R-3 Repository :: Browsing by Author "Engel, Paul S."

Browsing by Author "Engel, Paul S."

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Biodegradable, phosphate-containing, dual-gelling macromers for cellular delivery in bone tissue engineering
(Elsevier, 2015) Watson, Brendan M.; Vo, Tiffany N.; Tatara, Alexander M.; Shah, Sarita R.; Scott, David W.; Engel, Paul S.; Mikos, Antonios G.
Injectable, biodegradable, dual-gelling macromer solutions were used to encapsulate mesenchymal stem cells (MSCs) within stable hydrogels when elevated to physiologic temperature. Pendant phosphate groups were incorporated in the N-isopropyl acrylamide-based macromers to improve biointegration and facilitate hydrogel degradation. The MSCs were shown to survive the encapsulation process, and live cells were detected within the hydrogels for up to 28 days inﾠvitro. Cell-laden hydrogels were shown to undergo significant mineralization in osteogenic medium. Cell-laden and acellular hydrogels were implanted into a critical-size rat cranial defect for 4 and 12 weeks. Both cell-laden and acellular hydrogels were shown to degrade inﾠvivo and help to facilitate bone growth into the defect. Improved bone bridging of the defect was seen with the incorporation of cells, as well as with higher phosphate content of the macromer. Furthermore, direct bone-to-hydrogel contact was observed in the majority of implants, which is not commonly seen in this model. The ability of these macromers to deliver stem cells while forming in situ and subsequently degrade while facilitating bone ingrowth into the defect makes this class of macromers a promising material for craniofacial bone tissue engineering.
I. Dissociation and aromatization of a semibenzene. Reactions of triphenylmethyl and methyl isobutyryl radicals. II. The chemistry of alpha- and beta- azo radicals
(1993) Wang, Chengrong; Engel, Paul S.
Part I. Semibenzene methyl 7,7-diphenyl-p-mentha-1(7),2,5-triene-8-carboxylate (1), which can be regarded as the para recombination product of triphenylmethyl and methyl isobutyryl radicals (5), affords exactly these intermediates on thermolysis or, inefficiently, on direct photolysis. Whereas thiophenol or triplet 9-fluorenone aromatize 1, thermolysis in the presence of 1,4-cyclohexadiene allows trapping of 5 and oligomeric radicals. From the measured heat of aromatization, the C-C bond dissociation enthalpy of 1 and its analog lacking the side chain ("p-isotriphenylmethane") is calculated to be the lowest value known for any closed shell neutral hydrocarbon. Exposure of 1 to the atmosphere causes rapid autoxidation to the hydroperoxide. Part II. The thermolysis of 2,3-dimethyl-2,3-bis(phenylazo)butane (16a) leads exclusively to cleavage of the central C-C bond forming 2-(phenylazo)isopropyl radicals (17a). These radicals recombine to the C-N dimer 19a about 40 times faster than to the more stable C-C dimer 16a. The thermolysis of 2,3-dimethyl-2,3-bis(tert-butylazo)butane (16b) leads to C-N homolysis via a short lived $\beta$-azo radical 18b. A mechanistic study of 16b shows that C-N homolysis greatly dominates over C-C homolysis. Nevertheless, we estimate that the latter process would occur with $\Delta$G$\dagger$ (150$\sp\circ$C) $>$ 40.3 kcal/mol, which implies that the RSE of aliphatic hydrazonyl radical 17b is at least 6 kcal/mol below that of 17a, and it is little or no more than the RSE of the 1,1-dimethylallyl radical. The nature of the decomposition products of 2-cyclopropylazo-2-(phenylazo)-propane (26) suggests that the thermolysis of 26 mainly causes cleavage of one C-N bond on the cyclopropyl side, while photolysis of 26 leads to the predominant cleavage of the C-N bond on the phenyl side. Attempted determination of the heat of hydrogenation of the N=N double bond via catalytic hydrogenation of azo-n-butane (ANB) and oxidation of di-n-butyl hydrazine (DNBH) by HgO and O$\sb2$ gave varying values of $\Delta$H$\sb{\rm H}$ centering around 20 kcal/mol. 3-Methyl-1-phenyl-3-(phenylazo)-butene-1 (20) was prepared to study the thermolysis kinetics. The similarity of the BDH of 2a and 1a implies that the radical stabilization provided by the azo group is no better than that of the allyl group. (Abstract shortened by UMI.)
