Of chiral allyl boronates

Typical protocols for the preparation of chiral allyl boronates involve Matteson homologation of vinyl boronates 159 with halomethyl lithium 160 or the vinylation of halomethyl boronate 163 with vinyl Grignard 162 followed by transesterification with dialkyl tartrate 164 (Scheme 26) <1996JOC100>. 

The metal-catalyzed allylboration is efficient to control both diastereoselectivity and enantioselectivity. The Sc(OTf)3-catalyzed reaction of chiral allyl boronates resulted in 90-98% ee for representative aldehydes (Equation (158)).624 628 629 The first catalytic enantioselective allylboration and crotylboration was achieved by a chiral lanthanide catalyst (Equation (159)).630... 

By using allylation reaction of aldehydes with chiral allylic boronate A in Scheme 52, Lebreton synthesized (R)-35 and (S)-34 via ( )-B and (S)-C, respectively [78]. 

Over the past two decades, chiral allyl- and crotyl-boron reagents have proved to be extremely valuable in the context of acyclic stereoselection. The development of superior allyl-boron reagents, which can give enantio- and diastereoselectivities approaching 100%, has become both challenging and desirable.68... 

Systematic studies of the reactions of tartrate allyl-boronates with a series of chiral and achiral alkoxy-substituted aldehydes show that conformationally unrestricted a- and /f-alkoxy aldehyde substrates have a significant negative impact on the stereoselectivity of asymmetric allyl-boration. In contrast, con-... 

To improve the levels of selectivity in additions to chiral aldehydes, it is possible to resort to the tactic of double diastereoselection with the use of chiral allylic boranes and boronates (see section Double Diastereoselection ). Bis(isopinocampheyl) allylic boranes and the tartrate allylic boronates (see following section), in particular, are very useful in the synthesis of polypropionate natmal products by reaction with a-methyl and a-alkoxy functionalized aldehydes. 

The use of chiral Br0nsted acids is illustrated in Eq. 93 as a method for catalyst-controlled double diastereoselective additions of pinacol allylic boronates. Aside from circumventing the need for a chiral boronate, these additions can lead to very good amplification of facial stereoselectivity. For example, compared to both non-catalyzed (room temperature, Eq. 90) and SnCU-catalyzed variants, the use of the matched diol-SnCU enantiomer at a low temperature leads to a significant improvement in the proportion of the desired anti-syn diastereomer in the crotylation of aldehyde 117 with pinacolate reagent (Z)-7 (Eq. 93). Moreover, unlike reagent (Z)-ll (Eq. 91) none of the other diastereomers arising from Z- to E-isomerization is observed. 

Allyl boronates react very slowly with carbonyl compounds as compared to the corresponding allyldialkylboranes, due to the presence of two oxygen atoms on boron which diminish the Lewis acidity of boron. However, the activity of the allyl boronates can be enhanced by the addition of Lewis acid catalysts. There have been two complementary approaches described for the stereoselective allylation with allyl boronates, one involving the use of chiral Lewis acid, and the other involving chiral allyl boronates in conjunction with achiral Lewis acid catalyst. Several chiral fVsymmetric-based 1,2-diols 197 have been employed in combination with SnCLj as a Lewis acid and excellent level of enantioselectivity has been observed for the allylation to furnish homoallylic alcohols 198 with high ee (Equation 8) <2006AGE2426>. 

Alternatively, the allylboration of aldehydes 200 with chiral allyl boronates 199a-d (conveniently prepared from camphorquinone in four steps) also provided the optically active homoallylic alcohols 201a-d with high ee in the presence of achiral Lewis acid catalysts. These boronates are relatively unreactive with aldehydes at low temperatures in the absence of Lewis acid catalyst. However, they furnish low to moderate ee for the allylation at higher temperatures. Hall and co-workers were able to increase the reactivity of the allyl boronates at low temperatures by the addition of strong Lewis acids such as Sc(OTf)3 and obtained the homoallylic alcohols with high ee at low temperatures (Equation 9) <2003JA10160>. 

Unsaturated aldehydes are poor substrates for allylation because of their inherent instability toward alkene isomerization accordingly, very few reports describe the allylation of these substrates. However, Lautens et al. were able to overcome this problem via an indirect approach by the use of 2-vinyloxiranes 219 as surrogates for the above-mentioned aldehydes. The in situ formation of these otherwise unstable aldehydes is achieved via the Lewis acid treatment of vinyloxiranes to furnish aldehydes 221 which further undergo allylation with chiral allyl boronates 174 in moderate ee (Scheme 37) <20020L83>. The chiral induction could be further enhanced by the use of allylboranes derived from a-pinene. 

