Abstract
The lithiation of 2-(2,2-dimethyl-1,3-dioxolan-4-yl)ethyl diisopropylcarbamate (1) is achieved freely by sec-butyllithium in diethylether with high lk-diastereoselectivity: the bicyclic chelate complexes 3a and 3b are reacted with electrophiles to form optically active precursors 4a and 4b with >95% diastereoselectivity. In addition, tertiary diamines can undergo an external complexation in contest with the internal oxygen ligand, leading to improved stereoselectivities. The further reactions of lithiated carbamates with trans alkenyl-9-BBN derivatives after 1,2 metallate rearrangements, gave the key intermediate α-substituted allylic boranes 7. Subsequent allylboration of aldehydes gave (Z)-anti-homoallylic alcohols 8 in good yield and excellent d.r.
Highlights
Hoppe and co-workers investigated the lithiation of carbamates derived from non-racemic chiral primary alcohols generating organolithium intermediates which undergo electrophile-dependent stereodivergent substitution that often have remarkable configurational stability [1,2,3,4]
Two procedures were adopted for the deprotonation of the carbamate 1, one in the presence of external ligands and another with no external ligands involved (Scheme 1)
Very similar results were obtained as per Hoppe [3], but products more bench stable, and reactions are quicker, and give high yields and good d.r. This shows that the expulsion of an unhindered carbamate leaving group (OCb) could be quicker than that of a bulky carbamate (OCby)
Summary
Hoppe and co-workers investigated the lithiation of carbamates derived from non-racemic chiral primary alcohols generating organolithium intermediates which undergo electrophile-dependent stereodivergent substitution that often have remarkable configurational stability [1,2,3,4]. This stability is due to dipole stabilization and intramolecular chelation of the lithium counterion by the carbamoyl group [5]. Boronic esters and their reactions with sparteine-complexed lithiated carbamates [19] This protocol was further applied in the formation of penta-substituted tetrahydropyrans through the.
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