Handbook of Functionalized Organometallics Applications in S

10
2 Polyfunctional Lithium Organometallics for Organic Synthesis
are stable at ±78 C, because, taking into account Baldwin's rules along with the
microscopic reversibility principle, a b-elimination process (which would lead to a
olefin) is the reverse of the n-endo-trig cyclization. Thus, the stability of intermediates
15 decreases with increasing ring size [17]. In the case of other a-stannylated
hydroxycompounds, the corresponding transmetallation occurs only for their carbamate
derivatives [18] but not in the case of O-MOM protected systems [19]. The
reaction of intermediates 17 with aldehydes followed by reduction with AlH 3 gave
1,2-diols. The lithiation of enantiomerically enriched a-propargyloxy stannanes
proceeds with complete inversion of the configuration and has been applied to
study the Wittig rearrangement. In the case of compounds 18 the selectivity of
[2,3] versus [1,2] rearrangement is dependent of the nature of the R 2 substituent
on the acetylenic moiety [20].
Li
O
MOMO
Li
N
Pf Pf N
Li
O
R
O
O NEt2 Li
O Li
R 1
R 2
() n
14 15 (n = 0-2) 16 17 18
[Pf = 9-phenyl-9-fluorenyl]
The a-alkoxy alkyllithium derivative 20, prepared from compound 19 by tin±
lithium exchange with retention of the configuration, undergoes 5-exo-trig cyclization
to form first the cyclic compound 21 and finally the trans-disubstituted tetrahydrofuran
22 through a b-elimination process (Scheme 2.4) [21].
Ph
Scheme 2.4
H
SnBu3 n-BuLi, THF Ph
H
Li
O
-78ºC
O
19
OMe
20
Ph
O
Li
OMe
Ph
21 22 (56%)
O
OMe
The a- and b-configurated glycosyl anions 23±26 were prepared by tin±lithium
exchange. C-Glycosilated aminoacids were obtained by reaction of intermediate
23 with lactams [14], meanwhile the reaction of compounds 24 and 25 with electrophiles
led to a- and b-C-galactosides [22], respectively, intermediate 26 being
used in the stereocontrolled synthesis of carba-C-disaccharides [23].