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

676
Scheme 29
Scheme 30
Scheme 31
Scheme 32
to give 179 followed by protodesilylation (Scheme
30). 118
Allyl sulfone 181 can form R-product 182 via an
intramolecular transfer of an acyl group (Scheme
31). 119
The reaction of lithium and magnesium salts with
acetone in the presence of a chiral diamine 183 gives
R-substituted chiral sulfones 184. The enantioselectivity
is improved up to a value of 50% ee by
transmetalation from lithium to magnesium derivatives
(Scheme 32). 120
In the presence of HMPA the conjugate addition
to cyclic and acyclic enones proceeds exclusively via
1,4-R-attack to give 187. Without HMPA present,
with cyclic enones the 1,2-R-addition product 185 is
formed followed by a rearrangement to the 1,4-γadduct
186 (Scheme 33). 121 The R-regioselectivity for
acyclic enones (cf. 188) is not changed in the absence
Scheme 33
Scheme 34
of HMPA. 122 More details about the influence of
HMPA on the conjugate addition of γ-crotonolactone
to allyl sulfones, allyl sulfides, and allyl thiocarbamates
are given by Haynes. 122
The Michael addition of allyl sulfone 190 to aromatic
nitroolefins (cf. 189) yields predominantly
γ-(E)-adducts 192, whereas aliphatic nitroolefins (cf.
189) form exclusively the R-products 191, in which
the syn-isomer is mainly found. The presence of an
OH substituent in the sulfone (cf. 193) causes formation
of only (Z)-γ-adducts 195, whereas the presence
of a bromine substituent in the methyl group of
methylallyl sulfone 193 reverses the regiochemistry
to give 194 (Scheme 34). 123