A Route to Carbasugar Analogues - Jonathan Clayden - The ...

Chapter 1: Introduction
Unlike the dearomatising additions discussed in the previous section, the anti addition
of the electrophile is not due to the relative steric bulk of the nucleophile, but is due to
endo migration of the electrophile after alkylation of the chromium centre (76) and can
happen in two ways (Scheme 1.22). In the presence of a suitable ligand such as
acetonitrile, benzonitrile or CO, the electrophile migrates to one of the carbonyl
ligands to form acylated complex 77 before endo-migration delivers the acyl group
opposite to the nucleophile. In the absence of a suitable ligand, or if E is a poor
migratory group (benzyl, allyl, propargyl) non-acylated products 80 are isolated.
Again, the electrophilic quench is highly sensitive to the nature of the substrate and
nucleophile. This facility greatly enhances the scope of the methodology, allowing the
formation of two stereocentres, and the possibility for further functionalisation via the
enolate of acyl diene 78. Furthermore, olefins 79 have been obtained by the addition
of lithium dithiane to a benzene complex, followed by in situ hydrogenation. 52
1.4.1.a Asymmetric addition using Cr(CO) 3 complexes
Asymmetry has been successfully introduced by the use of substrate-bound chiral
auxiliaries (Table 1.13) and chiral ligands (Scheme 1.23). Oxazolines 81, derived
from amino alcohols are effective in directing the asymmetric addition to the benzene
nucleus; tert-leucine-derived oxazolines (entries d-f) react with greater
diastereoselectivity than for those derived from L-valine, consistent with Meyers’
observations of additions to naphthyloxazolines (vide supra). Electrophilic
substitution occurs via addition to the metal centre, without acylation since allyl is a
poor migratory group.
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