Measurement of the trapping rate constants and product studies of quinone derivative inhibitors with alpha-phenethyl radicals
(2009) Park, Hee Jung; Engel, Paul S.
Polymerization of olefins such as styrene, one of many industrially important monomers, is a troublesome problem during storage, transportation and processing. The problem is attenuated by addition of polymerization inhibitors such as quinone derivatives. Despite the importance of quinone type inhibitors, their chemistry is poorly understood. The goal of this research is to better understand the reaction mechanism by measuring trapping rates and product studies of quinone type inhibitors with alpha-phenethyl radicals. cis Azo-alpha-phenylethane was used as a new simple, unimolecular, low temperature source of alpha-phenethyl radicals. The absolute rate constants of five quinone type inhibitors (QIO, BQ, QM, DTBQ and DPQI) with alpha-phenethyl radicals generated from cis azo-alpha-phenylethane were measured by the radical dock method using the stable nitroxide SG1 as the dock. We found that the trapping rate decreased in the order QIO>BQ>QM>DTBQ=DPQI. Also product studies of QIO, BQ, QM, and DPQI with alpha-phenethyl radicals were done to augment previous results. A detailed product study revealed the formation of semi-stable cyclohexadienones in some cases as well as a temperature-dependent product distribution from 1,4-benzoquinone. The trapping of alpha-phenethyl radicals is sometimes reversible, dimishing the efficiency of quinone inhibitors at elevated temperatures.
Measurement of the trapping rate constants and product studies of quinone derivative inhibitors with phenethyl radicals
(2007) Park, Hee Jung; Engel, Paul S.
Styrene is one of many industrially important monomers. Polymerization of unsaturated monomers such as styrene is a troublesome problem during storage, transportation and processing. The problem is attenuated by addition of polymerization inhibitors such as quinone derivatives. Despite the importance of quinone type inhibitors, their chemistry is poorly understood. The goal of this research is to better understand the reaction mechanism and rates of alpha-phenethyl radicals with quinone type inhibitors. cis Azo-alpha-phenylethane was used as a new simple, unimolecular, low temperature source of alpha-phenethyl radicals. The absolute rate constants of five quinone type inhibitors (N-oxide, BQ, QM, tBQ and DPQI) with alpha-phenethyl radicals generated from cis azo-alpha-phenylethane were measured by the radical clock method using the stable nitroxide SG1 as the clock. We found that the trapping rate decreased in the order N-oxide>BQ>QM>tBQ>DPQI. Also product studies of BQ and QM with phenethyl radicals were done to augment previous results.
Mechanistic study of the 1-cyclopropyl-2,2-dimethyl-1,3-cyclopentanediyl biradical
(1994) Lowe, Kimberly Lorraine; Engel, Paul S.
The synthesis of the azoalkane precursor 21 was accomplished through a Diels-Alder route. This precursor provided a clean and convenient source of the 1,3-biradical 13 upon photolysis. Determination of the product ratio from the triplet biradical by gas chromatographic analysis yielded the rate of intersystem crossing of the biradical from the triplet to the singlet energy state. Structure confirmation of the cyclopropyl ring opening product was accomplished through co-injections of photolysis product and an authentic sample on two different GC columns. The lifetime of 13T was found to lie in the range of 0.4 ns to 18 ns. Comparison of this value to that of 6, whose lifetime is 59 ns, confirms that gem-dimethyls have the effect of increasing the intersystem crossing rate.
Part I. The radical chemistry of geminal bis-azoalkanes. Part II. The radical chemistry of vicinal bis-azoalkanes. Part III. Homolysis of a weak carbon-carbon bond
(1989) Chen, Yanqiu; Engel, Paul S.
