Boat-like conformation

The rearrangement of E and Z N-crotylamines 245 (R, E or Z=Me) gave the corresponding nitriles 247 with 82 and 68% yield, respectively. Disappointingly, the product was obtained as an inseparable mixture of synlanti diastereomers 247 indicating a low simple diastereoselectivity. Obviously, the intermediate ketene imine fitted neither a chair- nor a boat-like conformation. Hence, a low axis-to-center chirality induction was operative, and E and Z reactants gave a... 

Bicyclo[3,l,0]hexanes.-Hach has concluded his efforts to provide preparative-scale syntheses of pure (+)-thuj-3-one (30), and (-)-isothuj-3-one, and the four derived thujanols, viz. (+)-thujan-3-ol, (-)-isothujan-3-ol, (+)-neothujan-3-ol (31), and (-)-neoisothujan-3-ol (for earlier related work see Vol. 2, pp. 30, 37, 38 Vol, 4, p. 42 Vol. 7, p. 36). ° A conformational study on these four thujanols indicates that they have a slightly developed boat-like conformation, whereas the five-membered ring in (+)-thuj-3-one (30) and (-)-isothuj-3-one is flat, according... 

G=G bond approximately perpendicular to the coordination plane, with the triethylsilylmethyl group adopting an axial or pseudo-axial position about the hexenyl chelate. Gomplex 41 presumably adopts a boat-like conformation to avoid unfavorable 1,3-diaxial interaction between the triethylsilylmethyl group and one of the methoxycarbonyl groups. 

Neighboring-group participation reactions by sulfur and nitrogen functions have also been observed for deoxyhalogeno sugars. An example is the formation of the anhydride 146, which possesses an oxathiabicyclo[2.2.2]octane ring-system constrained in a boat-like conformation, on treatment of methyl 4-0-benzoyl-2-S-benzoyl-6-bromo-6-deoxy-2-thio-Q -D-altropyranoside (145) with methanolic... 

Protonated forms of the large-ring macrocycle [24]Ng02 (5) and related compounds have been shown to be active as synthetic phosphorylation catalysts in ATP synthesis. It is likely that in this case the substrate enters the macrocyclic cavity to some extent, or is enveloped by it. Evidence for this possibility comes from the crystal structure of the chloride salt of 5-6H (Figure 1) in which a chloride ion is enveloped within a cleft formed by the boat-shaped conformation of the macrocy-cle. The crystal structure of the nitrate salt of 5-4H has also recently been determined and the host again adopts a boat-like conformation as it interacts with the anion. The hydrochloride salt of the smaller [22]Ng binds two chloride anions above and below the host plane in a similar way to 1. Molecular dynamics simulations indicate that the pocket-like conformation for 5-6H is maintained in solution, although Cl NMR experiments demonstrate that halide ions are in rapid exchange between the complexed and solvated state. 

Dihydrobenzoxocinone (149) is produced by decomposition of the pyrazoline (148), from the reaction of 3-acetylcoumarin with excess diazoethane (74JCS(P1)66). The boat-like conformation (150) is assigned to the eight-membered ring. 

The CD spectra of six derivatives (60-65) of cedran-10-one devoid of a C(8)-C(i3) oxide bridge exhibit negative Cotton effects near 290 nm in accordance with the octant rule and a chair conformation of the six-membered ring151. The corresponding ketones with oxide bridge show a preference for a boat-like conformation and exhibit positive n —> jr Cotton effects152. 

Contrary to this bicyclic series, the equilibration of the a- and the b-cyano isomers derived from the steroid enones 92 (R=H or CH3) does not occur. It has also been impossible to produce the B-cyano isomer from the hydrocyanation reaction. This result was explained by the fact that in the steroid series, there is a strong steric interaction between the cyano group and the C-ll methylene group of ring C in the boat-like conformation 95 (cf 98). Consequently, addition to the B-face never occurs in these compounds. This was confirmed by the fact that 1a-cyanocholestanone (derived from 92, [KH3) readily incorporates labeled cyanide (13C=N) under the experimental conditions of the hydrocyanation reaction. 

According to ESR measurements, the intermediate glycosyl radical exists in the boat-like conformation [20], which was previously observed in some photoproducts of spirocyclization (Sect 2.2). This preferred conformation of the pyranosyl radical results from the stabilizing interaction of the single occupied p orbital with the a -Lumo of the adjacent p-C-OR bond (Scheme 6). 

Fig. 22 ORTEP views of (46dfppy)Pt(/z-pz)2Pt(46dfppy), showing the boat-like conformation that leads to short metal-metal separations. Reprinted with permission from [114]. (2005) American Chemical Society...

The 7-acetylbenzo[ ]naphtho[l,8-/,g]thiazocin-8(777)-one 556, as predicted by theoretical calculations (see Section 14.07.4.1) and confirmed by X-ray investigation (see Section 14.07.4.2), adopted a boat-like conformation <1996JST(365)93>. 

Fig.1 The molecular geometry of DHP analogues. This geometry was adopted by Gaudio et al. to calculate some quantum mechanical parameters, which they used in their QSAR study [16]. The 1,4-dihydropyridine ring is shown to have a boat-like conformation and the phenyl ring to be bound to it at a pseudoaxial position and approximately bisecting the pyridine ring. From [16]. 1994 John Wiley Sons, Inc., reprinted by permission...

Conformational effects also play an important role in determining IMDA diastereoselectivity. For example, trienes like (49) and (50) with two sp hybridized atoms within the connecting chain cyclize preferentially to ci s-fused rather than the trans-fuscd products that would be expected based tm the presence of the C(8)-methyl substituent (Figure 15). The benzo fusion forces the bridging chain to adopt boat-like conformations in the transition states, and the 1,3-diaxial interactions that the C(8)-methyl ordinarily experiences in the cif-fused, chair-like transition state are thus relieved (refer to Figure 12). The preferential formation of the cif-fused diastereomer may then be the consequence of a favored skew butene rotamer at C(6)—C(7) in (51) compared with a less favorable gauche butene rotamer at this position in the trans-fused transition state. This effect is discussed in more detail in Section 4.4.3.2 concerning relative diastereoselection in the IMDA reactions of substituted decatrienones. 

The neutral TTF has a boat-like equilibrium structure with C2V symmetry, while TTF and TTF have a planar D2/, symmetry <1999PGA1407>. Due to a small difference between planar and boat-like conformations, TTF is very flexible and can appear in various conformations depending on D-A and donor-donor interactions in the crystal. 

Unsymmetrical ester substitutions at C3 and C5 result in chirality at C4 of the dihydropyridines. Separation of pharmacological actions is seen in appropriately structures optical isomers of chiral dihydropyridines. Further, the availability of a boat-like conformation of the dihydropyridines has been proposed to facilitate the calcium channel blocking activity of this class of agents. 

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