Jt-Facial selectivity

Fukui applied the orbital mixing rule [1,2, 59] to the orbital hybridization or the deformation of the LUMO of cyclohexanone to explain the origin of the Jt-facial selectivity in the reduction of cyclohexanone. Cieplak [60] proposed that electron delocalization occurs from the bonds into the o orbital of the incipient bonds at the transition state. 

Fallis and coworkers studied Jt-facial selectivity in the reactions of series of 5-substituted 1,2,3,4,5-pentamethylcyclopentadienes Cp -X. They reported that the diene 5 (Cp -X X = SCH3) with maleic anhydride proceeded more slowly than that of the 5-oxygen substituted cyclopentadienes 6 and 7 (Cp -X X = OH, OCH3), where the HOMO of the diene 5 lies higher than those of 6 and 7 [7, 8] (Table 2). These results seemed to suggest that in the case of the reaction of 5 the NHOMO considerably contributed to the reactions. 

In the case of cyclopentadienes having halogen substituents at 5-positions, syn Jt-facial selectivity is expected since the dienes are classified into the case of... 

Table 1 Jt-Facial selectivity in the reactions of cyclopentadienes having the substituents of group 16 elements at 5-positions...

Table 6 Activation energies and Jt-facial selectivity in the Diels-Alder reactions of 5-X-cyclopentadienes with ethylene...

Kahn and Hehre straightforwardly extended this idea to the description of Jt-facial selectivity in Diels Alder reactions. They simply stated cycloaddition involving electron-rich dienes and electron-poor dienophiles should occur preferentially onto the diene face which is the more nucleophilic and onto the diene face which exhibits the greater electrophihcity (Scheme 40) [49],... 

Mataka and coworkers reported the studies of the Diels-Alder reactions of [3.3] orthoanthracenophanes 96 and 97, of which anthraceno unit, the potential diene, has two nonequivalent faces, inside and outside. The reactions of 96 with dien-ophiles gave the mixtures of inside and outside adducts with the ratios between 1 1 and 1 1.5. However, the ratio changes drastically, in favor of the inside adducts, when 97 reacts with dienophiles such as maleic anhydride, maleimide and naphto-quinone [55] (Scheme 46). Mataka suggested that the Jt-facial selectivity is controlled by an orbital interaction between the electron-poor dienophiles and the Jt-orbital of the facing aromatics, which would lead to a stabilization of the transition state, while Nishio suggested that the selectivity is due to the attractive k/k or CH/jt interaction [53]. 

The Jt-facial selectivity was explained by the Cieplak Effect due to back-donation of lone pair electrons on sulfur (Scheme 49). Mansuy and coworkers reported that in situ generated thiophene 1-oxide 99 could be trapped by 1,4-benzoqui-none to afford the corresponding syn attack product [58]. Tashiro and coworkers also reported that in situ generated thiophene 1-oxide derivatives 98,100-103 and... 

They reported that the DFT calculations of 114 at the B3LYP/6-31G level showed that the ji-HOMO lobes at the a-position are slightly greater for the syn-n-face than for the anti face. The deformation is well consistent with the prediction by the orbital mixing rule. However, the situation becomes the reverse for the Jt-LUMO lobes, which are slightly greater at the anti than the syn-n-face. They concluded that the iyn-Jt-facial selectivity of the normal-electron-demand Diels-Alder reactions... 

Dienes 581 and 583 undergo smooth Diels-Alder cycloaddition with a wide range of dienophiles affording the corresponding products. Their reaction with PTAD gives the respective products 582 and 584 with complete Jt-facial selectivity (Equations 82 and 83) <2005CEJ5136>. 

Cycloaddition of a-aryl-A-phenylnitrones to the C16-C17 n-bond in 16-dehydropregnenolone-3P-acetate (545) involves only the minor rotamer (A-form) of the nitrones. It proceeds regio-, stereo- and Jt-facial-selectively to give steroido[16,17-d]isoxazolidines (546) in high yield (Scheme 2.257), (Table 2.24) (760). Similarly the cycloaddition of a,N -diphenylnitrones proceeds with five-membered heterocyclic enones (761). 

Cyclo-reversions proceed readily in reactions of enantiopure D7i and D7i ent with cyclic (551) and acyclic (585) nitrones. The sulfinyl group in lactones D7i and D7i ent controls the Jt-facial selectivity and is also controller of the endo/exo selectivity (Scheme 2.274) (788). 

The Lewis acid-promoted [4+ 2]-cycloaddition reaction of the allenic ester 103 having a camphor-derived chiral auxiliary with cydopentadiene provided the adduct with excellent Jt-facial selection, leading to an enantioselective synthesis of (-)-/l-san-talene [92]. 

One of the factors directing the alkylation of an enolate is the Jt-facial selectivity. The differences in reactivity of the two diastereotopic faces of the enolate, due to steric and electronic features, contribute to the steric control of the alkylation (for extensive reviews, see refs 1, 4, and 30). Likewise, stereoelectronic features are important control elements for C- versus O-alkylation, as illustrated by the cyclization of enolates 1 and 3 via intramolecular nucleophilic substitution 39. 

Step b Jt-Facial selectivity may be determined by the stabilizing effect of the H-bond between the benzoate oxygen with the aldehyde hydrogen (see Tetrahedron Lett. 1997, 38, 33). 

Incorporation of the thiabutadiene moiety into a camphor framework resulted in the reaction with various dienophiles proceeding with complete Jt-facial selectivity and, in some instances, exo-selectivity. The stereochemistry of one product was confirmed by X-ray analysis. The (arylmethylene)thiocamphor compounds 429 were prepared by thionation of the corresponding ketones with Lawesson s reagent and exist as stable monomers (Equation 125) <1999TL8383>. 

As another example, the Diels-Alder reaction of 2-0-acryloyl derivative of methyl 4,6-di-O-benzyl-3-0-(f-butyldimethylsilyl)-a-D-glucopyranoside 117, prepared from methyl a-D-glu-copyranoside 96, with cyclopentadiene, was explored under thermal or Lewis acid-mediated conditions [94] (O Scheme 31). Under thermal (rt) conditions, the 2-0-acryloyl ester 117 provided one endo-addvLCi 118R with moderate endo/exo selectivity (74 26) and good r-facial selectivity (de = 82%). Under Lewis acid-promoted conditions, another enclo-adduct 118S was a predominant product with good jt-facial selectivity. 

Similarly, the pyrazine moiety of pyrazine-fused-isopropylideneorbomadiene 198 was prepared by condensation of 1,2-diketone compound 196 and ethylenediamine, followed by dehydrogenation of 197 in presence of nickel peroxide (overall yield 41%) <01JCS(P1)1372>, Then, compound 198 was treated with electrophilic reagents such m-CPBA, MBS, and 4-phenyl-l,2,4-triazole-3,4(4H)-dione (PTAD) to measure the effect of the pyrazine ring on the Jt-facial selectivity in these electrophilic reactions. 

Garcia Ruano et al. made good use of chiral sulfoxide 81 as a reactive chiral dieneophile. The sulfinyl-acrylonitrile showed virtually complete Jt-facial selectivity and high diastereoselectivity when reacted with furan. After 2 days at 25 °C with ftiran as the solvent, the en /o-cycloadduct 82 was isolated in 51% yield and the corresponding ejto-isomer 83 was isolated in 11% yield (Scheme 13.25) [55]. Only... 

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