A Pentalene

The PE spectra of cyclobutadiene 281192, tetra-t-butylcyclobutadiene 282193, 1,3,5-tri-t-butylpentalene 283194 and aceheptylene 284195, all of which are subject to second-order bond localization, have been described in the literature. Further examples are 1,3,5,7-tetra-t-butyl-s-indacene 285 (which exhibits a first double band 1,2 at I 2 = 6.75 eV, followed by two bands at I = 8.50 eV and I = 9.30 eV) and the tetracyclic hydrocarbon 1,3,6,8-tetra-t-butylpentaleno[2,l-a]pentalene 286 (the first two bands of which are observed at 6.40 eV and 7.65 eV)196. 

The results obtained in combination with the data on the transformations of cations 9-11, led to the conclusion that 1,2-shifts of methyl groups occur readily in carbocations having a pentalene fragment. On the basis of the data obtained for rearrangements of such carbocations, an alternative mechanism has been proposed (52) for the rearrangement of structurally related carbocation 16 having a bicyclo[3.3.0]octane skeleton described in (33) (Scheme 12). 

DOMIRO DIELS-ALDER REACTION 3,3a,3b.4.6a.7a-HEXAHYDR0-3,4,7-METHENO-7H-CYCLOPENTA[a]PENTALEN -7,8-DICARBOXYLIC ACID (3,4,7-Netheno-7H-c3rc1 openta[a]penta1 ene-7.8-dlcarbofyllc add, 3,3a,3b,4,6a,7a-hexahydro-)... 

Dimethyl 3,3a,3b,4,6a,7a-hexahydro-3,4,7-metheno-7H-cyclopenta[a]pentalene-7,8-d1carboxyl ate 3,4,7-Hetheno-7H-cyclopenta[a]pentalene-7,8-di carboxylic acid, 3,3a,3b,4,6a,7a-hexahydro-, dimethyl ester (9) (53282-97-6)... 

Last, it should be noted that some compounds which could be formally named as dithiolylidene ketones (14) are better represented by an ionic structure (15) or a pentalene structure (16). 

Dimethyl 3,3a,3b,4,6a,7a-hexahydro-3,4,7-metheno-7H-cyclopenta[a]pentalene-7,8-dicarboxylate 3,4,7-Metheno-7H-cyclopenta[a]pentalene-7,8-dicarboxylic acid, 3,3a,3b,4,6a,7a-hexahydro-, dimethyl ester (53282-97-6), 68, 198 Dimethyl (E)-2-hexenedioate (70353-99-0), 66, 52, 53, 59 N,0-Dimethylhydroxylamine hydrochloride Methylamine, N-methoxy-, hydrochloride Methanamine, N-methoxy-, hydrochloride (6638-79-5), 67, 69 N,N-Dimethylisobutyramide, 66, 117, 118... 

In Tables 1 and 2 the necessity to derive all structures from a pentalene skeleton leads to the fact that, for some compounds, the replacements or substitutions indicated, though unambiguous, do not correspond to an IUPAC recommended name. These deviations are... 

How many ABMOs are there in pentalene It is common knowledge that the best way to clarify the meaning of a theorem (especially this one, quoted without proof and, I regret, with some simplifications) is to give an example. As such an example, let us consider a pentalene molecule ... 

Some qualitative conclusions can easily be ( rawn from the results obtained, in particular, the tendency of the molecule to attract electrons to the NBMO. Indeed, a pentalene dianion is fairly stable. 

A Prins cyclization is also involved in the reaction of the cyclopropylenynes in equation (72) with AuCl or cationic Au(I) complexes to give tricyclic derivatives with an octahydrocyclobuta[a]pentalene skeleton. The formation of stereoisomers was unexpected and suggests the involvement of cationic intermediates (equation 72). 

In an attempt to synthesize fused aromatic systems of a pentalene-like structure, Boekelheid and Fedoruk (332) submitted the dicyanomethyl ylide of thiazole (77) to the addition reaction with dimethyl acetylenedi-carboxylate (DMA). They unexpectedly observed the formation of a fused six-membered (80) rather than a five-membered-ring (78). This ylide (77) was readily afforded by the reaction of thiazole (73) with tetracyanoethylene oxide and then put into reaction with DMA. The initially formed thiazolopyrrole derivative (78) is strongly polarized by the gcm-dicyano group, and its pyrrole ring is spontaneously cleaved with proton elimination. The ring dosure of the intermediate (79) leads to the final stable derivative of 5-FT-thiazolo[3,2-a]pyridine (80). More recently. 

While reaction 119bhas been used for a (+/—) hirsutene synthesis, the annelation according 119c was elaborated for a (+/—) pentalene preparation 2Z9b). 

The fully unsaturated linear and angular tetraquinanes 99-103, the C13-fenes-tranes 104,105 and the bisangular tetraquinanes 107,108 with two quaternary carbon atoms each (Fig. 8), resemble hydrocarbon skeletons consisting of four five-membered rings. While saturated congeners of 99 (see Note added in proof, p. 165) and 100 are known as skeletons of natural products [1, 2], none of the fully unsaturated compounds has even been tackled. While 99 and 100 consist of a pentalene and an added fulvene unit each, and 102 is a cyclopentene-annela-ted acepentalene, they have no chance to be stable at ambient temperature. Compounds 101,107 and 108, however, should be less strained. 

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