Spiro-1,3-cyclopentadienes

The cycloadditions of cyclopentadiene 1 and its spiro-derivatives 109 and 110 with quinones 52, 111 and 112 (Scheme 4.20), carried out in water at 30 °C in the presence of 0.5% mol. of cetyltrimethylammonium bromide (CTAB), gave the endo adduct in about 3 h with good yield [72b]. With respect to the thermal Diels-Alder reaction, the great reaction rate enhancement in micellar medium (Scheme 4.20) can be ascribed to the increased concentration of the reactants in the micellar pseudophase where they are also more ordered. 

Ohwada extends his theory, unsymmetrization of n orbitals, to Orbital Phase Environment including the secondary orbital interaction (Chapter Orbital Phase Environments and Stereoselectivities by Ohwada in this volume). The reactions between the cyclopentadienes bearing spiro conjugation with benzofluorene systems with maleic anhydride exemplified the importance of the phase environment. The reactions proceed avoiding the out-of-phase interaction between dienophile LUMO and the HOMO at the aromatic rings. The diene 34 with benzo[b]fluorene favored syn addition with respect to the naphtalene ring, whereas the diene 35 with benzo[c]fluorene showed the reverse anti preference (Scheme 22) [28]. 

The availability of cyclopentenones from butanolides allows the lactone annulation to facilitate the synthesis of cyclopentyl natural and unnatural products. An example that highlights the latter is dodecahedrane (178) for which 179 constitutes a critical synthetic intermediate 136,137). Lateral fusion of cyclopentenones as present in 179 can arise by acid induced reorganization and dehydration of 180. While a variety of routes can be envisioned to convert a ketone such as 182 into 180, none worked satisfactorily137 On the other hand, the cyclobutanone spiro-annulation approach via 181 proceeds in 64 % overall yield. Thus, the total carbon cource of dodecahedrane derives from two building blocks — cyclopentadiene and the cyclopropyl sulfonium ylide. 

Nieman and Keay198 reported the use of c/.v,c/s-spiro 4,4]nonanc-1, 6-diol as a new chiral auxiliary to be used in asymmetric Diels-Alder reactions. Their best results in a series of reactions between chiral acrylates and cyclopentadiene were obtained when the pivalate ester of ds,ds-spiro[4,4]nonane-l,6-diol was used as the chiral auxiliary. When 318 was treated with cyclopentadiene, the expected endo adduct 319 was obtained with more than 97% de (equation 88). 

Some representative examples of the [3+2] annulatlon are listed in Table 1. Both cyclic and acyclic allenophiles participate in the reaction, a-Alkylidene ketones undergo annulation to provide access to spiro-fused systems, and acetylenic allenophiles react to form cyclopentadiene derivatives. The reactions of (E)- and (Z)-3-methy1-3-penten-2-one illustrate the stereochemical course of the annulation, which proceeds with a strong preference for the suprafacial addition of the allene to the two-carbon allenophile. The high stereoselectivity displayed by the reaction permits the stereocontrol led synthesis of a variety of mono- and polycyclic systems. 

An extension of this reaction principle to other complexes related to 41 and other dienes is limited. However, 1,3-cyclopentadiene (lh) or 1,3,5,7-cyclooc-tatetraene substitute the tj6-1,3,5-cycloheptatriene ligand, and [Cr(CO)2(tj5-C5H5)(t/3-C5H7)] (5) and [Cr(CO)3(t/6-CgH8)] (72), are formed, respectively, in high yield (73). In order to prevent the formation of 5, spiro[4.2]hepta-1,3-diene and spiro [4.4] nona-1,3-diene (lu) (74) were used instead of lh. The diene lu should not readily form derivatives of t]5-cyclopentadienyl ligands. While spiro [4.2] hepta-1,3-diene does not form well-defined products, lu adds to 41 in a smooth reaction (73) [Eq. (22)]. The constitution of 43 was... 

Common to these molecules with their cyclopentadiene moieties is the so-called fulvene subunit 27. The first fulvenes, 6,6-dialkylfulvenes, were prepared as early as 1906 by Thiele et al. from sodium cydopentadienide and ketones [16]. The parent hydrocarbon 27 and many other derivatives have been thoroughly studied since the 1960s [17-19]. Diazocyclo-pentadiene (28), which is also easily prepared from cydopentadienide, is a heteroanalogue of fulvene. It has frequently been used as a precursor to other theoretically interesting molecules containing annelated cydopentadiene moieties, because its irradiation readily generates the cyclopentadienylidene 29. This carbene has, for example, been trapped with alkynes to form spiro-annelated cydopentadiene derivatives 30 (Scheme 5) [20]. It has been proved by UV spectroscopy [21] and supported by calculations [22] that these spiro[2.4]heptatrienes (so-called [1.2]spirenes) 30 experience a spedal kind of electronic... 