Part I. The radical chemistry of geminal bis-azoalkanes. Two geminal bis-azoalkanes have been employed to generate carbon centered radicals adjacent to azo substituents. The activation free energies for C-N bond homolysis of 2,2-bis-(phenylazo)propane (1) and 2-methylazo-2-phenylazopropane (2) were 8 kcal/mol lower than for the corresponding model compounds 3-methyl-3-phenylazo-1-butene (4) and 3-methyl-3-methylazo-1-butene (5). This large rate enhancing effect of an $\alpha$-phenylazo substituent is ascribed to the high energy of azo functional groups. 2-(Phenylazo)-2-propyl radical was easily trapped by good hydrogen donors such as thiophenol to afford a hydrazone. The interconversion of geminal bis-azoalkane 2 and tetrazene 3 by low temperature photolysis has been observed. Part II. The radical chemistry of vicinal bis-azoalkanes. Three vicinal bis-azoalkanes have been synthesized by oxidation of hydrazones. In thermolysis, substituents on the azo group dramatically influence the bond homolysis pattern, which varies from pure C-C homolysis when the substituent is phenyl to pure C-N homolysis in the case of a t-butyl substituent. Phenylazo accelerates $\beta$ C-C bond homolysis by approximately 3.2 kcal/mol more than the methylazo group. This result supports the delocalized $\pi$ electronic structure of the 2-(phenylazo)-2-propyl radical. The photochemical behavior of vicinal bis-azoalkanes resembles that of mono azoalkanes except for 9, which underwent both C-C and C-N bond homolysis. $\beta$-Azo radicals from stepwise C-N bond homolysis have been clearly trapped. Part III. Homolysis of a weak C-C bond. The formal para-recombination product of 2-methoxycarbonylpropyl and trityl radicals (53) was readily air oxidized to hydroperoxide through a radical chain mechanism. The weak C-C bond in 53 was cleaved homolytically in thermolysis, photolysis, and triplet sensitized photolysis generating methyl isobutyrate radical and triphenylmethyl radical. This result rules out McElvain's concerted thermolysis mechanism. Triphenylmethyl radical underwent intramolecular cyclization under the photolysis conditions. The facile aromatization of 53 through a radical mechanism was observed.
Part I.~Reduction of azoalkanes by benzhydryl radicals. Part II.~Thermolysis and photolysis of an alpha-aminoazoalkane, 2-tert-butylazo-2-dimethylaminopropane. Part III.~A reinvestigation of the photochemistry of trifluoromethylazocyclopropane
(1989) Wu, Wen-Xue; Engel, Paul S.
Benzhydryl radicals donate a hydrogen atom rapidly to the less hindered nitrogen atom of aliphatic and aromatic azo compounds, leading to the corresponding hydrazines. When the initially formed hydrazyl radical possesses a weak $\beta$ bond, it undergoes scission before receiving a second hydrogen atom. An unsymmetrical $\alpha$-aminoazoalkane, 2-tert-butylazo-2-dimethylaminopropane (3c) has been prepared by reaction of 2-tert-butylazo-2-chloropropane (4c) with dimethylamine. A photolysis and thermolysis study revealed that this azoalkane is unusual because it is sensitive to acid and protic solvents. Normal homolysis of 3c in nonpolar solvents allows assessment of the radical stabilizing ability of the dimethylamino group. The solution phase photolysis of trifluoromethylazocyclopropane (1) has been reinvestigated. The results showed that this azocyclopropane undergoes the same primary photoreactions as the other azocyclopropanes that have been studied. It is concluded that the original work on 1 is seriously in error. (Abstract shortened with permission of author.)
Photochemical studies of single-walled carbon nanotube ozonides and alpha-azoxy ketones
(2007) Tsvaygboym, Konstantin; Engel, Paul S.
