The generality of the [4+1] annulation methodology in the context of natural product synthesis was demonstrated by extending its applicability to the heteroatom (nitrogen) case thus allowing access to the alkaloid field. This novel methodology involved the intramolecular union of a hypovalent nitrogen atom equivalent and a conjugated diene to afford a pyrroline ring. The flexibility of this strategy was exemplified by the formal synthesis of ring-A oxygenated pyrrolizidine alkaloids platynecine 8, turneforcidine 9, hastanecine 10, and dihydroxyheliotridane 11. The key features of this technology involved preparation of azidodiene ill, its cyclization, via the intermediate triazoline which was not isolated, to vinylaziridines 234, and the vinylaziridine-pyrroline rearrangement of several derivatives of 234 to pyrrolizidines 239, 241, and 242. A study of the thermal decomposition of oxygenated azidodienes such as 196 and 233 was carried out. Conclusive results regarding the stereochemical control of the C-7 substituent were attained and used for the formal stereospecific syntheses of pyrrolizidinediols 8, 9, 10, and 11. The possibility of asymmetric induction was also investigated, and was realized in the microbial reduction of only one of the enantiomers of alcohol ill protected as ester 248, providing potential access to either enantiomeric series of pyrrolizidine diols. [see document for diagram of chemical reaction] / Ph. D.

The objective of this study was to develop new synthetical routes to natural and industrial products utilizing ketene cycioaddition reactions. The cycioaddition of diphenylketene with α,β-unsaturated imines yields (2+2) cycioaddition products, g-lactams. However, electron donating groups, such as dimethylamine, in the 4-position of the α,β-unsaturated imines result in (4+2) cycloaddition products, ∂-lactams. Dichloroketene reacted with α,β-unsaturated imines to yield (4+2) cycloaddition products, g-lactams. Large substituents in the 4-position of a, ^-unsaturated imines resulted in a (2+2) cycioaddition product, β-lactam. The ∂-lactams derived from dichloroketene are easily dehydrochlorinated to the corresponding 2-pyridornes.

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

by Ming Lok Yeung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1994. / Includes bibliographical references (leaves 54-56). / ACKNOWLEDGMENT --- p.i / ABBREVIATION --- p.ii / ABSTRACT --- p.iii / CONTENTS --- p.iv / Chapter I. --- INTRODUCTION --- p.1 / Chapter II. --- RESULTS AND DISCUSSION --- p.8 / Chapter II-1 --- [3+2] CYCLOADDITION OF FISCHER CARBENE COMPLEXES WITH NITRONES --- p.8 / Chapter II-2 --- KINETIC STUDIES OF THE [3+2] CYCLO ADDITION --- p.18 / Chapter III. --- CONCLUSION --- p.35 / Chapter IV. --- EXPERIMENTAL --- p.36 / Chapter V. --- APPENDIX --- p.49 / Chapter V. --- REFERENCES --- p.54 / Chapter VI. --- LIST OF SPECTRA --- p.57 / Chapter VII. --- SPECTRA --- p.58

Radical cyclizations have been proven to be extremely important in organic synthesis. However, their reliance on toxic trialkyltin hydrides has precluded their practical applications in pharmaceutical manufacturing. Many tin hydride substitutes have been suggested but none of them are adequate alternates to the traditional tin reagent. Transition-metal hydrides have been shown to catalyze the hydrogenation and hydroformylation of unsaturated carbon-carbon bonds. Theses reactions begin with a Hydrogen Atom Transfer (HAT) from a metal to an olefin, generating a carbon-centered radical. The cyclization of that radical is an effective route to five- and six-membered rings. The HAT will be fastest if the M–H bond is weak. However, making the reaction catalytic will require that the hydride can be regenerated with H2. HCr(CO)3Cp has proven to be a good catalyst for such cyclizations, but it suffers from air sensitivity. The yield of the cyclization product depends on how the rate of radical cyclization compares with the rates of side reactions (hydrogenation and isomerization), so special substituents on a substrate are best installed to increase the cyclization rate. In attempting to improve the efficiency of radical cyclization I have studied the effect of substituents on the target double bond on the rate of cyclization. A single phenyl substituent has proven to stabilize a radical better than two phenyls. This stabilization leads to faster cyclizations and a higher cyclization yield. I also have found that Co(dmgBF2)L2 (L = THF, H2O, MeOH…) under H2 is an effective hydrogen atom donor. I have monitored by NMR the catalysis by the system of the hydrogenation of stable radicals (trityl radical and TEMPO radical) and found the rate-determining step to be the activation of hydrogen gas by CoII. The reactive form of the complex is five-coordinated cobalt complex Co(dmgBF2)2L. The Co/H2 system can also transfer hydrogen atom to C=C bonds, thus initiate radical cyclizations. The resting state of the cobalt is the CoII metalloradical, so a cycloisomerization is obtained. Such a reaction neither loses nor adds any atom and has 100% atom economy.

