Chirality allylic stannanes

Chiral allylic stannanes and chiral aldehydes. Pairwise combination of these chiral allylstannane reagents with chiral a- and jS-alkoxy aldehydes revealed a... 

Scheme 5.2.56 Examples of reactions involving transmetalations with InCIs 5.2.12 Reactions of Chiral Allylic Stannanes...

In an inventive approach to higher-sugar lactones (Scheme 2), Marshall and Beaudoin treated the tartrate-derived enal 9 with the chiral allyl stannane shown to give the threo-adivtct 10, after funher silylation of the initially fwmed alcohol. This bis-allylic ether was hydroxylated with high erythro-selectivity to 11, convertible to the octonolactone 12. A similar sequence was used to extend di-O-isopropylidene-a/dehydo-D-arabinose to the Cn-lactone 13, an intermediate previously used in a synthesis of hikizimycin. ... 

Cram erythro-products" (G.E. Keck, 1984 A, B, C). [3-(Silyloxy)allyl]stannanes and O-pro-tected a- or y -hydroxy aldehydes yield 1,2,3- or 1,2,4-triols with three chiral centres with high regio- and diastereoselectivity (G.E. Keck, 1987). 

With chiral aldehydes, reagent approach is generally consistent with a Felkin model.163 This preference can be reinforced or opposed by the effect of other stereocenters. For example, the addition of allyl stannane to l,4-dimethyl-3-(4-methoxybenzyloxy)pentanal is strongly in accord with the Felkin model for the anti stereoisomer but is anti-Felkin for the syn isomer. 

Enantioselective Addition Reactions of Allylic Stannanes. There have been several studies of the enantiomers of a-oxygenated alkenyl stannanes. The chirality of the a-carbon exerts powerful control on enantioselectivity with the preference for the stannyl group to be anti to the forming bond. This is presumably related to the stereoelectronic effect that facilitates the transfer of electron density from the tin to the forming double bond.182... 

An alternative route to nonracemic a-alkoxy stannanes entails the reduction of acyl stannanes with chiral hydrides61 62. Accordingly, conjugated stannyl enones yield (S)-a-alkoxy allylic stannanes by reduction with (J )-(+)-BINAL-H. As expected, (S)-(—)-BINAL-H gives rise to the enantiomeric (7 )-a-alkoxy allylic stannanes (equation 29)61. Upon treatment with Lewis acids, these stannanes undergo a stereospecific anti 1,3-isomerization to the (Z)-y-alkoxy allylic stannanes61. 

A second route to nonracemic /-oxygenated allylic stannanes utilizes an enantioselective deprotonation of allylic carbamates by BuLi in the presence of (—)-sparteine. The configurationally stable a-lithio carbamate intermediate undergoes enantioselective S/,-2 reaction with Bu3SnCl and Mc SnCI (Scheme 28)65. Once formed, the /-carbamoyloxy stannanes can be inverted by successive lithiation with. s-BuLi and stannation with R3SnCl (Scheme 29)65. The former reaction proceeds with S/.-2 retention and the latter by Sf2 inversion. Nonracemic allylic carbamates can also be used to prepare chiral stannanes. Deprotonation with. s-BuLi TMEDA proceeds stereospecifically with retention (Scheme 29)65. 

Certain S- and e-oxygenated allylic stannanes have been found to transmetallate with SnCU to give chiral pentacoordinated chloro stannane intermediates which add stereos-electively to aldehydes (Scheme 31)74. These reactions proceed with net 1,5-and 1,6-asymmetric induction. 

The effectiveness of various substituted BINOL ligands 12-16 in the Zr(IV)-or Ti(IV)-catalyzed enantioselective addition of allyltributyltin to aldehydes was also investigated by Spada and Umani-Ronchi [21], The number of noteworthy examples of asymmetric allylation of carbonyl compounds utilizing optically active catalysts of late transition metal complexes has increased since 1999. Chiral bis(oxazolinyl)phenyl rhodium(III) complex 17, developed by Mo-toyama and Nishiyama, is an air-stable and water-tolerant asymmetric Lewis acid catalyst [23,24]. Condensation of allylic stannanes with aldehydes under the influence of this catalyst results in formation of nonracemic allylated adducts with up to 80% ee (Scheme 3). In the case of the 2-butenyl addition reac-... 

Jorgensen et al. reported that C2-symmetric bis(oxazoline)-copper(II) complex 25 also acts as chiral Lewis acid catalyst for a reaction of allylic stannane with ethyl glyoxylate [37]. Meanwhile, p-Tol-BINAP-CuCl complex 26 was shown to be a promising chiral catalyst for a catalytic enantioselective allylation of ketones with allyltrimethoxysilane under the influence of the TBAT catalyst [38]. Evans and coworkers have developed (S,S)-Ph-pybox-Sc(OTf)3 complex 27 as a new chiral Lewis acid catalyst and shown that this scandium catalyst promotes enantioselective addition reactions of allenyltrimethylsilanes to ethyl glyoxylate [39]. But, when the silicon substituents become bulkier, nonracemic dihydrofurans are predominantly obtained as products of [3+2] cycloaddition. 

