TiCl(4)-promoted Baylis-Hillman reactions of substituted 5-isoxazolecarboxaldehydes with cycloalkenones).
The Baylis-Hillman (BH) reaction of substituted 5-isoxazolecarboxaldehydes with cyclohexenone in the presence of TiCl(4) invariably lead to the formation of hemiacetals beside the BH adducts. A similar reaction in the presence of DABCO, DBU, or 3-HQN yielded minor quantities of phenols in addition to the usual BH adducts. Similar to 5-isoxazolecarboxaldehydes, the TiCl(4)-mediated BH reaction of cyclohexenone with various benzaldehydes also furnishes hemiacetals in considerable yields. The reaction mechanism involving the formation of alpha-chloromethyl enone as an intermediate has been proposed. The synthesis of hemiacetals 5 and 14 from compound 4 in the presence of cyclohexenone and cyclopentenone, respectively, under acidic conditions indicates that enolization and aromatization of the cyclohexene ring are the key steps in the reaction mechanism.
Patra
Arundhati
A
Medicinal ChemistryDivision, Central Drug Research Institute, Lucknow 226 001, India.
Batra
Sanjay
S
Joshi
Bhawani S
BS
Roy
Raja
R
Kundu
Bijoy
B
Bhaduri
Amiya P
AP