Synthesis from Other Rings

The formation of this product and some kinetic data were taken as evidence for valence isomerization of the dithiins. The minor product (27) from the methyl-ation of 3-nitro-2,5-diphenyl-l,4-dithiin appears not to be stable, and it rearranges via (28) and (29) to (30a), which is methylated to give the isolated product (30b).Upon photolysis of cycloalkene-thiadiazoles (31), thiophens (32) are formed, in yields of 7—31%, together with 1,4-dithiins, 1,3-dithioles, and 1,2,4,5-tetrathians.  

---

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. 

SYNTHESIS BY TRANSFORMATION OF ANOTHER RING 4.07.10.1 Synthesis From Other Heterocycles... 

The various possibilities for the first two types of syntheses are listed schematically in Table 4, and the possibilities for synthesis from other heterocycles are listed in Table 5. In each case the method is discussed in more detail in the following sections Tables 4 and 5 indicate the section(s) in which a particular reaction is described. In the case of sulfur-containing heterocycles, 5-oxides and 5,5-dioxides are treated alongside the basic parent structures, since for 1,3,2-dioxathiolane, 1,2,3-oxadithiolane, and 1,2,5-oxadithiolane the unoxidized rings are unknown. 

Combination of a C-S Unit with a C-C-C Unit. 4 Ring Synthesis by Formation of Three Bonds. 5 Ring Synthesis from Other Heterocyciic Compounds. 5.1 From Three- and Four-Membered Heterocyciic Compounds. 5.2 From Five-Membered Heterocyciic Compounds... 

Ring Synthesis from Other Heterocyciic Compounds... 

The reaction sequence of Scheme 3 might well be classified as an imidazole synthesis from other heterocycles, but it seems more logical to treat this as a cyclization involving formation of the 1,5-bond. The enaminoketone condensation products (12) of 3-amino-l,2,4-oxadiazoles and 1,3-dicarbonyl compounds are cyclized by base to imidazoles (13) in 60-80% yields." Such a reaction makes use of the well-established general attack of a nucleophilic center in the side chain on N-2 of the oxadiazole ring. Benzamidine combines with 2-amino-3-phenacyl-l,3,4-oxadiazolium bromides to produce 1-acyl-amino-2-benzimidoylamino-4-arylimidazoles. ... 

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

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... 

RING SYNTHESIS FROM OTHER HETEROCYCLIC COMPOUNDS... 

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... 

The structure of 82 was established by alkaline ring cleavage to benzilic acid amide and by hydrogenolysis to (C6H5)2CH—CONH— COCfiHs. These reactions also served to eliminate 83 as the structure of the 169° compound. The other possible isomeric structure, (C6H5)2C(CN)0C0C6H5, which could have formed after 0-acylation, was ruled out by its independent synthesis from bromodiphenyl-acetonitrile and silver benzoate. 

---