BAYLIS - HILLMAN Vinyl alkylation

BAYLIS - HILLMAN Vinyl alkylation Amine catalyzed converelon of acrylates to a-(hydroxyalkyl) acrylates or of vinyl ketones to a-(hydroxyalkyO vinyl ketones. 

Fluorochromenes result from the reaction of the Cs salts of salicylaldehydes with the phosphonium triflate 37 (Scheme 24) <0OJCS(Pl)103> and 3-acylchromenes are formed from salicylaldehydes and alkyl vinyl ketones in a chemoselective Baylis-Hillman reaction (Scheme 25) . 

Hydroxybenzaldehydes react with alkyl vinyl ketones in the presence of l,4-diazabicyclo[2.2.2]octane (DABCO) to yield 3-acyl-277-chromenes (Scheme 19) <2000J(P 1)1331 >. The reaction proceeds via a Baylis-Hillman type pathway to form the zwitterionic intermediate 61 with subsequent cyclization and dehydration to afford 2-acyl-2H-chromenes (Scheme 19) <20030BC1133>. 

Portnoy and Goren immobilized M-alkylated imidazoles on polystyrene-bound polyether dendrons via three different synthetic routes (Scheme 15.43). All these systems catalyzed the Baylis-Hillman addition of methyl vinyl ketones to aromatic aldehydes, displaying a very strong positive dendritic effect and a significant enhancement effect of water as a cosolvent. Remarkably, substrates that did not undergo the reaction with the nondendritic immobilized or soluble M-alkyl imidazole catalysts underwent a smooth reaction with one of the catalytic second-generation dendrons. 

Chalcogen-based catalysts for the Baylis-Hillman reaction have been further developed. 2,6-Diphenyl-4//-chalcogenopyran-4-(thi)ones (61 X = S, Se Y = O, S) catalyse the addition of an appropriate alkene to an aldehyde in the presence of titanium tetrachloride in dichloromethane. The first step is proposed to involve Y-(rather than X-)alkylation, driven by aromatization. The resultant zwitterion (62) then attacks the aldehyde. Although (62) has not been directly observed, the reaction of methyl vinyl ketone with the selenopyranone catalyst (61 X = Se, Y = O) shows NMR evidence of an D-alkylation product and of a titanium alkoxide (63 Z = CH2CH2COMe and TiCb, respectively). 

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