This thesis contributes to two disparate problems in chemistry: studying properties of carbon nanotube ozonides and products of their decomposition and determining behavior of alpha-azoxy radicals. This work demonstrates that interaction of ozone with single-walled carbon nanotubes (SWNT) results in formation of 1,2,3-trioxolanes (SWNTO3). Their formation rate was found to be on the order of subseconds at room temperature for diluted SWNT---1% aqueous SDS suspensions. SWNTO3 decayed to SWNT epoxides (SWNTO) with release of molecular oxygen. Gas evolution measurements performed on dry ozonated SWNT showed oxygen release to follow a simple exponential rise with rates approximately 1.5 - 2 min-1 at r. t. The lifetime of SWNTO3, with a dissociation activation energy of approximately 0.7 eV, depends on temperature and SWNT type. At room temperature, it is less than two minutes for small-diameter SWNTs suspended in water. Ozonides exhibited extreme quenching of SWNT fluorescence and substantial bleaching of NIR absorption. The maximum number of 1,2,3-trioxolanes forming on the surface of SWNT at any given time was found to be less than 4% of the theoretical value, indicating a saturation point. Reaction of ozonated nanotubes with excess ozone is limited by the SWNTO3 decomposition rate. Thinner tubes exhibited faster ozonide decay rates resulting in greater oxidation levels over time in excess of ozone. Ozonation with small quantities of ozone did not result in a D-band increase in the Raman spectra, both for solid and liquid state experiments, though substantial decrease of the G band was observed. IR absorbance kinetics of SWNT films revealed exponential intensity drift over time with rates close to those in fluorescence and NIR absorbance techniques. Ozonated SWNTs were found to abstract electrons from amines and thiols, thus resulting in covalent attachment of nucleophiles to the sidewall. The azoxy functional group greatly stabilizes an attached carbon-centered radical, but the chemistry of such alpha-azoxy radicals is unclear. This work reports that generation of alpha-azoxy radicals by irradiation of alpha-azoxy ketones PhCO-C(Me)2-N=N(O)-R causes ketone rearrangement to azoester compounds PhCOO-C(Me)2-N=N-R. This study proposes a mechanism for this rearrangement.
Photolysis of azoalkanes. Reactions and kinetics of the 1-cyclopropyl-1,3-cyclopentanediyl biradical and 1-cyclopropylcyclopentyl radical
(1991) Culotta, Anne Marie; Engel, Paul S.
The 1-cyclopropylcyclopentyl radical (114a) and 1-cyclopropylcyclohexyl radical (114b) were generated in the presence of PhSH via 366 nm photolysis of azo-1-cyclopropylcyclopentane and azo-1-cyclopropylcyclohexane respectively. These radicals undergo cyclopropylcarbinyl (CPC) rearrangement to primary homoallylic radicals 115a and 115b, respectively, at rates of 1.45 $\times$ 10$\sp7$ s$\sp{-1}$ and 1.1 $\times$ 10$\sp7$ s$\sp{-1}$ based on competitive trapping with PhSH. These rates are 6-9 times slower than the parent CPC radical. E$\sb{\rm a}$ for 114a$\to$115a was found to be 7.0 kcal/mol and log A = 12.3. Deuterium labeling studies show that homoallylic radical 115a recloses to 114a with k$\sb{\rm rr}$ = 5.5 $\times$ 10$\sp4$ s$\sp{-1}$. Unsymmetrical triplet biradical 1-cyclopropyl-1,3-cyclopentanediyl (104T) is produced from the triplet sensitized photolysis of 1-cyclopropyl-2,3-diazabicyclo (2.2.1) hept-2-ene (123). Biradical 104T undergoes CPC rearrangement to a pair of 1,6-biradicals 105E and 105Z the latter of which undergoes rapid intramolecular disproportionation to Z-3-propylidenecyclopentene (108Z). On account of its geometry, 105E cannot lead directly to a stable product; hence, it recloses to 104T (k$\sb{\rm ra}$ = 1.2 $\times$ 10$\sp5$ s$\sp{-1}$) but interestingly, not to 104S. If the CPC rearrangement rate of 104T is taken as that of 114a, we calculate from the product distribution that the lifetime ($\tau$ = 1/k$\sb{\rm isc}$) of 104T is 59 ns. This figure is only one half the lifetime of the parent 1,3-cyclopentanediyl and is close to $\tau$ = 42 ns for 1,3-dimethyl-1,3-cyclopentanediyl, showing that the effect of the cyclopropyl group is similar to that of methyl. ISC of 104T displays linear Arrhenius behavior over 3.4-61.0$\sp\circ$C. Activation parameters for ISC of 104T were determined via competition with the CPC rearrangement and were found to be E$\sb{\rm a}$ (ISC) = 1.6 kcal/mol and log A = 8.4. Thus 104T is shown to be a ground state triplet and the low value of log A is characteristic of spin-forbidden ISC.
