Olefinic esters selective reduction

The selective reduction of the carbonyl group in the olefination product of keto-esters using Na2HP04 and NaBH4 leads to the synthesis of a-alkylmelhylene-y-butyrolactones (see Eq. 7.130).176... 

The selective reduction of the 8-hydroxy- 3-keto ester 4 with Me4NBH(OAc)3 afforded the corresponding l,3-anft -diol in 87% yield and 14 1 diastereoselectiv-ity. The 1,3-anti -diol was protected as the acetonide 5, followed by a Pd-catalyzed coupling reaction with the vinyl iodide 6 to provide the diene 7 in 69% yield. Reduction of the ester, Swem oxidation, and finally, Wittig olefination afforded the (Z)-vinyl iodide 8. 

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. ... 

A very elegant stereoselective synthesis of juvenile hormone has been achieved by Johnson and co-workers,who employed the olefinic ketal Claisen reaction to great advantage. Thus, the hydroxy-ester (15), on treatment with the olefin-ketal (16) in acidic medium was converted into the ester (17). Sodium borohydride reduction to the corresponding allylic alcohol and a second Claisen-reduction sequence as described above yielded the trienol-ester (18). Chlorination under Sni conditions and selective reduction of the resultant primary allylic chloride produced the well-known triene-ester (19) which was converted to juvenile hormone (11). 

Indole can also be used as a carbon nucleophile to react with MBH acetates to give trisubstituted olefins with ( )-selectivity for ester- and acetyl-containing substrates, whereas (Z)-isomers for nitrile-substituted acetate were obtained via S N2 -type allylic substitution catalyzed by InBrs or 12 (Scheme 3.104). The resulting products generated from 2-nitro-MBH adduct can proceed via a one-pot reductive cyclization to form indolylquinoline derivatives 263 in good yields (Scheme 3.104). ... 

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). 

The selective reduction of a,3-unsaturated esters to saturated esters [(75) (76)] in the presence of isolated olefins using... 

The synthesis of several terpenoid natural lactones has been described. Butyrolactones are formed efficiently by the cyclization of hydroxy-acids, and acid olefin cyclization. The diester (123) has been converted independently into the isomeric lactones cinnamolide (124), and drimenin (125) (Scheme 71). Selective reduction of the a,/3-unsaturated ester was used in the latter case, whereas selective oxidation of the corresponding allylic alcohol with manganese dioxide was used in the former. Manganese dioxide has also been... 

Selective reduction of the olefin unit in conjugated ketones and esters was observed. However, the reactions are often accompanied by side reactions and the reactivity of the complexes toward common functional groups limits their applicability. 

Perlmutter used an oxymercuration/demercuration of a y-hydroxy alkene as the key transformation in an enantioselective synthesis of the C(8 ) epimeric smaller fragment of lb (and many more pamamycin homologs cf. Fig. 1) [36]. Preparation of substrate 164 for the crucial cyclization event commenced with silylation and reduction of hydroxy ester 158 (85-89% ee) [37] to give aldehyde 159, which was converted to alkenal 162 by (Z)-selective olefination with ylide 160 (dr=89 l 1) and another diisobutylaluminum hydride reduction (Scheme 22). An Oppolzer aldol reaction with boron enolate 163 then provided 164 as the major product. Upon successive treatment of 164 with mercury(II) acetate and sodium chloride, organomercurial compound 165 and a second minor diastereomer (dr=6 l) were formed, which could be easily separated. Reductive demercuration, hydrolytic cleavage of the chiral auxiliary, methyl ester formation, and desilylation eventually led to 166, the C(8 ) epimer of the... 

Synthesis of a C(8)-C(18) segment of the larger fragment of lb using the same basic strategy is depicted in Scheme 25. Here, hydroxy ketone 176 was subjected to syn-selective (dr of crude product=90 10) reductive amination [42] with sodium cyanoborohydride and benzylamine followed by tetrahydro-oxazine formation using aqueous formaldehyde. The resulting heterocycle 182 was then converted to unsaturated ester 184 by successive desilylation, oxidation, and entirely (Z)-selective Horner-Wadsworth-Emmons olefination. Re-... 

Retrosynthetic considerations reveal an approach (Scheme 1.2.6) in the first step based on disconnections of the C -Cy and C12-C13 double bonds. Those can be built up using highly B-selective Wittig olefinations between allyltributylphos-phorous ylides derived from the corresponding allylic bromides 29 and 31 [31]. The aldehyde 30 is accessible from the keto ester 33, which can be prepared in high enantiomeric purity by a biocatalytic enantioselective reduction of a... 

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