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Asymmetric allylstannane

Effective 1,7-asymmetric induction was observed in the analogous reactions between C-hydroxy-allylstannanes and aldehydes, although in this case the free hydroxy group was required126. [Pg.378]

Asymmetric catalysis of BINOL-Ti complexes in the reaction of aliphatic and aromatic aldehydes with an allylstannane has also been reported independently by Umani-Ronchi [54] and Keck [55]. The former group has suggested that a new complex generated by the reaction of the BINOL-Ti complex with allylstannane is the catalytic species that provides remarkably high enantioselectivity (Scheme 8C.23). It is interesting that no reaction occurs if dry MS 4A... [Pg.559]

Stereo specific generation and reactions of allylic alkali and alkaline earth metals have been reviewed121 and solvent-mediated allylation of carbonyl compounds with allylstannanes has been explored.122 Chiral phosphoramides derived from (5 )-proliiie have been used to catalyse asymmetric allylation of aromatic aldehydes by allylic trichlorosilanes.123... [Pg.344]

Chiral (salen)chromium(III) complexes catalyse the asymmetric allylation of a range of aldehyde types, using allylstannane reagents.171... [Pg.22]

All the examples described above involved the reaction of diazoacetate derivatives with silver salts to initiate the formation of a putative silver carbene however, other pathways exist. For example, Porcel and Echavarren have reported an intramolecular cyclization of an allylstannane to a pendent alkyne (Scheme 8.22) that involves the intermediacy of a silver carbene.52 As can be seen in Table 8.12, the reactions proceeded in moderate to excellent yield, providing the dienylstannane, while in some cases, reductive destannylation occurred. Several asymmetric reactions were reported with substrate ( )-145d, leading to the formation of the expected adduct in reasonable enantioselectivities (ee = 73-78%) in a preliminary screen with a number of different ligands. [Pg.253]

Catalytic asymmetric allylations of aldehydes or ketones are roughly classified into two methods, namely, those using chiral Lewis acid catalysts and those using chiral Lewis base catalysts. The former method uses less reactive allylsilanes or allylstannanes as the allyl source. The latter method requires allyltrichlorosi-lane or more reactive allylmetals. Both processes are applicable to the reactions with substituted allylmetal compounds or propargylation. [Pg.116]

OXAZOLIDINECARBOXYLATE has previously been described in Volume 70 of Organic Sytheses. An alternative procedure for the preparation of this compound is presented in this volume along with its use in a dia-stereoselective addition reaction with 2-TRIMETHYLSILYLTH1AZOLE to provide a compound bearing a 2-amino-1,3-diol substructure that appears in a variety of natural products. The conversion of abundantly available isosorbide into OSO ISOPROPYLIDENE-l ti-DIANHYDRO-d-GLUCITOL provides a potentially useful carbohydrate-deri ved material for the use in complex tetrahydrofuran synthesis. Finally, asymmetric reduction of an a,j9-unsaturated acylstannane with (R)-BINAL provides access to (S,E)-l-(METHOXYMETHOXY)-l-TRIBUTYLSTANNYL-2-BUTENE, an a-alkoxy allylstannane that has been used in enantioselective vicinal diol synthesis amongst other transformations. [Pg.8]

The chiral titanium complex BINOL-T1CI2 also catalyzes the asymmetric carbonyl addition reaction of allylic silanes and stannanes. Thus the addition reaction of glyoxylate with (E)-2-butenylsilane and -stannane proceeds smoothly to give the syn product in high enantiomeric excess (eq 12). The syn product thus obtained can be readily converted to the lactone portion of verrucaline A. The reaction of aliphatic and aromatic aldehydes with allylstannane is also catalyzed by BINOL-TiC to give... [Pg.93]

Carbonyl Allylation and Propargylation. Boron complex (8), derived from the bis(tosylamide) compound (3), transmeta-lates allylstannanes to form allylboranes (eq 12). The allylboranes can be combined without isolation with aldehydes at —78°C to afford homoallylic alcohols with high enantioselectivity (eq 13). On the basis of a single reported example, reagent control might be expected to overcome substrate control in additions to aldehydes containing an adjacent asymmetric center. The sulfonamide can be recovered by precipitation with diethyl ether during aqueous workup. Ease of preparation and recovery of the chiral controller makes this method one of the more useful available for allylation reactions. [Pg.302]

Umani-Ronchi et al. [56] investigated the asymmetric allylation of aldehydes with allylstannane in the presence of chiral bis(oxazoline) ligands and several metal salts (Sch. 31). Combination of zinc halides and the bis(oxazoline) ligand 70 gave the ally-lated product 71 with moderate enantioselectivity (40 % ee), while other metal salts afforded either no product or racemic products. Because the formation of an allyl-zinc-bis(oxazoline) species was excluded on the basis of the NMR experiments, the reaction was considered to proceed by a Lewis acid-mediated pathway. [Pg.81]

