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

698
Scheme 115 Scheme 117
usual improvement of yield of R-product with ZnCl2).
Lithiated H2CdCH-CH(CN)(NMePh) yields γ-products
when reacted with RX, RCHO, and RCOR while
both R- and γ-adducts are obtained with Ph-CHd
CH-CLi(CN)(NR2). 332 The regioselectivity of the
reaction of the former is remarkably high considering
the exclusive R-attack for the oxygen analogues (Ph-
CHdCHLi(CN)(OSiMe3), Ph-CHdCH-CH2-CLi-
(CN)/(OEE).
3. 2-Cyano-∆ 3 -piperideines (CdC−C(CN)(NR2))
Carbanions 633 produced by deprotonation of 2-cyano-∆
3 -piperideines give regioselective R-products
634 333 with D2O, methyl iodide, benzyl, and isopropyl
bromide and the highly hindered tert-butyl chloride.
However, with bulkier molecules, e.g., pivaloyl chloride,
C4-substituted product 635 is afforded (Scheme
116). 334 The acyclic aminonitriles of Ahlbrecht undergo
γ-alkylation to give 627 (Scheme 114). 332 However
633 cannot form a metallocycle with Li situated
at the C4-position, a situation which is possible for
acylic analogues.
Scheme 116
C. Oxygen with Sulfur or Nitrogen
1. 1-Alkoxy-1-(alkylthio)-2-propenes (CdC−C(OR)(SR))
R-Products are generated in the alkylation reaction
of 1-methoxy-1-(alkylthio)allyl sulfide anions. 335,336
Hence, oxathio-substituted crotyllithium 637 reacts
with alkyl halides predominantly at the R-site to give
639, whereas aldehydes and ketones undergo nondiastereoselective
reaction at the γ-terminus, giving
641 via a six-membered transition state 638 where
lithium at the R-site coordinates the carbonyl oxygen
leading to the γ-product 640 which readily cyclizes
to spiro compound 641 (Scheme 117). 337 The comparison
of the orientation of electrophilic attack of
the oxythio-substituted reagents with those at the
analogous monosubstituent derivatives (alkoxy allyl
anion 642ssee also section II.B and lithiumallyl
sulfide 643ssee also section II.C) (Scheme 117)
shows a similarity to the latter (S) and suggests that
stabilization by sulfur is more important for the
prediction of the site of electrophilic attack than the
oxygen.
2. N-(R-Ethoxyallyl)-1-benzotriazole and -1-triazole
(CdC−C(OR)(NR2))
The oxygen- and nitrogen-substituted allyl anion
645 obtained upon deprotonation of N-(R-ethoxyallyl)benzotriazole
644 undergoes exclusive R-product
646 formation with alkyl halides, aldehydes, most
ketones, and R,�-unsaturated esters while exclusive
γ-attack to form 647 is observed with sterically
hindered electrophiles, e.g., bulky ketones such as
2,4-dimethyl-3-pentanone. 647 was hydrolyzed to
give γ-lactone 648 (Scheme 118). 338-343 This work has
recently been reviewed in detail. 344
Scheme 118