Conformational transition vinyl ethers

The classical reaction with chlorosulfonyl isocyanate has been extended to it-vinyl sulfide 449 to give a 2.5 1 diastereomeric mixture of 4-(phenylthio)azetidin-2-ones 343 and 450 (Equation 180) <2000MI935>. The facial selectivity in the cycloaddition has been explained by the conformational preference of the allylic groups in the transition structure. A similar reaction with styrene resulted into synthesis of the racemic 4-aryl-azetidin-2-one (Equation 181) <2000TA2351>. The divinyl ether 451 reacted with acid-free chlorosulfonylisocyanate to form 4-vinyloxyazetidin-2-one 452 (Equation 182) <1996SL895, 1997TA2553, 1998TL8349>. Most of the results in the reactions of isocyanate with vinyl ethers could be rationalized by a -conformational preference of the ether in... 

As pointed out earlier, the aliphatic Claisen reairangement and its variants are powerful means to effect stereocontrolled C,C-bond formation. Highly ordered transition states effect a reliable transfer of stereochemistry from starting materials to products. Naturally, the geometry of the vinyl ether bond and the conformation of the transition state are crucial parameters in this process. The former issue is strongly dependent on the Claisen variant that is employed, whereas the transition state geometry is controlled by both steric and electronic features of the Claisen system. Additionally, the choice between... 

The a-substituted 1-methyl allyl vinyl ether was shown to isomerize with a strong preference for the irons product (i.e., 95 % irons, 5 % cfe) . This corresponds to a conformational preference for equatorial methyl (as opposed to axial) in the chair transition state of about 2.4 kcal.mole . The identical value was calculated from the irons product preference in the allyl ester Claisen rearrangements (see a-methyl allyl acetate and a-trifluoromethyl allyl trifluoroacetate). 

Finally the results recently reported by DUBIN and Strauss are certainly worth mentioning, Potentiometric titration of alternating copolymers of maleic anhydride and n-butyl ether indicates that there occurs a conformational transition which is not exhibited by a copolymer of maleic anhydride and ethyl vinyl ether (Fig. 7). The anomalous behavior of the butyl copol3mier is clearly cormected with the establishment of hydrophobic interactions among the butyl groups. Dubin and Strauss conclude that the maleic anhydride and n-butyl vinyl ether copolymer is hypercoiled in dilute aqueous solution. These authors also point out that the behavior of the latter copolymer is identical with that previously found for highly charged polysoaps. 

C6 stereocenters. R.K. Boeckman Jr. and co-workers had to overcome the stereochemical preference of the thermal rearrangement by using a bidentate Lewis acid promoter (TiCU) that coordinated to both the oxygen of the vinyl ether and the ester. This coordination enforced a boatlike conformation for the existing six-membered ring in the transition state. The rearrangement itself took place via a chairlike transition state. 

The thermal rearrangement of vinyl ethers of secondary allylic alcohols (type A) yield disubstituted alkenes with E configuration due to chairlike transition state conformations with the R substituents in pseudoequatorial positions. 

In a further example, the diastereomeric vinyl ethers 3 and 5 arc selectively transformed either to product 4 or 6. These examples show that the conformational preference for equatorial versus axial methyl in the transition state overrules the usual preference for the new C - C bond to be formed at the less-hindered face of the allyl double bond394. Rearrangement of 7 applied in the synthesis of pilocarpine produces a 2 1 mixture of 8 and 9395. 

In studies on hydrophobic interactions and conformational transitions of selected hydrolyzed maleic anhydride-alkyl vinyl ether copolymers (I, 2), we found that under certain conditions neutralization by base caused phase separation in aqueous solutions of the hexyl and octyl copolymers. Both the hexyl and octyl copolymers are insoluble at low pH and require partial neutralization by a base for complete dissolution. Under certain conditions, these copolymers will again precipitate as their degree of neutralization is raised with more base. Insolubility at low pH has been reported for other polyacids (3, 4). However, the phase separation at high pH contradicts the wealth of data indicating that neutralization increases the water affinity of polyacids. 

On the other hand, copolymers of maleic acid with various vinyl monomers had been investigated in studying the influence of local charge density on polyelectrolyte properties, and some alternating copolymers of maleic acid with n-alkyl vinyl ethers were found to undergo the conformational transitions from the compact form to the extended coil upon ionization of primary carboxyl groups in maleic acid. " The hydrophobic interaction seems to be responsible for stabilization of the compact form. Viscometric, potentiometric and calorimetric measurements have been used to study the transition mechanisms. 

The modified titration curves of the fractionated (MA-St)n aqueous NaCl at various ionic strengths from 0.009 to 0.27 resembled the potentiometric behaviors of the maleic acid copolymer with n-butyl vinyl ether and poly(methacrylic acid) in aqueous salts. There was no precipitation or turbidity in the deionized polymer solution, except in aqueous NaCl of 0.27 M. The conformational transition from the compact to extended coil form was seen in diagp ams of the intrinsic viscosity vs. qp and the reduced viscosity in the range 0.1 < aj < 0.4 was independent of polymer concentration. Both the potentiometric and viscometric titration data showed that the pH-induced conformational transition of (MA-St)n in aqueous NaCl is due to intramolecular interaction. The unperturbed molecular dimensions were calculated from the viscosity data and the acid compact form of this copolymer was found to be not so compact as the conformations of globular proteins. Dilatometry did also indicate existence of the transition,and the transition was observed in aqueous solutions of various salts chlorides, bromides, perchlorates and thiocyanates of Na" ", K" ", Rb" ", Cs" " and... 

In the transition state for the asymmetric Claisen rearrangement of allyl vinyl ether 7c with (J )-ATBN 5a, the one conformation of the ether substrate is the matched system for the Cj-symmetric molecular cleft of the chiral aluminum reagent 9, which gives the (S)-product (Fig. 2.3). Increased enantioselectivity using... 

Proton NMR studies of the D2O hydrolyzed -butyl vinyl ether copolymer have been investigated.A sharp decrease in the linewidths of side-chain resonances was noted at a neutralization degree of approximately 0.2. This is the same region where potentiometric titrations show a conformational transition to occur. A similar effect was not noted for the methyl vinyl ether copolymer. 

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