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

706
Scheme 149
The alkoxyallylic sulfone 792 is deprotonated and
R-alkylated twice with MeI to form 793. To avoid the
elimination of the ethoxy group, HMPA is required
for the second alkylation (Scheme 150). 393 A similar
reaction takes place with 2-oxo-substituted 1-sulfonylallyl
anion 794, whereby upon irradiation with
300-nm light in benzene the dialkylated sulfone 795
undergoes a quantitative 1,3-shift to give 796 (Scheme
150). 394
Scheme 150
5. 5-Methylene-1,3-dioxanes
5-Methylene-1,3-dioxane 797 anions 798 form R-adducts
799 preferentially by reaction of these with
carbonyl compounds. 395 In the presence of Zn 2+ as the
counterion, the R-product 799 is exclusively obtained,
similar to the reaction of the oxyallyl anion. However,
γ-adducts 800 are predominantly synthesized upon
alkylation or trimethylsilylation (Scheme 151). 396
F. General Discussion
The orientation of reactions with electrophiles of
allyl anions stabilized by two nonidentical and nongem-heteroatoms
is summarized in Scheme 152.
Scheme 151
Scheme 152. Two Non-Gem-Heteroatoms a
a Allyl anions with identical heterosubstituents are not in this
scheme.
This general discussion deals with sections V.A
through V.D. Obviously, when two identical heteroatom
linked groups are in the 1- and 3-positions of
an allyl anion, no regioisomers can arise. When the
groups are different, then the most strongly R-directing
group dominates. Thus, RSO2 dominates over
both OR and SR, and also over benzotriazolyl (unpublished
work by A.R.K.). Group RSO dominates
over Cl. Benzotriazolyl and SR each dominate over
NR2 and OR. As regards R3Si, it dominates over NR2
but is itself dominated by OSiR3.