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

674
Scheme 20
Scheme 21
the “natural stereoisomer” 136 is obtained in the
presence of BF3‚Et2O. The reaction proceeds via a
chelate-controlled mechanism whereby the ketone is
precomplexed or preactivated by BF3‚Et2O followed
by addition to the allyl sulfide anion to give R-adduct
136.
To achieve regioselective γ-acylation of CH2dC(Me)-
CH2-SPh, the 2-methyl-substituted allylic sulfide
138 needs to be first converted to the R-silylallylic
sulfide 139 and hence to 140 (Scheme 22). 89
(Methylthio)- and (tert-butylthio)allyllithium (cf.
141) give, with cyclopentenone 142, the R-1,2-adduct
Scheme 22
Scheme 23
Scheme 24
143 and the γ-1,2-adduct 144, while the phenyl
analogue of 141 in addition gives small amounts of
1,4-R- and γ-addition products. The formation of the
1,4-R-addition product 145 with hardly any 1,4-γattack,
can be enhanced by the presence of 1 equiv
of HMPA (Scheme 23). 90,91
Conjugate addition to allyl phenyl sulfides, 92 e.g.,
146 also gave R-products 147 and 148 (Scheme 24).
This route can eventually furnish a prostaglandin,
whereby the key step involves the reaction of the
lithium enolates of 147 and 148, produced in the
initial conjugate addition step, with triphenyltin
followed by alkylation 93-95 or a stereoselective steroid
synthesis by subsequent trapping of the enolates with
benzyl bromide. 96
3. Allyl Sulfoxides (CdC−C−SOR)
Chiral sulfoxides can easily be introduced into a
molecule and this is an important strategy for asymmetric
synthesis. Allylic sulfoxides 149 were reported
to form R- 150 (major) and γ-products 151 (minor)
upon alkylation 76 (Scheme 25). The ratio of the
mixture depends on both the type of allyl sulfoxide
and the electrophile. 97,98 Solladié 99 reported mainly