Regioselectivity of the Reactions of Heteroatom-Stabilized Allyl ...

670
located to cases where extra stabilization of the anion
is provided by the formation of a pentadienyl anion.
1. 1-Alkoxy-2-propenes (CdC−C−OR)
1-Alkoxy-2-propenes 39 can easily be metalated
with sec-butyllithium at -65 °C in THF. At higher
temperatures they tend to undergo 1,2-Wittig rearrangement
43 f 44 (Scheme 6). 23-33 Various types
of 1,2-Wittig rearrangements are classified according
to the substituent on the oxygen as shown in Scheme
6.
Lithiated 1-alkoxy-2-propenes 40 react with alkyl
halides to give a mixture of enol ethers 41 (γ-attack)
and allyl ethers 42 (R-attack) (Scheme 6). The regioselectivity
is rather insensitive to change in solvent
or temperature, but is influenced by the oxygen
ligand R (steric effects). For example, the tert-butyl
allyl ether favors the γ-product with Z stereochemistry
determined by five-membered ring chelation. 34,35
The dianion 48 of 1-indanone 45 undergoes almost
exclusive γ-attack to give 47 upon addition of 1 equiv
of alkyl halide (Scheme 7), which can be explained
in terms of the electronic repulsion of the charged
oxygen which is reinforced by the aspiration to
maintain maximum charge stabilization by delocalization.
36
Reactions of heterosubstituted allylic anions with
carbonyl compounds as electrophiles frequently show
the opposite regioselectivity to that found for alkylation
reactions. Thus, lithiated trimethylsilyl allyl
ether (40, R) SiR3) and tert-butyl allyl ether (40, R
) t Bu) give predominantly R-products 49 with cyclo-
Scheme 6
Scheme 7
Scheme 8
Scheme 9
hexanone. However, the yield of the γ-product 50 is
increased by using a smaller group in the ether,
although even with methyl allyl ether some R-product
49 is observed beside γ-attack. 37
Replacement of the lithium counterion in 40 by
treatment with zinc chloride 34 to give 51 as does
CdCl2 38,39 results in exclusive formation of R-adducts
49 in subsequent reactions with aldehydes and
ketones. Similar R-direction is achieved upon replacement
of the lithium counterion with triethylaluminum
40,41 or via boron “ate” complexes 42 (Scheme 8).
Another way to achieve R-attack is to use (R-ethoxybutenyl)tributyltin.
43 Furthermore, highly regioselective
conjugate 1,2-R-carbonyl additions of allylzinc
34,35 and allylcadmium reagents 44 to enones is
observed.
To obtain predominantly γ-products, Mukaiyama 45
treated cinnamyl ether with s-BuLi, Weiss’ base,
Schlosser’s base or KDA (Seebach’s base) followed by
the reaction with the electrophile. This method has
been used in asymmetric synthesis. The yield and the
optical purity strongly vary with the solvent and the
type of base applied. An intramolecular example is
the cyclization of allyloxycarbanions of 52 and 54 to
vinyl oxetanes 53 and 55 (Scheme 9). 46
2. Silyl Allyl Ethers (CdC−C−OSiR3)
Allyloxy carbanions 57 of silyl allyl ethers 60 are
in rapid equilibrium with the corresponding silyl
alkoxides 58 (silyl-Wittig-rearrangement) (Scheme
10). 47-49 Hard electrophiles (TMSCl, chloroformates,
diphenyl carbonate, and protons) react exclusively or