Heterocyclics with 2 hetero atoms other

The optically active 1,3-oxathiane derivative 81, synthesized as a chiral auxiliary (cf. Scheme 30), has been reported. The isomers were assignend by X-ray diffraction in the solid state trans) and by NOE experiments in solution cis) (97CPB778). Rules for the specification of the absolute configuration of the enantiomers R or S) for 1,3-oxathianes and other heterocycles with at least two hetero atoms were proposed and proved... 

Most importantly, the scope of the Diels-Alder reaction is very high - not only allowing the synthesis of cyclohexenes and 1,4-cyclohexadienes using 1,3-butadienes and alkenes and alkynes, respectively, but also giving access to a multitude of different heterocycles by exchanging the atoms a-d in the butadiene as well as the atoms e and f in the alkene by hetero atoms such as oxygen, nitrogen and sulfur. However, also dienes and dienophiles with several other atoms as phosphorous, boron, silicone, and selenium have been described. Thus, many different heterodienes and heterodienophiles have been developed over the years (Tables 1-1 and 1-2). 

Heterocyclic compounds are organic ring structures (-cyclic) that include an atom or atoms other than carbon (hetero-) in the ring. Thus, although compounds such as benzene and cyclohexane form organic rings, they do not include atoms other than carbon, and therefore do not qualify as heterocyclic. The particular number of elements in the ring varies, with... 

Exchange of the bromine atoms in 142 and 145 for other halides or hetero-atomic groups is similarly facile [100]. Among those listed in Scheme 31, the triazides 154 and 158 are interesting because of their further conversion. Reduction of 154 with lithium aluminium hydride furnishes the C3v-symmetrical triamine 155 [105] (cf. Scheme 33 for the Cs-symmetrical isomer). The homologue 158 was converted into unusual heterocycles by thermolysis [100] (cf. Scheme 43). Three-fold bridgehead ethers such as 159 and 160 were obtained by alcoholysis of 142, and Ag1 ion assisted reaction of 142 with tert-butyl hydroperoxide afforded the tris(peroxide) 161 [100]. 

For reversible additions, for example of a proton, the position of equilibrium depends on the pA of the heterocycle, and this in turn is influenced by the substituents present on the ring electron-releasing groups enhance the basicity and electron-withdrawing substituents reduce the basic strength. The p of simple pyridines is of the order of 5, while those for 1,2- and 1,3-azoles depend on the character of the other hetero atom pyrazole and imidazole, with two nitrogen atoms, have values of 2.5 and 7.1 respectively. 

The main factor giving increased acidity of heterocyclic C-hydrogen relative to benzenoid C-hydrogen is the inductive effect of the hetero atom(s) thus metallation occurs at the carbon a to the hetero atom, where the inductive effect is felt most strongly, unless other factors, with varying degrees of importance, intervene. These include the following. 

Some of the components shown in these examples have two electrophilic centres and some have a nucleophilic and an electrophilic centre in other situations components with two nucleophilic centres are required. In general, components in which the two reacting centres are either 1,2- or 1,3-related are utilised most often in heterocyclic synthesis, but 1,4- ( e.g. HX-C-C-YH) (X and Y are hetero atoms) and 1,5-related ( e.g. 0=C-(C)3-C=0) bifunctional components, and also reactants which provide one-carbon units (formate, or a synthon for carbonic acid - phosgene, Cl2C=0, or a safer equivalent) are also important. Amongst many examples of 1,2-difunctionalised compounds are 1,2-dicarbonyl compounds, enols (which first react in a nucleophilic sense at carbon and then provide an electrophilic centre (the carbonyl carbon), Hal-C-C=0, and systems with HX-YH units. Amongst often used 1,3-difunctionalised compounds are the doubly electrophilic 1,3-dicarbonyl compounds and a,P-unsaturated carbonyl compounds (C=C-C=0), doubly nucleophilic HX-C-YH (amidines and ureas are examples), and a-amino- or a-hydroxycar-bonyl compounds (HX-C-C=0), which have an electrophilic and a nucleophilic centre. 

Heterocyclic compoimds have ring structure(s) that, imlike isocyclic compounds, contain atoms of elements other than carbon (see Figure 2.1). There are millions of heterocyclic compoimds in existence, which can be divided into six groups, as defined by Beilstein (see Section 2.2 above) and as listed in Table 2.7. The groups are characterised by the number of noncarbon (= hetero ) atoms they contain. Commonly these are atoms of oxygen or nitrogen, which form stable cyclic combinations with carbon, especially in 5- and 6-membered rings. 

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