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

1.4 – Metal complexation
R
R
R
O
N
O
N
O
N
i) NuLi
ii) allyl Br, HMPA
Nu
+
Nu
Cr(CO) 3
81
82a
82b
Entry R Nu 82 / % 82a : b
a i-Pr Me 61 96 : 4
b i-Pr n-Bu 54 95 : 5
c i-Pr Ph 48 95 : 5
d t-Bu Me 69 >99 : 1
e t-Bu n-Bu 62 >99 : 1
f t-Bu Ph 51 95 : 5
Table 1.13 – use of chiral oxazoline auxiliaries 53
Asymmetric additions to prochiral arene chromium species 84 has been achieved with
chiral ligand 73 (Scheme 1.23), the same ligand used by Tomioka in the asymmetric
addition to naphthylesters and imines (Scheme 1.20). This ligand is proposed to work
by coordinating either the organolithium nucleophile or the arene-chromium complex.
Whilst an advantage of using a chiral ligand rather than an auxiliary would normally
be that little or no trace of activation remains, the need for a ortho-directing group
seems to negate this.
i) RLi Ph
Ph
O N
OMe O N
73 OMe 2 eq
R
ii)
Br
Cr(CO) 3
84
54
Scheme 1.23 – use of an asymmetric ligand
51-67%
65 - 93% e.e.
The above methods have been utilised by Kündig et al. in the recent synthesis of both
enantiomers of aceteoxytubipofuran via the key intermediates (–)-86 and (+)-86. In
the key step, two stereocentres, one of them quaternary, are formed (Scheme 1.24).
44