Cyclopentenes 2 + 2 + 2 cycloaddition reactions

At this point the catalytic process developed by Dotz et al. using diazoalkanes and electron-rich dienes in the presence of catalytic amounts of pentacar-bonyl(r]2-ds-cyclooctene)chromium should be mentioned. This reaction leads to cyclopentene derivatives in a process which can be considered as a formal [4S+1C] cycloaddition reaction. A Fischer-type non-heteroatom-stabilised chromium carbene complex has been observed as an intermediate in this reaction [23a]. 

Although 1-vinylnaphthalene thermally reacts with 4-acetoxy-2-cyclopenten-1-one (98) to regioselectively afford 99, the isomer 2-vinylnaphthalene gives the same thermal cycloaddition with low yield (30 %) and reacts satisfactorily only with 98 at 10 kbar (Scheme 5.10). Both products 99 and 101 were converted into the cyclopenta[a]phenanthren-15-one (100) and cyclopenta[c]phenanthren-l-one (102) isomers. Acetoxyketone 98 acts as a synthetic equivalent of cyclo-pentadienone (114 in Scheme 5.14) in cycloaddition reactions [33]. 

Utilizing an alternate mode of Diels-Alder reactivity, Harman has examined the cycloaddition reactions of 4,5-T -Os(II)pentaammine-3-vinylpyrrole complexes with suitably activated dienophiles <96JA7117>. For instance, cycloaddition of the p-vinylpyrrole complex 58 with 4-cyclopentene-l,3-dione, followed by DDQ oxidation affords 59, possessing the fused-ring indole skeleton of the marine cytotoxic agent, herbindole B. 

Tam and coworkers [311] developed a method for the synthesis of 1,3-disubsti-tuted cyclopentanes 6/4-124 and cyclopentenes. Thus, reaction of the condensed isoxazolines 6/4-123, easily obtainable by a 1,3-dipolar cycloaddition, gave 6/4-124 in good yields using Mo(CO)6 (Scheme 6/4.30). 

The introduction of heterodienes has extended the synthetic versatility of cycloaddition reactions in organic synthesis.150 Denmark and coworkers have developed the use of nitroalkenes as dienes in [4+2] cycloaddition. Nitroalkenes react with simple alkenes in the presence of SnCl4 as a promoter. For example, the reaction of nitrocyclohexene with cyclopentene gives three products. The major product is awh-isomer, which arises from an exo approach of cyclopentene toward nitrocyclohexene (see Eq. 8.94).151... 

The meso-ionic l,3-dithiol-4-ones (134) participate - in 1,3-dipolar cycloaddition reactions giving adducts of the general type 136. They show a remarkable degree of reactivity toward simple alkenes including tetramethylethylene, cyclopentene, norbomene, and norbor-nadiene as well as toward the more reactive 1,3-dipolarophilic olefins dimethyl maleate, dimethyl fumarate, methyl cinnamate, diben-zoylethylene, A -phenylmaleimide, and acenaphthylene. Alkynes such as dimethyl acetylenedicarboxylate also add to meso-ionic 1,3-dithiol-4-ones (134), but the intermediate cycloadducts are not isolable they eliminate carbonyl sulfide and yield thiophenes (137) directly. - ... 

Cyclazinylium salts are also accessible from 3,4-bismethylenepyrrolizinium salts (248b) via cycloaddition reactions. Treatment with ethyl vinyl ether and (V-morpholino-cyclopentene in boiling ethanol yielded (296a) and (296b) respectively (79TL3405). 

SCS-MP2 and the new perturbative B2-PLYP density functional methods provide accurate reaction barriers and outperform MP2 and B3-LYP methods when applied to the 1,3-dipolar cycloaddition reactions of ethylene and acetylene.39 Phosphepine has been shown to catalyse the asymmetric 3 + 2-cycloaddition of allenes with a variety of enones (e.g. chalcones) to produce highly functionalized cyclopentenes with good enantiomeric excess.40 The AuPPh3SbF6 complex catalysed the intramolecular 3 + 2- cycloaddition of unactivated arenyne- (or enyne)-yne functionalities under ambient conditions.41 A review of the use of Rh(I)-catalysed 3 + 2-cycloadditions of diaryl-and arylalkyl-cyclopropenones and aryl-, heteroaryl-, and dialkyl-substituted alkynes to synthesise cyclopentadienones for use in the synthesis of natural products, polymers, dendrimers, and antigen-presenting scaffolds has been presented.42... 

