Heterocycles with Different Heteroatoms

Heterocycles are known to have O—S, S—N, and O—N bonds. The cyclization of peri-hydroxy and peri-amino-substituted naphthalene sulfonic acids gives rise to 1,8-naphthosultone 246 and 1,8-naphthosultam 247, respectively (33MI2 63MI1). On gas phase photolysis of naphtho[/ c]- 

Formation of an N—O bond, together with the isooxazole ring closure, proceeds on thermal decomposition of 1-azidoanthraquinones and leads to 

Isothiazoloanthrone 255 is formed on heating 1-thiocyanoanthraquinone 253 with ammonia or on reacting l-chloro and 1-sulfoanthraquinones 254 (X = Cl, SO3H) with ammonia, sodium sulfide, and sulfur [76-IJC(B)625 79HOU4I4 83MI1]. 

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The preparation of five-membered rings in solid-phase organic chemistry has been reported in several publications. Versatile syntheses of these heterocycles with different numbers and kinds of heteroatoms have been described. The synthesis of five-membered rings containing one nitrogen atom (Fig. 3.6) as pyrrolidines (231) [311-316] pyrroles (232) [317-320] pyrrolidinones (233) [321-323] pyr-rolinones (234) [324—326] 2,5-pyrrolidinediones (235) [327-329] 2,4-pyrrolidine-diones (236) [330-332] 2,5-pyrrolinediones (237) [333] or heterocycles with one oxygen or one sulfur atom like tetrahydrofurans (238) [334—336] 2,5-dihydrofurans (239) [337], furans (240) [338, 339], yS-lactones (241) [340-343], 2,5-dihydrofura-nones (242) [344] (Scheme 3.35) and thiophenes (243) [345, 346] can be accomplished on solid supports. 

Thus far only eight different types of five-membered heterocycles with two heteroatoms, one of these being tellurium, are known. Of special interest are those containing S,Se, or Te as the second heteroatom in the ring. Such compounds afford electron-donor components of charge-transfer complexes or radical cation salts, exhibiting properties of the so-called organic metals. 

All four-membered heterocycles with three heteroatoms can be classified into four types compounds with three similar heteroatoms 1 symmetrical 2, and unsymmetrical 3 heterocycles with two different heteroatoms and derivatives with three different heteroatoms 4 <1996CHEC-II(1B)1189>. 

Table 1 Differences in total energies ( tot) of five-membered heterocycles with one heteroatom and cationic ir-complexes formed on their protonation for ot- (d ) and /i-positions AEp), as well as values of AE p = AE —AEp (all in kcal/mol) ...

Now let us examine rather different types of heteroaromatic compounds those with five-membered rings. Furan, pyrrole, and thiophene are important five-membered ring heterocycles with one heteroatom. 

Although four-membered rings are highly strained and not very stable heterocycles, there are some examples (Fig. 3.5) for the preparation of heterocycles of this type on solid supports. Different /9-lactams (229-230) have been synthesized employing different synthetic pathways [133, 275, 306-309] such as the addition of ketenes (Scheme 3.34) [306], or ester enolates to imines [133]. Also the synthesis of four-membered rings with two heteroatoms has been reported [310]. 

Unsaturated and saturated compounds are considered in the same sections since the number of five-membered heterocycles with three and more heteroatoms (and especially their structure types) is limited and their reactivity does not differ fundamentally. 

Heterocycles treated in this section belong to several structure types. The metal atoms in compounds under discussion can exist not only in the tetravalent but also in the carbenoid divalent state and the heterocycles differ not only in the nature and number of metal atoms but also in the nature of ring bonds of the latter. Thus, besides conventional organometallic compounds having only M—C bonds there are heterocycles with M—M bonds and those with C—M—X or X—M—Y fragments (X and Y—common heteroatoms like O, S, N). The last type is the most numerous and important group of five-membered Ge-, Sn- and Pb-heterocycles in which the metals are involved in M—X and M— Y bonds similar to those in respective metal alkoxides, thiolates and amides. This feature not only affects the structural parameters of these compounds but determines their chemical properties, synthetic routes and applications. 

Naturally the redox properties of the general structures A-C ould strongly be influenced by different heteroatoms X. Since this report deals mainly with Weitz type systems, X is incorporated into a heterocyclic ring. 

The following types of dipolarophiles have been used successfully to synthesize five-membered heterocycles containing three heteroatoms by [3 + 2]-cycloaddition of thiocarbonyl ylides azo compounds, nitroso compounds, sulfur dioxide, and Al-sulfiny-lamines. As was reported by Huisgen and co-workers (91), azodicarboxylates were noted to be superior dipolarophiles in reactions with thiocarbonyl ylides. Differently substituted l,3,4-thiadiazolidine-3,4-dicarboxylates of type 132 have been prepared using aromatic and aliphatic thioketone (5)-methylides (172). Bicyclic products (133) were also obtained using A-phenyl l,2,4-triazoline-3,5-dione (173,174). 

The solution thus consists of different particles denoted as contact ion pairs, solvent-separated ion pairs and free ions. The fraction of the individual particles depends on the type of salt, type of solvent, polymerization system, temperature, and salt concentration. The catalytic effect of these particles may be very different as is evident in anionic polymerization of vinyl monomers. For instance, free polystyryl anion is 800times more reactive than its ion pair with the sodium counterion 60 . From this fact it follows that, although the portion of free ions is small in the reaction system, they may play an important role. On the other hand, anionic polymerization and copolymerization of heterocycles proceeds mostly via ion pairs. This is due to a strong localization of the negative charge on the chain-end heteroatom which strongly stabilizes the ion pair itself62. Ionic dissociation constants and ion contributions to the reaction kinetics are usually low. This means that for heterocycles the difference between the catalytic effect of ion pairs and free ions is much weaker than for the polymerization of unsaturated compounds. This is well documented by the copolymerization of anhydrides with epoxides where the substi-... 

This chapter covers bicyclic ring systems containing two five-membered rings where each ring contains one heteroatom and neither heteroatom is situated at the position of the ring junction. These 5 5 fused heterocyclic systems are represented by the general structures 1-4, wherein X and Y may be the same or different heteroatoms and represent O, NH, S, Se, and Te. The fully conjugated title compounds have a central C-C bond and are isoelectronic with the 10-7i-electron pentalene dianion 5 (Fig. 1). 

Since oxiranes are representative heterocyclic monomers containing an endo-cyclic heteroatom, and the most commonly polymerised of such monomers, they have been subjected to copolymerisations with heterocyclic monomers containing both an endocyclic and an exocyclic heteroatom. Coordination copolymerisations of heterocyclic monomers with different functions are focused on oxirane copolymerisation with cyclic dicarboxylic acid anhydride and cyclic carbonate. However, the statistical copolymerisation of heterocyclic monomers with an endocyclic heteroatom and monomers with both endocyclic and exocyclic heteroatoms have only a limited importance. Also, the block copolymerisation of oxirane with lactone or cyclic dicarboxylic acid anhydride is of interest both from the synthetic and from the mechanistic point of view. Block copolymerisation deserves special interest in terms of the exceptionally wide potential utility of block copolymers obtained from comonomers with various functions. It should be noted, however, that the variety of comonomers that might be subjected to a random, alternating and block polymerisation involving a nucleophilic attack on the coordinating monomer is rather small. 

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