Allyl selectivity

PCC can be modified to show selectivity for the oxidation of allylic alcohols in stnoidal systems. A solution of PCC in dichloromethane with 2% pyridine at ca. 2 C was found to be an effective and selective oxidant (Table 6). In contrast to chromate oxidations of saturated alcohols in rigid systems. Parish and coworkm found that quasiequatorial allylic alcdiols woe oxidized faster than axial ones. Similar xoperties woe also found for solutions of PCC and 3,3-dimethyIpyrazole (2%) in dichloromethane. In addition, Parish and coworkos also eiamined sevoal otho aromatic amines for the ability to promote allylic selectivity. 2,2 -Bipyridine, pyrazine, pyridazine, s-lriazine and 2,4,6-tri diaiy yridine all had S( ne effect, but their cacy tqtpeared to be sutetrate dependent Most recoitly the combination of PCC... 

Ester enolates. Oppolzer showed in 1983 that the Z(Gj-dienolate shown in Scheme 5.30a adds to cyclopentenone with 63% diastereoselectivity [160]. Additionally, the enolate adduct can be allylated selectively, thereby affording (after purification) a single stereoisomer having three contiguous stereocenters in 48% yield. The transition structure illustrated is not analogous to any of those illustrated in Scheme 5.29 because cyclopentenone is an s-trans-Z-enone, whereas the enones in Scheme 5.29 are s-cis-E. In 1985, Corey reported the asymmetric Michael addition of the EfOj-enolate of phenylmenthone propionate to -methyl crotonate as shown in Scheme 5.30b [161]. The product mixture was 90% syn, and the syn adducts were produced in a 95 5 ratio, for an overall selectivity of 86% for the illustrated isomer. The transition structure proposed by the authors to account for the observed selectivity is similar to that shown in Scheme 5.29c, but with the enone illustrated in an s-trans conformation. Intramolecular variations of these reactions were reported by Stork in 1986, as illustrated in Scheme 5.30c and 5.29d [162]. Two features of... 

A synthesis of castanospermine has been described commencing with the chiral lactam 268. A -Allylation, selective primary benzyl-acetyl exchange, deacylation, oxidation and Wittig homologation gave 269. Metathesis reaction gave the... 

Hayashi S, Hirano K, YOTunitsu H, Oshrma K (2006) Palladiiun-catalyzed stereo- and regiospecific allylation of aryl halides with hrunoallyl alcohols via retro-allylation selective generation and use of o-allylpalladiurtt. J Am Chem Soc 128(7) 2210-2211. doi 10.1021/... 

Selective oxidation of a,p-unsatutrated (allylic, benzylic, acetylenic) alcohols. 

Substituted epoxides are attacked by organocopper reagents at the least hindered carbon atom and form alcohols (C.R. Johnson, 1973A). With a, 9-unsaturated epoxides tram-allylic alcohols are produced selectively by 1,4-addltion (W. Carruthers, 1973 G.H. Posner, 1972). 

A conceptually surprising and new route to prostaglandins was found and evaluated by C.R. Johnson in 1988. It involves the simple idea to add alkenylcopper reagents stereo-selectively to a protected chiral 4,5-dihydroxy-2-cyclopenten-l-one and to complete the synthesis of the trisubstituted cyclopentanone by stereoselective allylation of the resulting enolate. 

The allyl bromides formed by method (A) contain 25% of the undesired (Z)-isomer. The selectivity of the palladium-catalyzed amination can be steered by the application of polymer-bound systems (see section 2.6.3 B. M. Trost, 1978),... 

Hard carbon nucleophiles of organometallic compounds react with 7r-allyl-palladium complexes. A steroidal side-chain is introduced regio- and stereo-selectively by the reaction of the steroidal 7T-allylpalladium complex 319 with the alkenylzirconium compound 320[283]. 

It is possible to prepare 1-acetoxy-4-chloro-2-alkenes from conjugated dienes with high selectivity. In the presence of stoichiometric amounts of LiOAc and LiCl, l-acetoxy-4-chloro-2-hutene (358) is obtained from butadiene[307], and cw-l-acetoxy-4-chloro-2-cyclohexene (360) is obtained from 1.3-cyclohexa-diene with 99% selectivity[308]. Neither the 1.4-dichloride nor 1.4-diacetate is formed. Good stereocontrol is also observed with acyclic diene.s[309]. The chloride and acetoxy groups have different reactivities. The Pd-catalyzed selective displacement of the chloride in 358 with diethylamine gives 359 without attacking allylic acetate, and the chloride in 360 is displaced with malonate with retention of the stereochemistry to give 361, while the uncatalyzed reaction affords the inversion product 362. 