Alkoxy-substituted allylstannanes are transmetalated on treatment with Sn(IV) halides to give allyltin trihalides which react with aldehydes with effective remote asymmetric induction [62]. For example, the 5-benzyloxy-4-methylpent-2-enylstan-nane gives predominantly the 1,5-anti products with useful stereoselectivity (Eq. 37) [62b]. Allylstannanes and 5-benzyloxy-4-methylpent-2-enylsilanes also react with aldehydes with modest levels of 1,5-induction after treatment with SnCU. Trans-metallation to give intermediate allyltin trihalides might be involved (Eq. 38) [63]. [Pg.410]

Yamamoto, Y, Kobayashi, K, Okano, H, Kadota, I, Asymmetric synthesis of syn-l,2-diols via the reaction of aldehydes with chiral "y-(tetrahydropyranyloxy)allylstannanes, J. Org. Chem., 57, 7003-7005, 1992. [Pg.501]

Keck, G. E., Yu, T. Catalytic Asymmetric Allylation Reactions Using BITIP Catalysis and 2-Substituted Allylstannanes as Surrogates for P-Keto Ester Dianions. Org. Lett. 1999,1, 289-291. [Pg.612]

The first example of the Lewis acid-catalyzed asymmetric addition of achiral allylstannanes to achiral aldehydes was reported by Marshall in 1992 using Yamamoto s chiral (acyloxy)borane (CAB) catalyst [87]. In initial studies with this catalyst, both aliphatic and aromatic aldehydes could be employed with substituted... [Pg.337]

Marshall s chiral allenylmetal reagents have been utilized in double asymmetric reactions with chiral aldehydes for the synthesis of polypropionate natural products. All four dipropionate diastereomers are accessible from the reactions of chiral allenylmetal reagents with a-chiraI-y5-alkoxy aldehydes 97 (153, 158, 276, 277]. The BF3-OEt2-catalyzed addition of allylstannane (l )-218a to aldehyde 97a occurs in high yield and diastereoselectivity to give the xyn.syn-dipropionate 395, presumably through either the synclinal or antiperiplanar Felkin transition states 396 and 397 (Eq. (11.31)). [Pg.465]

The efficient transmetalation of allylic stannanes to allylboron reagents has generated an attractive methodology for asymmetric allylation. Corey and coworkers first described the use of enantiomers of bromoborane 228 (Scheme 5.2.51) for mild and quantitative transmetalation of allylstannane to yield the allylboron reagent 229. i The asymmetry in the bis-toluenesulfonamide of 228 is derived from l,2-diamino-l,2-diphenylethane, and both antipodes are readily available in high optical purity, by resolution of the starting diamines producing (R,R)- and (5, 5 )- Stein chiral auxiliaries in transmetalation product 229. [Pg.538]

The preparation of enantioenriched Q -(alkoxy)allylstannanes has been advanced by the asymmetric Noy-ori reduction of the stannyl ketone 264 with 2,2 -dihydroxy-l,T-binapthyl lithium aluminum hydride (BINAL-H) reagents affording the non-racemic (5 )-265 with > 95% ee, upon O-alkylation with common protecting groups (Scheme 5.2.57, top). °... [Pg.541]

A chiral phosphine-silver(I) complex generated from BINAP and AgOTf-catalyzed asymmetric allylation of aldehydes with allylstannanes, resulting in high enantioselectivity. With 2-butenylstannane, the anti adduct was obtained preferentially irrespective of the double-bond geometry of the stannane (Scheme 12.28) [76]. [Pg.635]

Allylation of ketones is a fundamental and important transformation, and therefore, efficient catalysts promoting addition of allylstannanes to ketones have been investigated [89]. Enantioselective allylation of ketones is a very challenging topic. It has been disclosed that asymmetric allylation of ketones with allylstannanes was promoted by addition of BINOL/TiCl2(OiPr)2 catalyst [90] or by premixing of BINOL with tetraallyltin [91]. In these reactions, however, enantioselectivity was not sufficient for practical purposes (acetophenone <65% ee). It was recently discovered that acetophenone was allylated by a mixture of tetraallyltin and an alkyl-triallyltin in the presence of monothiobinaphthol to furnish the desired chiral homoallyl alcohol with high enantioselectivity (Scheme 12.35) [92]. [Pg.638]


See other pages where Asymmetric allylstannane is mentioned: [Pg.307]    [Pg.198]    [Pg.346]    [Pg.346]    [Pg.325]    [Pg.125]    [Pg.738]    [Pg.125]    [Pg.39]    [Pg.51]    [Pg.264]    [Pg.360]    [Pg.167]    [Pg.259]    [Pg.90]    [Pg.147]    [Pg.803]    [Pg.125]    [Pg.833]    [Pg.473]    [Pg.654]    [Pg.883]    [Pg.236]    [Pg.453]    [Pg.375]    [Pg.637]   
See also in sourсe #XX -- [ Pg.337 ]




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