Page 874 - Advanced Organic Chemistry Part B - Reactions & Synthesis

850 shift at both the transmetallation and addition steps, resulting in restoration of the
original allylic structure.
CHAPTER 9
Carbon-Carbon R SiOCH CH CHCH CH O (n-C 4 9 3 2 H
H ) SnCH
3
2
2
2
Bond-Forming Reactions
of Compounds of Boron, CO C(CH ) + CO CH
2
3 3
Silicon, and Tin CH 3 2 3
O
CH SPh
2
2
Cl Cl CH SPh
H CH 3 Cl CH SPh CH O
2
Sn O OH 3
R O
CO 2 CH 3
R
CO CH 3
2
H
Entry 10 was used in conjunction with dihydroxylation in the enantiospecific
synthesis of polyols. Entry 11 illustrates the use of SnCl with a protected polypro-
2
pionate. Entries 12 and 13 result in the formation of lactones, after MgBr -catalyzed
2
additions to heterocyclic aldehyde having ester substituents. The stereochemistry of
both of these reactions is consistent with approach to a chelate involving the aldehyde
oxygen and oxazoline oxygen.
R R R
N N N O
Ph CO CH 3 Ph CO 2 CH 3 Ph
2
O O H O O
H H H
Mg O Mg O
9.3.2.6. Allenyl Stannanes. Allenyl stannanes are a useful variation of the allylic
stannanes. 192 They can be made in enantiomerically pure form by S 2 displacements

N
on propargyl tosylates. 193
R CH R R H S
OSO CH 3 Bu SnLi R S 3 C Bu SnLi OSO CH 3
2
2
3
R 3 C CH R
CH H Bu Sn 3 CH
2
H 3 CuBr .SMe Bu Sn 3 CuBr .SMe 2 H 3
3
2
2
The allenic stannanes react with aldehydes under the influence of Lewis acids such
as BF and MgBr . Unbranched aldehydes are not very stereoselective, but branched
3
2
aldehydes show a strong preference for the syn adduct.
CH 3
R CH 3 BF 3 CHR′ 2
C + O CHCHR′
Sn H 2
Bu 3 R OH
With -benzyloxypropanal, using MgBr as the Lewis acid, chelation control is
2
observed. The stereospecificity is determined by an anti orientation of the C–Sn bond
192 J. A. Marshall, Chem. Rev., 96, 31 (1996).
193
J. A. Marshall and X. Wang, J. Org. Chem., 56, 3211 (1991).