Dimethyl- Open-Chain Compound

In the fused compounds (241) and (242) the furan ring fails to react as a diene and Diels-Alder reaction with dienophiles occurs on the terminal carbocyclic rings. However, (243) and (244) afford monoadducts with dimethyl fumarate by addition to the furan rings (70JA972). The rates of reaction (Table 2) of a number of dehydroannuleno[c]furans with maleic anhydride, which yield fully conjugated dehydroannulenes of the exo type (247), have been correlated with the aromaticity or antiaromaticity of the products (76JA6052). It was assumed that the transition state for the reactions resembled products to some extent, and the relative rates therefore are a measure of the resonance energy of the products. The reaction of the open-chain compound (250), which yields the adduct (251), was taken as a model. Hence the dehydro[4 + 2]annulenes from (246) and (249) are stabilized compared to (251), and the dehydro[4 ]annulenes from (245) and (248) are destabilized (Scheme 84). 

The special types of bonding in three-membered ethyleneimine rings (41—43) have been studied using microwave spectroscopy (44—47), electron diffraction (48), and photoelectron spectroscopy (49—51), and have occupied theoretical chemists up to the present day (52). These studies reveal that ethyleneimine has a distincdy shortened C—C bond of 0.148 nm (as compared to 0.154 nm in open-chain compounds) and a noticeably lengthened C—N bond of 0.149 nm (compared to 0.146 nm). Because of the high s character of the free electron pair on the nitrogen, ethyleneimine also shows a lower basicity (p Ka = 7.98) than noncyclic aliphatic amines such as dimethyl amine (p Ka = 10.7) (53). 

The condensation of 3,4-diaminopyridine with glyoxal hydrate in alcohol gives a 50% yield of the parent heterocycle m.p. 97°. This method is more convenient than the use of the bisulfite compound in aqueous acetic acid, which provides only a low yield of the required compound. Benzil and diacetyl and l,4-dibromobutane-2,3-dione provide the expected 2,3-disubstituted products 4 in good yields. Dimethyl oxalate reacts with 3,4-diaminopyridine in refluxing M hydrochloric acid to give the open-chain compound 5." This may be cyclized to the dioxo compound 6 by heating at 230°. 

In open-chain compounds, where rearrangement products are more easily obtained, 2-bromo-2,4-dimethyl-3-pentanone gives the a-alkoxyketone as a major product even when the reaction is carried out under aprotic conditions.Consequently two factors seem to be decisive in inhibiting cyclopropanone formation and yielding the a-alkoxyketone even in an aprotic solvent. The first is the degree of substitution, which can play a role in open-chain or cyclic compounds. The second operates only in cyclic systems and is ring strain which can be calculated by force field techniques. This kind of strain can be due to valence angle deformations or to transannular nonbonded interactions or bonded interactions. ... 

A careful examination of the results given in Tables 7 and 8 reveal that with the exception of 2,3-dimethyl-2-butene (in the case of 47) only cyclic guest molecules are taken up into the lattices of the host compounds 46-48, but not the respective open-chain analogues. Saturated 2,3-dimethylbutane, as a compound for comparison, is not accommodated either, either by 46 or by 47. Moreover, only cycles with distinct ring sizes (five- to eight-membered rings) are effective, indicating the presence... 

It was found (616) that the course of the heterogeneous reaction of 1-hydroxy-5,5- dimethyl-2,4- diphenyl-3-imidazoline-3-oxide with PhLi depends on the crystalline phase of the starting compound, which can be obtained, predominantly, in cyclic or open chain tautomeric forms. 

Compounds 16 and 19 each deliver the expected six alcohols after reduction of the primarily formed hydroperoxide mixtures as a result of an oxygen attack on the trisubstituted A1 double bonds of these molecules. The ratio of tertiary/secondary hydroperoxides (or alcohols) is about 44 56, as has also been found with 1-methylcyclohexene (30)13S while open-chain olefins such as trimethylethylene (S3), 1,1-dimethyl-2-ethylethylene (id), 2,6-dimethyl-2-octene (39), myrcene (42), / -citronellol (45), linalool (48), and l,l-dimethyl-2-benzylethylene (51) give ratios of tertiary/secondary hydroperoxides between 54 46 and 60 40.104-1 7 7 1 79 The slight deviations from 1 1 ratios in all these cases are probably due to stereochemical rather than electronic effects exerted by the olefins on the reaction with oxygen. 

Methyl-2-methylenecyclopropane (1) and 1,1 -dimethyl-2-methylenecyclopropane (4) give rise to the formation of monospiro and dispiro dimers, 3, 6 and 2, 5, respectively. In the case of 1,1,2,2-tetramethyl-3-methylenecyclopropane (7), the dispiro compound 9 is formed along with an open-chain monomer 8. With 1 as substrate, several other dimers and trimers are additionally obtained. On the other hand, with 7, phenylated ring-opened products, such as 10 and 11 arising from transfer of the ring-cleaved MCP to the solvent benzene, are formed in 4% (L = EtsP) to 34% (L = i-Pr P) yield. ... 

Reduced pyridazines have also been irradiated. The dihydropyridazine 290 gives in high yield the open-chain product 291,582 whereas dimethyl l,2-dihydropyridazine-l,2-dicarboxylate gives two isomeric compounds, 292 and 293 (the minor product).493 It is postulated that the latter is formed by... 

Dimethyl urea is prepared by the interaetion of urea and methylamine, whieh upon treatment with eyanoaeetie aeid yields an open-chain nitrite with the elimination of a molecule of water. This resulting compoimd undergoes cyclization in the presence of alkali. The cyclized compound on treatment with nitrous acid, followed by reduction, reaction with formic acid and subsequently with alkali gives rise to the formation of theophylline, which upon methylation finally yields caffeine. 

Synthetic methods for preparation of 1,2,4,5-tetroxanes have been reviewed recently <2001COR601, 2002RMC113>. The most general method involves acid-catalyzed addition of hydrogen peroxide to carbonyl compounds and subsequent cyclization of the hydroperoxide intermediates. The direct synthesis is carried out normally in the presence of either sulfuric, perchloric, or methanesulfonic acids and affords symmetrically substituted tetroxanes (Equation 26). In many cases, for example, where the carbonyl compound is unsubstituted in the a-position, tetroxanes are contaminated with hexaoxonanes and open-chain hydroperoxides. Selective removal of the more reactive hydroperoxides can be achieved with dimethyl sulfide or potassium iodide. Recrystallization usually removes residual hexaoxonanes but, failing that, heating the mixture with perchloric acid in acetic acid can convert hexaoxonanes to tetroxanes or convert the thermodynamically less stable hexaoxonanes to more water-soluble lactones, which may facilitate the purification process <2002RMC113>. 

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