Rh(II) metallopeptides for asymmetric catalysis
(2015-02-17) Sambasivan, Ramya; Ball, Zachary T.; Engel, Paul S.; Verduzco, Rafael
The development of peptides as chiral ligands for asymmetric Rh(II) catalysis is discussed in this work. Mother Nature’s solution to chiral ligand design is to make use of the naturally available chiral building blocks – amino acids. Polypeptides, built from amino acids, provide a diverse, modular and functional-group-rich framework for the development of selective transition- metal catalysts and enable facile ligand screening. The dirhodium core has a paddlewheel structure that can ligate readily to the side chain carboxylate of aspartate or glutamate in a peptide chain. When a peptide containing aspartates or glutamates in the i and i+4 position is complexed to dirhodium, the transition metal complex thus formed has a defined peptide secondary structure with retention of catalytic activity at the metal center. The first-generation approach used a solution-phase library of dirhodium bis-peptide catalysts for the enantioselective Si–H bond insertion and cyclopropanation. The parallel and anti-parallel orientation of bis-peptide catalysts was determined by pyrene fluorescence. Subsequently, parallel on-bead screening of catalyst libraries allowed significantly higher throughput by obviating the need for purification and isolation of individual catalysts. This enables the synthesis and screening of catalysts in 96-well plate format within a few days. More recently, a mono-peptide catalyst with a tridentate eq-eq-ax peptide ligand has been identified which exhibits differential behavior in solution and on bead. New insights into the development of immobilized homogeneous catalysts for stereoselective catalysis are discussed.
Single-walled carbon nanotubes: Induced decomposition of peroxides and non-covalent encapsulation into water-soluble PEG-eggs
(2009) Abmayr, David William, Jr; Engel, Paul S.
This thesis presents two studies aimed of furthering the understanding of single-walled carbon nanotube (SWNT) chemistry for potential applications in composite and biological systems. In composite systems, SWNTs are used as structural members, and bis-acyl peroxides are frequently used as reaction initiators to cure the surrounding matrix. The behavior of the peroxide is often critical to the performance of the final composite. In this study, SWNTs are shown to induce the decomposition of a series of bis-acyl peroxides by single electron transfer. Four bis-acyl peroxides are evaluated for decomposition rate in the presence of SWNTs via iodometry. The resulting SWNTs are analyzed for functionalization by Raman microscopy and X-ray Photoelectron Spectroscopy (XPS). Benzoyl peroxide (BP), p-methoxybenzoyl peroxide (pMBP), phthaloyl peroxide (PhP), and trifluoroacetyl peroxide (TFAP) have known decomposition characteristics and known sensitivities to electron sources. This study demonstrates that all four peroxides undergo induced decomposition in the presence of SWNTs. Of the four, phthaloyl peroxide exhibits the greatest increase, followed by TFAP, BP, and pMBP. This study also demonstrates that all but TFAP functionalize the SWNTs. The decomposition data may be used to design improved composite systems. In aqueous solution, the ability of SWNTs to heat up upon exposure to radiofrequency energy gives them potential uses in biological systems. SWNTs are not soluble alone in aqueous solution, so one approach is to use amphiphilic triblock copolymers to capture and isolate SWNTs in water. This study addresses the difficulties encountered in synthesizing these polymers reproducibly and controllably. Presented here are modifications to the Atom Transfer Radical Polymerization (ATRP) method that not only enable the reproducible synthesis of these triblocks, but also enable them to be made in a highly controlled manner with specific block lengths. The SWNTs encapsulated by the polymers made through this new approach are shown not only to retain their fluorescence but also to demonstrate fluorescence on par with the best surfactants in current use. These structures are expected to provide a new entry into the use of nonfunctionalized SWNTs in biological systems such as radiofrequency heating for the destruction of cancer cells.
