determined for 20-ketosteroids with a 16, 17 three-membered ring (X=S, NH, NAc, O, CH₂), and a linear relationship has been found between the -effects of X and the calculated changes in electron density on the C atoms C¹⁶ and C¹⁷; the - and -effects depend on the electronegativity of X. The C¹⁶ and C¹⁷ centers in the epoxysteroid (X=0) are more electrophilic than in the other analogs that were studied; this is consistent with the observed higher reactivity of the epoxide ring in reactions of cleavage by nucleophilic reagents.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2189–2194, October, 1987.     

Fluoro-artemisinins: When a gem-difluoroethylene replaces a carbonyl group

Exo gem-difluoromethylene-artemisinins (8) has been designed to mimic artemisinin. The classical Wittig olefination reaction applied to artemisinin failed. An alternative reaction involving the generation of an α-CF₃ carbanion, from the corresponding bromide 6, allowed the access to the target compound 8, and could also be exemplified in sugar series. The replacement of the carbonyl function by a difluoroethylene moiety resulted in a better antimalarial activity.     

Syntheses of group 7 metal carbonyl complexes with a stable N-heterocyclic chlorosilylene

Two structurally characterized manganese [L(2)Mn(CO)(4)](+)[Mn(CO)(5)](-) (1) and rhenium [L(3)Re(CO)(3)](+)[ReCO)(5)](-) (2) silylene complexes were prepared in one pot syntheses by reacting 1 equivalent of Mn(2)(CO)(10) with 2 equivalents of stable N-heterocyclic chlorosilylene L {L = PhC(NtBu)(2)SiCl} and 1 equivalent of Re(2)(CO)(10) with 3 equivalents of L in toluene at room temperature. Both complexes 1 and 2 were characterized by single-crystal X-ray structural analysis, NMR and IR spectroscopy, EI-MS spectrometry, and elemental analysis.     

Theoretical characterization of the chemical bonds of some three-membered ring compounds through QTAIM theory

Structural Chemistry - In this paper, we investigate the nature of the carbonyl and the intraring C–C, C–N, C–O, N–N, O–O and N–O bonds of cyclopropanone and the...     

CC Triple-bond stabilization in three-membered ring compounds: Boriryne,dilithiumcyclopropyne and berylliryne

Ab initio calculations have been performed on the title compounds. At 3-21G, boriryne is not a minimum on the potential energy hypersurface. The 3-21G optimized minimum of dilithiumcyclopropyne corresponds to dilithiumcyclopropenylidene with a planar tetracoordinated carbon atom and CC bond lengths intermediate between single and double bonds. Berylliryne is calculated to be a stable molecule, being the global minimum at the C₂ Be energy hypersurface at MP2/6-31G*//6-31G* with a CC bond length of 1.258 Å.     

Cyclopentadienyl ring metalation in cyclopentadienyliron carbonyl complexes

The cyclopentadienyl ligands in Cp(CO)₂FePh, Cp(CO)₂FeCH₂Ph and [Cp(CO)₂Fe]₂ were metalated by BuLi in THF at ?78°C. With Cp(CO)₂FePh it was possible to employ the reaction for the synthesis of cyclopentadienyl carbonyl complexes substituted on the Cp ligands.     

Thietane ring as a novel protecting group

A novel protecting group for NH functionality of heterocycles, a thietane ring, was proposed. It can be readily introduced by alkylation of NH-heterocycles with 2-chloromethylthiirane. Removal of the thietane protecting group is performed via oxidation to thietane 1,1-dioxide with hydrogen peroxide in acetic acid and subsequent treatment with sodium alkoxide.