Another common synthesis of cyclic boron compounds involves transesterification. For example, the chiral allyl boronates 155 can be synthesized via the reaction of dioxaborolane 329 with dialkyl tartrate 330 in high yield. The transacetalization affords an attractive alternative to the formation of these chiral boronates, which are otherwise difficult to prepare (Equation 13). 

The addition to 1,3-dienes afforded a new class of allylboron compounds. The platinum(0)-phosphine catalysts stereoselectively yielded or-1,4-addition products 131233 234 and 133216 235 for 2,3-disubstituted butadiene, 1,3-cyclohex-adiene, and 1,3-pentadiene by Txr-coordination of a diene to a platinum catalyst (Equations (30) and (31)). In contrast, phosphine-free Pt(dba)2 resulted in the selective formation of a 1,2-addition product 134216 for 1,3-pentadiene (Equation (31)). The corresponding chiral allyl boronates were synthesized by diboration of dienes with 123 or 124.234 235... 

Diastereoselective allylation of optically pure sulfinyl dienal complexes using tributyl allyltin can be obtained (Scheme 138). 2,4-Hexadien-1,6-dial iron tricarbonyl complex (88) undergoes nucleophilic addition reactions with diaUcylzincs in the presence of a catalytic amount of an optically active amino alcohol (Scheme 139). Very high enantio-and diastereoselectivity is observed. Related reactions of (88) with chiral allyl boronic esters give allylated alcohols in very high enantiomeric excess. 

The coupling of two enantiomerically pure fragments to form an alkene of fixed geometry in the centre of ebelactone allows a convergent synthesis.50 One fragment, the vinyl iodide 285 is ultimately derived from a coupling between a chiral aldehyde and a chiral allyl boronate but more immediately by silyl-cupration of an alkyne 284 and iodination. 

In order to explain the chemistry of allylic metals, the reactions of allylic boron compounds [8,12-14] are covered in detail. The boron chemistry is divided into four parts simple enantioselectivity (addition of CH2=CHCH2-, creating one new stereocenter), simple diastereoselectivity of crotyl additions (relative configuration after CH3CH=CHCH2- addition, where neither reagent is chiral), single asymmetric induction with chiral allyl boron compounds (one and two new stereocenters), and double asymmetric induction (both reactants chiral, one and two new stereocenters). Then follows a brief discussion of other allyl metal systems. 

The direct access to a-chiral allylic boronates and silanes [9] from linear precursors by copper(l)-catalyzed asymmetric allylic displacement with boron and silicon nucleophiles is another major focus of this chapter. A separate section is devoted to direct enantioconvergent transformations of racemic allylic acceptors. 

Synthesis of a series of novel functionalized achiral and chiral allyl boronates has been recently reported by Ramachandran via nucleophilic SNj -type addition of copper boronate species (generated from the boronates 37, 141, 142 under Miyaura conditions) [115,116] to various functionalized allyl acetates that tvere prepared either via vinylalumination or by Baylis-Hillman reaction with various aldehydes [117]. The resulting allylic boronates bearing an ester moiety (X=OR) were subsequently used for the synthesis of a-alkylidene-/3-substituted-y-butyrolactones by allylboration of aldehydes (Scheme 3.76). 

Reagent control This involves the addition of a chiral enolate or allyl metal reagent to an achiral aldehyde. Chiral enolates are most commonly formed through the incorporation of chiral auxiliaries in the form of esters, acyl amides (oxazolines), imides (oxazolidinones) or boron enolates. Chiral allyl metal reagents are also typically joined with chiral ligands. 

The use of tartrates as chiral auxiliaries in asymmetric reactions of allenyl bor-onic acid was first reported by Haruta et al.69 in 1982. However, it was not for several years that Roush et al.,70 after extensive study, achieved excellent results in the asymmetric aldol reactions induced by a new class of tartrate ester based allyl boronates. 

Table I. Representative Diastereofacial Selectivities (anti syn) in Reactions of Allyl Boronates and Chiral Aldehydes ...

If the presence of sensitive functional groups poses problems of chemose-lectivity in the use of hard allylic metal reagents, allylboronate derivatives also can be accessed by a milder transmetalation of allylic tin species with boron halides.This approach has been used by Corey in the synthesis of chiral bis(sulfonamido)boron reagents such as the medially 1 reagent 15 (Eq. 19) (see section Chiral Boronate Derivatives ). ... 

Chiral Boronate Derivatives. A large number of chiral auxiliary reagents based on allylic boronates has been reported. This section provides a brief overview of the historically important ones, but it focuses mainly on the most popular systems and the emerging ones (Fig. 4). 

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