Ho Ho-On. / Thesis submitted in: August 2004. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references. / Abstracts in English and Chinese. / Thesis Committee --- p.i / Abstract --- p.ii / Acknowledgements --- p.iv / Table of Contents --- p.v / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- The Gaussian-3 Method --- p.1 / Chapter 1.2 --- The G3 Method with Reduced MΦller-Plesset Order and Basis Set --- p.2 / Chapter 1.3 --- Calculation of Thermodynamical Data --- p.2 / Chapter 1.4 --- Remark on the Location of Transition Structures --- p.3 / Chapter 1.5 --- Natural Bond Orbital (NBO) Analysis --- p.3 / Chapter 1.6 --- Scope of the Thesis --- p.4 / Chapter 1.7 --- References --- p.4 / Chapter Chapter 2 --- The Important Basic Concepts of Ab Initio Calculations and Their Application to Pericyclic Reactions --- p.6 / Chapter 2.1 --- Potential Energy Surfaces --- p.6 / Chapter 2.2 --- Ab Initio Method --- p.6 / Chapter 2.2.1 --- Basic Sets --- p.7 / Chapter 2.2.2 --- Correlation Methods --- p.8 / Chapter 2.3 --- Pericyclic Reaction --- p.10 / Chapter 2.4 --- The Perturbation Theory of Reactivity --- p.10 / Chapter 2.5 --- References --- p.11 / Chapter Chapter 3 --- Ab Initio Study of the Cycloaddition Reaction between Ethylene and Butadiene as well as That between Ethylene and Hexatriene --- p.13 / Chapter 3.1 --- Introduction --- p.13 / Chapter 3.2 --- Methods of Calculation --- p.15 / Chapter 3.3 --- Results and Discussion --- p.15 / Chapter 3.3.1 --- Reaction between ethylene and butadiene --- p.30 / Chapter 3.3.2 --- Reaction between ethylene and hexatriene --- p.34 / Chapter 3.3.3 --- Electrocyclic reaction of hexatriene --- p.37 / Chapter 3.4 --- Conclusion --- p.40 / Chapter 3.5 --- References --- p.41 / Chapter Chapter 4 --- A G3(MP2) Study on the Electrocyclic Reactions of [12]annulene --- p.43 / Chapter 4.1 --- Introduction --- p.43 / Chapter 4.2 --- Methods of Calculation --- p.44 / Chapter 4.3 --- Results and Discussion --- p.45 / Chapter 4.4 --- Summary --- p.51 / Chapter 4.5 --- Conclusion --- p.52 / Chapter 4.6 --- References --- p.52 / Chapter Chapter 5 --- A G3(MP2) Study on the Cycloaddition Reactions between Ethylene and Azines as well as Other Related Systems --- p.54 / Chapter 5.1 --- Introduction --- p.54 / Chapter 5.2 --- Methods of Calculation --- p.55 / Chapter 5.3 --- Results and Discussion --- p.55 / Chapter 5.3.1 --- Addition of ethylene to azines --- p.55 / Chapter 5.3.2 --- "Addition of ethylene to quinolene, isoquinolene and 1,8-naphthyridine" --- p.64 / Chapter 5.4 --- Conclusion --- p.70 / Chapter 5.5 --- References --- p.70 / Chapter Chapter 6 --- Conclusion --- p.72 / Appendix A Energetic and Bonding Investigation of Phosphabenzene and Arsabenzene: A Gaussian-3 Study --- p.73 / Chapter A.1 --- Introduction --- p.73 / Chapter A.2 --- Methods of Calculation --- p.73 / Chapter A.3 --- Results and Discussion --- p.74 / Chapter A.4 --- Conclusion --- p.77 / Chapter A.5 --- References --- p.77 / Appendix B Energetic and Bonding Study of Hexamethylenetetramine (HMT) and Fourteen Related Cage Molecules: A G3(MP2) Investigation --- p.79 / Chapter B.1 --- Introduction --- p.79 / Chapter B.2 --- Methods of Calculation --- p.80 / Chapter B.3 --- Results and Discussion --- p.80 / Chapter B.4 --- Conclusion --- p.87 / Chapter B.5 --- References --- p.87 / Appendix C The Gaussian-3 Theoretical Models --- p.89 / Chapter C.1 --- The G3 Theory --- p.89 / Chapter C.2 --- The G3(MP2) Theory --- p.90 / "Appendix D Calculation of Enthalpy at 298 K, H298" --- p.91

The kinetics and mechanism of the reactions of (tmpa)W-(CO)^ and (tmen)W(CO)^ (tmpa = N,N,N',N'-tetramethy1-1,3-diaminopropane and tmen = N,N,N1,N1-tetramethylethylenediamine) with four phosphorus donor ligands (triisopropyl phosphite, triphenyl phosphite, triphenylphosphine and "constrained phosphite", 4-methyl-2,6,7-trioxa-l-phosphabicylo[2.2.2]octane) in xylene have been investigated in detail. These reactions were found to take place by the ring-opening of the bidentate ligand in a reversible step which leads to the formation of a five-coordinate intermediate of the type [(h^-tmpa)W(CO)or [(h^-tmen)W(CO). The intermediate then reacts with one molecule of phosphorus ligand, L, to form a six-coordinate intermediate, which can either expel the bidentate ligand and react with another molecule of L leading to the formation of a new disubstituted tungsten tetracarbonyl or go through a ring-reclosure step to form a seven-coordinate activated com-2 2plex or intermediate of the type [(h -tmpa)W(CO)^(L)] or [(h - tmen)W(CO)^(L)] which then regenerates the substrate through the expulstion of the L molecule. This mechanism is consistent with the observed rate behavior in these systems. For the reaction of (tmpa)W(CO)^ with the "constrained phosphite", an intermediate of the type [(h1-tmpa)W(CO)4P(OCH2)3CCH3] was isolated and identified.

Thesis (M.S.)--University of North Carolina at Wilmington, 2003.