Asymmetric synthesis using diisopropyl tartrate modified (E)- and (Z)-crotylboronates: preparation of the chiral crotylboronates and reactions with achiral aldehydes

Synthesis of (S,S)‐Diisopropyl Tartrate (E)‐Crotylboronate and Its Reaction with Aldehydes: (2R,3R,4R)‐1,2‐Dideoxy‐2‐Ethenyl‐4,5‐O‐(1‐Methylethylidene)‐Xylitol

“…5 The reactions of (E)-crotylboron reagents with aldehydes is particularly useful, in that these reagents provide 2,3-anti diastereomers typically with greater selectivity than can be obtained by using propionate enolates via aldol reactions. To date, many efficient and practical chiral crotylmetal reagents or crotylmetallation procedures have been reported, 5 including those by Roush,2,6 Brown, 7 Corey,8 Leighton, 9 Denmark, 10 Soderquist, 11 Hall 12 and Krische, 13 among others. The DIPT modified (E)-crotylboronate reagent developed by Roush and his coworkers, and which is illustrated in the present procedure, is one of the most widely adopted reagents due to the ease of preparation, storage and handling, and the high selectivity obtained in crotylboration reactions of chiral aldehydes-especially in the matched double asymmetric mode.…”

“…2,6c-f The current procedure for preparation of (S,S)-diisopropyl tartrate (E)-crotylboronate (2) follows the protocol previously reported by the Roush group. 2 This procedure involves the metallation of trans-2-butene with n-BuLi and t-BuOK in THF at −50 ºC for 25 min to generate (E)-crotylpotassium 1 with high isomeric purity (>98%). Treatment of 1 with (iPrO)3B followed by aqueous hydrolysis and esterification with (S,S)-diisopropyl tartrate (DIPT) furnished the (S,S)-DIPT (E)-crotylboronate reagent, 2.…”

“…2 (R,R)-DIPT (E)-crotylboronate 9 can be prepared from trans-2-butene and (R,R)-DIPT according to the same procedure as described here. 2 The preparation of (Z)-crotylboronates 5 and 10 by using cis-2-butene and the appropriate enantiomer of DIPT is analogous to the procedure described for (E)-crotylboronates with the exception that the metallation of (Z)-2-butene is performed at −20 to −25 ºC for 30-45 min, which ensures high conversion in the metallation step to form (Z)-crotylpotassium. In comparison to preparation of (E)-crotylpotassium, the temperature control is less critical since (Z)-crotylpotassium is highly favored at equilibrium (>99:1).…”

“…2 In some cases, the enantioselectivity can be increased to 91% ee when crotylboration is performed at −95 ºC (entry 4). The reactions with pivalaldehyde, α,β-unsaturated aldehydes and aromatic aldehydes are less enantioselective (entries [8][9][10][11][12][13].…”

Allylation of Aldehyde and Imine Substrates with In Situ Generated Allylboronates – A Simple Route to Enatioenriched Homoallyl Alcohols

“…[1][2][3][4][5] Furthermore, tartrate based chiral allylboronates react with aldehydes with good to high level of enantioselectivity. [4,6,7] Although, the parent allylboronate and its alkyl-substituted analogs are easily available reagents, the limited access to properly functionalized and/or chiral allylboronates often limit the synthetic scope of the allylation reactions. Recently, we have communicated [8] an efficient one-pot allylation procedure based on in situ generation of allylboronates (Scheme 1).…”