Fragmentation of heterocycles

Heterocyclic fragmentations which formally lead to arylnitrenes are far less important synthetically than the methods described above. Photolysis of N-aryloxaziridines in a quartz apparatus is thought to give a nitrene  

Photolysis of 2-alkylindazoles (aza analogs of anthranils) in aqueous media gives either azepinones (7) or 2-amino-5-hydroxyacetophenones (8) depending upon the pH — a nitrene nitrenium ion equilibrium is thought to be involved.  

---

Enzymatic reactions of heterocycles 85WCH85 83WCH449. Fragmentation of heterocycles 8OYGK853. 

Most of the above examples demonstrate that the amino-alcohol moiety can be synthesised with a high degree of stereocontrol. Furthermore, the formation of heterocyclic intermediates, in particular oxazolidinones and oxazolines above, is often a crucial factor in exercising this control. Hence, ring fragmentations of, -heterocycles, produced by methods other than allylic functionalisation, persist as an important area within amino-alcohol synthesis. 

Fragmentations of heterocycles have played an important role in the preparation of amide derivatives. Moderate to good yields of a-hydroxy-amides were obtained on reaction of a-hydroxy-acid aceto-nides with primary amines.N-Alkyl-2-methyl-2-oxazolinium salts (obtained by mixing alkyl halides and 2-methyl-2-oxazoline in dichloromethane) were found to react with sodium benzeneselenolate to yield -(2-phenylselenoethyl)-t -alkylacetamides, which after oxidation to ] -vinyl analogues with sodium metaperiodate in methanol, gave secondary amides on sequential treatment with mercuric acetate in aqueous tetrahydrofuran and sodium borohydride/3M... 

The mass spectral fragmentation of the fully saturated parent heterocycles has been studied on a comparative basis (65JA2920) and the principal processes are indicated in Figure 10. All of the compounds exhibit appreciable molecular ions. The M-1 ions (v)... 

Transformations of heterocycles with participation of functionalized polymers including heterocyclic fragments 97S1217. 

Asymmetric Pauson-Khand reaction in syntheses of heterocycles fused with five-member carbocyclic fragment 980PP121. 

N-Heterocycles as fragments of artificial supramolecules capable of multiple interactions 97YGK357. 

A mass spectrometric study was carried out to establish tbe structure of compoimd 69. Its mass spectrum contains tbe molecular ion peak m/z 252 (16.98%) and a base peak (100%) at m/z 210, corresponding to 2-(2-hydroxypbenyl)benzimidazole (70). A tendency towards decreasing the heterocycle size is characteristic of the mass spectrometric behavior of 1,5-benzodiazepin-2-ones [61] and consequently the mass spectra of these compounds contains intense peaks of the corresponding benzimidazoles. It is also known that the mass spectrometric fragmentation of 1,5-benzodiazepines is similar to their thermal or acid decomposition. In fact, refluxing compound 69 in concentrated sulfuric acid yields benzimidazole 70 as the main product. 

In view of the reaction behavior of l,2 i.5-oxaphosphetanes (22), treated above, it appears fitting to reconsider the mechanism of the hydroxylion induced fragmentation of p-bromophosphinic acid 6443). It was assumed that formation of the phosphinate 65 is followed by that of the four-membered heterocycle 66, which spontaneously decomposes to benzalacetophenone and phenyldioxophosphorane the latter then adds water to give the phosphonie acid 43 ... 

Intermolecular cyclization of allyloxycarbonylphosphines in the presence of azabisisobutyronitrile also results in the formation of heterocycles containing the P—C—O fragment (11) [Eq. (9)] (82ZN965). 

A series of heterocyclic compounds, containing the P—C—N and o-substituted aminophenylene fragment have been synthesized [Eq. (36)]. These are benzazaphospholenes (71), benzazaphosphorinanes (72), and benzazaphosphepanes (73) (70-71 Mil 74MI1 76MI1). 

Chemical properties and structure of heterocycles containing P—C—O and P—C—N fragments are determined by the interaction between the heteroatoms. This interaction is most clearly pronounced in heterocyclic compounds also possessing an acceptor functional group (B(III), O—B(III), N—B(III), etc.). 

An analysis of the principal methods for construction of compounds with 1,2,5-oxadiazole heterocyclic units was published in CHEC(1984) and CHEC-II(1996) < 1984CHEC(6)393, 1996CHEC-II(4)229>. In this chapter only reactions that lead to the formation of 1,2,5-oxadiazole cyclic fragments are considered. The functionalization or replacement of substituents of heterocyclic ring as well as oxidation or deoxidation of nitrogen atoms are described in Section 5.05.4. 

The 1,3-dipolar addition to terminal alkenes of nitrile oxides, generated from nitromethylene derivatives of bicycloheptane, provides 9,ll-ethano-13,15-isoxazolinoprostanoids, PGH analogs, with alkyl, phenyl, or additional heterocyclic fragment in the oo-chain (461). Chemical transformations of 9,11-ethano-13,15-isoxazolinoprostanoids furnish prostanoids with bifunctional fragments of P-hydroxyketone and a-aminoalcohol in the oo-chain. The reaction of P-hydroxy ketones with methanesulfonyl chloride gives rise to prostanoids with an enone component in the oo-chain. 9,ll-Ethano-16-thiaprostanoids have been prepared, for the first time, by nucleophilic addition of thiols to the polarized double bond in the oo-chain. The 1,3-dipolar addition to terminal alkenes of nitrile oxides, generated from nitromethylene derivatives of bicycloheptane provides 9,ll-ethano-13,15-isoxazolinoprostanoids with an alkyl, phenyl, or additional heterocyclic fragment in the oo-chain (462). 

---