We have developed palladium(0)/monophosphine-catalyzed
trans-selective alkylative cyclization reactions of alkyne- and allene-aldehydes with organoboron reagents leading to 3-substituted 2-cycloalken-1-ols and 3-cycloalken-1-ols, respectively. Three-component reaction involving secondary aliphatic amines as the third component affords the corresponding tertiary amines via
in situ generated iminium ions. These cyclization reactions allow a combinatorial synthesis of biologically important indenes bearing three different substituents at 1,2,3-positions from available
o-ethynylbenzaldehyde derivatives. 6-
Endo-trig cyclizations of alkynyl- and allenyl-iminium ions,
in situ prepared from 3-butynylamine and 2,3-butadienylamine with formaldehyde, afford 1,4-disubstituted 1,2,3,6-tetrahydropyridines. The remarkable
trans-selectivity of these processes would result from the novel reaction mechanism involving “
anti-Wacker”-type oxidative addition. Changes of the cyclization mode when enones were employed as electrophiles suggest that low tendency of palladium(0) catalyst to form π-complex with carbonyls and iminium ions cause the
trans-selectivity.
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