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

Scheme 10
Scheme 11
predominantly at the oxygen to give, for example, 59
(Scheme 10). 50-52 However, Still and Macdonald 35
suggest that the alkoxy silane is the major anionic
species in solution at low temperatures where alkylation
results in the formation of only C-alkylated
products furnishing predominantly γ-attack 61 with
primary halides. 35 In general, the analogous reaction
with aldehydes or ketones gives predominantly R-products
for example 62 53 (Scheme 10). Siloxyallylbarium
compounds 63, obtained by reaction of the corresponding
lithium derivatives with BaI2, react with
carbonyl compounds and alkyl halides to give exclusively
the (Z)-γ-products 64 54 (Scheme 10)
The reaction of deprotonated 2-[1-(triisopropylsiloxy)allyl]-N-methoxymethylimidazole
65 with ketones
and aldehydes yields regioselectively the γ-product
66 (enoyl silyl ethers of 2-acylimidazoles) which
eventually furnish γ-lactones 67 (Scheme 11). 55
3. 3-[(Trialkylsilyl)oxy]-1,4-pentadiene
Lithiated 3-(trialkylsilyloxy)-1,4-pentadiene 70
shows a γ-selective reaction to 68 with carbonyl
compounds, whereas a mixture of R- 72 (minor) and
γ-adduct 71 is obtained upon alkylation (Scheme
12). 56 Products of γ-reaction 71 are often preferred
with primary and alkenyl halides, but the nature of
the leaving group is important. Thus, dominant
R-substitution to 69 is observed with tosyl and triflate
as leaving groups, whereas additives such as HMPA
or TMEDA and transmetalation by zinc or potassium
do not significantly change the γ/R ratio in the
alkylation of the anion. 57
γ-Selective sulfenylation to give 73 followed by
further deprotonation to 74 and alkylation, which
now is directed by the methylthio substituent, gives
Scheme 12
entirely γ-product 76 with respect to the silyloxy
group (see also section V.B.3). 58
4. 2-Alkenyl Carbamates (CdC−C−O−CO−NR2)
671
Anions of 2-alkenyl carbamates 77 undergo regioselective
γ-reactions with carbonyl compounds to
afford γ-hydroxyenol carbamates 79 (Scheme 13)
which can be converted into lactols and oxidized on
to lactones. 59,60 The γ-selectivity is enhanced with
increasing γ-substitution, decreasing R-substitution,
or decreasing reactivity of the carbonyl compounds.
At the same time the number of alkyl groups should
not exceed three, as the kinetic acidity of the 1,3,3trimethyl
derivative is already too low to be lithiated.
61 The regiochemistry of alkylation and silylation
is dependent on the position of the alkyl groups in
the allylic system. A six-membered transition structure
78 is postulated, 59 in which the lithium cation
is held at the R-carbon atom by the oxygen of the
carbamate. Crotyl carbamates are also used for
diastereoselective homoaldol reactions. The anti diastereoselectivity
is improved by application of organotitanium
or organoaluminum reagents. 62,63
(-)-Sparteine complexes 81 of lithiated primary
O-2-alkenyl carbamates 80 undergo carboxylation
with inversion of configuration to produce 39% R- 83
and 30% γ-adduct 84 (Scheme 14). Transmetalation
of 1-lithio-2-butenyl-N,N-diisopropylcarbamate 82
with (TiO i Pr)4 to 85, followed by carboxylation,
proceeds via inversion and the addition of the aldehydes
occurs in an anti SE′ process; 64,65 homoaldol
adducts 87 and the corresponding γ-lactones 88 are
synthesized. Asymmetric synthesis is carried out
using chiral starting materials which are stannylated
and undergo enantioselective homoaldol addition
under the influence of TiCl4. 66 There are other
examples for asymmetric synthesis leading to ho-