Unprecedented One-Pot Stereoselective Synthesis of Knoevenagel-Type Derivatives­ via in situ Condensation of N-Methyleniminium Salts of Ethylenethiourea­ and Ethyleneurea with Active Methylene Reagents

Abstract

A facile stereoselective synthesis of Knoevenagel-type compounds 8 and 9 was accomplished through a one-pot two-step procedure. The reaction of ethylenethiourea (1) and ethyleneurea (2) with benzoyl chloride-N,N-dimethylformamide complex in N,N-dimethylformamide gave the corresponding isolable ternary N-methyleniminium chlorides 3 and 4, along with benzoic acid (5) as a byproduct. Hydrolysis of salts 3 and 4 yielded the N-formyl derivatives 6 and 7, thus confirming the N-methyleniminium structure. Salts 3 and 4 condensed in situ at 120 °C with a variety of acyclic active methylene reagents Y-CH₂-Z (Y = or ≠ Z), in the presence of 5 (Procedure A) or of 5 and triethylamine (1.5 equiv) (Procedure B), afforded α,β-unsaturated compounds 8 and 9. The overall yields ranged from poor to good. Mechanistic hypotheses concerning the formation of 3 and 4 and the origin of the stereoselectivity are also discussed.

Key words

ternary iminium salts - active methylene reagents - stereoselective synthesis - Knoevenagel reaction

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