From Other Ring Systems

The cleavage of 6-methoxy-3-methyl4-oxo-3,4-dihydropteridine (37) to 2-amino-3-carboxy-5-methoxypyrazine (432) [with further data (783)] and of 6-methoxy-2-methyl4f/-pyrazino[2,3- /][l,3]oxazin-4-one (38) to 2-acetamido-3-amidinocarbamoyl-5-methoxypyrazine (432) has been discussed in Section II.5. 

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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-... 

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. 

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... 

Syntheses of Thiophens from other Ring Systems.—A new synthesis of thiophens starts from meso-ionic 1,3-dithiolones (65)—(67) (see Chapter 8), which as illustrated by resonance hybrid (66) can be considered as thiocarbonyl ylides. They... 

Syntheses of Thiophens from other Ring Systems.— There has been continued interest in the use of 1,3-oxathiazolium salts for the synthesis of thiophens in high yields. From the 2-aryl-substituted derivatives (56), 3-amino-thiophens (57) are obtained in the reaction with malononitrile in the presence of base, and 3-hydroxythiophens (58) and (59) with methyl cyanoacetate and... 

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