Pericyclic reactions cycloadducts

Very few pericyclic reactions of carbene complexes have been studied that are not in the cycloaddition class. The two examples that are known involve ene reactions and Claisen rearrangements. Both of these reactions have been recently studied and thus future developments in this area are anticipated. Ene reactions have been observed in the the reactions of alkynyl carbene complexes and enol ethers, where a competition can exist with [2 + 2] cycloadditions. Ene products are the major components firom the reaction of silyl enol ethers and [2 + 2] cycloadducts are normally the exclusive products with alkyl enol ethers (Section 9.2.2.1). As indicated in equation (7), methyl cyclohexenyl ether gives the [2 -t- 2] adduct (84a) as the major product along with a minor amount of the ene product (83a). The t-butyldimethylsilyl enol ether of cyclohexanone gives the ene product 9 1 over the [2 + 2] cycloadduct. The reason for this effect of silicon is not known at this time but if the reaction is stepwise, this result is one that would be expected on the basis of the silicon-stabilizing effect on the P-oxonium ion. 

A complex sequence of pericyclic reactions, intramolecular and intermolecular cycloadditions and cycloreversions, was studied in an attempt to readily achieve bicyclic five-membered heterocycles, the methyl 4,6-dihydrothieno- and methyl-4, 6-dihydrofuro[3,4-b]-furan-3-carboxylates 146 and 147. The results give further evidence of the potential of intramolecular Diels-Alder based multiple processes [129], 2-Substituted furans and thiophenes 148 and 149, heated in the presence of 3,6-di(pyridin-2 -yl)-,y-tetrazine, underwent intramolecular and intermolecular cycloadditions. The cycloadducts underwent double cycloreversion reactions with the loss of a nitrogen and dipyridyldiazine as illustrated in Scheme 2.55. The electron-deficient dipyridyltetrazine reacts with the isolated, electron-rich olefinic bond rather than with the bond conjugated with the methylcarboxylate. 

Similarly, Tsuge et al. reported the reaction of triafulvene 315 with thiazolium Af-phenacylide 325 in THF at room temperature for 4 days to give the cage compounds, 6,8-thiazapentacyclo[6.3.1.0i-i .0 . 0 ]dodecenes 326 and 327 in 51% and 36% yield, respectively (Scheme 7.72) [82]. The polycycles 326 and 327 were produced by a pericyclic reaction of the initially formed ew io [3-1-2]cycloadduct 328, followed by a 1,3-hydrogen shift of 329. 

Pericyclic Diels-Alder reactions are suprafacial reactions and this manner of bond formation preserves in the cycloadduct the relative stereochemistry of the substituents at Ci and C4 and at Ci and C2 of the parents diene and dienophile, respectively (Scheme 1.7). The relative stereochemistry of the substituents in the... 

A combined system formed from Co(acac)3, 4 equiv of diethylalu-minum chloride, and chiral diphosphines such as (S,S)-CHIRAPHOS or (/ )-PROPHOS catalyzes homo-Diels-Alder reaction of norbomadiene and terminal acetylenes to give the adducts in reasonable ee (Scheme 109). Use of NORPHOS in the reaction of phenylacetylene affords the cycloadduct in 98.4% ee (268). It has been postulated that the structure of the active metal species involves noibomadiene, acetylene, and the chelating phosphine. The catalyzed cycloaddition may proceed by a metallacycle mechanism (269) rather than via simple [2+2 + 2] pericyclic transition state. 

Cyclocondensation with aldehydes (11, 332-333). The cyclocondensation of aldehydes with trimethylsilyloxydienes to provide 2,3-dihydro-4-pyrones has been studied in the most detail with this diene. Two pathways have been identified. In reactions catalyzed by ZnCl2, MgBr2, and a lanthanide shift reagent such as Eu(fod)3,2 a pericyclic pathway is suggested by isolation of the initial cycloadduct 2 with the expected endo-selectivity. In the presence of acid, this adduct is converted into a cis-2,3-dihydro-4-pyrone (3). Only traces of the trans-isomer (4) are formed. In contrast, the BF3-catalyzed reaction results... 

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