William H. Hersh

Synthesis of Trisubstituted Isoxazoles by Palladium(II)-Catalyzed Cascade Cyclization–Alkenylation of 2-Alkyn-1-one O-Methyl Oximes

A palladium-catalyzed, cascade 5-endo-dig cyclization-alkenylation synthesis of isoxazoles has been developed. The addition of 1 equiv of n-Bu(4)NBr significantly increases the yield of the desired 4-alkenyl-3,4,5-trisubstituted isoxazoles. A variety of trisubstituted isoxazoles are prepared in moderate to excellent yields. One example of the synthesis of a naphthoisoxazole is reported by a cascade cyclization-alkenylation-Heck reaction.

Antimalarial activity of ruthenium( ii ) and osmium( ii ) arene complexes with mono- and bidentate chloroquine analogue ligands

Eight new ruthenium and five new osmium p-cymene half-sandwich complexes have been synthesized, characterized and evaluated for antimalarial activity. All complexes contain ligands that are based on a 4-chloroquinoline framework related to the antimalarial drug chloroquine. Ligands HL(1-8) are salicylaldimine derivatives, where HL(1) = N-(2-((2-hydroxyphenyl)methylimino)ethyl)-7-chloroquinolin-4-amine, and HL(2-8) contain non-hydrogen substituents in the 3-position of the salicylaldimine ring, viz. F, Cl, Br, I, NO2, OMe and (t)Bu for HL(2-8), respectively. Ligand HL(9) is also a salicylaldimine-containing ligand with substitutions in both 3- and 5-positions of the salicylaldimine moiety, i.e. N-(2-((2-hydroxy-3,5-di-tert-butylphenyl)methyl-imino)ethyl)-7-chloroquinolin-4-amine, while HL(10) is N-(2-((1-methyl-1H-imidazol-2-yl)methylamino)ethyl)-7-chloroquinolin-4-amine) The half sandwich metal complexes that have been investigated are [Ru(η(6)-cym)(L(1-8))Cl] (Ru-1-Ru-8, cym = p-cymene), [Os(η(6)-cym)(L(1-3,5,7))Cl] (Os-1-Os-3, Os-5, and Os-7), [M(η(6)-cym)(HL(9))Cl2] (M = Ru, Ru-HL(9); M = Os, Os-HL(9)) and [M(η(6)-cym)(L(10))Cl]Cl (M = Ru, Ru-10; M = Os, Os-10). In complexes Ru-1-Ru-8 and Ru-10, Os-1-Os-3, Os-5 and Os-7 and Os-10, the ligands were found to coordinate as bidentate N,O- and N,N-chelates, while in complexes Ru-HL(9) and Os-HL(9), monodentate coordination of the ligands through the quinoline nitrogen was established. The antimalarial activity of the new ligands and complexes was evaluated against chloroquine sensitive (NF54 and D10) and chloroquine resistant (Dd2) Plasmodium falciparum malaria parasite strains. Coordination of ruthenium and osmium arene moieties to the ligands resulted in lower antiplasmodial activities relative to the free ligands, but the resistance index is better for the ruthenium complexes compared to chloroquine. Overall, osmium complexes appeared to be less active than the corresponding ruthenium complexes.

A non-Karplus effect: evidence from phosphorus heterocycles and DFT calculations of the dependence of vicinal phosphorus-hydrogen NMR coupling constants on lone-pair conformation.

In contrast to literature reports of a Karplus-type curve that correlates (3)J(PH) with phosphorus-hydrogen dihedral angle, a recently reported glycine-derived 1,3,2-oxazaphospholidine (7c) has two hydrogen atoms on the ring with identical PNCH dihedral angles but measured coupling constants of ∼6 and 1.5 Hz. DFT calculations were in accord with these values and suggested that the smaller coupling constant is negative. Experimental evidence of the opposite signs of these coupling constants was obtained by analysis of the ABX NMR spectrum of the new glycine-derived N-p-toluenesulfonyl phosphorus heterocycle 6c. DFT calculations on 6c and on Me(2)NPCl(2) and t-BuPCl(2) were also in accord with NMR data and allowed confirmation of unusual features including a lone pair effect on (3)J(PH), the negative coupling constant, temperature-dependent chemical shifts due to rotation about the sulfonamide S-N bond, and vicinal phosphorus-hydrogen coupling constants over 40 Hz. Calculation of phosphorus-hydrogen coupling constants both as a function of PYCH dihedral angle θ (Y = O, N, C) and lone pair-PYC dihedral angle ω shows similar θ,ω surfaces for (3)J(PH) with a range of (3)J(PH) from -4.4 to +51 Hz and demonstrates the large non-Karplus effect of lone-pair conformation on vicinal phosphorus-hydrogen coupling constants.

