Twist-boat conformer

In addition to the chair conformation of cyclohexane, a second arrangement called the twist-boat conformation is also nearly free of angle strain. It does, however, have both sleric strain and torsional strain and is about 23 kj/mol (5.5 kcal/mol) higher in energy than the chair conformation. As a result, molecules adopt the twisl-boat geometry only under special circumstances. 

Twist-boat conformation (Section 4.5) A conformation of cyclohexane that is somewhat more stable than a pure boat conformation. 

A further test of the stereoelectronic theory of reactivity of phosphate esters has been attempted using measurements of the rates of displacement of 4-nitrophenate from the esters (23) and (24), their phosphorus epimers, and also (25), in aqueous methanol the introduction of the 4a-Me group into the system would, it was hoped, reduce the the flexibility of the bicyclic structures and so possibly eliminate the participation of twist-boat conformations. The presence of the 4a-Me group has no effect of the rate of displacement of the axial ArO group... 

Dialkyl zinc compounds form phosphine complexes of formula RZnP(SiMe3)2 on addition of one equivalent of bis(trimethylsilyl)phosphine. Solution and solid-state studies showed that the complexes are dimeric or trimeric in solution or the solid state. Bis(trimethylsilyl)phosphido-methylzinc crystallizes as a trimeric molecule with a Zn3P3 core in the twist-boat conformation. Bis(trimethylsilyl)phosphido- -butylzinc, shows a similar central Zn3P3 fragment. The sterically more demanding trimethylsilylmethyl substituent forms a dimeric species of bis(trimethylsilyl) phosphido-trimethylsilylmethylzinc. Solution studies of bis(trimethylsilyl)phosphido- .vo-propyl-zinc demonstrate a temperature-dependent equilibrium of the dimeric and trimeric species and the crystalline state contains a 1 1 mixture of these two oligomers. A monomeric bis(trimethyl-silyl)phosphido-tris(trimethylsilyl)methylzinc has also been synthesized.313... 

The above-mentioned [Hg(P—Z-Bu2)2]2 has been obtained in a phosphide-transfer reaction between HgMe2 and Ga(P-z-Bu2)3. If CdMe2 and Ga(P—/-Bu2)3 are reacted, the product is the trimeric [MeCd(P-/-Bu2)2]3 with a six-membered Cd3P3 ring in twist-boat conformation and exocyclic Cd—Me bonds thus, Cd achieves near-planar, three-coordinate geometry. The Cd—P bond lengths in the ring are all similar (rav(Cd—P) 258.5 pm) in solution these Cd—P bonds... 

Interestingly, we were intrigued by the ESI mass spectrum of the compound, as the observed base peak consisted of [M-S02+Na]+. This led us to explore a thermal retro-Diels-Alder reaction that could afford the desired enone 69. It is noteworthy that the chemistry of cyclic enol-sulfites would appear to be an under-explored area with a few references reporting their isolation being found [57]. At last, we were also able to prepare epoxy ketone 70 from 69 in three steps, albeit epoxidation did not take place unless the TES group was removed. Spartan models reaffirmed our initial conformational assessment of enone 69 and epoxy ketone 70, which contain sp3-hybridized C8a and s/r-hybridized C8b (p s e u d o-. v/r - h y b r i d i zed C8b for 70) at the AB-ring junction (Fig. 8.12) and displayed the desired twisted-boat conformation in A-ring. 

Refer to Figure 24-9 for the chair and twist boat conformations of cyclohexane. 

The final entries in Table 7 concern eight-membered monocyclic dienes and polyenes. The unconjugated 1,5-cyclooctadiene was observed to have twist-boat conformation and C2 symmetry. In accordance with what is very often the case in GED studies of cyclic... 

The [6]radialenes are normally observed to have chair conformations164,168 181,182, although a twist-boat conformation has been observed for a very highly substituted... 

The six-membered heterocyclic ring has twisted boat conformation for Y = S and is nearly planar for Y = O. 

Alternatively, lactols react with benzenesulfinic acid in the presence of CaCl2 to yield the sulfones, again at room temperature [307,309]. In the axial series, the bulk of the sulfone group is such that the 4Cj chair is not always the preferred conformation, and it has been shown that a twist-boat conformer is adopted in at least one instance [305]. Nevertheless, equilibration studies have shown that the sulfonyl group has a small anomeric effect and that the axial anomer is preferred [310],... 

Figure 2. History of the ring torsion angle C1-C2-C3-C4 calculated from a molecular dynamics simulation of the motions of an a-D-glucopyranose molecule in vacuum which began in the C conformation and which subsequently underwent a transition to a twist-boat conformation. (Reproduced from Ref. 9. Copyright 1986 American Chemical Society.)...

Also, 2,2,3,6-tetra-Me-5-Cl-l,3-dioxan, due to 1,3-diaxial interactions, prefers the 2,5-twist-boat form (76BSF563) the same conformation was reported for the stereoisomeric 2-Ph-4-(2 -furyl)-5-N02-6-Me-l,3-dioxanes and 2,2,6-tri-Me-4-(2 -furyl)-5-N02-l,3-dioxanes (75MI2), for 2-Alk-2,4,4-tri-Me-l,3-dioxane derivatives (78KGS1172) and for the cis isomers of 2-OR-4-Me-l,3-dioxane (R = Et, nPr, /Pr, nBu, n-CsHn) (81DOK116). The corresponding trans isomers adopt the chair conformation with di-eq substitution. The isomeric 2-OR-4,4-di-Me-l,3-dioxanes also prefer the 1,4-twisted-boat conformer (81DOK116). 

Slightly twisted-boat conformer with the 3-substituents in equatorial position. 

In the case of 2,4,4,6-tetra-alkyl substitution (due to steric hindrance) the corresponding 1,4-twist-boat conformer 19c was identified by H NMR spectroscopy (85ZOR96). 

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