2-Hydroxyallyl

Stabilization of 34 by a hydroxy group is smaller, as is suggested by the small energy difference of 1.3 kcal mol between a-hydroxycyclopropyl and 2-hydroxyallyl cation in favour of the latter cation (see Figure 25). Experimentally, the a-methoxycyclopropyl cation appears to be a stable intermediate in substitution reactions in solution, and there is evidence for the independent existence of both the a-methoxycyclopropyl and 2-methoxyallyl cations in the gas phase245. 

A carboxylic acid is even more stable than an a-hydroxyketone, which is why the thermodynamic factor accounts for 86% of the activation energy lowering of 2-hydroxyallyl vinyl ether (2-hydroxy-AVE) ... 

The reactions of protic acids and other electrophiles at remote Lewis basic functionality have been investigated as a pathway to both transformations and interconversions of carbonyl-substituted metallacycles. Irida-, pallada-, and platinacyclobutanones react reversibly with protic acid to yield 73-2-hydroxyallyl complexes (Equation 36), modulating between metallacyclobutane and hydroxyallyl structures (and further discussed in Section 2.12.9.3.5) <1993CC1039, 1995JOM143, 19970M1159>. 

Upon thermolysis, l.l -disubstituted dicyclopropyl ethers expel cyclopropanone, which oligomerizes rapidly (major pathway). This process is accompanied by some ring opening to a 2-hydroxyallyl cation, thus accounting for the ultimate formation of substituted acetones. 

Normally, the a-methoxyketone is obtained with retention of configuration. In the presence of a symmetrical intermediate such as 2-hydroxyallyl, derivatives methoxylated in a and a can be formed this is the case for the derivatives of 19a. However, steric factors can orient the substitution selec-... 

The formation of the protonated zwitterion (the 2-hydroxyallyl cation) has been proved by nmr spectroscopy of the cyclopropanone in super acid medium " see footnote p. S50. 

Cramer and Barrows investigated computationally the intermolecular cycloaddition between s-cis butadiene and various oxyallyl dienophiles [4]. The four studied in order of increasing electrophilicity are oxyallyl (neutral) < sodium oxyallyl cation < lithium oxyallyl cation < 2-hydroxyallyl cation. Computations showed that the more electrophilic oxyallyl cations showed a higher preference for a stepwise pathway, while the weakly electrophilic oxyallyl dienophiles followed concerted cycloadditions. 

Figure 1.3. Frontier orbital energies (eV) and confidents for acrolein and protonated acrolein. In the latter case the upper numbers refer to the situation where bond lengths and angles correspond to those of acrolein. The lower numbers are more suitable for a hydroxyallyl cation. The actual situation is assumed to be intermediate. The data are taken from ref. 104.

The diisopropyl 2-dimethylphenylsilyl-2-propenyl- and 2-[cyclohexyloxy(dimethyl)silyl]-l,3,2-dioxaborolane-4,5-dicarboxylate reagents 7 and 8 were recently introduced for the enantiose-lective a- and y-hydroxyallylation of aldehydes86. 

Asymmetric induction has been observed in reactions of e-methoxy- and t-hydroxyallyl-stannanes and aldehydes promoted by tin(IV) bromide 25. 

Strategies centered on reductive introduction of the fluoroolefin via a geminal difluoro allylic array have been reviewed [66]. In an introductory example to this synthetic approach, Okada et al. [67] developed a completely stereoselective synthesis of Z)-2,5-syn 2-alkyl-4-fluoro-5-hydroxy-3-alkenoic acids through the Cu(l)-mediated allylic substitution reaction of trialkylaluminum with the (E)-4,4-difluoro-5-hydroxyallylic alcohol derivative (61) (Scheme 21). Reaction... 

Y. Nakamura, M. Okada, A. Sato, H. Horikawa, M. Koura, A. Saito, T. Taguchi, Stereoselective synthesis of (Z)-fluoroalkenes directed to peptide isosteres Copper mediated reaction of trialkylaluminum with 4,4-difluoro-5-hydroxyallylic alcohol derivatives, Tetrahedron 61 (2005) 5741-5753. 