Transformations involving chiral catalysts most efficiently lead to optically active products. The degree of enantioselectivity rather than the efficiency of the catalytic cycle has up to now been in the center of interest. Compared to hydrogenations, catalytic oxidations or C-C bond formations are much more complex processes and still under development. In the case of catalytic additions of dialkyl zinc compounds[l], allylstan-nanes [2], allyl silanes [3], and silyl enolethers [4] to aldehydes, the degree of asymmetric induction is less of a problem than the turnover number and substrate tolerance. Chiral Lewis acids for the enantioselective Mukaiyama reaction have been known for some time [4a - 4c], and recently the binaphthol-titanium complexes 1 [2c - 2e, 2jl and 2 [2b, 2i] have been found to catalyze the addition of allyl stannanes to aldehydes quite efficiently. It has been reported recently that a more active catalyst results upon addition of Me SiSfi-Pr) [2k] or Et2BS( -Pr) [21, 2m] to bi-naphthol-Ti(IV) preparations. 

Oxygenated (E)-allylic stannanes can also be formed by 1,4-addition of a BuaSn cyanocuprate to enals and in situ trapping of the enolate with TBSCl (Eq. 39) [59]. Tlie corresponding (Z) isomers are not produced in these reactions. Thus far the method has only been applied to the synthesis of racemic stannanes a suitable chiral catalyst for the cuprate addition has not been found. 

Table 39. Additions of a y-oxygenated allylic stannane bearing a chiral ether auxiliary to achiral aldehydes. OH OH...

Transmetalations of Chiral Oxygenated Allylic Stannanes 10.12.1 SnCU... 

In 1996 Yanagisawa, Yamamoto, and their colleagues first reported the asymmetric allylation of aldehydes with allylic stannanes catalyzed by a BINAP silver(I) complex [29]. The chiral phosphine-silver(I) catalyst can be prepared simply by stirring an equimolar mixture of BINAP and silver(I) compound in THF at room tempera-... 

Marshall, J A, Seletsky, B M, Luke, G P, Synthesis of protected carbohydrate derivatives through homologation of threose and erythrose derivatives with chiral y-alkoxy allylic stannanes, J. Org. Chem., 59, 3413-3420, 1994. 

Marshall, J A, Chiral allylic and allenic stannanes as reagents for asymmetric synthesis, Chem. Rev., 96, 31-47, 1996. 

In a demonstration of the synthetic utility of chiral y-alkoxy stannane reagents, Marshall and co-workers applied this methodology to the synthesis of the gypsy moth pheromones (+)- and (-)-disparlure [287]. The synthesis required the production of the [y-(alkoxy)allyl]stannane reagent 440 (Scheme 11-32). 

Allylation reactions can be designed to effect high stereoselectivity in the case of chiral /3-alkoxy aldehydes, in which the ether oxygen provides for effective coordination with a Lewis acid. Multi-valent, oxophilic Lewis acids serve to pre-organize the aldehyde substrate in a six-membered chelation complex. As in the examples of a-chelation control, an open transition state is deployed with synchnal or antiperiplanar orientations based upon the consideration of steric interactions with placement of the small (hydrogen) vinyl substituent of the allylic stannane over the preformed metallocycle. Several examples are illustrated in Scheme 5.2.20. i... 

Williams " has reported an enandocontroUed approach by low temperature formation of a six-membered coordination complex of chiral Af-enoyl-l,3-oxazolidin-2-ones with ZrCU- Conjugate Se additions of a variety of substituted allylic stannanes occur with moderate to high stereoselectivity based on the oxazolidinone auxiliary, as shown by the conversion of 176 to 177 (Scheme 5.2.38). The precise role of the chiral auxiliary for determining the facial selectivity in the reaction is not fully understood. 

Benzaldimines have been reported to give high yields of condensation products with allylic stannanes, in the presence of palladium catalysts, under neutral conditions. Studies support transmetalation of the stannane to yield a bis-7r-allylpalladium complex, which binds imine 190 for allyl transfer. In the case of the catalyst 191, the chiral allyl ligand is not transferable, but determines facial selectivity in the optically emiched amine 192 (80% ee) (Scheme 5.2.42). 

Yamamoto and coworkers have demonstrated intra-molecular examples of the addition of allylic stannanes to imininm ions with good to excellent yields. The incorporation of chirality using (/ )-(+)-1-phenylethylamine provides for facial selectivity in the Lewis acid-promoted ring closure of 193. Trans-tetrahydropyranyl amine 195 is obtained as the major product using a variety of Lewis acids resulting from the synclinal arrangement 194, which leads to diequatorial substitution (Scheme 5.2.43). 

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. 

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