Asymmetric synthesis: Chiral auxiliaries, methods of asymmetric induction – substrate, reagent and catalyst controlled reactions; determination of enantiomeric and diastereomeric excess; enantio-discrimination. Resolution – optical and kinetic.
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1.The major product formed in the following reaction is
In first step NaHMDS generates an Enolate and in second step attack of Enolate on epoxide ring in SN2 manner give the product
2.The major product formed in the following reaction is
The reagent 1mol% K2OsO2(OH)4; (DHQ)2-PHAL give syn product on reaction with MeSO2NH2, K3Fe(CN)6, diol produces. We know that with the largest alkene substituent in the lower left corner as drawn the DHQ based ligand system promotes dihydroxylation from the lower (α) face where as the lop (β) face reacts with the DHQD-based ligand system.
3.The major product formed in the following reaction sequence is
Boronic esters RB(OR’)2 react with methoxy(phenylthio)methyllithiumLiCH(OMe)SPh to give salts, which, after treatment with HgCl2 and then H2O2, yield aldehydes. This synthesis has been made enantioselective, with high ee values (> 99%), by the use of an optically pure boronic ester.
4.The absolute configurations of the chiral centres of starting ketone in the following reaction is
(a) 3R, 6S
(b) 3S, 6S
(c) 3R, 6R
(d) 3S, 6R
This is an application of Cram’s rule. LAH being smaller nucleophile result in the formation of transaldol product as the major one.
(Reference : Organic Chemistry, Clayden)
5.The major product formed in the following transformation is
This is a problem of diastereoselectivity in which the use of smaller ligands (such as n-butyl) and a good leaving group (such as OTf) on a boron atom, combined with hindered amine (such as iPr, Net ) give rise to cis-enolate. Thus the corresponding aldol formed is cis-aldol.
6.The major product B formed in the following reaction sequence, and overall yield of its formation are