CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

To confirm the 3,5-syn stereochemistry of 132, it was converted to the acetonide
derivative 132b (Scheme IV.2.3). Thus, compound 132 was desilylated to the diol 132a
and subsequently converted to the cyclic acetal 132b by reacting with 2,2´dimethoxypropane
(DMP) in the presence of pyridinium p-toluene sulphonate (PPTS). Its
13 C NMR spectrum showed peaks at δ 19.6, 29.8 and 97.7 ppm, which indicated the 3,5-
syn stereochemistry of 132. 156 The 1 H and 13 C NMR spectra of 132b are shown in Fig.
IV.2.4 and Fig. IV.2.5 respectively.
The observed selectivity in the reduction of compound 131 could be explained as
follows. Presumably, the reduction proceeded through a six-membered chair like transition
state A or B, both possessing the R 1 group in an equatorial position (Scheme IV.2.4). Of
these, the bottom side approach of the H - in the transition state B that would furnish the
anti-diol is effectively hindered by the steric bulk of the axial protecting group R 3 . Thus,
the reaction would proceed through the transition state A producing the syn-diol, as
observed in the present case. Such type of intramolecular activation of the carbonyl by a
tricoordinated boron atom followed by intramolecular hydride delivery has been reported,
and also the propensity of such β-hydroxy ketone boron aldolates to reduce with the
opposite sense of asymmetric induction was established earlier. 157 Apparently, a bulky
protecting group (R 3 ) like the TBDPS group, present in 131 was responsible for the
exclusive formation of the syn-product 132.
153