A,ft-Unsaturated esters

A highly diastereoselective aziridination of 8-phenylmenthol-derived a,ft -unsaturated esters (38) with 3-acetoxyamino-2-ethylquinazolinone (39) has been reported. The (g) yields of the resulting aziridines (40) were greatly improved in the presence of hexamethyldisilazane.51... 

In addition to the diastereoface-differentiating 1,2-addition of Grignard reagents to keto aminals, highly stereoselective 1,4-addition of Grignard reagents to a,ft-unsaturated esters having an aminal moiety was also examined. Thus, the aminal 14, easily prepared from the diamine la and fumaraldehydic acid methyl ester, was... 

The infrared spectrum of IV indicates an enol (3200 and 1650 cm"1), an a, ft unsaturated ester (1675 cm"1), and a normal carbomethoxy carbonyl group (1725 cm"1). The NMR spectrum substantiates the enolic character of the molecule with a singlet (8 = 11.3 ppm). It also shows two doublets ( 4.40 and 4.84 ppm, J = 16.5 cps) which can be assigned to two methylene protons (similar chemical shift to analogous protons in III), and singlets at 3.78 ppm and 5.16 ppm. The ratio of protons was shown to be 1 1 1 6 1 in the order given. Such a spectrum is consistent with either the enol form of III (Illb shown below) or with structure IV. 

Because of their previous findings that a,/l-unsaturated thioesters were more reactive than their ester counterparts in Diels-Alder reactions85, Hart and coworkers86 performed a systematic study of the cycloaddition reactions of a,/l-unsaturated thioesters and a,ft-unsaturated selenoesters with several dienes. Thermal reactions were compared with Lewis acid catalyzed reactions at room temperature (equation 28 and Table 2). The results clearly demonstrated that use of a Lewis acid enhanced the regioselectivity (entries 1 vs 2, 3 vs 4, 5 vs 6 and 7 vs 8) as well as the endo (with respect to the thioester or selenoester group) selectivity (entries 5 vs 6 and 7 vs 8). 

Complex hydrides have been used rather frequently for the conjugate reduction of activated dienes92-95. Just and coworkers92 found that the reduction of a,ft-unsaturated ketene 5,5-acetals with lithium triethylborohydride provided mixtures of 1,4- and 1,6-reduction products which were transformed into enals by treatment with mercuric salts (equation 27). Likewise, tetrahydro-3//-naphthalen-2-ones can be reduced with L-Selectride to the 1,6-reduction products93 -95 this reaction has been utilized in the stereoselective synthesis of several terpenes, e.g. of (/ )-(—)-ligularenolide (equation 28)95. Other methods for the conjugate reduction of acceptor-substituted dienes involve the use of methylcopper/diisobutylaluminum hydride96 and of the Hantzsch ester... 

Cross-metathesis of conjugated electron-deficient alkenes such as a,ft-unsatur-ated esters, ketones, aldehydes, and amides often give high cross-product/dimer ratios due to the slow rate of dimerization of these substrates (Eq. 171). When this occurs, the cross-product is dominant even when the reactions are performed with a 1 1 stoichiometry of the reactants.330 When one of the alkene partners homodimerizes slowly, such as happens with electron-deficient and sterically hindered alkenes, the reaction is driven to the cross-product. With respect to the stereochemistry of the reaction, the E-isomer is obtained with electron-deficient alkenes (Eq. 171), and the E/Z ratio may also vary depending on the types of substituents present on the reactants. 

Asymmetric Diels-Alder reactions of a, ft-unsaturated acids.2 The acrylate ester (2) of this alcohol (1) undergoes Lewis acid-catalyzed reactions with cyclopentadiene in >97 3 diastereoselectivity. 

Pyrolysis at 190° of the resulting diastereomeric Al-pyrazolines (8) and (11) leads to elimination of nitrogen and formation of the cis- and trans-cyclo-propanecarboxylates (9) and (12), respectively. Thermal decomposition of the A -pyrazoline (13) affords methyl tiglate (14) in addition to the cyclopropane derivative (15) in a ratio 2 1, while A2-pyrazolines such as (3) give only a,ft- or / ,y-unsaturated esters, and no cyclopropane derivatives. 

The conjugate addition of cyclic or acyclic a-substituted ft-keto esters to cy,/3-unsaturated ketones can be achieved with good diastereo- and enantio-selectivity (<98% ee) by using derivatives of Cinchona alkaloids, such as (104), a chiral organo-catalysts.155... 

The predominant conformation of a,P-unsaturated carbonyl compounds (aldehydes, ketones, esters) can vary when Lewis adds are coordinated to the carbonyl group, ft has been experimentally shown that the s-trans conformer of acrolein is more stable than the s-ds conformer, in the absence of any Lewis add. The two conformers are of almost equal stability in the case of acrylic acid or methyl acrylate 83] (Figure L21). The barriers between these conformers are low (4-9 kcal mol ). Interconversion is rapid, and the Curtin-Hammett principle ( 1.3) can be applied. 

TLC is commonly used for the separation of different classes of wax components or for analysis of monomers from cutin and suberin depolymerization. A typical separation is shown in Fig. 6.12. By such methods, it is possible to separate hydrocarbons, wax esters, primary alcohols, secondary alcohols and /8-diketones from plant waxes (von Wettstein-Knowles, 1979). Products of hydrogenolysis from cutin can be separated by TLC into alkan-l-ols, alkane-a,ft>-diols, Cis triols, Ci6 triols and Cis tetrols (Kolattukudy, 1980). Unsaturated components can be resolved by argentation-TLC (Tulloch, 1976) and threo or erythro diastereoisomers separated by boric acid/silica gel TLC (Eglinton and Hunneman, 1968). Straight-chain compounds can be preferentially removed from branched compounds as their urea complexes (Kolattukudy, 1980). 

Cooper and Anders (20) reported the HPLC analysis of unsaturated C l8 and C20 fatty acids. Since the methylene-interrupted polyunsaturated acids show no specific UV absorption, the 2-naphtacylesters were prepared for UV detection at 254 nm [the column was a 3-ft X 0.07-in.-lD stainless steel tube packed withCORASIL-Ci8, methanol/water(85 15) served as the eluent, and a flow rate of 12 ml/h was obtained at a pressure of 300 psig]. The lower detection limit was 4-90 ng of ester. 

In 2005, the groups of List and McMillan simultaneously described excellent results in the asymmetric reduction of a,/ -unsaturated aldehydes with a prochiral center in the ft position [14, 15]. (For experimental details see Chapters 14.22.1 and 14.22.2). In both cases the catalyst used was a chiral imidazolidinone (6 or 8), and some representative examples are listed in Tables 11.1 and 11.2. The reactions were run at 10-20 mol% of catalyst, at moderate temperature (13 °C or 4 °C) over several hours. The hydride source (Hantzsch ester) was utilized in stoichiometric quantities, and the chemical yields and enantiomeric excesses proved to be... 

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