Some applications of calorimetric techniques
(1984) Grow, Robert Thomas; Margrave, John L.; Wilson, Lon J.; Engel, Paul S.
Four calorimetric techniques -- Oxygen Bomb, Fluorine Bomb, Solution, and Levitation Calorimetry -- were applied to obtain new thermochemical data for a variety of organic and inorganic compounds. Oxygen bomb calorimetry measurements were performed on the following compounds and the enthalpies of formation obtained for 2,2,4-trimethylpentane, meta- and parabenzodicyclopentene, 1,5-diphenylnaphthalene, and 2,3-diazabicyclo[2.2.2]oct-2-ene are: -258.28 + 5.18, -53.6 ± 1.62, -9.18 ± 1.7, 216.12 ± 5.87, and 13.39 ± 1.55 kJ mole, respectively. Enthalpy of formation measurements made on coal model compounds (naphthalene, biphenyl, phenanthrene, 2-methylnaphthalene, 1,2,3,4-tetrahydronaphthalene, and 2-ethylnaphthalene) gave the following values: 77.63 ± .75, 99.66 ± .67, 116.19 ± 1.66, 32.87 ± .71, -64.14 ± 2.17, and -76.92 ± 1.76 kJ mole, respectively. Fluorine bomb calorimetry was used to obtain high precision values for the enthalpies of formation of SFg(g) and SiF4(g) as -122.83 ± .64 and -1611.6 ± 1.32 kJ mole, respectively. The compounds molybdenum diselenide and tetraarsenic tetrasulfide were burned in fluorine to obtain enthalpies of formation of -236.4 ± 1.31 and -132.92 ± 6.4r respectively. Solution calorimetry was performed on the compounds: lithium oxide, cesium bromide, and potassium acid phthalate to establish their enthalpies of solution as -131.746 ± .14, 26.143 ± .43, and -156.5 ± .59 kJ mole-1, respectively. Levitation calorimetric studies of liquid tungsten indicate the enthalpy of fusion of tungsten to be 52. ± 2.3 kJ mole.
Synthesis and Characterization of Injectable, Biodegradable, Phosphate-Containing, Chemically Cross-Linkable, Thermoresponsive Macromers for Bone Tissue Engineering
(American Chemical Society, 2014) Watson, Brendan M.; Kasper, F. Kurtis; Engel, Paul S.; Mikos, Antonios G.
Novel, injectable, biodegradable macromer solutions that form hydrogels when elevated to physiologic temperature via a dual chemical and thermo-gelation were fabricated and characterized. A thermogelling, poly(N-isopropylacrylamide)-based macromer with pendant phosphate groups was synthesized and subsequently functionalized with chemically cross-linkable methacrylate groups via degradable phosphate ester bonds, yielding a dual-gelling macromer. These dual-gelling macromers were tuned to have transition temperatures between room temperature and physiologic temperature, allowing them to undergo instantaneous thermogelation as well as chemical gelation when elevated to physiologic temperature. Additionally, the chemical cross-linking of the hydrogels was shown to mitigate hydrogel syneresis, which commonly occurs when thermogelling materials are raised above their transition temperature. Finally, degradation of the phosphate ester bonds of the cross-linked hydrogels yielded macromers that were soluble at physiologic temperature. Further characterization of the hydrogels demonstrated minimal cytotoxicity of hydrogel leachables as well as in vitro calcification, making these novel, injectable macromers promising materials for use in bone tissue engineering.
