Asymmetric allylboration of 2-N,3-O-isopropylidene-N-Boc-L-serinal: Diastereoselective synthesis of the calicheamicin γ1(I) amino sugar

Article abstract of DOI:10.1021/jo00109a008

Syntheses of the calicheamicin amino sugar 6 and its erythro diastereomer 7 have been completed by a sequence involving the asymmetric allylboration of N-Boc-serinal acetonide L-8 with the tartrate ester modified allylboronates (R,R)-9 and (S,S)-9, respectively. The reaction of (R,R)-9 and L-8 in toluene provides 14 with 89:11 selectivity, whereas the reaction of (S,S)-9 with L-8 in Et₂O provides the diastereomer 15 with 90:10 selectivity. It is shown that the relatively modest diastereoselectivity of these double asymmetric reactions is compromised by the low enantiomeric purity of 8 (86-87% ee), and data are provided indicating that these reactions should be highly diastereoselective (≥95:5 in each case) if performed with enantiomerically pure aldehyde. The two diastereomeric homoallylic alcohols, 14 and 15, are easily elaborated into the targeted amino sugars 6 and 7 via the acetamide-substituted pyranosides 22 and 26. Methyl pyranosides 22a and 22e were shown to adopt preferentially the unexpected conformations B and D, with axial acetamide substituents, in nonpolar solvents, while the expected conformations A and C were strongly favored in d₆-DMSO because of hydrogen bonding interactions with the solvent. The syntheses of 6 and 7 reported herein are expected to facilitate the design and synthesis of analogs of the calicheamicin aryl tetrasaccharide 3, which should prove useful in further analysis and applications of oligosaccharides as DNA binders.