Lactones protonated

The first step of the reaction is the oxypalladation of the triple bond with PdCl2 as shown by 228 to form the alkenylpalladium species 229, and the Pd is displaced with proton to regenerate Pd(TI) species and the lactone 224. The alkenylpalladium species 229 can be utilized for further reaction. When allyl chloride (230) is added, double bond insertion is followed by elimination of... 

Unsubstituted 3-alkyl- or 3-aryl-isoxazoles undergo ring cleavage reactions under more vigorous conditions. In these substrates the deprotonation of the H-5 proton is concurrent with fission of the N—O and C(3)—-C(4) bonds, giving a nitrile and an ethynolate anion. The latter is usually hydrolyzed on work-up to a carboxylic acid, but can be trapped at low temperature. As shown by Scheme 33, such reactions could provide useful syntheses of ketenes and /3-lactones (79LA219). 

Figure 4.9 Mechanisms of the reactions catalyzed by the enzymes mandelate racemase (a) and muconate lactonizing enzyme (b). The two overall reactions are quite different a change of configuration of a carbon atom for mandelate racemase versus ring closure for the lactonizing enzyme. However, one crucial step (red) in the two reactions is the same addition of a proton (blue) to an intermediate of the substrate (red) from a lysine residue of the enzyme (E) or. In the reverse direction, formation of an intermediate by proton abstraction from the carbon atom adjacent to the carboxylate group.

The NMR experiments 55 are obtained from actinomycin D in order to check the amino acid sequence, to assign proton-proton and some carbon-proton connectivities, and to deduce informations concerning proton distances and the spatial structure of both cyclopentapeptide lactone rings. Conditions CDCI3, 10 mg per 0.3 ml, 25 °C, 500 MHz H), 125 MHz ( C). (a) HH COSY plot ... 

The proposed mechanism for the conversion of the furanone 118 to the spiro-cyclic lactones 119 and 120 involves electron transfer to the a -unsaturated methyl ester electrophore to generate an anion radical 118 which cyclizes on the /3-carbon of the furanone. The resulting radical anion 121 acquires a proton, giving rise to the neutral radical 122, which undergoes successive electron transfer and protonation to afford the lactones 119 and 120 (Scheme 38) (91T383). 

When a solution of 25 in a 1 1 mixture of methanol and methylene chloride is exposed to periodic acid, the dithiane group is cleaved oxidatively to give, after treatment of the crude product with camphorsulfonic acid (CSA) in methanol, bisacetal 12 as a 2 1 mixture of C-12 anomers in a yield of 80% (Scheme 3). Although the conversion of 12 into 10 could be carried out on the mixture of anomers, it was found to be more convenient to carry each isomer forward separately. When 12 is treated with lithium diethylamide, the methine hydrogen adjacent to the lactone carbonyl is removed as a proton to give an enolate which is then oxidized in a completely diastereoselective fashion with Davis s oxaziridine18 to afford 11. 

From a study of the decompositions of several rhodium(II) carboxylates, Kitchen and Bear [1111] conclude that in alkanoates (e.g. acetates) the a-carbon—H bond is weakest and that, on reaction, this proton is transferred to an oxygen atom of another carboxylate group. Reduction of the metal ion is followed by decomposition of the a-lactone to CO and an aldehyde which, in turn, can further reduce metal ions and also protonate two carboxyl groups. Thus reaction yields the metal and an acid as products. In aromatic carboxylates (e.g. benzoates), the bond between the carboxyl group and the aromatic ring is the weakest. The phenyl radical formed on rupture of this linkage is capable of proton abstraction from water so that no acid product is given and the solid product is an oxide. 

The cycloaddition of glyoxylic acid with cyclopentadiene in water at pH 6 and 60 °C is slow and occurs with low yield and low diastereoselectivity [18] (Scheme 6.17). Proton (pH = 0.9) [18], copper salts [27] and Bi(OTf)3 [28] accelerate the reaction and increase the diastereoselectivity. The lactones 28 and 29 originate from endo and exo cycloadducts 27, respectively. The proposed rearrangement is depicted in Scheme 6.17 for the major endo adduct 30. A competitive ene reaction that originates 28 and 29 cannot be excluded [28]. 

The hetero-COSY spectrum of 7-hydroxyfrullanolide, C15H20O3, a sesquiterpene lactone, along with the H-NMR and C-NMR data are shown. Assign the C-NMR shifts to various protonated carbons based on the cross-peaks in the hetero-COSY spectrum. 