Synthesis and Characterization of Novel, In Situ-Forming, Biodegradable Hydrogels for Cellular Delivery in Craniofacial Bone Tissue Engineering
(2014-08-01) Watson, Brendan M; Mikos, Antonios G.; Grande-Allen, K. Jane; Engel, Paul S.; Kasper, F. Kurtis
Extensive trauma, tumor resection or congenital abnormalities can lead to craniofacial bone defects that require intervention for repair. These defects are often treated with autologous grafts, which are limited in supply, often result in donor site morbidity, and are difficult to mold to appropriately match craniofacial contours. This thesis focused on the synthesis and characterization of a novel, in situ-forming biodegradable hydrogel for cellular delivery in craniofacial bone tissue engineering. To generate these hydrogels, a novel, thermogelling macromer containing pendant phosphate groups was synthesized and subsequently modified by the attachment of chemically cross-linkable methacrylate groups. The components of these macromers varied to evaluate the effects of differing components on macromer properties. This information was used to select two formulations of macromers that were shown to remain in solution at room temperature and undergo dual thermo and chemical gelation to form stable hydrogels at physiologic temperature. The hydrogels formed from these macromers were characterized for cytotoxicity and rate of degradation, which was found to be accelerated in the presence of alkaline phosphatase, an enzyme that is upregulated in newly formed bone. Furthermore, they were shown to support encapsulated mesenchymal stem cells for up to 28 days and undergo cellular induced mineralization under osteogenic conditions. These gels were then evaluated for their ability to generate bone in a critical-size rate cranial defect. The hydrogels demonstrated degradation within the defect and subsequent bone growth into the defect. Finally, the effects of incorporation of inorganic hydroxyapatite nanoparticles into the gels on cellular interaction and hydrogel physiochemical properties were evaluated. This research demonstrated the potential for thermogelling macromers to be designed to form degradable hydrogels that can be used to deliver cells into craniofacial bone defects.
Synthesis of functionalized naphthalenes
(1983) Lewis, Linda Kay; Billings, W. E.; Engel, Paul S.; Lewis, Edward S.
1,2 and 2,3-disubstituted naphthalenes were synthesized by carbene addition to substituted indenes. The indene precursors were formed by two methods: Grignard addition to indanones with elimination and by the reaction of indanones with phosphorus pentahalide. The carbene addition to indene was facilitated fcy the use of a micellar phase transfer catalyst, hexadecyltrimethyl ammonium branide. This catalyst allowed carbene addition to double bonds with both electron donating and withdrawing substituents. The naphthalenes formed by this method include: 2-chloro-3-methylnaphthalene, 2-chloro-3-phenylnaphthalene, 1,2-dichloronaphthalene, 2-chloro-l-phenylnaphthalene, 2-brano-3chloronaphthalene and 2,3-dichioronaphthalene.
Synthesis of Phenanthroizidine Alkaloids and a Practical Total Synthesis of (208)-(+)-Camptothecin
(1997) Roschangar, Frank; Ciufolini, Marco A.; Engel, Paul S.; Rudolph, Frederick B.
Chapter I describes the total syntheses of the representative phenanthroizidine alkaloids tylophorine, antofine and of their seco congeners, septicine and julandine, which have been accomplished through the CiufoliniByrne pyridine synthesis. Chapter II discusses the evolution of our strategy for the synthesis of (208)-( + )-Camptothecin.
Synthesis of poly(propylene fumarate) by acylation of propylene glycol in the presence of a proton scavenger
(2002-03-12) Peter, Susan J.; Suggs, Laura J.; Engel, Paul S.; Mikos, Antonios G.; Rice University; United States Patent and Trademark Office
High molecular weight linear poly(propylene fumarate) having a relatively low polydispersity index by utilizing a relatively pure intermediate and a method for making same. Fumaryl chloride and propylene glycol are reacted in the presence of potassium carbonate. The potassium carbonate present in the reaction solution prevents the acid by-product from catalyzing reactions at the fumarate double bond. The bis(propyl fumarate) produced according to this technique can be transesterified using conventional processing steps to yield P(PF). The P(PF) produced from bis(propyl fumarate) produced according to the present method has a higher molecular weight and is purer than P(PF) produced using previously known techniques.
