CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

fast and gave a significantly better yield (83%) of the products. Both Zn- and Fe-mediated
reactions produced the alcohols, 83c and 83d in almost 1:1 ratio.
In this case also, the observed excellent 2,3-anti selectivity suggested that the
reactions followed the Felkin Anh model, 24b,112 irrespective of the metal chosen.
Apparently the α-chelate attack was not favored due to hydration of metals in the moist
reaction environment. Interestingly, of the four cases, the Cu-mediated addition produced a
higher amount of the 3,4-anti product. The formation of significant amounts of both 3,4-
syn- and 3,4-anti-products gave ample evidence of a considerable amount of E,Z
equilibration of the allylic intermediate during carbon-carbon bond formation.
As evident from the above, none of the above protocols were suitable for the
preparation of 83a and 83b, which are also useful chiral intermediates, and may be
required for the synthesis of suitable target compounds. Since both these compounds
possess a β-hydroxy function, in principle, the previously described oxidation-reduction
strategy might be suitable for their synthesis. Hence, for their preparation (Scheme
III.1.8), compounds 83c and 83d were separately oxidized with PCC to afford the
respective ketones 84c and 84d in good yields. Formations of these compounds were
confirmed from the appearance of sharp carbonyl IR board band at ~1715 cm -1 in place of
the hydroxyl band. Reduction of these ketones with K-selectride ® furnished the desired
2,3-syn products 83b and 83a in good yields, almost exclusively. As expected, the IR
spectra of these compounds showed broad bands at ~3500 cm -1 and absence of the CO
peak. The mechanistic rationalization of the steric course of the reduction established the
2,3-syn stereochemistry of 83a and 83b, confirming the 2,3-anti relationship in their
progenitors, 83c and 83d. Thus, the studies provided efficient routes to prepare all the four
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