To O-heterocyclics

Cycloaddition approaches to O-heterocycles in synthesis of natural products 99JHC1391. 

Reactions of 2,3-allenols and their derivatives, particularly those leading to O-heterocycles 06CJO1468. 

Transition metal-catalyzed reactions using alkynes as precursors of car-bene and vinylidene complexes used in cyclizations leading to O-heterocycles 05CL1068. 

Trifluoroacetic acid G-Acylation of 0-heterocyclics Ring closure to O-heterocycles... 

Potassium hexacyanoferrate(lll) Oxidative pairing of phenolic radicals Ring closure to O-heterocyclics and reverse reaction... 

Annelation increases the complexity of the spectra just as it does in the carbocyclic series, and the spectra are not unlike those of the aromatic carbocycle obtained by formally replacing the heteroatom by two aromatic carbon atoms (—CH=CH—). Although quantitatively less marked, the same trend for the longest wavelength band to undergo a bathochromic shift in the heteroatom sequence O < NH < S < Se < Te is discernible in the spectra of the benzo[Z>] heterocycles (Table 17). As might perhaps have been anticipated, the effect of the fusion of a second benzenoid ring on to these heterocycles is to reduce further the differences in their spectroscopic properties (cf. Table 18). The absorption of the benzo[c]... 

Michael addition of aromatic or heterocyclic aldehydes (via cyanohydrins) to o. unsaturated systems. Also addilion of aliphatic aldehydes catalyzed by thiazoFium yKds... 

As mentioned in the introduction, l-heterobut-l-en-3-ynes, RXCH=CHC=CH (X = RN, O, S R = organic radical), are the nearest and most important diacetylene derivatives readily formed by nucleophilic addition of amines, alcohols, and thiols to diacetylene. In many heterocyclization reactions (especially those leading to fundamental heterocycles) l-heterobut-l-en-3-ynes behave as diacetylene synthetic equivalents, but unlike diacetylene, they are nonhazardous. Therefore, the syntheses of heterocycles therefrom are often more attractive in preparative aspect. 

As was suggested in the preceding discussion, most of the arene complexes isolated by metal-atom techniques are benzene derivatives. However, heterocyclic ligands are also known to act as 5- or 6-electron donors in transition-metal 7r-complexes (79), and it has proved possible to isolate heterocyclic complexes via the metal-atom route. Bis(2,6-di-methylpyridine)Cr(O) was prepared by cocondensation of Cr atoms with the ligand at 77 K (79). The red-brown product was isolated in only 2% yield the stoichiometry was confirmed by mass spectrometry, and the structure determined by X-ray crystal-structure analysis, which supported a sandwich formulation. 

Nitro compounds have also been reported to undergo photocyclizations. The intermediacy of an isoxazoline in the photorearrangement of o-nitro-benzaldehyde to o-nitrosobenzoic acid is now in doubt,318 but intramolecular hydrogen abstraction by an excited nitro group in nitrobenzene derivatives can result in the formation of heterocycles. 4-tm-Butyl-3-methoxy-2,6-dinitrotoluene (384) on irradiation in methanolic sodium hydroxide solution... 

Dr. S. T. Reid has contributed the first of two sections on photochemistry of heterocycles, the present one dealing with that of /V-heterocycles. This updates part of his own earlier review The Photochemistry of Heterocycles published in 1970 in Volume 11. A further contribution, which will extend the updating to O- and 5-heterocycles, should be published shortly. Dr. A. B. Hornfeldt s review, entitled Selenophenes, also deals with a subject that was previously reviewed in Volume 12 of this series by N. N. Magdesieva. 

The l,3-oxazin-2-ones and 1,3-oxazine-2-thiones previously synthesized were used to prepare various N- and O-heterocyclic systems fused with 1,3-oxazine rings.172 For example, furan-l,3-oxazin-2-one or furan-l,3-oxazine-2-thiones (190a,b) and pyran-l,3-oxazin-2-ones or pyran-l,3-oxazine-2-thiones (190c,d) were prepared in very good yields, ranging from 83% to 90%, by montmorillonite K-10 clay-catalyzed cyclodehydration of 189a,b and 189c,d, respectively (Scheme 35). 

One of the possible synthetic ways to obtain heterocyclic phosphines is the insertion of carbonyl compounds into the P—E (E = Si, Ge) bond of sila- and germa-phospholanes. Thus, the enlargement of the ring takes place and the P—C—O—E fragment is formed (9) [Eq. (7)] (74MI1 75JOMC35 77JOM35). The heterocyclic phosphepanes are obtained as a mixture of stereoisomers. 

Drew et al. [18-20] showed that alterations to the heterocyclic part of the hydrazide system effectively blocked the chemiluminescent reaction. O- and N-methyl analogs, the /V-amino imide, and the corresponding aminoquinazoline-2,3-dione were not chemiluminescent under the reaction conditions used in the oxidation. 

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