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

1.3 – Nucleophilic dearomatisation
Additions to 2-naphthyloxazoline 37 were also successful (Scheme 1.10) but have
been studied less. Oxazolines were also found to promote dearomatisation of
anthracenes but not phenyloxazolines. 29,30
37
O
N
i) NuLi, −45 °C
ii) MeI
Nu O Me
N Nu = n-Bu, 90%
s-Bu, 91%
t-Bu, 90%
Scheme 1.10 - additions to 2-naphthyloxazoline 26
1.3.2.c Addition to chiral naphthyloxazolines
As with chiral pyridyloxazolines, high levels of asymmetry could be introduced to the
already highly diastereoselective addition. 31 Enantiomerically pure oxazolines 38 were
subject to the same conditions, with the initial addition proceeding in high
diastereoselectivity followed by exclusively trans-addition for alkyl electrophiles
(Table 1.6). Oxazoline cleavage of the major diastereomer (39 to 40) also proceeded
with high fidelity and is discussed further in section 3.2. A comprehensive list of
results has been compiled by Ortiz et al. in their recent review. 1
Ph
O
N
OMe
i) NuLi, 2-4 hr
ii) E
Ph
O
OMe
N
E
Nu
i) MeOTf
then NaBH 4
ii) H +
OHC
38 39 40
NuLi E T / °C 39 / % d.r.* 40 † / % e.e. † / %
a n-BuLi MeI –45 97 94 : 6 – –
b n-BuLi (PhS) 2 –45 91 94 : 6 – –
c n-BuLi ClCO 2 Me –45 99 97 : 3 88 88
d MeLi (PhS) 2 –20 56 86 : 14 62 >99
e PhLi MeI –45 99 83 : 17 89 >99
f 32 vinyl-Li MeI –60 79 94 : 6 – –
g 32 vinyl-Li MeI –60 65 89:11 – –
* ratio of adduct 39 to that of trans addition from opposite face † from 39
31
Table 1.6 – selected nucleophilic additions to chiral 1-naphthyloxazolines
E
Nu
28