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Stereochemistry allylation reactions

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). [Pg.294]

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... [Pg.816]

Carboxylates behave as an O-nucleophile and are allylated. Reaction of AcONa with cyclopentadiene monoxide (139) proceeds with retention of stereochemistry as shown by 140, to give the 3,5-c -disubstituted cyclopentene 141 [74], Although alcohols are rather unreactive nucleophiles for the Pd-catalysed allylation, the alkoxide anions generated by the treatment of silyl ethers with TBSF are easily allylated. Desilylated alcohol from 142 reacts intramolecularly with the cis and trans vinyl epoxides to give the cis and trans pyrans 143 and 144 regio- and stereoselectively, and... [Pg.124]

The corresponding /i-amino aldehydes are reduced in situ and the corresponding amino alcohols are isolated in good yield with up to >99 % ee. The Mannich reactions proceed with excellent chemoselectivity and inline formation occurs with the acceptor aldehyde at a faster rate than C-C bond-formation. Moreover, the one-pot three-component direct asymmetric cross-Mannich reaction enables aliphatic aldehydes to serve as acceptors. The absolute stereochemistry of the reaction was determined by synthesis and reveled that L-proline provides syn /i-amino aldehydes with (S) stereochemistry of the amino group. In addition, the proline-catalyzed direct asymmetric Mannich-type reaction has been connected to one-pot tandem cyanation and allylation reaction in THF and aqueous media affording functional a-amino acid derivatives [39, 42]. [Pg.369]

Several indium-mediated intramolecular carbonyl allylation reactions have been investigated, and it has been found that these reactions provide an easy access to a variety of cyclic compounds. The intramolecular cyclization of 49a-c mediated by indium in aqueous media proceeds smoothly to afford carbocyclic systems containing y-hydroxy-Q -methylene esters 50a-c, which either spontaneously or readily cyclize to give fused o -methylene-y-butyrolactones 51a-c (Scheme 52). The same cyclization of 49d is too slow to compete with the side-reaction, in which the bromide is substituted by a hydroxy group. The ring junction stereochemistry of fused lactones 51 has been found to be cis in all cases. Of the two possible transition states, the one leading to the m-fused compounds is preferred, because the chair-chair conformation is favored over the chair-boat conformation.209... [Pg.679]

Ligation of the two carbonyl groups in acylformamide to TiCL is probably responsible for the stereochemistry of its allylation reaction shown in Eq. (76) [208]. The analogous allylation of the corresponding a-ketoiminium cation intermediate generated in the presence of TiCL afforded an inferior results (9 1 selectivity) [209]. [Pg.685]

In their synthesis of olivin, the aglycon segment of olivomycin A, Roush and coworkers used a highly diastereoselective substrate-directed y-alkoxy allylation reaction to set the C(l ) stereocenter [80]. Thus, reaction of the aldehyde 90, derived from L-threonine, with the [(Z)-y-methoxyallyl]boronate 91 resulted in the highly diastereoselective formation of adduct 92. The stereochemistry of 92 is consistent... [Pg.415]

The diastereoselectivity of these reactions is consistent with product formation occurring through transition state 137, where the reactive conformation of the aldehyde in the transition state (corresponding to the normal Felkin-Anh model) minimizes steric interactions with the allylstannane as well as the 1,3-dipole interactions of the aldehyde and the /(-alkoxy group. The allylation reaction of the 2,3-syn aldehyde 138, however, with allyltri-n-butylstannanes 98, generates the anti-Felkin adducts 139 preferentially (Eq. (11.9)) [93], The stereochemistry of these reactions is consistent with product formation occurring preferentially through transition state 140, in which 1,3-dipole interactions of the aldehyde and the P-... [Pg.421]

The synthesis of 145 (Scheme 11-4) began with the allylation adduct 121, obtained through a chelate-controlled addition of crotyltri- -butylstannane to the a-benzyloxy aldehyde 55 (see Eq. (11.4)). Adduct 121 was converted in five steps to aldehyde 146, which subsequently underwent a highly diastereoselective chelate-controlled allylation reaction with allyltri-n-butylstannane 98. The stereochemistry of the resulting adduct, 147, is consistent with formation through a chelate transition state analogous to 101 (Table 11-7). [Pg.423]

The Sakurai allylation works with a variety of Lewis acid catalysts such as TiCU, AICI3, SnCl4, LtAICL, in addition to BF3 OEt. Reviews (a) Denmark, S. E. Almstead, N. G. In Modem Carbonyl Chemistry Otera, J., Ed. Wiley-VCH Weinheim, 2000 Chapter 10 Allylation of Carbonyls Methodology and Stereochemistry, pp. 299-402. (b) Chemler, S. R. Roush, W. R. In Modern Carbonyl Chemistry, Otera, J., Ed. Wiley-VCH Weinheim, 2000 Chapter 11 Recent Applications of the Allylation Reaction to the Synthesis of Natural Products, pp. 403-490. (c) Demnark, S. E. Fu, J. Chem. Rev. 2003,103, 2763-2793. [Pg.160]

