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At least 307 records · Page 17Linked to original sources
PubMed2003

3-Boronoacrolein as an exceptional heterodiene in the highly enantio- and diastereoselective Cr(III)-catalyzed three-component [4+2]/allylboration.

This Communication reports the optimization of the first catalytic enantio- and diastereoselective hetero[4+2]/allylboration reaction to provide efficient access to alpha-hydroxyalkyl pyran derivatives. The key substrate 3-boronoacrolein pinacolate appears to be an exceptionally favorable heterodiene for use in Jacobsen's enantioselective reverse electron demand hetero[4+2] reaction with enol ethers, catalyzed by the tridentate (Schiff base)chromium complex 1. This one-pot three-component reaction was successfully applied to a concise total synthesis of (5R,6S)-6-acetoxy-5-hexadecanolide (2), the oviposition attractant pheromone of the female Culex mosquito capable of transmitting the West Nile virus.

Acrolein↗
PubMed2004

Electron-deficient bonding in rhomboid rings.

The bonding environment of boron is usually thought about in terms of localized 2c-2e/3c-2e bonding (as in diborane) or completely delocalized polyhedral bonding (as in B(12)H(12)(2)(-)). Recently, a number of boron compounds having a rhomboidal B(4) framework have been synthesized; these show an amazing variation in their skeletal electron count, one that cannot be interpreted in familiar ways. In this report, we systematically explore the origin of the range of electron counts in these compounds. We find that four skeletal MOs are primarily responsible for keeping the B(4) skeleton together. As a subunit in a macropolyhedral environment, termed rhombo-B(4), such an arrangement of B atoms deviates from Wade's rule by three electron pairs (if treated as a distorted arachno system derived from B(6)H(6)(2)(-)). Aided by this analysis, we examine the nature of bonding in Na(3)B(20), where the rhombo-B(4) unit forms linear chains fusing closo-B(7) units. Theory suggests that this structure requires one more electron per formula unit for optimal bonding. Finally, we study the nature of bonding in beta-SiB(3), where silicon atoms also adopt the rhomboid framework.

Journal Article↗
PubMed2004

In vitro demonstration of the heavy-atom effect for photodynamic therapy.

Photodynamic therapy (PDT) is an emerging treatment modality for a range of disease classes, both cancerous and noncancerous. This has brought about an active pursuit of new PDT agents that can be optimized for the unique set of photophysical characteristics that are required for a successful clinical agent. We now describe a totally new class of PDT agent, the BF2-chelated 3,5-diaryl-1H-pyrrol-2-yl-3,5-diarylpyrrol-2-ylideneamines (tetraarylazadipyrromethenes). Optimized synthetic procedures have been developed to facilitate the generation of an array of specifically substituted derivatives to demonstrate how control of key therapeutic parameters such as wavelength of maximum absorbance and singlet-oxygen generation can be achieved. Photosensitizer absorption maxima can be varied within the body's therapeutic window between 650 and 700 nm, with high extinction coefficients ranging from 75,000 to 85,000 M(-1) cm(-1). Photosensitizer singlet-oxygen generation level was modulated by the exploitation of the heavy-atom effect. An array of photosensitizers with and without bromine atom substituents gave rise to a series of compounds with varying singlet-oxygen generation profiles. X-ray structural evidence indicates that the substitution of the bromine atoms has not caused a planarity distortion of the photosensitizer. Comparative singlet-oxygen production levels of each photosensitizer versus two standards demonstrated a modulating effect on singlet-oxygen generation depending upon substituent patterns about the photosensitizer. Confocal laser scanning microscopy imaging of 18a in HeLa cervical carcinoma cells proved that the photosensitizer was exclusively localized to the cellular cytoplasm. In vitro light-induced toxicity assays in HeLa cervical carcinoma and MRC5-SV40 transformed fibroblast cancer cell lines confirmed that the heavy-atom effect is viable in a live cellular system and that it can be exploited to modulate assay efficacy. Direct comparison of the efficacy of the photosensitizers 18b and 19b, which only differ in molecular structure by the presence of two bromine atoms, illustrated an increase in efficacy of more than a 1000-fold in both cell lines. All photosensitizers have very low to nondeterminable dark toxicity in our assay system.

Aza Compounds↗
PubMed2006

Design and synthesis of a chemically cleavable fluorescent nucleotide, 3'-O-allyl-dGTP-allyl-bodipy-FL-510, as a reversible terminator for DNA sequencing by synthesis.

We report the construction of a novel chemically cleavable fluorescent labeling system based on an allyl group to modify nucleotide for DNA sequencing by synthesis (SBS). We have found that an allyl moiety can be used successfully as a linker to tether a fluorophore to a 3'-O-allyl-modified nucleotide, forming a chemically cleavable reversible terminator, 3'-O-allyl-dGTP-allyl-Bodipy-FL-510, for application in SBS. The fluorophore and the 3'-allyl group on a DNA extension product, which is generated by incorporating 3'-O-allyl-dGTP-allyl-Bodipy-FL-510, are removed simultaneously in 30 s by Pd-catalyzed deallylation in aqueous buffer solution. The resulting DNA product is successfully used as a primer for further extension in polymerase reaction. This one-pot dual-deallylation reaction thus allows the reinitiation of the polymerase reaction and increases the SBS efficiency.

