Allylic phosphonates reduction

A method for the preparation of alkenes has been described by Tunemoto and coworkers, whereby regioselective a-alkylation of y-substituted allyl phosphonates, followed by reduction with LAH, gives the ( )-alkenes (Scheme 14). 

Single P C bond cleavage has been described. For example, phosphonium salts 2.56 are reduced to phosphines by LAH in THF under reflux the cleaved bond corresponds to reduction of the most stable carbanion [H2] (Figure 2.28). The P—C bonds of allylic phosphonates 2.57 or phosphonium salts can also be cleaved by... 

Initially, the reduction of ester 45 proved to be problematic (Scheme 14). When THF was used as solvent, a large excess of DIBAL was required (10 equiv.) and the yield for the desired alcohol was still very low, 50%, with the remainder being unreacted 45. Use of methylene chloride as solvent led to complete reduction along with unwanted TES removal. Finally, it was found that the use of ether with 2.5 equivalents of DIBAL provided the desired alcohol in 91% yield. Oxidation gave the enal intermediate, which was converted to the diene ester 47 with allyl phosphonate 46. The selectivity was 19 1 E,Z Z,Z) and the yield was an excellent 93%. Previously, the more common methyl ester corresponding to 47 was employed. Hydrolysis to the desired acid with this substrate was... 

Hayashi and Midorika (191) used a [3,3]-sigmatropic rearrangement of an allylic dithiocarbamate to stereospecifically generate -olefins (Scheme 81). Reduction of alkylated allylic phosphonates with lithium aluminium hydride by Kondo et al. (192) exclusively gave E-alkenes (Scheme 82). 

The sulfone moiety was reductively removed and the TBS ether was cleaved chemoselectively in the presence of a TPS ether to afford a primary alcohol (Scheme 13). The alcohol was transformed into the corresponding bromide that served as alkylating agent for the deprotonated ethyl 2-(di-ethylphosphono)propionate. Bromination and phosphonate alkylation were performed in a one-pot procedure [33]. The TPS protecting group was removed and the alcohol was then oxidized to afford the aldehyde 68 [42]. An intramolecular HWE reaction under Masamune-Roush conditions provided a macrocycle as a mixture of double bond isomers [43]. The ElZ isomers were separated after the reduction of the a, -unsaturated ester to the allylic alcohol 84. Deprotection of the tertiary alcohol and protection of the prima-... 

Conversion of allylic acetates of allylstannanes by treatment with Et2AlSnBus and Pd° catalysts proceeds by addition of the tin to the metal, followed by reductive elimination (equation 204).309 The corresponding ally phosphonate, however, showed some loss of stereochemical integrity (equation 205).310 The Pd°-catalyzed reaction of allylic acetates, trialkyltin chlorides and Smh produced allylstannanes with no stereospecificity.311... 

Mulzer (Scheme 8 upper left) obtained the a,/(-unsaturated ester 33 with Z configuration from aldehyde 26a via a Still-Gennari olefination with phosphonate ester 34. Reduction of the ester with DIBAH and application of L-imidazole-PPhj gives allylic iodide 35. This acts as electrophile on the -anion of sulfone 36. After reductive removal of the phenylsulfone, group 28b is obtained [23]. 

Allylphosphine oxides may be used to make allyl alcohols by the reductive removal of the Ph2PO group with LAH. The reaction involves transposition of the double bond (equation Li(Bu 0)3AlH breaks selectively the C—P phosphonate bond, and this reagent is chemoselective and can... 

Diethyl P-keto-co-alkenylphosphonates, prepared by y-alkylation of the corresponding diethyl jj-ketophosphonates with allyl or homoallyl bromide, undergo ozonolysis at low temperature in CH2CI2 to give acyclic dicarbonyl phosphonates. Cyclization is accomplished with the aid of TsOH and EtjN as catalysts. Similarly, the diethyl 3-hydroxy-4-formylbutylphosphonate is prepared by allylation of diethyl 2-formylethylphosphonate using allyl bromide and zinc in a protic medium followed by ozonolysis and reductive workup. ... 

The synthesis of Cbz-protected D-valine methyl ester (296) (Scheme 40) begins with addition of an organometallic reagent to the ester function of 282. The resulting phosphonate 290 undergoes a Wittig reaction with isobutyraldehyde to afford 291. Chelation-controlled reduction of the ketone with zinc borohydride furnishes the a /-alcohol 292 (98% de). A [3,3] rearrangement of trifluoroacetimidate 293 produces allylic amine 294. Elaboration of the olefin to an ester furnishes the D-valine derivative 296 with 85% ee [101]. 

Stereoselective syntheses of (ff)-olefinic derivatives have been developed by alkylation of allyl diethyl phosphonates followed by reduction with lithium aluminium hydride.Both E)- and (z)-olefinic derivatives have been obtained from acetylene precursors by reduction with either lithium aluminium hydride or in presence of Lindlar catalysts. ... 

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