There has been some interest in the stereochemistry of reactions at C-6 in morphine derivatives. Borohydride reduction of the narcotic antagonist naloxone (101 R = allyl) gave a mixture of alcohols consisting mainly of the 6a-hydroxy-derivative, identical with the principal metabolite in vivo. The 6/3-hydroxy epimer, forming 10% of the mixture, was isolated by liquid chromatography and was also obtained via solvolysis of the 6a-tosyloxy-derivative. Reduction of naloxone and naltrexone (101 R — cyclopropylmethyl) with thiourea dioxide (formamidinesul-phinic acid) in aqueous alkali is reported, on the other hand, to give only the 6j8-hydroxy-derivatives. It has been reported that the major urinary metabolite of naltrexone administered to humans is the 6/3-hydroxy-epimer, while a chicken... [Pg.136]

Allyltributyltin (5) is the most commonly used reagent for carrying out allylation reactions via a free radical fragmentation process [5]. Keck reported the first practical use of allyltributyltin for free radical allylation reactions in 1982 in the context of a synthesis of perhydrohistrionicotoxin [6]. Heating bromide 4 with allyltributyltin in the presence of AIBN as a radical initiator gave the allylated derivative 6 (Scheme 3) in high yield with complete control of stereochemistry. Similar transformations had proven to be very difficult by standard ionic reactions. [Pg.52]

Few examples of chelation-controlled allylation reactions involving bicyclic chelates can be found in literature. Nagano and Azuma have shown that the allylation of a dialkoxy-substituted radical adjacent to a dimethyl acetal proceeds, in the presence of MgBr2 OEt2 (2.5 equivalents), with excellent stereocontrol independent of the synjanti stereochemistry of the radical precursor (Scheme 7) [24]. To best explain the high anti ratio obtained, they proposed bicyclic transition state M in which the allylstannane attacks from the side of the pyramidalized radical chelate. Surprisingly, when the same reaction was conducted at a lower temperature (0°C), poor selectivity was observed. No explanation was proposed to account for this decrease. It should be noted that the allylation gave no selectivity in the absence of Lewis acid. [Pg.449]

Curran, D.P and Kuo, L.H. (1994) Altering the stereochemistry of allylation reactions of cyclic a-sulfmyl radicals with diarylureas. The Journal of Organic Chemistry, 59, 3259-3261. [Pg.292]

The combined carbocupration of ynol ether followed by a zinc homologation in the presence of aldehydes has provided, in a single-pot operation, stereodefined allylic vicinal diol substmctures in good isolated yields and stereoselectivities (up to 92 8 dr) The stereochemistry of the final adducts suggests that the reaction 0 proceeds through a chairlike transition state in which the substituent of the aldehyde preferentially occupies a pseudoaxial position to avoid gauche interactions with an -configurated alkoxy-substituted aUylmetal species. A similar stereochemical outcome has been reported for the allylation reaction of 3,3-disubstituted allylzinc species with aldehydes. ... [Pg.354]

Many studies of the addition of nucleophiles to palladium-allyl complexes have been conducted. Hayashi has shown that the additions of stabilized anions, such as malonate anions or amine nucleophiles, to chiral, non-racemic allyl complexes occur with inversion of configuration.Addition of excess phosphine and either diethyl malonate or dimethylamine to a chiral, non-racemic allyl complex results in nucleophilic attack with nearly complete inversion. The reaction with sodium dimethylmalonate is shown at the right of Equation 11.40. In contrast, nonstabilized carbanions such as allyl or phenyl magnesium chloride react with the same Ti -allylpalladium complex with retention of configuration as shown at the left of Equation 11.40. The stereochemistry from reaction of the Grignard reagents likely results from nucleophilic attack at the metal, followed by reductive elimination. [Pg.437]

Couplings WS Allyl Intermediates- Excellent reviews cover selectivities and mechanism in palladium catalysed allyl reactions. General examples of NaCH(CX)2R)2 additions to allyl acetates, carbonates, and chlorides have appeared. Trifluotoacetale is the preferred leaving group for retention of (E)/( stereochemistry in the case of substituted allyls. Ketomalonates are susceptible to diallylation under PdL2 catalysis especially in the presence of diacetates whidi prove effective routes to bicyclic systems (Equation 6). ... [Pg.376]

The stereochemistry of the allylation reaction in aqueous medium is somewhat analogous to that in organic medium. In terms of diastereoselectivity, two types of situation prevail (Type A and Type B) (Scheme 4.12). The Type A situation usually gives an anti-diastereoselectivity that is independent of the stereochemistry of the double bond in the allyl bromide moiety. The diastereoselectivity (anti/syn ratio) is governed by the steric size of the substituent on the aldehydes. The anti/syn ratio increases as the size of the aldehyde R group increases (Scheme 4.13). [Pg.109]

It was in 1994 that urea derivatives were employed as catalysts for the radical addition reactions, where addition of urea increased the reaction rate and also altered the stereochemistry in the allylation reaction of the sufinyl radical with... [Pg.64]


See other pages where Stereochemistry allylation reactions is mentioned: [Pg.248]    [Pg.614]    [Pg.1343]    [Pg.1348]    [Pg.1343]    [Pg.1348]    [Pg.31]    [Pg.71]    [Pg.423]    [Pg.151]    [Pg.159]    [Pg.130]    [Pg.138]    [Pg.252]    [Pg.1240]    [Pg.229]    [Pg.13]    [Pg.432]    [Pg.975]    [Pg.377]    [Pg.571]    [Pg.9]    [Pg.19]    [Pg.168]    [Pg.101]   
See also in sourсe #XX -- [ Pg.559 , Pg.561 ]




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