The SECSY spectrum of the coumarin presents cross-peaks for various coupled nuclei. These cross-peaks appear on diagonal lines that are parallel to one another. By reading the chemical shifts at such connected cross-peaks we arrive at the chemical shifts of the coupled nuclei. For instance, cross-peaks A and A exhibit connectivity between the vinylic C-4 and C-3 protons resonating at 8 7.8 and 6.2, respectively. The C-4 methine appears downfield due to its )3-disposition to the lactone carbonyl. Similarly, cross-peaks B and B show vicinal coupling between the C-5 and C-6 methine protons (8 7.6 and 7.1, respectively) of the aromatic moiety. The signals C and C represent the correlation between the oxygen-bearing C-11 (85.4) andC-12 (84.6) methine protons in the side chain. These interactions are presented around the structure. 

The IR spectra of carbohydrazide 9 showed absorption bands at 3317 cm (OH,Hydrazide NH2), 3269 cm (aromatic CH), 1711 cm (CO stretching), and 1621-1640 cm (CO-NH-NH2 groups). The H NMR spectra exhibited a singlet due to the CONHNH2, NH proton at 9.32 ppm. Methylene protons resonated as a singlet at 4.23 ppm. The structures of the products lOa-1 were inferred from their analytical and spectral data. Thus, their IR spectra showed characteristic absorption bands at 3400-3240 cm (NH,OH), 1710-1700 cm (lactone CO), and NHCO at 1650-1600 cm . ... 

The vanillin ethers 36 and 39 exhibited the IR band of the lactone carbonyl group at 1710-1720 cm whereas the aldehydic carbonyl stretching was observed in the range of 1680-1690 cm In the NMR spectra all the protons resonated at expected fields. The aldehydic proton appeared downfield around 9-10 aromatic protons in the range of 7-8 and the C3 - H of coumarin around 6.5. The methylene, methoxy, and methyl protons resonated around 5, 3.8, and 2.2, S respectively. 

Intramolecular lactonization can also be carried out with DCCI and DMAP. As with most other macrolactonizations, the reactions must be carried out in rather dilute solution to promote the intramolecular cyclization in competition with inter-molecular reaction, which leads to dimers or higher oligomers. A study with 15-hydroxypentadecanoic acid demonstrated that a proton source is beneficial under these conditions and found the hydrochloride of DMAP to be convenient.130... 

The stereochemistry of the C(3) hydroxy was established in Step D. The Baeyer-Villiger oxidation proceeds with retention of configuration of the migrating group (see Section 12.5.2), so the correct stereochemistry is established for the C—O bond. The final stereocenter for which configuration must be established is the methyl group at C(6) that was introduced by an enolate alkylation in Step E, but this reaction was not very stereoselective. However, since this center is adjacent to the lactone carbonyl, it can be epimerized through the enolate. The enolate was formed and quenched with acid. The kinetically preferred protonation from the axial direction provides the correct stereochemistry at C(6). 

This polymeric lipid can first be polymerized by free radical initiator in organic solutions before making the vesicles. The proton NMR spectrum of the polymerized lipid shows that vinyl protons of the cyclic acrylate between 85.00 ppm and 86.00 ppm disappeared from the spectrum, compared with that of monomeric lipid. Also in the IR spectrum (Figure 6) the absorption peak at 1670 cm"1 for the cyclic acrylate carbon carbon double bond disappeared as the result of polymerization. The carbonyl absorptions of the esters at 1740 cm 1 and the lactone at 1805 cm"1 still remain in the spectrum. 

The hydrolysis of the cyclic acetal, which was used as the connecting group between the polymer chain and the lipid, was confirmed both by the IR and the proton NMR spectra of the lipid recovered from the vesicular system after standing for 3 weeks at room temperature. The lactone absorption at 1805 cm-1 disappeared from the IR spectrum (Figure 6) as the result of hydrolysis. Furthermore, a new aldehyde absorption band at 1705 cm 1 was observed in the spectrum, which is related to the substituted benzaldehyde group of the hydrolyzed product. The proton NMR spectrum (Figure 10) also clearly showed the formation of the benzaldehyde, as indicated by the peak at 810.20 ppm. 

Compounds (99) and (100) are thought to be formed by addition of cyanide ion to the ring ortho to the carbonyl, followed by protonation at oxygen, aromatization by tautomerization, hydrolysis of the nitrile, and lactonization upon acidification. The photolysis of 2-methoxyacetophenone, on the other hand, results in rearrangement to 3-methoxyacetophenone ... 

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