Addition of allylstannane

The diastereoselectivity is lower in the additions of allylstannanes and allyl Grignards. 

Use of di-( -butyl)stannyl dichloride along with an acyl or silyl halide leads to addition of allylstannanes to the aldehydes.1728,172 Reaction is also promoted by butylstannyl trichloride.173 Both SnCl4 and SnCl2 also catalyze this kind of addition. 

The key step in the synthesis of A-ring fragment 50 [56] is the chelation-controlled addition of allylstannane 53 to aldehyde 52, which sets the C7 stereocenter and introduces the C8 gem-dimethyl moiety. Aldehyde 52 is itself prepared from 1,3-propanediol using the author s protocol for titanium-catalyzed enantioselective allylstannation [57], which sets the C5 stereocenter, followed by chelation-controlled Mukaiyama aldol addition [58] to establish the C3 stereocenter (Scheme 5.6). 

Conversion of the A-ring aldehyde 50 to hydroxyallylsilane 57 is accomplished in a three-step sequence involving addition of allylstannane 49 followed by oxidation-reduction for diastereomeric enrichment at Cll. With both hydroxyallylsilane 57 and aldehyde 51 in hand, intermolecular Prins pyran annulation proceeds smoothly to furnish the tricyclic compound 58 in 61 % isolated yield. 

Scheme 6.15. Zr-catalyzed enantioselective addition of allylstannanes to aldehydes is more facile than the corresponding Ti-catalyzed processes.

In the palladium-catalyzed addition of allylstannanes to allyl acetates, complete allyl inversion of the allylstannane is observed.137 Symmetrical coupling is also possible, for example, by treatment of cinna-myl acetate with hexa-n-butyldistannane.137 In this reaction, cinnamyl tri-n-butylstannane is generated in situ by Pd catalysis and it then couples with the allyl acetate with allyl inversion (equation 32). 

Methanol promotes addition of allylstannanes to aldehydes and ketones, to give homoallylic alcohols without added catalyst.86 Aldehydes are significantly more reactive. It is suggested that the primary activating influence is hydrogen bonding to the carbonyl. 

The enantioselective addition of allylstannanes to glyoxylates and glyoxals, and also simple aromatic and aliphatic aldehydes, catalysed by chiral (salen)Cr(III) complexes, has been studied.133 The reaction proceeded smoothly for the reactive 2-oxoaldehydes and allyltributy 1 tin in the presence of small amounts (1-2 mol%) of (salen)Cr(III)BF4 under mild, undemanding conditions. The stereochemical results have been rationalized on the basis of the proposed model. 

Enantioselective addition of allylstannane to straight-chain aldehydes has been achieved using a chromium-salen catalyst.186... 

Very recently, a limited survey of the CAB and Keck BINOL methodology with crotyltributyltin was conducted by Marshall and Palovich (Table 3) [50b]. A modified CAB, prepared from the 2,6-dimethoxybenzoic ester of (l ,jR)-tartaric acid, and 1.5 equiv. BH3-THF was used in the addition of crotyltributyltin and allyltributyltin to representative achiral aldehydes in the presence of 2 equiv. (Cp3C0)20. Addition to crotyltin proceeded with good to excellent diastereoselectivity and enantioselectivity to give syn adducts in 70-93 % ee as major products (78 22-92 8). The addition of allylstannane to cyclohexanecarboxaldehyde afforded the (R) adduct in 55 % ee. In contrast, the use of Keck s BINOL catalyst gave an allyl adduct in 87 % ee. Addition of crotylstannane to cyclohexanecarboxaldehyde with this catalyst led, however, to a 65 35 mixture of syn and anti adducts 43 (R = Me) and 44 (R = Me) in 95 % and 49 % ee. 

Table 3. Comparison of BINOL- and CAB-promoted addition of allylstannanes to cyclohexanecarboxaldehyde.

The titanium-promoted 1,4-addition of allylstannanes to a,(8-unsaturated ketones is also a known transformation (Eq. 112) [262],... 

The addition of allylstannanes to glycal a-oxides provides an entry to P-C-glycosides. The reaction is catalyzed by BUjSnOTf. 

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)). 

Intramolecular addition of allylstannanes and allylsilanes to 2,3-epoxy ethers has also been reported <89JOC3ll4>. These reactions give rise to mixtures of products, resulting from 6-exo and 1-endo attack, respectively (Scheme 8). 

AIBN is effective as a radical initiator for the allylstannylation of alkenes (Scheme 5.7.21). Methyl acrylate, which is a representative substrate, accepts the addition of allylstannanes having an electron-withdrawing substituent, such as ester and cyano groups, at their (3 -position. The reaction of allylstannanes... 

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]. 

Coupling reactions catalyzed by Pd complexes of the Se-C-Se pincer type are accelerated when thep-position to the metal atom is substituted by an electron donor (e.g., MeO group). Allylation. In the addition of allylstannanes to aldehydes and A-tosylimines, 3 and 4 act in the promoter role, respectively. Noteworthy is that the pincer complex 4 features two different but synergistic donor groups. 

Carbonyl condensations. The highly syn-selective addition of allylstannanes to protected amino aldehydes is mediated by MgBr . 

Allylation. A chiral zirconate complex is prepared from BINOL and commercial. -PrOI Zri-PrOH. The catalyst promotes the enantioselective addition of allylstannanes to aldehydes. 

In the field of synthesis of natural macromolecular carbocycles, macrocyclization through formation of a carbon-carbon bond has been intensively investigated. We will give just a few examples the alkylation of carbanions (Fig. 1 la, I lb) the addition of allylstannane to aldehyde (Fig. 11c) and the intramolecular Wittig reaction (Fig. 1 Id). ... 

Evans DA, Trotter BW, Cote B (1998) Addition of allylstannanes to glycal epoxides. A diastereoselective approach to p-C-glycosidation. Tetrahedron Lett 39 1709-1712... 

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