Handbook of Functionalized Organometallics Applications in S

BnO
BnO
OBn
O
AcLiN
23
BnO
OBn
Li BnO
O
OLi
Li
2.2 a-Functionalized Organolithium Compounds
BnO
BnO
OBn
O
LiO Li
BnO
BnO
24 25 26
As commented above, this kind of functionalized organolithium compounds
can also be prepared through deprotonation processes. The deprotonation can be
also performed in a diastereoselective way, for instance, in the presence of
(±)-sparteine [24]. Treatment of O-alkyl carbamate 27 with s-BuLi in the presence
of (±)-sparteine at ±78 C gave the organolithium compound 28 with high ee,
which upon carboxylation and acidic hydrolysis led to (R)-pantolactone 29
(Scheme 2.5) [25].
OCONR 2
O
O
Scheme 2.5
N
O
s-BuLi, (-)-sparteine
Et2O, -78ºC
OCONR 2
Li
O
N
1. CO2,-78ºC tort
2. 2 M HCl
3. 5 M HCl, 90ºC
27 28
29 (80%, >95% ee)
The organolithium compound 30 was also prepared by an enantioselective deprotonation
in the presence of (±)-sparteine. It is configurationally stable, but
when reacting with electrophiles retention or inversion of its configuration is observed
[26]. The deprotonation can also be performed in the presence of a chiral
bisoxazolidine to give benzylic anions 31±33, which reacted with electrophiles to
yield the corresponding reaction products with good stereoselectivity. In this case,
the stereoselectivity was explained by a dynamic thermodynamic mechanism
instead of through an enantioselective deprotonation [27]. The a-lithiated O-protected
propargyl alcohol 34 was prepared by deprotonation with n-BuLi and used
in the synthesis of a cyclic enediyne related to maduropeptin chromophor [28].
O Li Me
Li
O
O
O
Ph O Ph
Ph OMe
Li
Li
Li
Me3Si 30 31 32 33
34
O
O
OBn
O
Li
OMe
Lithiated epoxides [29] of general structure II (X = O, n = 0) were first postulated
as intermediates by Cope, and many of them act as carbenoid species: they
can undergo (a) b-elimination to give an enolate, (b) a-ring opening followed by
O
11
O
OH