Reduction, of conjugated esters

Enantioselective 1,4-reductions of conjugated esters with a PMHS, CuCl, BINAP mixture (eq 10) are high yielding and allow asymmetric construction of /3-keto stereogenic centers (80-92% ee). ... 

D. Conjugate Reduction of Conjugated Aldehydes, Ketones, Acids, Esters and Nitriles... 

Reduction of the ester group in (31) requires protection of the ketone the double bond wanders during this step but returns to conjugation in (27). 

Reduction of conjugated carbonyl compounds using stoichiometric amounts of the ammonium salt shows little advantage over the sodium salt in acidic methanol [11] with both reagents producing allylic alcohols (58-88% for acyclic compounds and 15-64% for cyclic compounds) by selective 1,2-reduction of the conjugated systems. Aldehydes, ketones and conjugated enones are also reduced by tetra-n-butylammonium cyanoborohydride in HMPA [11, 12], whereas haloalkanes and alkanesulphonic esters are cleaved reductively under similar conditions [13]. 

The reduction of unsaturated esters encompasses the reduction of esters containing double and triple bonds, usually in positions, and/or aromatic rings. Such esters may be converted to less saturated or completely saturated esters, to unsaturated or less unsaturated alcohols or to saturated alcohols. Presence of aromatic rings in conjugation with the multiple bonds facilitates saturation of the bonds. Aromatic rings are hydrogenated only after the saturation of the double bonds. 

Reduction of conjugated diunsaturated acids.3 a,/ y, 5-Diunsaturated acids are reduced by sodium dithionite in an alkaline medium (NaOH, NaHC03) to a mixture of (Z)- and (E)-/J,y-unsaturated acids (40 75% yield). A similar reduction of the diunsaturated esters is possible under phase-transfer conditions (Adogen 464, C6H6-HzO). 

Scheme 11.4 Catalytic reduction of conjugated aldehydes with ethyl Flantzsch ester [12].

Asymmetric Organocatalytic Reduction of Olefins 395 Table 11.1 Organocatalytic reduction of conjugated aldehydes by Hantzsch ester 4 [14]. 

Many other functional groups are reduced by Sml. For example, sulfonyl halides are converted to disulfides. The reduction of nitro compounds by this reagent enables synthesis of 1,2-diamines based on a reaction sequence initiated by nitronate addition to aldimines, and in the presence of nitriles it furnishes amidines, and that of isothiocyanates in the presence of conjugated esters, the half thioamide derivatives of succinic esters. 

P-Arylthwpropanoic esters. Reduction of arenesulfonyl chlorides in the presence of conjugated esters is followed by Michael reaction. 

The sequence can also be applied to the enolate anion formed by conjugate addition of organometallic reagents to a,/8-unsaturated ketones. Thus addition of diethyl phosphorochloridate to a mixture of A4-cholestene-3-one (4) and dimethyl-copperlithium gives the diethyl enol phosphate (5) in 55% yield. Reduction of the ester gives the olefin (6) in high yield. 

Inanaga et al. achieved the hydrodimerization of conjugated esters and amides with the SmI2/THF-HMPA system in the presence of a proton source [74].(Scheme 31). However, inexplicably, Alper et al. achieved reductions (not reductive dimerization) of the double bond of a, 3-unsaturated esters and amides with the same system [75]. In the latter paper, however, the presence of a proton donor is not mentioned. 

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