Heterocyclic compounds ring opening

A new approach to the synthesis of annulated sulfur heterocycles based on triflic anhydride-promoted electrophilic cyclization of the hetaryl-containing alkyl sulfides, including 1409, was elaborated <2003S1191>. The proposed method includes intermediate formation of sulfonylsulfonium salts 1410 followed by electrophilic attack on the aromatic ring (Scheme 271). Smooth demethylation of initially formed cyclic sulfonium salts 1411 by treatment with EtsN afforded a number of five- 1412 and six- 1413 membered fused sulfur heterocycles. Unexpected ring opening with formation of compound 1414 took place in the reaction of diethylamine with five-membered sulfonium salt 1411. 

A large fraction of the chemical reactions known are used to form heterocyclic compounds. Displacement reactions and cycloadditions are particularly important, and their rates are therefore of great practical interest. The same is true for the rates of reverse reactions — ring opening by displacements or retrocycloadditions. It was realized over the last 40 years that... 

R" = H, R = H) gives similar butadienethiolate complexes. Tr-Complexation of thiophene in compounds of the type 123 activates the heterocycle relative toward other nucleophiles, such as OMe , SMe , SEt , S(i-Pr) , andCH(COOMe)2. The products 127 include the ring-opened butadiene-thiolate framework. 

The simple ring opening of tetrahydro-1,3-oxazine derivatives is not the only possible reaction of these heterocyclic compounds catalyzed by mineral acids. An interesting rearrangement of 6-aryl-6-alkyltetrahydro-l,3-oxazines when warmed with concentrated hydrochloric acid was found by Schmiedle and Mansfield ... 

The diazotization of amino derivatives of six-membered heteroaromatic ring systems, particularly that of aminopyridines and aminopyridine oxides, was studied in detail by Kalatzis and coworkers. Diazotization of 3-aminopyridine and its derivatives is similar to that of aromatic amines because of the formation of rather stable diazonium ions. 2- and 4-aminopyridines were considered to resist diazotization or to form mainly the corresponding hydroxy compounds. However, Kalatzis (1967 a) showed that true diazotization of these compounds proceeds in a similar way to that of the aromatic amines in 0,5-4.0 m hydrochloric, sulfuric, or perchloric acid, by mixing the solutions with aqueous sodium nitrite at 0 °C. However, the rapidly formed diazonium ion is hydrolyzed very easily within a few minutes (hydroxy-de-diazonia-tion). The diazonium ion must be used immediately after formation, e. g., for a diazo coupling reaction, or must be stabilized as the diazoate by prompt neutralization (after 45 s) to pH 10-11 with sodium hydroxide-borax buffer. All isomeric aminopyridine-1-oxides can be diazotized in the usual way (Kalatzis and Mastrokalos, 1977). The diazotization of 5-aminopyrimidines results in a complex ring opening and conversion into other heterocyclic systems (see Nemeryuk et al., 1985). 

Grubbs and coworkers [238] used the ROM/RCM to prepare novel oxa- and aza-heterocyclic compounds, using their catalyst 6/3-15 (Scheme 6/3.9 see also Table 6/3.1). As an example, 6/3-35 gave 6/3-36, by which the more reactive terminal alkene moiety reacts first and the resulting alkylidene opens the five-membered ring. In a similar reaction, namely a domino enyne process, fused bicyclic ring systems were formed. In this case the catalyst also reacts preferentially with the terminal alkene moiety. 

The retro-1,3-dipolar cycloaddition of imidazo[l,5- ][l,2,4]oxadiazoles 40, promoted by reaction with triphenylphos-phine at reflux in THF, gives the cyclic nitrones 187 (unreported yields) (Equation 15) <1997T13873>. The ring opening of compounds 40 leading to heterocycles 187 (Equation 15) can also be achieved thermally in the condensed phase under vacuum <1997TL2299>. 

There are several reports dealing with the use of tetrahydropyrrolo[l,4]oxazinones derived from natural proline or prolinol as chiral auxiliaries for the synthesis of enantiomerically pure compounds. The preparation of the heterocycle is described in Scheme 33 (Section 11.11.7.4). The presence of a rigid bicyclic skeleton allows stereoselective introduction of different substituents. The final ring opening of the system (generally by hydrolysis) provides enantiomerically pure compounds with the possibility of recycling the starting chiral auxiliary. 

Addition of lithiated heterocycles to aldonolactones yields carbon-linked nucleosides (56). Thus, the reaction of 2,3 5,6-di-O-isopropylidene-L-gu-lono-1,4-lactone (9b) or 2,3-O-isopropylidene-D-ribono-l,4-lactone (16a) with various lithiated heterocycles gave gulofuranosyl derivatives 53a-g or ribofuranosyl derivatives 54b,c. Gulonolactols 53a-g and ribonolactols 54b,c were acetylated with acetic anhydride in pyridine to yield their acetyl derivatives. The stereochemistry of compounds 53a-g and 54b,c was discussed in terms of the Cotton effect of circular-dichroism curves of the ring-opened alcohols formed upon reduction by sodium borohydride. The configuration at C-l of 53g was proved by means of X-ray analysis (57,58). 

As far as the polymerisation of heterocyclic compounds with one hetero-atom is concerned (cyclic ethers and their analogues) there seems little doubt at present that the propagation involves a displacement at the positive propagating centre. The ring which is part of this -onium ion is opened between the charged atom and a carbon atom next to it, and this becomes attached to the hetero-atom of the monomer ... 

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