Allyl deoxygenation

The next step was reduction of the ester and O-benzyl ether groups in 6 with lithium in liquid ammonia. Note how the allylic alcohol survived these reduction conditions. Sometimes, allylic deoxygenation can... 

Allylation. Deoxygenative allylation of allylic and benzylic alcohols with allyItrimethylsilane is promoted by (FS02),NH. 

A preparative method for allylamines is based on the Pd-catalyzed substitution reaction of /V-allylbenzotriazoles by amines. Formic acid is a convenient hydride source for allylic deoxygenation of Baylis-Hillman adducts via their derived acetates, thus the reaction is the conclusion of a three-step process for access to trisubstituted alkenes which contain one electron-deficient group. 

Reaction with sodium borohydride was equally stereoselective, the stereochemistry of the resulting diol being evident from the ready formation of the cyclic carbonate in F9. An additional advantage of this derivative arose from a timely report by Sutherland [72] that allylic deoxygenation with compounds such as these proceeded more smoothly than with the corresponding acetates [73]. Indeed, reaction of F9 with a Pd(0) catalyst led smoothly to FIO and, as expected on the basis of steric and conformational grounds, epoxidation occurred exclusively from the convex surface to give Fll. 

The next key step in the sequence was the macroexpansion, smoothly executed by treatment of 142 with potassium hydride and 18-crown-6 in THF at room tenperature. Reduction of the resultant ketone and selective reduction of the least hindered olefin afforded 144. An allylic deoxygenation and deprotection of the isopropenyl group with acetyl mesylate gave the target natural product 145. 

Carbonyl reduction begins to compete with olefin saturation when the double bond is hindered or the carbonyl is aromatic or an aromatic vinylog. In conjugated systems, deoxygenation may occur via an intermediate allylic alcohol (51). 

The coupling product is converted to the allyl mesylate 6. This is reductively cyclized to yield bis(cyclopentano)annulated methylenecyclooctanol 7a, which furnishes the hydrocarbon 7b on reductive deoxygenation. 

An effective deoxygenation using enantiomerically pure epoxides from primary allylic alcohols ( Sharpless epoxides ) [44] to give enantiomerically pure secondary allylic alcohols was described by Yadav [45]. This approach circumvented a kinetic resolution of secondary allylic alcohols that implies a maximum yield of 50% ( Scheme 5). 

Allyl Alcohols. Secondary cyclic allylic alcohols are reduced with the combination of Et3SiH and ethereal LiC104, even in the presence of a tertiary alcohol (Eq. 35) or ketal function.173 Primary allylic alcohols do not undergo deoxygenation under similar conditions.173... 

Iodotrimethylsilane formed in situ from the reaction of chlorotrimethylsilane and sodium iodide, also effects the conversion of 2-ene-l,4-diols to 1,3-dienes (equation 16)46. Allylic thionocarbonates on heating with triphenylphosphite undergo deoxygenation (Corey-Winter reaction) to generate olefins47. This procedure has been used for making hexatrienes (equation 17)47b. 

Another interesting application of the deoxygenation reaction is shown in Scheme 12.6. Sharpless epoxides are transformed to enantiomerically pure allylic alcohols [14]. It should be noted that the disadvantage of the loss of one-half of the allylic alcohol, as in the case of kinetic resolutions of allylic alcohols, is not a problem when this protocol is employed. 

Cleavage of allyl carboxylates (general procedure) f To a solution of NaHTe (prepared in situ from tellurium powder (1.30 g, 1.0 mmol) and NaBH4 (0.90 g, 2.4 mmol) in EtOH (20 mL) buffered with deoxygenated HOAc (1.2 mL in 5 mL EtOH)) is added the allyl carboxylate (0.005 mol). The mixture is refluxed under N2 for 2 h, then filtered, the filtrate... 

Hydroalumination of allylic alcohols or ethers. llydroalumination of these substrates under usual conditions (9. 276) proceeds poorly because of deoxygenation associated with titanium catalysts.3 The desired reaction, however, can be effected in 50 80" yield with ZrC l4 in the case of alcohols and Cp.ZrCL in the case of cllieis (equations 1 and II). 

Deoxygenation of allyttc alcohols.3 A method for conversion of allylic alcohols to l-alkenes is outlined in equation (I). The first step is an allylic rearrangement of an Oallyl xanthate to 2. The second step is an allyl transfer from sulfur to tin with tri-n-butyltin hydride to give the allylic stannane (3). The last step, destannylation, is a well-known mule to terminal alkcnes.4... 

---