The presence of five-membered rings such as cyclopentanes, cyclopentenes, and dihydrofurans in a wide range of target molecules has led to a variety of methods for their preparation. One of the most successful of these is the use of trimethylenemethane [3 + 2] cycloaddition, catalysed by pal-ladium(O) complexes. The trimethylenemethane unit in these reactions is derived from 2-[ (trimethylsilyl)methyl]-2-propen- 1-yl acetate which is at the same time an allyl silane and an allylic acetate. This makes it a weak nucleophile and an electrophile in the presence of palladium(0). Formation of the palladium 7t-allyl complex is followed by removal of the trimethylsilyl group by nucleophilic attack of the resulting acetate ion, thus producing a zwitterionic palladium complex that can undergo cycloaddition reactions. 

Besides the common oleftnic dipolarophiles, other unsaturated systems have been evaluated in cycloaddition reactions of zwitterionic TMM-Pd complexes, including polyenes and acetylenes. While acyclic electron-poor dienes generally gave mixtures of five- and seven-membered rings [48], a limited number of selective [3 + 4] and [3 + 6] cycloaddition reactions have been achieved with cyclic polyenic substrates as illustrated by formation of cycloadducts 41 and 42 from pyrone [49] and tropone [50], respectively (Scheme 16). On the other hand, activated alkynes have failed to produce the corresponding cyclopentene derivatives [51]. 

Products 6 were formed by a concerted [3+2] cycloaddition via a helical TS of type 47 however, it was unexpected that the cycloaddition should show this regioselectivity rather than that shown in structure 48. The only example to show the expected regioselectivity was the case of 46e in which the formation of 6e was observed (28%) and the major product was the indazole 49 (59%) (Scheme 11). It is worthy to mention that diazo compounds in which the cyclopentene was not present are stable at 25 °C and gave compounds of type 18 as the main products. The presence of the cyclopentene ring serves in some way to expedite the cycloaddition reaction through the TS 47 <2000J(P1)1139>. 

Surprisingly, some Diels-Alder cycloaddition reactions show no variation in endojexo product ratio with changes in solvent phase. Ordered liquid-crystalline solvents are not able to differentiate between endo- and exo-activated complexes in the Diels-Alder reaction of 2,5-dimethyl-3,4-diphenylcyclopentadienone with dienophiles of varying size (cyclopentene, cycloheptene, indene, and acenaphthylene), when it is carried out in isotropic (benzene), cholesteric (cholesteryl propionate), and smectic liquid-crystalline solvents at 105 °C [734]. 

The importance of the ethylene ketal described above with respect to stereocontrol of the de-Mayo reaction is emphasized by later published works of two other teams. Fetizon and co-workers obviously followed a similar concept and carried out photocycloaddition reactions with 96a in model studies (Scheme 23) (Si). As can be seen from retroaldol product 98, exclusive a attack of the cyclohexene has taken place. Thus, the relative stereochemistry of the BC ring connection is opposite to that of taxane. Totally comparable results were obtained by Berkowitz et al. in the course of cycloaddition reactions of cyclopentene, cyclohexene, or those of a cyclohexenone ketal to the camphor derivative 96b (.84). 

The [2 -I- 2] cycloaddition reaction of l-(trimethylsilyloxy)cyclopentene (63) and acet-ylenecarboxylate, in the presence of ZrCU, was accompanied with desilylation to afford bicyclo[3.2.0]heptene carboxylate 64 (Eq. 27) [28],... 

Another modification of a cyclopentene moiety was achieved by the cycloaddition reaction of the 1,3-dithiolane derivative 483 with singlet oxygen O2 to form the endoperoxide 484, which upon treatment with either triethyl-amine, triphenylphosphine, or bromine gave the corresponding hydroxy ketone 485, a mixture of the epoxide 486 and the enonc 487, or a mixture of isomeric adducts 488 and 489, respectively (Scheme 69) <1995JOC1333>. 

The thermal expansion of a vinylcyclopropane to a cyclopentene ring is a special case of a [l,3]-sigmatropic migration of carbon, although it can also be considered an internal [ 2 + 2]-cycloaddition reaction (see 15-63). It is known as a vinylcyclopropane rearrangementThe reaction has been carried out on many vinylcyclopropanes bearing various substituents in the ring or... 

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