A low molecular weight hydroxyallyl modified telechelic PIB was used as poly(alcohol) for the synthesis of the composites. Several procedures for the synthesis have been described involving two steps and as well as single step. The synthesis is schematically shown in Figure 6.5. 

OL, -Unsaturated acylsilanes (3). Allyl trimethylsilyl ethers (1) on metalla-tion with /-butyllithium (- 78 to - 30°) rearrange to organolithiums that on protonation form (a-hydroxyallyl)silanes (2). These silanes are thermally unstable, but can be oxidized by DMSO-oxalyl chloride or by NCS-S(CH3)2 to a,p-unsaturated acylsilanes (3).1... 

It has been demonstrated by Olah et al.420 that a,/3-unsaturated ketones are O-protonated in HF-SbF5 to form hydroxyallylic cations, which were directly observed by NMR spectroscopy. Jacquesy and Coustard have found indirect evidence for diprotonation of a,/3-unsaturated ketones (enones) by trapping the dication with CO.421 The resulting acylium ion centers are then quenched with methanol or benzene. An interesting synthetic method was therefore developed for carboxylation of bicyclic enones in superacid media at atmospheric pressure [Eqs. (5.158) and (5.159)]. 

SYNS I -HYDROXYSAFROLE Q 1 -METHYLENEDIOXY-4-(l-HYDROXYALLYL)BENZENE a-VINYLPIPERONYL ALCOHOL... 

METHYLENEDIOXY-4-(l -HYDROXYALLYL)BENZENE see BCJOOO 7,8-METHYLENEDIOXYISOQUINOLINE see MJR775... 

Because of the intermediacy of radical anions and/or hydroxyallyl free radicals in dissolving metal reductions of enones, dimerization may compete with simple reduction. Scheme 7 shows the three types of dimers that may be produced. 

The dimerization products shown in Scheme 7 are generally the major ones obtained in electrochemical reductions (vide infra) or reductions at metal surfaces, - in which radical anion intermediates must diffuse to a surface before further electron transfer can occur. In metal-ammonia solutions, however, simple reduction is generally favored over dimerization. These solutions provide high concentrations of available electrons, favoring the probability of the radical ion or hydroxyallyl radical accepting a second electron. 

The electrochemical reduction of a,p-unsaturated ketones and related compounds in aprotic media in the absence of metal cations can, in some cases, lead to relatively stable anion radicals.However, in the presence of proton donors the latter are protonated to form hydroxyallyl radicals, which tend to dimerize more rapidly than they diffuse back to the electrode to undergo further reduction (Scheme 17). Although these allyl radicals prefer to dimerize by coupling at the -position, if this position is sterically hindered, as in the case of cholest-4-en-3-one, coupling at the carbonyl carbon may be observed, yielding pinacols. ... 

The reaction of the hydroxyallyl ylide Ph3As=CHCH=CHCH20H with benzal-dehyde takes place under phase-transfer conditions and gives a mixture of E and Z 3-hydroxyallyloxiranes it gives a higher yield in acetonitrile in the presence of potassium fluoride and aluminium oxide than in the presence of solid potassium hydroxide . ... 

SCHEME 10.57 The stereochemical outcomes of a-hydroxyallylations of aldehydes can be influenced by sugar-derived auxiliaries. 

Chika, J I, Takei, H, Arabinose as a chiral auxiliary for the asymmetric a-hydroxyallylation of aldehydes to syn-l,2-diols. Tetrahedron Lett., 39, 605-608, 1998. 

Reactions of platinum-allenyl derivatives (5), the transformations of 7] -propargyl and t) -allenyl ligands into 7] -hydroxyallyls on mono- and binuclear complexes (6), the cyclization of diynes via metal-allenyl intermediates (7), and the oxidation of [Pt(Br)(PPh3)2(i) -CH=C=CMe2)] to [Pt(Br)(PPh3)2 i9 -C=CMe2(OOH) ] (8) have also been described. 

Roush, W. R., Grover, P. T., Lin, X. Diisopropyl tartrate modified (E)-y-[(cyclohexyloxy)dimethylsilyl]allylboronate, a chiral reagent for the stereoselective synthesis of anti 1,2-diols via the formal a-hydroxyallylation of aldehydes. Tetrahedron Lett. 1990, 31,7563-7566. 

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