Allyl Compounds↗
PubMed1996

Synthesis and evaluation of a boronated nitroimidazole for boron neutron capture therapy.

We postulated that nitroimidazoles, previously used for radiosensitizing solid tumors, may be interesting templates as carriers of 10B for boron neutron capture therapy. To test this hypothesis, we synthesized a 10B-enriched nitroimidazole, 1-2[(undecahydro-closo-dodecaborato)thio]ethyl]-2- methyl-5-nitroimidazole (imidocaptate), by coupling the Cs salt of BSH (Cs2-10B12H11SH) with 1-(2-bromoethyl)-2-methyl-5-nitroimidazole followed by purification of the adduct. Imidocaptate was taken up by V-79 cells in culture and showed no inherent toxicity under euoxic conditions up to 1.05 mM (126 micrograms of 10B/mL of culture medium). Imidocaptate showed a dose-dependent decrease in D0 when the treated cells were irradiated with a thermal neutron beam. At the highest dose tested (126 micrograms of 10B/mL of culture medium), the ratio of control to sample D0 values was 2.6 for both linear quadratic and single-hit multitarget models. At 33 micrograms of 10B/mL, imidocaptate showed a control/treated D0 ration (1.5) equal to that observed with the disulfide form of BSH at 28 micrograms of 10B/mL. Compared to BSH and its disulfide, the reduced toxicity and equipotency of imidocaptate suggest that this agent may be useful for boron neutron capture therapy of cancer.

Animals↗
PubMed1999

Synthesis and biological evaluation of boron-containing polyamines as potential agents for neutron capture therapy of brain tumors.

New boron-containing spermidine/spermine (SPD/SPM) analogues have been synthesized: N5-[4-(2-aminoethyl-o-carboranyl)butyl] and N5-{4-[(2,3-dihydroxypropyl)-o-carboranyl]butyl} SPD/SPM derivatives (ASPD-5, ASPM-5, DHSPD-5, and DHSPM-5) as well as N5-{[4-(dihydroxyboryl)phenyl]methyl}spermidine (BBSPD-5). These boronated polyamines retain their ability to displace ethidium bromide from calf thymus DNA and are rapidly taken up in vitro by F98 rat glioma cells. The in vitro toxicities of ASPD-5, ASPM-5, DHSPD-5, and DHSPM-5 are lower than those previously reported for N5-[4-(o-carboranyl)butyl] SPD/SPM derivatives (SPD-5 and SPM-5) but similar to those of native SPD and SPM. Very low toxicity was also observed for BBSPD-5. In vivo studies of ASPD-5 and BBSPD-5 were performed in mice bearing intracerebral implants of the GL261 glioma and subcutaneous implants of the B16 melanoma. The biodistribution data found in both tumor models suggest that the polyamines synthesized to date do not appear to be suitable boron agents for BNCT.

Animals↗
PubMed2000

Asymmetric allylboration and ring closing alkene metathesis: a novel strategy for the synthesis of glycosphingolipids.

A novel strategy for the synthesis of D,L-glucosylceramide 1, a member of the glycosphingolipid class of natural products is described. Reagent-controlled asymmetric Brown allylboration gave excellent stereochemical control in the construction of adjacent stereocenters in the sphingoid base portion of the molecule. The trans-configured double bond was obtained as a single geometrical isomer by use of silicon-tethered olefin metathesis employing the Schrock carbene [(CF3)2MeCO]2Mo(=CHCMe2Ph)(=NC6H3-2,6-i-Pr2++ +) and in situ PhLi-induced ring-opening of the intermediate 5,6-dihydro-2H-1,2-oxasiline followed by protodesilylation with TBAF in DMSO. The synthesis was completed by long chain amide formation and global deprotection.

Alkenes↗
PubMed2001

Suzuki reaction of vinyl triflates from six- and seven-membered N-alkoxycarbonyl lactams with boronic acids and esters.

The Pd(0)-catalyzed reaction of vinyl triflates from N-alkoxycarbonyl lactams with different boron compounds has been studied. The coupling reaction of alkenylboronates and arylboronic acids with six- and seven-membered lactam-derived N-alkoxycarbonyl vinyl triflates was feasible under very mild conditions in THF-water employing (Ph3P)2PdCl2 as a catalyst and Na2CO3 as a base, which provided in high yields the corresponding 6- or 7-substituted N-alkoxycarbonyl-3,4-dihydro-2H-pyridines and N-alkoxycarbonyl-2,3,4,5-tetrahydroazepines. Allylboronates reacted slower but, with vinyl triflates from delta-valerolactam, still gave acceptable yields of the coupling product. Alkylboronic acids required different reaction conditions, in particular the presence of Ag2O together with a base in anhydrous toluene and (dppf)PdCl2 as a catalyst, affording the corresponding 6-alkyl-N-alkoxycarbonyl-3,4-dihydro-2H-pyridines in high yields.

