Handbook of Functionalized Organometallics Applications in S

218
6 Polyfunctional Tin Organometallics for Organic Synthesis
reported that c,c-disubstituted allyltins reacted stereospecifically, (E)-reagent giving
a syn adduct and (Z)-reagent giving an anti adduct [179]. The initial proposal
was then completed by the synclinal acyclic transition state [180] to account for
the diastereoselectivity change. This reaction was applied to various c-substituted
allyltins in the total synthesis of complexframeworks [181] with excellent diastereoselectivities
(Scheme 6.20). It is worth noting that a-substitution on the allyltin
reagent may affect dramatically the syn selectivity as well [182]. Thus, the syntheses
of optically active a-oxygenated allyltins and their subsequent rearrangement
to c-alkoxyallyltins [183] allowed an easy access to stereocontrolled 1,2 syn diols
further applied to synthetic purpose [184].
C10H21 H
O
H
MOMO OTs
O
OBn
MOMO SnBu3 H
BF3.OEt2, CH2Cl2, -78ºC
80%
C10H21 H
O
H
MOMO OTs
Scheme 6.20
OH
OMOM
In addition to the amount of work done with aldehydes as substrates, there is
some evidence of diastereoselective addition of c-substituted allyltins to ketones,
leading to tertiary homoallylic alcohols withTiCl 4 or SnCl 2 as Lewis acid [185].
Diastereoselectivity induced by chiral aldehydes: the substrate plays an important
role in the facial diastereoselection, particularly when there is an asymmetric center
adjacent to the carbonyl group. In the general case, the approach of the allyltin
is assumed to follow the Felkin±Anh model giving the syn adduct preferentially.
This induction was used for the synthesis of natural products [186] even as complexas
Ciguatoxin or Laulimalide [187].
Chelation control: however, a reversal of the diastereofacial selectivity may arise
when the substrate has, in the a or b position of the side chain, a group prone to
complexation with the Lewis acid. Then, the use of bidentate Lewis acids (Mg II ,
Zn II ,Sn IV or Ti IV ) allows the reaction to proceeding under a ªchelation controlº,
model preferentially provides the syn adduct for a 1,4-chelation. Various a-alkoxy
aldehydes [188] were used in carbohydrate chemistry. Similarly, a-amino aldehydes
were used as precursors for b-amino alcohols (Scheme 6.21) [189]. On the
contrary, a 1,5-chelation control gives predominantly the anti adduct, so that b-alk-
O
H
N
BOC
O
Scheme 6.21
CH 2Cl 2
SnBu 3
BF 3.OEt 2,-78ºC
MgBr 2.OEt 2,-20ºC
OH
N
BOC
O
+
OH
N
BOC
O
OBn
12 : 88 84% (Felkin-Anh)
83 : 17 87% (chelated)