Benzoic acid and naphthoic acid are formed by the oxidative carbonylation by use of Pd(OAc)2 in AcOH. t-Bu02H and allyl chloride are used as reoxidants. Addition of phenanthroline gives a favorable effect[360], Furan and thiophene are also carbonylated selectively at the 2-position[361,362]. fndole-3-carboxylic acid is prepared by the carboxylation of 1-acetylindole using Pd(OAc)2 and peroxodisulfate (Na2S208)[362aj. Benzoic acid derivatives are obtained by the reaction of benzene derivatives with sodium palladium mal-onate in refluxing AcOH[363]. 

Alcohols are oxidized slowly with PdCh. Oxidation of secondary alcohols to ketones is carried out with a catalytic amount of PdCh under an oxygen atmo-sphere[73.74]. Also, selective oxidation of the allylic alcohol 571 without attacking saturated alcohols is possible with a stoichiometric amount of PdfOAc) in aqueous DMF (1% H OifSll],... 

In addition to the catalytic allylation of carbon nucleophiles, several other catalytic transformations of allylic compounds are known as illustrated. Sometimes these reactions are competitive with each other, and the chemo-selectivity depends on reactants and reaction conditions. 

The allyl-substituted cyclopentadiene 122 was prepared by the reaction of cyclopentadiene anion with allylic acetates[83], Allyl chloride reacts with carbon nucleophiles without Pd catalyst, but sometimes Pd catalyst accelerates the reaction of allylic chlorides and gives higher selectivity. As an example, allylation of the anion of 6,6-dimethylfulvene 123 with allyl chloride proceeded regioselectively at the methyl group, yielding 124[84]. The uncatalyzed reaction was not selective. 

The allylic esters 189 and 191 conjugated with cyclopropane undergo regio-selective reactions without opening the cyclopropane ring. The soft carbon nucleophiles are introduced at the terminal carbon to give 190, and phenylation with phenylzinc chloride takes place on the cyclopropane ring to form 192[120]. 

Hydroxylysine (328) was synthesized by chemoselective reaction of (Z)-4-acet-oxy-2-butenyl methyl carbonate (325) with two different nucleophiles first with At,(9-Boc-protected hydroxylamine (326) under neutral conditions and then with methyl (diphenylmethyleneamino)acetate (327) in the presence of BSA[202]. The primary allylic amine 331 is prepared by the highly selective monoallylation of 4,4 -dimethoxybenzhydrylamine (329). Deprotection of the allylated secondary amine 330 with 80% formic acid affords the primary ally-lamine 331. The reaction was applied to the total synthesis of gabaculine 332(203]. 

Silyl ethers serve as preeursors of nucleophiles and liberate a nucleophilic alkoxide by desilylation with a chloride anion generated from CCI4 under the reaction conditions described before[124]. Rapid intramolecular stereoselective reaction of an alcohol with a vinyloxirane has been observed in dichloro-methane when an alkoxide is generated by desilylation of the silyl ether 340 with TBAF. The cis- and tru/u-pyranopyran systems 341 and 342 can be prepared selectively from the trans- and c/.y-epoxides 340, respectively. The reaction is applicable to the preparation of 1,2-diol systems[209]. The method is useful for the enantioselective synthesis of the AB ring fragment of gambier-toxin[210]. Similarly, tributyltin alkoxides as nucleophiles are used for the preparation of allyl alkyl ethers[211]. 

When allylic compounds are treated with Pd(0) catalyst in the absence of any nucleophile, 1,4-elimination is a sole reaction path, as shown by 492, and conjugated dienes are formed as a mixture of E and Z isomers[329]. From terminal allylic compounds, terminal conjugated dienes are formed. The reaction has been applied to the syntheses of a pheromone, 12-acetoxy-1,3-dode-cadiene (493)[330], ambergris fragrance[331], and aklavinone[332]. Selective elimination of the acetate of the cyanohydrin 494 derived from 2-nonenal is a key reaction for the formation of the 1,3-diene unit in pellitorine (495)[333], Facile aromatization occurs by bis-elimination of the l,4-diacetoxy-2-cyclohex-ene 496[334],... 

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