Allylation using tartrate allylboronates

Results of reactions of chiral a-methyl aldehydes and several chiral crotyl- and allyl-boron reagents are summarized in Tables 8 and 9. It is apparent from these data that the Brown (Ipc)2B(crotyl) and (Ipc)2B(allyl) reagents (51), (52) and (219) consistently give excellent results for the synthesis of each product diastereomer (Table 8, entries 3-6, 11, 16, 20, and 24 Table 9, entries 1,2, 10 and 18). This is true also for their reactions with chiral a- and 3-alkoxy aldehydes (Scheme 49).i. i4S-i50 Thg tartrate crotylboronates (18) and (19) also display excellent selectivity in the synthesis of crotyl diastereomers (136), (137) and (139) (Table 8, entries 7,10,13,17,25 and 28), but are much less selective for the syndesis of crotyl diastereomer (138), especially from -alkoxy-substituted aldehydes such as (253). Tartrate allylboronate (224) is also less effective than (Ipc)2Ballyl (219) for the synthesis of (257) and (258) in Table 9, and of (266) and (267) in Scheme 49.Substantial improvements in selectivity have been realized by using the taitramide-based allylboronate (228), and the results with this reagent (Table 9, entries 4, 7, 9, 12, 14, 17, 20 and 22) compare very favorably with those obtained with (219). The data... 

Roush, W. R., Walts, A. E., Hoong, L. K. Diastereo- and enantioselective aldehyde addition reactions of 2-allyl-1,3,2-dioxaborolane-4,5-dicarboxylic esters, a useful class of tartrate ester modified allylboronates. J. Am. Chem. Soc. 1985,107, 8186-8190. 

The chiral allyl- and 2-butenylboronates derived from tartrate esters (Chart 10-5) have been used in combination with a wide variety of chiral aldehydes to produce homoallylic alcohols in high yield and moderate to high enantioselectivity [124], The results obtained from reaction of selected chiral aldehydes (Chart 10-6) with the tartrate-modified allylboronates 195 and 197 (Chart 10-5) are shown in Table 10-20. As with the achiral aldehydes, the highest enantioselectivities are obtained when the chiral aldehydes are combined with allylboronate 197. A strong reagent-induced selectivity is apparent, but is nevertheless dependent on the intrinsic bias of the aldehyde. 

Scheme 11-10). It should be noted that glyeeraldehyde derivatives are outstanding substrates for the tartrate ester-modified allylboronates [118]. Aldehyde 261, derived from 260 in two steps, underwent a highly stereoselective (selectivity = >99 1) allylation reaction with the Brown Ipc2BAllyl reagent 195 [112, 113] (an in-depth discussion of the synthesis and use of this reagent appears in Section... 

By a similar method, the (Z)-crotylborate is synthesized from cA-2-butene in 70-75% yield with a 98% isomeric purity. The tartrate esters of allylboronic acids are an excellent reagent for asymmetric allylboration of carbonyl compounds. Allyl(diisopinocampheyl)borane [51] and the allylic boron derivatives of ester and amide, such as camphordiol [52], pinanediol [53], 1,2-diphenyl-1,2-ethylenediamine [54], have also been successfully used for asymmetric allylboration of carbonyls. 

The synthesis and use of tartrate-modified allylboronate 1 was first reported by Roush and co-workers in 1985. The synthesis and use of the corresponding (E)- and (. -crotylboronate reagents 2 and 3 was published by Roush and co-workers shortly thereafter. The ease of synthesis, stability and efficient reactivity of these reagents offers advantages over many other allyl- and crotylmetal reagents. Roush and co-workers have extensively explored the enantioselective allylations with achiral aldehydes as well as the... 

The allylation and crotylation of a-alkoxy aldehydes provide chiral 1,2-diol synthons which can be used in the synthesis of polyoj genated small molecules, for example, natural and unnatural sugars The tartrate-derived allylboronates I and 2 provide reagent-controlled selectivity in reaction with chiral glyceraldehyde acetonide 30. The intrinsic selectivity of the aldehyde is estimated by its reactions with pinacol allylboronates 33 and 34. The reagents 1 and 2 overcome the aldehyde s... 

---