CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

Allylation with 1-Bromo-4-(tert)-butyldiphenylsilyloxy-2Z-butene (82): In another
reaction, the γ-substituted allylic bromide 82 was chosen for coupling with 1 (Scheme
III.1.7). Compound 82 was derived from commercially available (Z)-but-2-en-1,4-diol
(80) via (i) monosilylation of its carbinol function with TBDPSCl in the presence of
imidazole as the base to afford 81, (ii) subsequent bromination of 81 to 82 in 71% overall
yield via mesylation and reaction with NaBr. Compound 82 was characterized from the
absence of IR hydroxyl band and the 1 H NMR resonances for the TBDPS group. As
earlier, the Barbier type addition of 82 to 1 was carried out using the bi-metallic strategy
(Scheme III.1.7) as well as following Luche’s procedure 3
to obtain a mixture of all
possible diastereomers 83a-d of the γ-addition product. The results of the study are
summarized in Table III.2.4. 121
In all the cases, the diastereoisomers, with 2,3-anti, 3,4-syn 83c and 2,3-anti, 3,4-
anti 83d stereochemistry, were produced predominantly. Fortunately the stereomers 83c
and 83d were separable by column chromatography. Formation of these isomers were
confirmed by the IR band at ~3500 cm -1 and also from the terminal olefinic signals at δ
5.00-5.23 (2H) and δ 5.80-6.00 (1H) for 83c, and at δ 5.11-5.23 (2H) and δ 5.70-6.02 (1H)
for 83d in their respective 1 H NMR spectra. Compounds 83a and 83b were produced only
in very small quantities and could not be separated from each other by column
chromatography.
97