Showing papers in "Progress in Heterocyclic Chemistry in 1996"

Chapter 4 Four-membered ring systems

Abstract: Publisher Summary The Four-membered heterocycles shows the continued development of azetidine chemistry, in part, because of the recognition of the ease of formation of l-azabicyclo butane and its conversion to azetidines. The volume of β-1actone chemistry has increased because of the occurrence of the ring system in potentially useful natural products and because of the increasing use of β-1actones as synthetic intermediates, including their use in the preparation of biodegradable polymers. This chapter discusses the chemistry of four-membered ring compounds containing one nitrogen atom of monocyclic azetidine, azetines, and azetes of their fused derivatives and its derivatives. The β-1actams have been divided into monocyclic β-1actams (including bicyclic spiro compounds) and fused β-1actams.

Chapter 5.7 Five-membered ring systems: With O & N atoms

Abstract: Publisher Summary This chapter discusses the five-membered ring systems with O & N atoms such as Isoxazoles, Isoxazolines, Isoxazolidines, Oxazoles, Oxazolines, Oxazolidines, and Oxadiazoles. An efficient procedure for the synthesis of isoxazoles suitable for a multi-kilogram scale is shown in the chapter. “Fluorous synthesis”, a strategic alternative to conventional solid-phase synthesis, is illustrated by the preparation of a small isoxazoline library. The isoxazolines are detached from the fluorous label by treatment with pyridine hydrofluoride. The chiral titanium cornplex promotes the exo-selective and enantioselective 1, 3-dipolar cycloaddition of the unsaturated acylsuccinimide to the nitrones to yield the isoxazolidines. N-Acylisoxazolin-5-ones lose carbon dioxide on thermolysis or photolysis to give transient iminocarbenes, which undergo cyclisation to yield oxazoles.

Chapter 6.1 Six-membered ring systems: Pyridine and benzo derivatives

Abstract: Publisher Summary This chapter discusses the six-membered ring systems—namely, pyridine and benzo Derivatives. Use of aliphatic nitro compounds to prepare a variety of pyridines, 1,4-dihydropyridines, and 2-pyridiones is reviewed in the chapter, along with the syntheses of pyridines by catalysis and subsequent derivatization reactions. Cyclotrimerization of alkynes and chiral nitriles gives chiral-substituted pyridines using Co(I) catalysis. Nekrasov reviewed the heterodiene approach to pyridines using cyanamides. Selective cyclotrimerization of alkynes with nitriles produced pentasubstituted pyridines with little formation of benzenoid products. Similar cyclotrimerizations have been accomplished with irradiation in 3 hours but in lower yield. Rhodium catalysis has been found to yield pyridines without irradiation in slightly longer times, but the unsymmetrical alkynes have led to isomeric products. 2-Pyridones with 5-phosphonate ester substituents can be made from alkynes and enamine derivative. Malononitrile condenses with 3-pyridinecarboxaldehyde and a ketone to form substituted bipyridines. Chloride anion has been used to effect dehydrochlorination of pyridone. The product is an important intermediate for agricultural compounds.

Chapter 1 Geminal diazides of heterocycles

Abstract: Publisher Summary This chapter discusses geminal diazides of heterocycles and presents a short historical review of the development of the chemistry of quinolines containing diazido malonyl moiety. The synthesis of dichloro malonyl heterocycles was reported over 30 years ago, and in the following years the exchange of the halogen atoms against nucleophiles (amines) was studied. A type of geminal diazide proved to be a good starting material for a number of transformations. Consideration of the mechanism has to account for the fact that hydrazoic acid and dinitrogen are liberated during the course of the reaction. Investigations of the reactivity of 5,5-diazidobarbitufic acid have led to somewhat different results. In many cases, N-substitued anilines or azomethines (resp. enamines) react with two equivalents of diethyl malonate to give pyrono derivatives of 2-quinolones or 2-pyridones in good yield. Chlorination of these compounds with sulfuryl chloride and aqueous work up yields dichloroacetyl derivatives of quinolones and pyridines, which in turn react with sodium azide to afford a variety of diazidoacetyl derivatives.

Chapter 2 Radical methods in the synthesis of heterocyclic compounds

Abstract: Publisher Summary This chapter discusses radical methods in the synthesis of heterocyclic compounds. Radical reactions have found increased application in the preparation of simple as well as complex heterocyclic compounds. Heterocycle syntheses, in which radical reactions are used for the key bond construction, are discussed in this chapter. An interesting method for the preparation of epoxides using radical methodology has been reported. Formation of five-membered heterocycles through radical methodologies has been investigated extensively. Inter-, intramolecular, and cascade reactions have been reported for the synthesis of heterocycles. In the case of intramolecular cyclizations, 5-exo pathway is the preferred mode of reactivity. Rate constant for the formation of pyrrolidine by radical ring closure has been reported. An efficient modification of the manganese (III) mediated malonate radical addition to styrene has been examined. Intramolecular radical cyclization reactions are extensively used for the formation of heterocycles. These reactions are generally carried out under reductive conditions using tin or silicon hydride reagents. The tin hydride reagent can be used catalytically.

Chapter 5.5 - Five-Membered Ring Systems: With N & S (Se) Atoms

Abstract: Publisher Summary This chapter discusses the five-membered ring systems with N and S (Se) atoms. The reaction of zirconium metallacycles is used to produce a variety of main group heterocycles through the metallacycle transfer of a carbon fragment. The scope and potential of this reaction for the synthesis of some different heterocycles, including heterocycles containing two heteroatoms, are described in the chapter. In the experiment described in the chapter, the treatment of aryl and heteroaryl o -azidocarbaldehydes with bis(trimethylsilyl)sulfide and hydrochloric acid gave fused isothiazoles through thionation of the formyl function followed by spontaneous decomposition at room temperature. Yield is dependent upon the nature of the heteroaromatic ring. The formation of a carbon–carbon bond in the 4-position of an isothiazole was carried out in moderate yield using the 4-magnesioisothiazole, which was prepared through metal halogen exchange reaction. 4-iodoisothiazole was reacted with ethylmagnesium bromide, then with electrophiles to give ethyl isothiazole-4-carboxylate and isothiazol-4-carbaldehyde. Secondary and tertiary alcohols were obtained from aromatic aldehydes and ketones.