CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

undergo in situ isomerization to the other transition state 67, which can also be generated
from the corresponding Z- bromide Z-65. This would result in the syn-product.
M 1 X + M 2
M 1 + M 2 X
R 1 Br
+ M
M 1 Br
1
R 1
64 65
H
R O R 1 M 1 Br R 1
+
O
H H
H R
E-65
66
R 1 = H, CH 3 , CH 2 OTBDPS, C 6 H 13
OH
M 1 Br
R
R 1
anti
R
H
O
H
+
R 1
Z-65
M 1 Br
H
R 1
H
R
67
O
M 1 Br
OH
R
R 1
syn
Scheme.III.1.1
To explore the viability of this approach, Zn was chosen as the reducing metal in
separate combinations with four commercially available salts viz. CuCl 2. 2H 2 O,
CoCl 2. 6H 2 O, FeCl 3 and SnCl 2 .2H 2 O. These were chosen in consideration of the redox
potentials of the following couples: E 0 Zn= Zn 2+ + 2e (+0.761 V), E 0 Co= Co 2+ + 2e (+ 0.280 V), E 0
Cu= Cu 2+ + 2e (-0.337 V), E 0 Fe= Fe 2+ + 2e (+ 0.441 V), E 0 Fe 2+ = Fe 3+ + e (- 0.771 V) and E 0 Sn= Sn 2+ + 2e
(+0.140 V). This implied that the reduction of Co(II), Cu(II), Fe(III) and Sn(II) salts can be
effected on treatment with metallic Zn to produce Co, Cu, Fe and Sn respectively in their
active states. Due to higher surface area, use of Zn powder is advantageous for the
reduction. For the studies, the α-oxygenated chiral aliphatic aldehyde 1 was chosen in
order to compare the stereochemical course of the new protocol with that observed earlier
by our group using Mg 29d and Zn 3 as the metal mediators. Thus, in the synthetic protocol, a
78