CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

and NaBH 4 reduction afforded the stereochemically pure 1,3-syn isomer of 18. The bulky
protecting group on the hydroxyl moeity was instrumental in controlling the 1,3-
asymmetric induction. Benzoylation of the carbinol function of 18, followed by
desilylation furnished 19. This was converted to the 1,3-anti diol 19 following a
oxidation-reduction protocol (using PCC as the oxidant and K-selectride ®
as the
reductant). Acid catalyzed deketalization of 19 furnished the diol 20. Cleavage of its α-
glycol unit with NaIO 4 followed by reaction of the resultant aldehyde with PhMgBr
furnished the required alcohol 21 with excellent enantioselectivity (ee 95%). Its
desilylation, conversion into acetonide and debenzoylation gave the target synthon 22.
7d
i, ii
O
O
OR 1
CHO
iii-v
O
O
OR 1
OH
vi, vii
O
O
OH OR 2
17. R 1 = TBDPS
18. R 1 = TBDPS 19. R 2 = Bz
iv, viii, i, ix
HO
OH
OR 1
OR 2
x, xi OH
vii, xii
Ph
OR 1 OR 2
Ph
O
O
OR 1
20. R 1 = TBDPS, R 2 = Bz 21. R 1 = TBDPS, R 2 = Bz 22. R 1 = Bz
i) TBDPSCl, Imidazole, CH 2 Cl 2 ,25 o C; ii) O 3 ,CH 2 Cl 2 ,-78 0 C; PPh 3 ; iii) Allyl bromide, Zn, THF, Satd aqueous
NH 4 Cl; iv) PCC, CH 2 Cl 2 ,25 0 C; v) NaBH 4 ,MeOH,O 0 C; vi) BzCN, Et 3 N, 0 0 C; vii) n Bu 4 N + F - ,THF,0 0 C; viii) K-
Selectride (R) , THF, -78 0 C; ix) Aqueous CF 3 CO 2 H, CH 2 Cl 2 , O 0 C; x) NaIO 4 , 25 0 C; xi) C 6 H 5 MgBr, THF, -20 0 C; xii)
2,2'-dimethoxypropane, PPTS, rt.
Scheme 8
For the synthesis of 25 (Scheme 9), compound 8d was benzoylated, subjected to
hydroboration-oxidation of its terminal olefin moiety, and the resultant alcohol oxidized
with PCC to yield the aldehyde 23. This on debenzoylation under alkaline conditions
xviii