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

Scheme 58
steric bulk of the carbonyl compound (highest for
benzaldehyde). 185
Secondary allyl amides 320 undergo dilithiation to
form N-lithio-R′-lithioallyl amides 321 which react
with alkyl iodides and H2O to give enamides 319 (γproduct,
Scheme 57). 186
Lithiated N-alkyl-N-allyl amides 323 react with
electrophiles (RX, RCHO, Me3SiCl, etc.) to give
mainly γ-products along with minor amounts of
R-products 325 (Scheme 57). 187 In the ∆ 3 -piperidinyl
amide 326 case, both R- 328 and γ-products 329 are
obtained, in ratios which depend on the electrophile.
A highly diastereoselective homoaldol reaction is
carried out with chiral N-allylureas 330 by lithiation
and then transmetalation with a Ti derivative and
reaction with carbonyl compounds resulting in γ-products
331. 188 With Li + as the counterion, R- and
γ-products are afforded with alkyl halides, aldehydes,
and ketones. Replacement of Li + by Mg 2+ ,Zn 2+ ,or
Cd 2+ normally leads to R-attack; however, on transmetalation
with Mg 2+ the γ-products are obtained to
give (Z)-enamides 332 (Scheme 58). 175
10. Pyrroline and Piperidine (Tetrahydropyridine)
Formamidines
The anion from tetrahydropyridine 333 is attacked
at the γ-carbon by electrophiles to give 334 (Scheme
59). 189,192,193 Similarly, the anion 337 of the tetrahydropyridine
amidine 335 forms predominantly γ-adducts
338 (Scheme 60). 189-191
However, R-products, e.g., 336 are synthesized in
the reaction of tetrahydropyridine formamidine 335
with 2,6-dimethylphenyl isocyanate (Scheme 60). 189,190
Similarly, Meyers 194 reported that the valine-based
chiral formamidine 339 gave upon alkylation predominantly
the R-product 340, in contrast to the
formation of 338 upon reaction with the tert-butylformamidine
of tetrahydropyridine. Pyrroline formamidines
341 195 and those derived from octahydroiso-
Scheme 59
Scheme 60
quinoline (cf. 343) 196 also give predominantly the
R-attack products 342 and 344, respectively (Scheme
60).
F. Phosphorus
Reflecting their different oxidation states, phosphonates
and phosphine oxides are discussed separately.
A further section deals with phosphonamides
which could alternatively have been considered with
the phosphinoxides. One study of allylic phosphines
is covered.
1. Allylphosphonates (CdC−C−PO(OR)2)
683
Allylphosphonates are used for the preparation of
dienes 197 and polyenes 198 via olefination with carbonyl
compounds. Most of the allylic phosphonates described
are stabilized by additional functionality in
the γ-position (see sections III and IV).
Allylphosphonates 346 are prepared by Arbuzov
phosphorylation from the corresponding allyl bromides
345. The reactions of lithiated 346 with
aromatic and aliphatic aldehydes 199 result predominantly
in R-threo-products 347 except for a few
examples in which mixtures of R- and γ-adducts (e.g.,
R 3 ) 4-Cl-C6H4;R 1 ) R 2 ) H) or γ-adducts (R 3 ) 2-Cl-
C6H4, 4-NO2-C6H4; R 1 ) R 2 ) H) are obtained. The
products 347 can be converted stereospecifically into
dienes 350 using DCC (dicyclohexylcarbodiimide)
activated by copper(II) as dehydrating agent (Scheme