Diastereoselective Bromocyclization of O-Allyl-N-tosyl-hydroxylamines

Hydroxylamines: From Synthetic Intermediates to Synthetic Targets

ConspectusSynergy between the teaching and research activities of a University should be a source of new ideas, each informing the other. A classroom discussion gave rise to our concept of using hydroxylamines as a form of "tethered nitrogen" for alkaloid synthesis. The "tether" temporarily connects a nucleophilic nitrogen atom to the substrate, rendering an intermolecular reaction intramolecular, thus providing stereo- and regiochemical control for C-N bond formation. In the context of the synthesis of 1,3-amino alcohols, this necessitated the synthesis of isoxazolidines. This concept led to the exploration of methods for the synthesis of these heterocycles beyond the well-established 1,3-dipolar cycloadditions. The first two methods developed based upon this concept were palladium catalyzed cyclocarbonylation and silver catalyzed allene cyclization. These new methods were applied to two syntheses of sedamine and the synthesis of three Nuphar alkaloids. Intramolecular aza-Michael addition, combined with the use of cross-metathesis to generate the substrates, gave access to both isoxazolidines and tetrahydro-1,2-oxazines. This new method made possible a synthesis of monomorine with high stereochemical control. The extension to more complex alkaloids required the incorporation of additional chemistry. Combining allene cyclization with iminium ion chemistry allowed the extension to the more complex piperidine alkaloids porantheridine and sedinine. In these cases, successful stereocontrol relied on the ability to predict the conformation of the intermediates and the trajectory of the nucleophilic attack, which may be sterically or stereoelectronically controlled. Inspection of our collection of methods revealed that we had good methods for cis-3,5-disubstituted isoxazolidines and trans-3,6-disubstituted tetrahydro-1,2-oxazines. Inspired by earlier work involving iminium ions, methods were developed to provide the complementary trans-isoxazolidines and cis-oxazines, using an allylation reaction. This, combined with our interest in applications of hydroformylation, led to the synthesis of 5-hydroxysedamine. Venturing away from piperidines, this method was successfully applied to the synthesis of the β-amino acid antibiotic negamycin. Use of N,O-heterocycles as intermediates naturally sparked an interest in N,O-heterocycles as synthetic targets, as there are many natural products that contain the hydroxylamine moiety, often incorporated into an isoxazolidine or 1,2-oxazine ring. As the presence of the hydroxylamine moiety cannot be fully demonstrated using the usual spectroscopic methods, total synthesis can be employed to bridge this logical gap. A synthesis of raistrickindole A, utilizing an intramolecular Mitsunobu reaction of a hydroxamic acid to form a 1,2-oxazine, confirmed the structure of this diketopiperazine natural product. A synthesis of preisomide, using an aza-Michael addition to form the required 1,2-oxazine, also confirmed the structure of this natural product.

Triflamidation of Allyl-Containing Substances:Unusual Dehydrobromination vs. Intramolecular Heterocyclization

Allyl halides with triflamide under oxidative conditions form halogen-substituted amidines. Allyl cyanide reacts with triflamide in acetonitrile or THF solutions in the presence of NBS to give the products of bromotriflamidation with a solvent interception, whereas in CH₂Cl₂ two regioisomers of the bromotriflamidation product without a solvent interception were obtained. The formed products undergo base-induced dehydrobromination to give linear isomers with the new C=C bond conjugated either with the nitrile group or the amidine moiety or alkoxy group. Under the same conditions, the reaction of allyl alcohol with triflamide gives rise to amidine, which was prepared earlier by the reaction of diallyl formal with triflamide. Unlike their iodo-substituted analogs, bromo-substituted amidines successfully transform into imidazolidines under the action of potassium carbonate.

Enantioselective Halolactonization Reactions using BINOL-Derived Bifunctional Catalysts: Methodology, Diversification, and Applications

A general protocol is described for inducing enantioselective halolactonizations of unsaturated carboxylic acids using novel bifunctional organic catalysts derived from a chiral binaphthalene scaffold. Bromo- and iodolactonization reactions of diversely substituted, unsaturated carboxylic acids proceed with high degrees of enantioselectivity, regioselectivity, and diastereoselectivity. Notably, these BINOL-derived catalysts are the first to induce the bromo- and iodolactonizations of 5-alkyl-4( Z)-olefinic acids via 5- exo mode cyclizations to give lactones in which new carbon-halogen bonds are created at a stereogenic center with high diastereo- and enantioselectivities. Iodolactonizations of 6-substituted-5( Z)-olefinic acids also occur via 6- exo cyclizations to provide δ-lactones with excellent enantioselectivities. Several notable applications of this halolactonization methodology were developed for desymmetrization, kinetic resolution, and epoxidation of Z-alkenes. The utility of these reactions is demonstrated by their application to a synthesis of precursors of the F-ring subunit of kibdelone C and to the shortest catalytic, enantioselective synthesis of (+)-disparlure reported to date.

Stereoselective Synthesis of Isoxazolidines via Copper-Catalyzed Alkene Diamination

A convenient copper-catalyzed intramolecular/intermolecular alkene diamination reaction to synthesize 3-aminomethyl-functionalized isoxazolidines under mild reaction conditions and with generally high levels of diastereoselectivity was achieved. This reaction demonstrates that previously underutilized unsaturated carbamates are good [Cu]-catalyzed diamination substrates. Sulfonamides, anilines, benzamide, morpholine, and piperidine can serve as the external amine source. This relatively broad amine range is attributed to the mild reaction conditions. Reduction of the N-O bond could also be achieved, revealing the corresponding 3,4-diamino-1-alcohols efficiently.

Isoxazolidine: A Privileged Scaffold for Organic and Medicinal Chemistry

The isoxazolidine ring represents one of the privileged structures in medicinal chemistry, and there have been an increasing number of studies on isoxazolidine and isoxazolidine-containing compounds. Optimization of the 1,3-dipolar cycloaddition (1,3-DC), original methods including electrophilic or palladium-mediated cyclization of unsaturated hydroxylamine, has been developed to obtain isoxazolidines. Novel reactions involving the isoxazolidine ring have been highlighted to accomplish total synthesis or to obtain bioactive compounds, one of the most significant examples being probably the thermic ring contraction applied to the total synthesis of (±)-Gelsemoxonine. The unique isoxazolidine scaffold also exhibits an impressive potential as a mimic of nucleosides, carbohydrates, PNA, amino acids, and steroid analogs. This review aims to be a comprehensive and general summary of the different isoxazolidine syntheses, their use as starting building blocks for the preparation of natural compounds, and their main biological activities.

Phosphine Oxide-Sc(OTf)3 Catalyzed Highly Regio- and Enantioselective Bromoaminocyclization of (E)-Cinnamyl Tosylcarbamates. An Approach to a Class of Synthetically Versatile Functionalized Molecules

A highly regio- and enantioselective bromoaminocyclization of (E)-cinnamyl tosylcarbamates catalyzed by a chiral phosphine oxide-Sc(OTf)3 complex is described. A wide variety of optically active aryl 5-bromo-1,3-oxazinan-2-ones can be obtained with high yield and enantioselectivity.

Stereoselective bromocyclization of allylated aldoxime ethers

The intramolecular bromoamination of allylated aldoxime ethers leads first to isoxazolidinium salts which then undergo a skeletal rearrangement to form bromo-5,6-dihydro-4H-1,3-oxazines. Aliphatic aldoxime ethers with α-protons undergo multiple brominations before rearrangement.