Tartrate allylboronates

Stereochemical Studies with Allylboronates. the Tartrate Allylboronates... 

Figure 12. Products of the reactions of tartrate allylboronates with achiral, unhindered aliphatic aldehydes.

The synthesis of aldehyde 48 proceeds in 16 steps from (S)-39 in 15% yield and 75% stereoselectivity. The brevity, efficiency, and selectivity of this synthesis rivals alternative acyclic diastereoselective approaches to the rifamycin ansa chain, (see footnote 4 in reference 3i), thereby providing a clear testimony to the potential of the tartrate allylboronates as reagents for complex synthetic problems. 

Figure 13. Reaction of D-glyceraldehyde acetonide with tartrate allylboronate 36.

Figure 20. Reactions of epoxyaldehydes 69 and 70 with enantiomers of tartrate allylboronate 36.

Figure 21. Reaction of tartrate allylboronate 36 with epojQ aldehyde enantiomers of 70.

If our mechanism is correct, we expected reagent 88 to be substantially more enantioselective than the parent tartrate allylboronate 36." ... 

Roush et al. discovered that the tartrate ester-modified allylboronates, such as diisopropyl tartrate allylboronate (.S, .S )-41, react with achiral aldehydes to give the homoallylic alcohols 42 in good yields and high levels of enantioselectivity of up to 87% ee when the reaction is carried out in toluene in the presence of 4-A molecular sieves20 (Scheme 3.1q). To rationalize the asymmetric induction realized by 41, two six-membered transition states were compared (Scheme 3.1r). It was reasoned that transition state A was favored over transition state B due mainly to the nonbonded electronic repulsive interactions of the lone-pair electrons of the aldehyde oxygen and the carbonyl oxygen of the tartrate ester. 

R,R)-dimethyl tartrate allylboronate and acetaldehyde showed that transition state A is more stable than B by 1.75 kcal/mol (Scheme 3.1s). The major force for the energy difference is an attractive Coulomb interaction between the ester oxygen and the boron-complexed aldehyde carbonyl group The distance between the two interacting charges is shorter in A (3.28 A) than in B (4.11 A). The authors concluded that the repulsive nln interaction proposed initially might play a lesser role than speculated previously. 

Reactions with Achiral Aldehydes. The reaction of tartrate allylboronates with achiral aldehydes proceeds with moderate to excellent enantioselectivity (60-92% ee) and high yield (80-90%). Simple aliphatic aldehydes give good enantioselectiv-ities (decanal 86% ee, CyCHO 87% ee, eq 2), while p-alkoxy and conjugated aldehydes give diminished selectivities (60-80% ee) (eq 3). The enantioselectivity is highly temperature and solvent dependent. Best results for reactions with the vast majority of aldehydes are obtained in toluene at —78 °C. 4°A molecular sieves are included to ensure that the reaction is anhydrous. Other tartrate esters (e.g. diethyl tartrate) may also be used without loss of enantioselectivity. 

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... 

Synthesis of (4J ,5iQ-Diisopropyl 2-Allyl-l,3,2-dioxaborolane-4,5-dicarboxylate [(i ,i )-Tartrate Allylboronate I] ... 

Figure 6Jt Model for absolute stereoinduction in additions of tartrate allylboronates to aldehydes.

---