Ethylene faciality

Tetraene 141 has been converted into various complex polycondensed adducts by reacting with a variety of dienophiles such as maleic anhydride, N-phenylmaleimide, N-phenyltriazolinedione,p-benzoquinone and tetracyano-ethylene carried out under thermal conditions. All cycloadditions occurred facial-diastereoselectively from an outside attack and provided monocycloadducts which had an exceptionally close relationship between diene and dieno-phile and then underwent intramolecular cycloaddition [125]. The reaction between 141 and p-benzoquinone is illustrated in Scheme 2.53. 

In principle, the diene can react with dienophiles at either of its faces. Anti 7t-facial selectivity with respect to the substituent at 5-positions was straightforwardly predicted on the basis of the repulsive interaction between the substituent and a dienophile, however, there were some counter examples. The first of them is the syn tr-facial selectivity observed in the reaction between 5-acetoxy-l,3-cyclopentadiene 1 and ethylene reported by Woodward and coworkers in 1955... 

Ab initio calculation of Diels-Alder reactions of a series of 5-heteroatom substituted cyclopentadienes Cp-X (65 X = NH, 50 X = NH, 64 X = NH3, 67 X = O", 54 X = OH, 68 X = OH3% 69 X = PH, 51 X = PH, 70 X = PH3% 71 X = S, 55 X = SH, 72 X = SH/) with ethylene at HF/6-31++G(d)//HF/6-31-i i-G(d) level by BumeU and coworkers [37] provided counterexamples of the Cieplak effect. The calculation showed that ionization of substituents has a profound effect on the n facial selectivity deprotonation enhances syn addition and protonation enhances anti addition. The transition states for syn addition to the deprotonated dienes are stabilized relative to those of the neutral dienes, while those for anti addition are destabilized relative to those of the neutral dienes. On the other hand, activation energies for syn addition to the protonated dienes are similar to those of the neutral dienes, but those for anti addition are very much lowered relative to neutral dienes (Table 6). 

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

In contrast with exo (top) facial selectivity in the additions to norbomene 80 [41], Diels-Alder reaction between isodicyclopentadiene 79 takes place from the bottom [40] (see Scheme 32). To solve this problem, Honk and Brown calculated the transition state of the parent Diels-Alder reaction of butadiene with ethylene [47], They pointed ont that of particular note for isodicyclopentadiene selectivity issue is the 14.9° out-of-plane bending of the hydrogens at C2 and C3 of butadiene. The bending is derived from Cl and C4 pyramidalization and rotation inwardly to achieve overlap of p-orbitals on these carbons with the ethylene termini. To keep the tr-bonding between C1-C2 and C3-C4, the p-orbitals at C2 and C3 rotate inwardly on the side of the diene nearest to ethylene. This is necessarily accompanied by C2 and C3 hydrogen movanent toward the attacking dienophile. They proposed that when norbomene is fused at C2 and C3, the tendency of endo bending of the norbomene framework will be manifested in the preference for bottom attack in Diels-Alder reactions (Schane 38). 

The most interesting conclusion made in the study (162) is that the facial attack of olefin on six-membered cyclic nitronate from the side of the C-5 atom (or from the side opposite to the C-6 atom) is much more preferable. This result was obtained by calculation methods for model reactions of nitronates (256) and (257) with ethylene (E=H, Scheme 3.171). 

The preference of the distal attack of olefin (ethylene) on nitronate (257) could be attributed to steric hindrances due to the presence of the axial alkoxy substituent at the C-6 atom, which shields the proximal attack. These hindrances are absent in nitronate (256). However, one could suggest that for nitronate (256) adopting a half-chair conformation, the approach of olefin from the side of the C-6 atom is more shielded even if the C-6 atom is unsubstituted because considerably deviates upward from the plane of the C=N bond of the dipole in comparison to the deviation of the C-5 atom in the opposite direction (see Sections 3.3.3 and 3.5.2). It could be worthwhile to combine this approach with a consideration of the facial preference for model nitronates substituted at the C-5 atom also. 

The only paper concerning catalysis by Lewis acids of the Diels-Alder reactions of these simple sulfinyl ethylenes was due to Ronan and Kagan [20], who studied the influence of TMSOTf in the reaction of compound (S)-l with cyclo-pentadiene and furane. In the first case, the reaction occurs at 0°C in 3 h, giving an 89 11 mixture of endo and exo adducts (overall yield 60%) with very high n-facial selectivity (de> 92%). The high efficiency of the catalyst increasing the reactivity of 1 also made possible its reaction with furan, which evolved with low endo/exo selectivity (55 45) and lower 7r-facial selectivity (de 70%) than that observed with cyclopentadiene. These excellent results were nevertheless, eclipsed by those reported in the same paper [20] concerning the activation of... 

In connection with the total synthesis of grandisol, an asymmetric addition of ethylene on chiral heterocyclic aminals and ketals was examined (Scheme 23). The selectivity can be high, with a preferred approach of ethylene from the less hindered side, especially when chiral pyrrolidone 97 or furanones 100 were used in place of cyclic enones [70]. The diastereoisomeric excess of 101 or 102 remains modest with 5-menthyloxy furanone, even if the dark addition of nucleophiles or radicals on 100 occurs with a total facial selectivity. From a detailed analysis of the dependence of the product ratio with temperature and substituents, it was proposed that a pyramidalization of the (3-carbon in the relaxed of the... 

Similarly photocycloaddition of ethylene with chiral 5-alkoxyfuranones 181 proceeds with facial diastereoselection [153], Although selectivity remains low, the reaction has been used for the enantiospecihc synthesis of grandisol because it is easy to separate the initial diastereoisomers [154]. Selectivity is greatly influenced by temperature and by substituents on the ethylenic bond of the furanone ring. It was concluded that steric hindrance of the alkoxy group and stereoelectronic effects of substituents on the conjugated system are responsible for the selectivity observed. It was also proposed that the diastereoselection of the photocycloaddition of ethylene with 181 was dominated by the approach of ethylene to the vibrationally relaxed (71,71 ) excited furanone rather than by the selectivity of the reactions of biradicals. 

Expanded poly(tetrafluoro ethylene) (ePTFE) CORE-TEX W.L. Core Associates, Flagstaff, USA Regenerative membrane, osteoconductive membrane, large diameter aortic, and carotid vascular grafts, tension-free repair of ventral incisional hernia orbital reconstruction, facial reconstruction, rhinoplasty... 

Figure 29 This Fe°-based tetrahedral cage is held together by ethylene te-cyanide bridging ligands with tridentate facial capping ligands completing the octahedral coordination of the metal centers. "...

Plotnick HB (1978) Carcinogenesis in rats of combined ethylene dibromide and disulfiram. JAMA 239 1609 Plotnick H, Birmingham DJ (1993) Disulfiram alcohol facial flush in rubber industry (abstract). Proceedings of the American Contact Dermatitis Society Annual Meeting, Washington, D.C. p 11... 

Self-assembly of synthetic facial amphiphile is, however, not restricted to assemblies with low aggregation numbers. For instance, very large and well-defined supramolec-ular assemblies are formed by the rigid-flexible macrocycle constructed from a rigid aromatic oligomer as the hydrophobic face and a flexible oligo (ethylene glycol) as... 

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