Alfred Hassner

AMINOPYRIDINES AS ACYLATION CATALYSTS FOR TERTIARY ALCOHOLS

The acylation of unreactive alcohols with acid anhydrides is greatly facilitated by the addition of a catalytic (0.02–0.1 equivalent) amount of a 4-dialkylaminopyridine. The reaction is faster in nonpolar than in polar solvents and acetyl chloride is not as effective as acetic anhydride. Several pyridine, pyridazine, and quinoline derivatives have been examined as potential acylation catalysts. Of the systems examined, only a few of the 4-substituted pyridines were found to be acylation catalysts, the most effective being 4-pyrrolidinopyridine 4 and 1,1,3,3-tetramethyl-4-(4-pyridyl)guanidine 8. The reaction of t-butanol with an isocyanate is also accelerated by the presence of 4 but not as much as in the case of acylations. The cause of the pronounced effect of these pyridine species in catalyzing acylation reactions seems to be a combination of the increased donor ability of the 4-substituent and the stabilizing effect that this substituent has on an acyl pyridinium intermediate.

Intramolecular oxime olefin cycloadditions. Stereospecific formation of functionalized pyrrolidines.

Abstract Allylamines possessing a properly positioned aldoxime or ketoxime chain undergo thermally induced dipolar cycloaddition to bicylic isoxazolidines, with stereospecific introduction of three stereo centers. This provides an entry into stereospecifically functionalized pyrrolidines.

Phenylselenium azide addition to alkenes. A new and stereospecific introduction of Se and N into organic molecules

Abstract The first examples of direct introduction of PhSe and N 3 functions by addition to olefins are reported. The addition of PhSeN 3 to simple alkenes, as well as to activated alkenes takes place in the absence of a catalyst. The reaction proceeds stereospecifically but not regiospecifically, apparently via a 3-membered selenonium ion intermediate.

Stereoselectivity in intramolecular 1,3-dipolar cycloadditions. Nitrile oxides versus silyl nitronates

Abstract Unlike nitrile oxides, silyl nitronates undergo highly stereoselective intramolecular cycloadditions to produce functionalized carbocyclic or heterocyclic rings.

Stereoselective Intramolecular 1,3-Dipolar Cycloadditions

An in depth account of intramolecular 1,3-dipolar cycloadditions involving dipoles such as nitrile oxides, silyl nitronates, H-nitrones, azides, and nitrilimines is presented with particular emphasis on the stereochemistry during the cycloaddition. Various methods employed for the generation of the dipoles and their applications to stereoselective synthesis are also discussed.

A novel amination of aromatic and heteroaromatic compounds

Abstract Vinyl azides act as NH+2 equivalents in reaction with aromatic or heteroaromatic lithium derivatives thus providing a direct method for amination of such compounds.

Synthetic methods : IX. Hydroboration of silylacetylenes. A new synthesis of α-silaketones

Abstract The first example of monohydroboration of silylacetylenes with dichloroborane is described. The reaction occurs regio- and stereo-specifically and leads after methanolysis to isolable vinylboronic esters (III). The latter undergo photo Z ⇌ E isomerization. Oxidation of III to the corresponding α-silaketone was accomplished in high yield using trimethylamine- N -oxide dihydrate. The overall method represents a useful conversion of acetylenes to α-silaketones.

Cycloaddition of vinyl aziridines with unsaturated substrates. A novel rearrangement of an unsaturated nitro compound.

Abstract Vinylaziridine 2 undergoes reaction with electrophilic acetylenes and olefins to produce 7-membered azepine derivatives. With β-nitro-styrene however, a novel rearrangement occurs, presumably via an ene reaction to form 10 , the structure of which is definitively shown by x-ray diffraction.

A route to pyrrolizidines, indolizidines and quinolizidines via intramolecular oxime olefin cycloadditions

Abstract A route to functionalized pyrrolizidines, indolizidines or quinolizidines is described. The reaction involves thermal cycloaddition of pyrrolidines or piperidines, possessing properly positioned aldoxime and alkene functions, and proceeds with stereospecific introduction of three stereo centers.

Formation of functionalized cyclic ethers by intramolecular nitrile oxide cycloadditions

Abstract The reaction of O-trimethylsilyl α-bromo aldoximes with unsaturated alcohols produces oximino ethers which can be readily oxidized with sodium hypochlorite. The transient nitrile oxide intermediate formed undergoes spontaneous cyclization affording fused isoxazolines. MM2 calculations help rationalize the observed stereoselectivity.