Synthesis of radioactive materials and compositions of same
(2009-11-24) Gadeken, Larry L.; Engel, Paul S.; Laverdure, Kenneth S.; Rice University; BetaBatt, Inc.; United States Patent and Trademark Office
The present invention relates generally to synthesis of radioactive material, such as a tritiated polymer, and an apparatus for generating electrical current from the nuclear decay process of a radioactive material. In one embodiment, the invention relates to an energy cell (e.g., a battery) for generating electrical current derived from particle emissions occurring within a radioactive material such as a tritiated polymer) on pore walls of a porous semiconductor. The radioactive material may be introduced into the energy cell by a wetting process.
The photochemistry of (alpha)-acetylenic ketones
(1977) Schroeder, Myron Eugene; Engel, Paul S.
An investigation of the photochemistry of several a-acetylenic ketones was instituted. The photochemical behavior of 2-octyne-4-one 22 was compared with that of 2-octene-4-one 2j. Whereas the latter underwent only cis-trans olefin isomerization, the former proceeded efficiently to y-hydrogen abstraction products. Both photochemical and spectroscopic properties of 22. reveal its similarity to valerophenone 58. 4,4-dimethyl-1hexyne-3-one 23 was synthesized in order to study a-methyl effects in the photochemistry of ynones. 4,4-dimethyllhexyne-5-ene-3-one 24^ was also synthesized and a preliminary inspection of the photochemistry of 23. and 24 was made.
The photochemistry of azocyclopropanes
(1988) Bodager, Gregory Allen; Engel, Paul S.
The photochemistry of a number of azocyclopropanes has been studied: cis-1-(methyl-trans-azo)-2-phenylcyclopropane, trans-10(methyl-trans-azo)-2-phenylcyclopropane, trans-azocyclopropane and cis-azocyclopropane. Direct irradiation into the lowest n $\rightarrow\pi$* band led to nearly exclusive cis-trans isomerization of the azo group. Minor processes for the phenylcyclopropanes were cyclopropyl ring isomerization, rig expansion of 1-methyl-5-phenyl-2-pyrazoline and formation of styrene. These processes suggest formation of a biradical intermediate which is apparently not formed in the case of trans- and cis-azocyclopropane. Singlet sensitization of cis-1-(methyl-trans-azo)-2-phenylcyclopropane with benzene increased the quantum yields of the processes competing with azo group isomerization. The increased efficiencies indicate that the second excited state of the azoalkane contributed to the observed photochemistry. On the other hand, the second excited state of trans- and cis-azocyclopropane is not accessible to benzene singlet sensitization and so led to exclusive interconversion of the two azo isomers. Direct irradiation into the short wavelength UV band of trans- and cis-azocyclopropane in pentane produced more decomposition products than n $\rightarrow\pi$* excitation. The efficiency of trans $\rightarrow$ cis azo group isomerization decreased at the expense of the C-N and cyclopropyl C-C homolyses. These two cleavages accounted for a large number of products including nitrogen, cyclopropane, ethylene and 1-cyclopropyl-2-pyrazoline. Cyclopropyl ring cleavage to a biradical intermediate followed by reclosure to regenerate the starting azoalkane was postulated as one of the major processes to rationalize the low quantum yield of products. Formation of products propene, allylcyclopropane and 1,5-hexadience upon irradiation at 193 nm suggested production of vibrationally excited cyclopropyl radicals which rearrange to allyl. Short wavelength irradiation of trans-azocyclopropane in the vapor phase led to a large number of decomposition products. Triplet-sensitization of cis-1-(methyl-trans-azo)-2-phenylcyclopropane produced both azo and cyclopropyl group isomerization products, but not styrene. The differing product distribution relative to direct irradiation indicates that the triplet state is not exclusively involved in the singlet photochemistry. The unusually high quantum yield found for triplet-sensitized isomerization of trans-azocyclopropane to the cis isomer was initially rationalized in terms of cyclopropyl ring cleavage to form a biradical intermediate. However, a stereospecifically labelled deuterium analog of trans-azocyclopropane, prepared to test the biradical hypothesis, showed that a such an intermediate is probably not formed upon triplet sensitization.

Browsing by Author "Engel, Paul S."

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