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

Scheme 110
Scheme 111
uents in the γ-position, such as CO2Et, favor γ-alkylation
to give 608. 316
Reaction of the metalated cyanohydrins of 609 with
R,�-unsaturated ketones gives conjugated R-1,4-addition
products 610 (Scheme 111). 317-321
The anion 612 of oxytrimethylsilyl cyanohydrin
611 gives exclusively the R-products 614 in reactions
with ketones or aldehydes at -78 °C accompanied 322
by 1,4-O,O-silyl rearrangement producing 613 (Scheme
112). 323 On warming to room temperature, the anion
species 615 undergoes 1,4-O,C-silyl rearrangement
to form the new O-anions 616 and 617 which again
can be trapped by silylation to give 620 and 621,
respectively (Scheme 112). In contrast to the reaction
of 615 with alkyl halides and carbonyl compounds,
treatment with trimethylchlorosilane does not proceed
regioselectively. A mixture of R- 618 and γ-adduct
619 is obtained (Scheme 112). 324
The ethoxyethylsilyl cyanhydrin anion 623 also
shows exclusive R-reactvity to give 622 at -78 °C.
However, upon treatment with aldehydes and ketones
γ-reactivity to form exclusively 624 is observed
if the reaction is carried out at 0 °C. 325,326 Metalation
of the R-product 622 at -78 °C followed by warming
to 0 °C yields exclusively γ-product 624 (Scheme 113).
2. R-Cyanoallylamines (CdC−C(CN)(NR2)
R-Cyanoallylamines 625 generally react at the
γ-carbon with alkylating agents to give 627 327,328 and
with R-enones to give predominantly 1,4-addition (cf.
626). An improvement of the yield of 1,4-adduct 626
is achieved by allowing the kinetically controlled
product mixture to equilibrate. 329 Furthermore, asymmetrical
alkylation at the γ-position was carried
out. 330 If NR2 in 625 is a morpholino group the
alkylation takes places either exclusively or predominantly
at the R-position producing 629 (Scheme
114). 331
Scheme 112
Scheme 113
Scheme 114
697
Treatment of allylic aminonitriles 630 with ketones
for 90 s at -78 °C in THF yielded R-condensation
products 631. When the same sequence is carried out
at0°Corat-78 °C and allowed to warm to 0 °C
only the γ-product 632 is observed (Scheme 115) 326
(cf. section IV.C.1). The overall yield of γ-adduct 632
can be improved by addition of anhydrous ZnCl2 (cf.