Olefinic esters reduction

The introduction of tritium into molecules is most commonly achieved by reductive methods, including catalytic reduction by tritium gas, PH2], of olefins, catalytic reductive replacement of halogen (Cl, Br, or I) by H2, and metal pH] hydride reduction of carbonyl compounds, eg, ketones (qv) and some esters, to tritium-labeled alcohols (5). The use of tritium-labeled building blocks, eg, pH] methyl iodide and pH]-acetic anhydride, is an alternative route to the preparation of high specific activity, tritium-labeled compounds. The use of these techniques for the synthesis of radiolabeled receptor ligands, ie, dmgs and dmg analogues, has been described ia detail ia the Hterature (6,7). 

A unique approach to the requisite C-ring fragment 51 is achieved through reductive cyclization of olefinic ester 55 by way of the titanium alkylidene, as described by Rainer and Nicolaou [59]. The olefinic ester 55 is prepared in ten steps from (R)-isobutyl lactate using consecutive chelation-controlled... 

The anion radical species formed by the electroreduction of aliphatic esters show interesting reactivities, and the reduction of olefinic esters gives bicyclic products with high regio- and stereoselectivity. The electroreduction of the ester in the presence of chlorotrimethylsilane affords a tricyclic product (Scheme 21) [35, 40]. The mechanism of this cyclization reaction seems to be the addition of anion radical species, formed by the reduction of the ester group, to the carbon-carbon double bond. 

Isopropyl anisole (171) was converted to bromide (172) by metalation, formylation and bromination. Alkylation with cyclopropyl ketoester produced (173) whose transformation to alcohol (174) was achieved by saponification, decarboxylation and reduction.. Its conversion to homoallylic bromide (175) was accomplished by the method of Julia et al. [56]. Alkylation of ethyl acetoacetate with bromide (175) furnished p-ketoester (176). It was subjected to cyclization with stannic chloride in dichloromethane. The resulting tricyclic alcohol provided the olefinic ester (177) by treatment with mesylchloride and triethylamine. Epoxidation followed by elimination led to the previously reported intermediate (146) whose conversion to triptolide (149) has already been described. 

Studies have been made on the influence of various groups on the rate of hydrogenation of the double bond. Reductions of olefinic alcohols (method 85), olefinic aldehydes (method 161), olefinic ketones (method 196), olefinic acids (method 267), olefinic esters (method 303), olefinic Cyanides (method 394), and olefinic amines (method 460) are treated separately. 

Olefinic alcohols are best prepared by the action of lithium aluminum hydride on the corresponding acid or ester as in the preparation of 3-penten-l-ol (75%). The double bond may be in the a,/S-position to the ester group, The Bouveault-Blanc procedure has also been used with success for reduction of nonconjugated olefinic esters. The addition of the sodium to an alcoholic solution of the ester is superior to the reverse addition of. the ester to sodium in toluene for the preparation of 2,2-dimethyl-3-buten-l-ol (62%). Selective catalytic hydrogenation is inferior. Large amounts of catalyst are required, and the products contain saturated alcohols. ... 

Over the last decade, a considerable number of reactions has been studied (11,35) (i) olefins oxidation (38,39), hydroboration, and halogenation (40) (ii) amines silylation (41,42), amidation (43), and imine formation (44) (iii) hydroxyl groups reaction with anhydrides (45), isocyanates (46), epichloro-hydrin and chlorosilanes (47) (iv) carboxylic acids formation of acid chlorides (48), mixed anhydrides (49) and activated esters (50) (v) carboxylic esters reduction and hydrolysis (51) (vi) aldehydes imine formation (52) (vii) epoxides reactions with amines (55), glycols (54) and carboxyl-terminated polymers (55). A list of all the major classes of reactions on SAMs plus relevant examples are discussed comprehensively elsewhere (//). The following sections will provide a more detailed look at reactions with some of the common functional SAMs, i.e hydroxyl and carboxyl terminated SAMs. 

Several convenient olefin syntheses have been reported. N-Carbalkoxysulfamate esters undergo elimination to olefins at room temperature. Epoxides afford olefins by reductive elimination under the influence of MgBr2 and magnesium amalgam. A twofold extrusion process for the construction of complex olefins has also been described. In the example shown, the highly hindered olefin shown was obtained in 80% yield. 

Several years later, Liu and coworkers reported another synthesis of cephalotaxine 172 that relied on a distinct [2,3]-Stevens rearrangement fScheme 1 S.4QL ° Proline derivative 173 was transformed into ammonium ylide 174 in the presence of allyl bromide and K2CO3. This zwitterion rearranged to a-allyl aminoester 175. Hydration of the olefin and reduction of the ester furnished diol 176, which was converted to aminoketone 177 via oxidation and aldol condensation. The assembly of this spirocyclic intermediate represented a formal synthesis of cephalotaxine 172. ... 

A novel insoluble green complex of palladium(II) and N,N -salicylideneethylenedi-amine (salen) functions as a selective heterogeneous catalyst, notably for the reduction of alkynes in the presence of alkenes, and of the latter in the presence of certain functional groups. E ciVEthylene from acetylene derivs. A soln. of startg. acetylene deriv. in EtOH hydrogenated for 17 min in the presence of the palladium(ll) complex (prepared from potassium tetrachloropalladate, salen, and triethylamine) cis-product. Y 100%. Reduction of alkenes is considerably slower however, terminal olefins may be reduced in the presence of internal olefins esters, oxo compds., dibenzyl ether, and iodobenzene were unaffected. F.e., also ar. amines from nitro compds. s. J.M. Kerr et al.. Tetrahedron Letters 29, 5545-48 (1988). 

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