Journal Article↗
PubMed2001

Bromo-boronolactonization of olefins.

Exposure of a variety of mono- and disubstituted ortho-alkenylarylboronic acids to NBS in THF/H(2)O under neutral conditions affords bromo-boronolactones, in some instances, with exceptional regiocontrol. The adducts, analogous to those formed by carboxylic acids, are shown to be useful synthetic intermediates.

Alkenes↗
PubMed2002

Application of the palladium-catalyzed borylation/Suzuki coupling (BSC) reaction to the synthesis of biologically active biaryl lactams.

The palladium-catalyzed, two-step, one-pot borylation/Suzuki coupling (BSC) reaction was developed to synthesize sterically hindered 2,2'-disubstituted biphenyl and phenyl-indole compounds in a short, simple, and efficient manner from two easily accessible aryl halides. High yields can be obtained by choosing properly both components according to their rough electronic properties. The illustration of the utility of this method was provided by the solution and solid-phase synthesis of seven- or eight-membered biphenyl lactams 5a-e, as well as paullone 3a. These compounds exhibit moderate albeit significant cytotoxicities and may serve as structural models for future medicinal chemistry developments.

Alkaloids↗
PubMed2003

Total synthesis of (+)-isoschizandrin utilizing a samarium(II) iodide-promoted 8-endo ketyl-olefin cyclization.

The 13-step synthesis of (+)-isoschizandrin reported herein features a samarium(II) iodide-promoted 8-endo ketyl-olefin coupling to assemble the eight-membered ring present in the target concomitantly with the required functionality and stereochemistry. In constructing (+)-isoschizandrin as a single atropisomer, the synthesis utilizes a kinetic resolution of a seven-membered lactone using a CBS-oxazaborolidine.

Alkenes↗
PubMed2005

Practical and efficient multigram preparation of a camphor-derived diol for the enantioselective Lewis acid catalyzed allylboration of aldehydes.

[reaction: see text] Chiral diols are important molecules with widespread use as chiral auxiliaries and ligands in enantioselective synthesis. Therefore, efficient and practical syntheses of highly dissymmetrical nonracemic diols are still a meaningful pursuit. Two new routes to access camphor-derived chiral diol 1 have been developed. One route employs camphorquinone (3) as the starting material, affording in only two steps the desired diol in 55% overall yield. The second route, from camphor (2), leads to the desired diol in an efficient four-step synthesis, with an overall yield of 55%.

Alcohols↗
PubMed2001

Novel synthetic approach to 19-nor-1alpha,25-dihydroxyvitamin D(3) and its derivatives by Suzuki-Miyaura coupling in solution and on solid support.

19-nor-1alpha,25-Dihydroxyvitamin D(3) was synthesized by the Suzuki-Miyaura coupling of the A-ring intermediate 3, which was efficiently prepared from readily available 5-(tert-butyldimethylsilyl)oxycyclohex-2-enone (5), with the boronate compound of the C,D-ring portion. The method could be applied to a solid-phase synthesis to prepare the des-C,D derivatives of 19-nor-1alpha,25-dihydroxyvitamin D(3). [reaction: see text]

Calcitriol↗
PubMed2004

Synthetic applicability and in situ recycling of a B-methoxy oxazaborolidine catalyst derived from cis-1-amino-indan-2-ol.

A procedure is described that greatly simplifies the use of an oxazaborolidine catalyst derived from (1R,2S) cis-1-amino-indan-2-ol. This B-OMe catalyst has been employed in the asymmetric reduction of a number of structurally diverse prochiral ketones, in particular the reduction of alpha-amino acetophenone and its derivatives. A method for reducing the effective catalyst loading by "in situ recycling" is also presented. [structure: see text]

Amino Alcohols↗
PubMed2007

Synthesis of boron cluster lipids: closo-dodecaborate as an alternative hydrophilic function of boronated liposomes for neutron capture therapy.

We succeeded in the synthesis of the double-tailed boron cluster lipids 4a-c and 5a-c, which have a B12H11S moiety as a hydrophilic function, by S-alkylation of B12H11SH (BSH) with bromoacetyl and chloroacetocarbamate derivatives of diacylglycerols for a liposomal boron delivery system on neutron capture therapy. Calcein encapsulation experiments revealed that the liposomes, prepared from the boron cluster lipid 4b, DMPC, PEG-DSPE, and cholesterol, are stable at 37 degrees C in FBS solution for 24 h. [reaction: see text].

Animals↗