As far as purely five-membered carbocyclic systems are concerned, spiro[4.4]nonatetraene (31) is the prototypical example of two -systems experiencing electronic interaction through their common central carbon atom. Based on symmetry considerations, it was proposed that spiroconjugation in molecules such as 31 should result in a splitting of the cyclopentadienyl HOMOs, whereas the LUMOs should be degenerate [22, 23]. As a consequence, the first electronic absorption (EA) band of cyclopentadiene in 31 was predicted to be split into a lower and a higher lying component of quite different absorptivities. 

Cyclopentadien Dimethyl-tetra-chlor- V/lc, 675, 678 Spiro[2.2]pentan 1-Chlor-l-(trichlor-ethenyl)- E19b, 741 (Cyclopropen-Ringspalt. + En)... 

Cyclopropan-(spiro-5)-hicyclo 2.2.I j hept-2-en - -6-chlor-6-methoxycarbonyl- EI7h, 1558 (1-Cl-l-COOR-en + Cyclopentadien)... 

C y clopro pan- (spiro-5 ) -bicy clo 2.2.1 ] hept-2-en-(6-spiro>-cyclopropan E17b, 1531 (Bi-cyclopropyli-den 4- Cyclopentadien) Cyclopropan-( spiro-5 ) -bicyclo [ 2.2.1 ] hept-2-cn-(7-spiro)-cyclopropan E17b, 1531 (Spiro[2.4]hepta-4,6-dien + En)... 

COOR + Cyclopentadien) Cyclopropau-(spiro-2>-bicyc-lo 4.3.0]nonan - -4,9-dioxo-E17b, 1567 (2-OSiR3 — 1,3-dien + cycl.-3-Oxo-en)... 

Cyclopentadien-<5-spiro-7>-bicyc-lo[4.1.0 hepta-2,4-dien Tetra-chlor- -trifluormethyl- E19b, 809 (Carben + Aren)... 

Bicyclo 5.1.0]octa-2,5-dien-<8-spiro-5)-cyclopentadien -1,2,3,4-Te-trachlor- V/Id, 391 f. 

The phenomenon of homoconjugation is related to cross-conjugation in that there are C=C units in close proximity, but not conjugated one to the other. Homoconjugation arises when the termini of two orthogonal 7i-systems are held in close proximity by being hnked by a spiro-tetrahedral carbon atom. Spiro[4.4]nonate-traene (25) is an example and it known that the HOMO (p. 1208) of 25 is raised relative to cyclopentadiene, whereas the LUMO is unaffected " Another example... 

The method described here is the direct adaptation of the original procedure developed by the late Professor Hans Schmid- As summarized in Scheme 1, the Schmid reaction has been successfully applied to [4 + 3] cycloadditions with cyclopentadiene, spiro[2.4]hepta-4,6-diene, and N-Boc-pyrrole. Additional examples can be found in references 8 and 9. Use of functionalized six-membered oxyallyls and synthetic applications of the [4 + 3] cycloadducts have also been described.8... 

The Diels-Alder reaction of methylenecyclopropane with cyclohexa-1,3-diene at 120°C gave the spirocyclopropanebicyclooctene in 50% yield.When cyclopentadiene and spiro[2.4]hepta-4,6-diene were used as dienes the respective spirocyclopropanenorbornenes were obtained. At 190°C, (chloromethylene)cyclopropane underwent [2 + 4] cycloaddition reactions with cyclopentadiene, furan and cyclohexa-1,3-diene to give the respective Diels-Alder adducts. The reaction of buta-1,3-diene and cyclohexa-1,3-diene with bicyclopropylidene as dienophile gave predominantly the [2 + 2] cycloadduct in addition to a small quantity of the Diels-Alder product. Cyclopentadiene, however, formed exclusively the dispirocyclo-propanenorbomene as result of a formal [2 + 4] cycloaddition. 

An example of the reduction of a nonactivated cyclopropane by lithium in liquid ammonia is the reduction of 1-methyl- and l,l-dimethylspiro[2.4]hepta-4,6-diene. Either cyclopropane bond at the spiro center was cleaved to give alkyl-substituted cyclopentadienes 1 and 2. ... 

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