Allyl formates, hydrogenolysis

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

The method has been successfully applied to steroids. The fnms AB ring junction in 612 can be generated cleanly from the. i-allylic formate 611 the cis junction in 614 is formed by the hydrogenolysis of the a-allylic formate 613... 

High regioselectivity is observed also in the hydrogenolysis of internal allylic formates. The hydrogenolysis of allylic formates 308 and 312 in the A ring of steroids... 

Ammonium formate hydrogenolysis of 7t-allyl-palladium(II) complexes formed via oxidative addition of Pd(0) to propargylic carbonate esters is a useful method for preparing disubstituted alkynes, as depicted below. This method of deoxygenation avoids the use of expensive and toxic reagents often associated with the free-radical Barton-McCombie deoxygenation. °... 

The hydrogenation of unsaturated aldehydes IV can be a complex transformation, as depicted in Scheme 2. Although the desired reactions are normally either the formation of allylic alcohol V, or saturated aldehyde VI, by 1,2 addition of hydrogen across the functional group, 1,4-addition across the conjugated functions can provide the enol, VII. Over-hydrogenation can result either in further saturation or, for allylic alcohols, hydrogenolysis to the alkene VIII (which can, in turn, be further saturated). 

Hydrogenolysis of allylic formates. The Pd(0) catalyst generated from Pd(OAc)2 and Bu2P effects hydrogenolysis of allylic acetates or carbonates with ammonium formate to afford 1-alkenes. Allylic formates are also converted to 1-alkcncs by this Pd(0) catalyst without use of ammonium formate. 

Regio- and stereoselective hydrogenolysis of allylic formates in rigid cyclic systems offers a useful method for stereoselective construchon of ring junctions. As... 

Deprotection of allyl ester in 467 with HC02H/Et3N was highly chemoselec-tive. In the coexistence of a 2,4-enyne system in 467, at first 7r-allylpalladium carboxylate 468, formed from the allyl ester, attacked the triple bond to form the butenolide 469 without reacting with formate. Hydrogenolysis with formate took place only at the final step to afford 470 chemoselectively. Also, the alkenyloxirane group, which is allylic epoxide, stayed intact [175],... 

In 1973, Hey and Arpe first reported Pd-catalyzed hydrogenolysis of aUyUc esters, phenyl ether, and amine using formic acid as a hydride source. In their attempt to prepare allylic formates from various allylic acetates by Pd-catalyzed transesterification, they obtained a mixture of alkenes instead of the expected allylic formatesf (Scheme 2). 

In addition to the preparation of l-alkenes, the hydrogenolysis of allylic compounds with formate is used for the protection and deprotection of carboxylic acids, alcohols, and amines as allyl derivatives (see Section 2.9). 

Various terminal allylic compounds are converted into l-alkenes at room temperature[362]. Regioselective hydrogenolysis with formate is used for the formation of an exo-methylene group from cyclic allylic compounds by the formal anti thermodynamic isomerization of internal double bonds to the exocyclic position[380]. Selective conversion of myrtenyl formate (579) into /9-pinene is an example. The allylic sulfone 580 and the allylic nitro compound... 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

Reduced furans are formed in two types of oxypalladation processes on substrate a,ft- and y,<5-unsaturated alcohols. In an unusual reaction, allyl alcohol is converted into 4-methylenetetrahydrofurfuryl alcohol, among other products (Scheme 57)97 the formation of propene is thought to arise by reductive hydrogenolysis of allyl alcohol. 

The same basic strategy was applied to the synthesis of the smaller fragment benzyl ester 28 as well (Scheme 4). In this case, aldehyde 22 prepared from (S)-2-hydroxypentanoic acid [9] was allylated with ent-10 and tin(IV) chloride, and the resulting alcohol 23 was converted to epimer 24 via Mitsunobu inversion prior to phenylselenenyl-induced tetrahydrofuran formation. Reductive cleavage of the phenylselanyl group, hydrogenolysis of the benzyl ether, oxidation, carboxylate benzylation, and desilylation then furnished ester 28. 

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