Sulfurization of Dinucleoside Phosphite Triesters with Chiral Disulfides

Sixteen chiral analogues of phenylacetyl disulfide (PADS) and 5-methyl-3H-1,2,4-dithiazol-3-one (MEDITH) were used to sulfurize five dithymidine phosphite triesters, each incorporating a β-cyanoethoxy or siloxy group. Each mixture of SP:RP phosphite triester diastereomers was combined with approximately one fourth of an equivalent of each of the sulfurizing reagents, and the RPS:SPS diastereomer ratios of the resulting phosphite sulfides or phosphorothioates were determined by reverse-phase HPLC. Diastereoselectivities and corresponding diastereomeric excess (de) values were calculated by correcting for the starting triester diastereomer ratios. The highest de values for RPS and SPS phosphorothioates were 14.7% and 7.9%, respectively, both using MEDITH analogues.

Auto-Tandem Palladium Catalysis: From Isoxazole to 2-Azafluorenone

An auto-tandem palladium catalysis from halogen-substituted isoxazoles and Michael acceptors is described. It involves two mechanistically distinct palladium-catalyzed reactions, a Heck reaction and a rearrangement, leading to 2-azafluorenones. It is the first example of palladium-catalyzed ring opening of isoxazoles and rearrangement of the β-imino ketone ring-opening product.

Synthesis of chiral disulfides: potential reagents for enantioselective sulfurization

Synthesis of chiral phosphorothioates for use as antisense oligonucleotides might benefit from the use of chiral disulfides. This paper reports the synthesis of chiral analogs of phenylacetyl disulfide and of 5-methyl-3H-1,2,4-dithiazol-3-one from the same set of 2-arylalkanoic acids. The X-ray crystal structures of the disulfides derived from (R) and [S]-2-phenylpropanoic acid establish the stereochemistry and the helicity of these materials, and density functional theory calculations suggest that the high specific rotations can be due to preferred retention of this helicity in solution. Chiral HPLC showed that the final products were formed with enantiomeric purities from 86.1% to>99.9%.

Synthesis of dinucleoside acylphosphonites by phosphonodiamidite chemistry and investigation of phosphorus epimerization

Summary The reaction of the diamidite, (iPr2N)2PH, with acyl chlorides proceeds with the loss of HCl to give the corresponding acyl diamidites, RC(O)P(N(iPr)2)2 (R = Me (7), Ph (9)), without the intervention of sodium to give a phosphorus anion. The structure of 9 was confirmed by single-crystal X-ray diffraction. The coupling of the diamidites 7 and 9 with 5′-O-DMTr-thymidine was carried out with N-methylimidazolium triflate as the activator to give the monoamidites 3′-O-(P(N(iPr)2)C(O)R)-5′-O-DMTr-thymidine, and further coupling with 3′-O-(tert-butyldimethylsilyl)thymidine was carried out with activation by pyridinium trifluoroacetate/N-methylimidazole. The new dinucleoside acylphosphonites could be further oxidized, hydrolyzed to the H-phosphonates, and sulfurized to give the known mixture of diastereomeric phosphorothioates. The goal of this work was the measurement of the barrier to inversion of the acylphosphonites, which was expected to be low by analogy to the low barrier found in acylphosphines. However, the barrier was found to be high as no epimerization was detected up to 150 °C, and consistent with this, density functional theory calculations give an inversion barrier of over 40 kcal/mol.

Syntheses and Reactions of Heterodinuclear Alkoxycarbyne Complexes

The alkoxycarbyne ligand is an isomer of the more common acyl and alkyl carbonyl moieties. We describe here the synthesis of two heterodinuclear methoxycarbyne complexes, and their decomposition to mononuclear methyl complexes. Reaction of CpFe(CO)2 −Na+ with MeCpMn(CO)2(THF) or of MeCpFe(CO) 2 − with CpCr(CO)(NO)THF gives the heterodinuclear monoanions [Cp(CO)Fe(μ-CO)2Mn(CO)MeCp]− Na+ and [MeCp(CO)Fe(μ-CO)2Cr(NO)Cp]−Na+, respectively, in good yield. The X-ray crystal structure of the Ph3PCH 3 + salt of the FeCr anion confirms both the structure and the location of the NO ligand. Alkylation of the FeMn anion with CH3OSO2CF3 and of the FeCr anion with (CH3)3O+BF 4 − gives the methoxycarbynes 1 and 2, respectively. Carbyne 1 is obtained in 62% yield as a mixture of cis and trans isomers that interconvert rapidly at room temperature, while 2 is obtained as chromatographically separable cis (2c) and trans (2t) isomers, each in 10% yield.