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Containing Two or More Heteroatoms

There have been few studies of the reactivity of molecules in this class. Calculations (ir density) indicate that the positional order in naphtho-[2,3-c]-l,2,5-thiadiazole (8.129 X = S), is 4 6 5, whereas localization energies and frontier electron densities predict 4 5 6 [66TCA(5)401] the high reactivity of the 4-position follows from the benzenoid character generated in the transition state (8.130). CNDO/2 calculations for naph-tho[2,3-c]-l,2,5-oxa-, -thia-, and -selena-diazoles (8.129, X = O, S, Se) predict the order 6 4 5 (73CHE1331). For the corresponding [1,2-c] [Pg.251]

Calculations predict the most reactive sites in thieno[2,3-b]quinoline (8.132) to be the expected 2- or 3-positions [77ZN(B)1331], but there is little agreement on the positional reactivity order in 6-methylindolo-[2,3-6]quinoxaline (8.133) (84CHE687). tt Densities predict the order 7 9 1 (Huckel) or 7 9 3,4 (CNDO/2), localization energies (Huckel) predict 4 7 1, superdelocalizabilities predict 4 1 7, and frontier electron densities predict 1 4 3 (Huckel) or 4 1 2 (CNDO/2). The observed nitration and bromination at the 9-position serve to underline the inadequacy of all these theoretical methods. Localization energies predict that pyrrolo[l,2- j]quinoxaline (8.134) should have a posi- [Pg.251]


Scheme 1 General reaction schemes tor the synthesis of five-membered heterocycles containing two or more heteroatoms by aldol-related reactions... Scheme 1 General reaction schemes tor the synthesis of five-membered heterocycles containing two or more heteroatoms by aldol-related reactions...
However, five-membered rings containing two or more heteroatoms necessarily possess both a pyridine-like heteroatom and a pyrrole-like heteroatom. Thus their chemistry shows similarities to both those of the six-membered rings and of the five-membered rings with one heteroatom this is dealt with in Part 4. [Pg.4]

The final chapter in the present volume is concerned with the tautomeric equilibria of five-membered rings containing two or more heteroatoms and has been written by Professors V. I. Minkin and A. D. Gamovskii (Rostov University, Russia), together with Drs. J. Elguero, A. R. Katritzky, and O. V. Denisko. [Pg.327]

A range of the title compounds were prepared by treating the amines with sulfinyl chloride in benzene. Purification by vacuum distillation led to violent explosions of some of those with 5 membered rings containing two or more heteroatoms on a number of occasions. It is essential to keep the distillation temperature below... [Pg.191]

From five-membered heterocycles containing two or more heteroatoms 92... [Pg.67]

There are few examples of the preparations of heterocyclic compounds containing two or more heteroatoms which involve cyclization with formation of a bond between two heteroatoms. The best known instances of this type of reaction, all of which are [6 + 0] reactions, are the preparations of benzocinnolines as outlined in equations (l)-(4). A similar type of approach to that outlined in equation (4) has been used for the direct preparation of the di-N-oxide (2) from the dioxime (1 equation 5). The naphthotriazine betaine (4) is obtained as one of the products of the thermal decomposition of the azidoazo compound (3 equation 6). 1,2-Dithiins and their dibenzo derivatives have been prepared by oxidation of appropriate dithiols and related starting materials as outlined in equation (7). All of these reactions are, however, somewhat specialized and there has been essentially no systematic study of the preparation of six-membered heterocycles via formation of a bond between two heteroatoms. [Pg.69]

Some of the ring expansion reactions discussed in Section 2.03.3.3.1 can be extended to five-membered heterocycles containing two or more heteroatoms. Reaction of imidazoles and pyrazoles with dichlorocarbene, for example, gives chloropyrimidines together with small amounts of chloro-pyrazines or -pyridazines, and oxidative ring expansion of 1-aminopyrazole with nickel peroxide gives 1,2,3-triazine (this, in fact, constitutes the only known synthesis of the unsubstituted triazine). There are, however, a number of interesting and useful transformations which are unique to five-membered polyheteroatom systems. [Pg.92]

There are two important general types of reaction by which six-membered heterocycles containing two or more heteroatoms can be transformed into other six-membered heterocyclic systems, namely reactions which involve an ANRORC mechanism, and reactions which proceed by a Diels-Alder/retro-Diels-Alder type of mechanism. Transformation of 1,3-oxazinones and -thiazinones into pyrimidones (equations 193 and 194) has been extensively used, especially in the conversion of isatoic anhydride into quinazolinones (e.g. equation 195). 2,4-Diaryl-l,2,3,5-oxathiadiazines, which are readily accessible by reaction of sulfur trioxide with aryl isocyanates, are useful precursors to pyrimidines and 1,3,5-triazines (equation 196). [Pg.96]

A selection of these physical constants for pyrroles, furans and thiophenes is included in Table 32 of Chapter 2.4 and trends are discussed there (Section 2.4.4.1.1), together with data for five-membered rings containing two or more heteroatoms. A computer-assisted QSPR method has been proposed for predicting the normal boiling point for new furans, tetrahydrofurans, and thiophenes (91JCI301, 98JCI28). [Pg.79]

Cycles containing two or more heteroatoms obviously give rise to many conformational possibilities some of which have been investigated (e.g., [91,92,112-114]). [Pg.41]


See other pages where Containing Two or More Heteroatoms is mentioned: [Pg.112]    [Pg.173]    [Pg.96]    [Pg.36]    [Pg.339]    [Pg.1]    [Pg.48]    [Pg.220]    [Pg.251]    [Pg.126]    [Pg.112]    [Pg.1]    [Pg.1]    [Pg.27]    [Pg.45]    [Pg.112]   


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