Alkenes reductive coupling with esters

For the one-step conversion of oxiranes to alkenes a variety of reducing metals have been used, including Li, Ti°, low-valent Nb, Cr , and Sm . ° In the latter case, HMPA and N,N-dimethylami-noethanol facilitate the reaction. Reaction of 2 equiv. of Bu"Li with WCle produces a reagent which effects efficient deoxygenation of oxiranes, as exemplified in equation (8). This reagent is particularly valuable for the deoxygenation of tri- and tetra-substituted oxiranes and is compatible with ethers and esters, although aldehydes and ketones reductively couple with it. 

Functionalized cyclopropanes. The reductive coupling of esters with 1-alkenes by a Grignard reagent and titanium alkoxide gives rise to cyclopropanols (the Kulinkovich reaction). With the use of ethylene carbonate, the condensation gives rise to 2-substituted cyclopropanone hemiacetals. An intramolecular version delivers bicyclic products. ... 

The reductive coupling of ketones or aldehydes with electrophilic alkenes such as conjugated esters (Scheme 52) has been described independently by Inanaga et al. [38] and Fukuzawa et al. [123,124]. The reaction was performed in THF either in the presence of HMPA [38] or an alcohol [123,124]. The mechanistic aspects of this reaction have been discussed [ 124]. Presumably, the ketyl radical or its protonated form adds to ethyl acrylate with formation of the C-C bond. An efficient enantioselective synthesis of v-butyrolactones has been derived by using AT-methylephedrinyl acrylate or crotonate [125]. One example (synthesis of 33) is indicated in Scheme 52. 

Several contributions have appeared in which intermolecular reductive couplings between ketones or aldehydes and electron-deficient alkenes have been described [30h, 53]. Reactions of aldehydes and ketones with unsaturated esters result in the formation of lactones. Diastereoselectivities are normally modest in these cases, but can be quite high for certain substitution patterns (Eq. 45). 

Organotitanium and -zirconium chemistry already has an established place in organic synthesis and many reactions are covered elsewhere in the Practical Approach series. Examples include reductive coupling of carbonyl compounds with low valent titanium to form 1,2-diols or alkenes methylenation of ester carbonyl groups with titanocene methylidene (Cp2Ti=CH2) zirconium-catalysed methylalumination of alkynes and hydrozirconation of alkynes and alkenes with the Schwartz reagent, Cp2ZrHCl. ... 

Deoxygenation. This substance reduces epoxides to alkenes in 85-95% yield, generally with retention of configuration, particularly in reduction of trans-tpox-ides. This reduction proceeds more readily than that of carbonyl or ester groups, but a,(3-enals are coupled reductively by 1 at 25° to diallylic ethers. 

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