Condensation-Type Ring-Closure Reactions

Reviews of the coordination chemistry of this type of ligand have been published for pyrazoles24 and imidazoles,25 and to a lesser extent for the triazoles26 and tetrazoles.27 Many azole ligands are commercially available or are easily synthesized by a variety of condensation and ring-closure reactions.4-6... 

Reactions of selected metal complexes of multidentate amines with formaldehyde and a range of carbon acids (such as nitroethane) have led to ring-closure reactions to yield a series of three-dimensional cage molecules (see Chapter 3). Condensations of this type may also be used to produce two-dimensional macrocycles (Comba et al., 1986) - see [2.20], In such cases, it appears that imine intermediates are initially produced by condensation of the amines with formaldehyde as in the Curtis reaction. This is followed by attack of the conjugate base of the carbon acid on an imine carbon. The resulting bound (new) carbon acid then reacts with a second imine in a cis site to yield chelate ring formation. 

The reaction of phosphorus pentasulfide with a-acylamino carbonyl compounds of type Ilia also yields thiazoles. Even more commonly, a mercaptoketone is condensed with a nitrile of type IVa or a-mercaptoacids or their esters with Schiff bases. This ring closure is limited to the thiazolidines. In the Va ring-closure type, /3-mercaptoalkylamines serve as the principal starting materials, and ethylformate is the reactant that supplies the carbon at the 2-position of the ring. These syntheses constitute the most important route for the preparation of many thiazolidines and 2-thiazohnes. In the Vb t3fpe of synthesis, one of the reactant supplies only the carbon at the 5-position of the resultant thiazole. Then in these latter years new modern synthetic methods of thiazole ring have been developed (see Section 7 also Refs. 515, 758, 807, 812, 822). 

Benzoxazoles and other condensed oxazoles are obtained by the oxidative ring-closure of phenolic Schiff s bases (equation 103) (78H(10)57). The formation of phenanthrooxazoles by the action of benzylamine or other amines on phenanthraquinone (equation 104) is a reaction of this type. 

The photolytic ring closure of stilbene type compounds has been widely employed as a versatile method in the preparation of condensed aromatic and heteroaromatic compounds [67]. According to orbital symmetry conservation rules [68], the reaction is a reversible conrotatory [ 6] electrocyclization of the czs-isomer (e.g. czs-stilbene (1), Scheme 12) to the trans-4 a,4 -dihydrophenan-threne (lb). Because the cist trans-isomerization of stilbenes is sufficiently fast under the reaction conditions applied, the trans-isomer of the stilbenes can also be used. Depending on concentration, structure, and solvent, the stilbene type compounds can also undergo a [2 + 2] cycloaddition reaction leading to aryl-substituted cyclobutanes [69]. 

Ring-closure of Ce. The reaction of (13) with LDA led to a 2,4-diamino-thiophen, which according to n.m.r. exists in the imino-form (14). Compounds of type (14a), prepared by the condensation of methyl aryl ketones with phenyl isothiocyanates in the presence of sodium hydroxide followed by alkylation, gave the aminothiophens (14b) if is COPH or COMe, without additional catalysis by a base. On the other hand, if R is C02Et or CONH2, the thiazolidones (14c) were obtained and if R is CH2Br, (14d) was the product. 

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