From Other Rings

From Other Rings. Instead of preparing 5-aryl-2-hydroxythiophens (32) by allowing 4-aryl-4-oxobutanoic acid to react with PaSs, it is considered better to prepare first the butenolide (31) by treatment of the acid with acetic anhydride followed by sodium hydrosulphide. The crude (32) can be transformed into (33) in a one-pot procedure by its reaction with an aromatic aldehyde and hydrogen chloride. A patent describes the synthesis of more 

The strange metal-organic heterocycle (34), upon treatment with cerium(IV) salts, gave tetramethyl thiophentetracarboxylate in almost quantitative yield.  

---

Pyridine ring syntheses (48) can be classified into essentially two categories ring synthesis from nonheterocyclic compounds, and synthesis from other ring systems. The synthesis of pyridine derivatives by transformations on the pyridine ring atoms and side-chain atoms have been considered in the previous section. 

Synthesis From Other Ring Systems. These syntheses are further classified based on the number of atoms in the starting ring. Ring expansion of dichlorocyclopropane carbaldimine (53), where R = H and R = ryl, on pyrolysis gives 2-arylpyridines. Thermal rearrangement to substituted pyridines occurs in the presence of tungsten(VI) oxide. In most instances the nonchlorinated product is the primary product obtained (63). 

These reviews often contain tables of compounds together with the routes used to prepare them and in many cases reference in the present work will be directly to the appropriate chapter. Nevertheless, there are in most cases direct references to all the principal routes of synthesis in such a restricted format a completely comprehensive account is impossible. The section on synthesis from other ring systems is weighted towards the work of the last 20 years. 

Thieno[2,3-d]pyrimidines can also be synthesized by cyclic transformations from other ring systems. Thieno[2,3-d]oxazin-4-ones 123 were transformed into 3-substituted thieno[2,3-d]pyrimidin-4-ones 19a-d by treat-... 

In 1-alkylimidazoles the ring protons adjacent to an IV-methyl group can be differentiated from other ring protons by a characteristic shift in S with variation in solvent. In non-polar solvents H-5 appears at higher field than H-4, with the order being reversed in polar solvents (see Table 4). 

Syntheses from Other Ring Systems 4.17.10.1 Syntheses from Organotin Heterocycles... 

Thiophenes, synthesis by cyclization reactions and from other ring systems (general review) 85HC(44,1)1. 

From Other Rings. 2,5-Dialkylfurans and also furans with functionalized side-chains, such as alcohols, ketones and esters, react with hydrogen sulphide in alcohols in the presence of... 

Ch. 1 Preparation of Thiophenes by Ring Closure Reactions and from Other Ring Systems Ch. 2 Theoretical Calculations on Thiophenes Ch. 3 Naturally Occurring Thiophenes... 

The fnrther classification used here and earlier into (1) ring closures and (2) preparations from other rings is also arbitrary and often appears inappropriate in that a useful relationship such as that between furfural and the acyclic 2-oxoglutaconaldehyde (XII-5) may not be apparent. ... 

---