Volume 76, Issue 11

Tandem methodology for heterocyclic synthesis represents a powerful approach for the rapid buildup of molecular complexity from potentially simple starting materials. Work from our laboratory has shown that the rhodium(II)-catalyzed cyclization cascade of alpha-diazo imides represents an effective method for the synthesis of a variety of heterocyclic systems. As an extension of these studies, we became interested in using a linked Pummerer/ N -acyliminium ion cyclization sequence since we felt that this combination offers unique opportunities for the assemblage of complex target molecules. A synthetic method that combines transformations of different reaction types significantly broadens the scope of such procedures in synthetic chemistry. Alpha-thiocarbocations generated from the Pummerer reaction of beta-phenylsulfinylmethyl-alpha, beta-unsaturated amides can be intercepted by the adjacent amido group to produce transient amino-substituted furans which undergo subsequent Diels–Alder cycloadditions. Using this domino amido Pummerer/Diels–Alder cascade, we were able to assemble novel polycyclic systems in a single operation. The key step in the process involves the generation of a reactive N -acyliminium ion by fragmentation of an amino-substituted [4+2]-cycloadduct. The successful synthesis of a number of alkaloids by this sequence of reactions reveals the usefulness and importance of this unique domino cascade. Application of the process for the preparation of the stenoma alkaloid stenine was recently carried out in our laboratory.

2-, 3-, and 4-Pyridylketenes have been generated in CH 3 CN by photochemical Wolff rearrangements and identified by their ketenyl absorption in the infrared at 2127, 2125, and 2128 cm –1 , respectively. Reaction of these pyridylketenes with n -BuNH 2 results in the formation of intermediate amide enols from the 3- and 4-pyridylketenes, which are then converted to the corresponding pyridylacetamides. However , 2-pyridylketene forms a long-lived 1,2-dihydropyridine intermediate stabilized by an intramolecular hydrogen bond, and this is converted to the 2-pyridylacetamide with a rate constant 10 7 less than those for the conversion of the amide enols from the 3- and 4-pyridylketenes to amides. Hydration of the pyridylketenes results in the formation of an acid enol intermediate in the case of the 3-isomer, while the 2- and 4-isomers form longer-lived dihydropyridines. The pyridylketenes react with the stable free radical tetramethylpiperidinyloxyl (TEMPO,TO) forming 1,2-diaddition products ArCH(OT)CO 2 T.

Domino reactions are defined as processes of two or more bond-forming reactions under identical conditions, in which the subsequent transformation takes place at the functionalities obtained in the former transformation. They allow the efficient synthesis of complex molecules from simple substrates in an ecologically and economically favorable way. A very powerful domino process is the domino Knoevenagel–hetero-Diels–Alder reaction, in which an aldehyde or an beta-ketoester is condensed with a 1,3-dicarbonyl compound or a heteroanalog to give a 1-oxa-1,3-butadiene, which can undergo an inter- or intramolecular hetero-Diels–Alder reaction with dienophiles such as enol ethers or alkenes. The products are dihydropyrans, which can be transformed in a variety of ways. Thus, an extension of the process is the synthesis of highly substituted pyrrolidines, piperidines, and azepanes using aminoaldehydes. The process has also been employed for the enantioselective total synthesis of a variety of alkaloids, such as indol- and ipecacuanha alkaloids. In another domino process, erythrina and homoerythrina alkaloids have been prepared from simple phenylethylamines and ketoesters.

The document defines the terms most commonly encountered in the field of polymer blends and composites. The scope has been limited to mixtures in which the components differ in chemical composition or molar mass and in which the continuous phase is polymeric. Incidental thermodynamic descriptions are mainly limited to binary mixtures although, in principle, they could be generalized to multicomponent mixtures. The document is organized into three sections. The first defines terms basic to the description of polymer mixtures. The second defines terms commonly encountered in descriptions of phase domain behavior of polymer mixtures. The third defines terms commonly encountered in the descriptions of the morphologies of phase-separated polymer mixtures.

This report presents the results from IUPAC Working Party IV.2.2 of the global trial within the framework of IUPAC Commission IV.2, “Characterization of Commercial Polymers”. The results were compared on the basis of molecular weight obtained by size exclusion chromatography (SEC)using different techniques practiced in participating laboratories, the majority of which were materials suppliers. The practical methodologies used different solvents for the polymers, in particular, benzyl alcohol, 1,1,1,3,3,3 hexafluoropropan-2-ol and tetrahydrofuran; the latter solvent was used after chemical modification of the polyamides, in general with trifluoroacetic anhydride. Eight laboratories participated in the trial. The repeatability for molecular weight in each laboratory was good, whatever technique was used, the relative standard deviation averaged over all laboratories being around 3%. The deviations in distribution of molecular weights with different experimental methodologies were broader, but were reasonably good despite the diversity of methods. The differences in the distribution correspond to a confidence interval of about 30% in molecular weight.

A comparative experimental study of shear-flow rheological properties of thermotropic polymer liquid crystals by eight different laboratories is described. The materials involved four different liquid-crystalline polyesters (LCPs), a glass- fiber-filled liquid-crystalline polyester,hydroxypropyl cellulose (HPC), and two non-liquid-crystalline high-temperature polymers, a poly(etheretherketone) (PEEK), and a polyarylate (PAR). Studies were made in both steady shear-flow and dynamic oscillatory experiments. The data from the various laboratories involved were compared. The level of agreement in the data was much less for most liquid-crystalline polymers than for similar isotropic melts. The Cox –Merz rule is valid for PEEK and PAR, but not for the LCPs and HPC. The occurrence of low levels of extrudate swell and high levels of uniaxial orientation in extrudates of the LCPs and HPC is described.