Hydroxy coumarins 812 -methylene compounds

Use of the Knoevenagel reaction (67OR(l5)204), in which a benzaldehyde reacts with an activated methylene compound in the presence of an amine, goes some way to overcoming the inherent difficulties of the Perkin synthesis of coumarins (see later). In order to obtain the coumarin rather than the usual cinnamic acid, a 2-hydroxy substituent must be present... 

Two mechanisms have been proposed for the Knoevenagel reaction. In one, the role of the amine is to form an imine or iminium salt (378) which subsequently reacts with the enolate of the active methylene compound. Under normal circumstances elimination of the amine would give the cinnamic acid derivative (379). However, when an o-hydroxy group is present in the aromatic aldehyde intramolecular ring closure to the coumarin can occur. The timing of the various steps may be different from that shown (Scheme 118). 

A basic ionic liquid, l-butyl-3-methyl imidazolium hydroxide, [bmImjOH, was found to catalyze the Knoevenagel condensation of aliphatic aldehydes and ketones with active methylene compounds elSciently in the absence of any organic solvent (Scheme 5.58). Coumarins have been obtained in one step from the reaction of o-hydroxy aldehydes following this procedure. ... 

Nanoparticulate ZnO was used as an efficient catalyst for the synthesis of cou-marins (84) by the reaction of o-hydroxy benzaldehydes (82) and 1,3-dicarbonyl compounds (83) via Knoevenagel condensation under microwaves and thermal conditions (Scheme 9.24) in moderate to excellent yields (Kumar et al. 2011). This protocol differs from the previous methods for the synthesis of coumarins (84) in terms of simplicity and effectiveness. The application of ZnO/MgO in ionic liquid [bmim] [BF4] was carried out successfully for the synthesis of 4//-pyrans (85) and coumarins (88) at ambient temperature via Knoevenagel condensation reaction of aldehydes (8) or 2-hydroxybenzaldehyde derivatives (86) with active methylene compounds (16, 43, 87) (Schemes 9.25 and 9.26) (Valizadeha and Azimib 2011). The method has several advantages in terms of mild reaction conditions, reusability of the catalyst, high yields of the products, and short reaction times. In comparison with methods mentioned in the literature for the synthesis of 4F(-pyrans (85) and coumarins (88), this protocol has better yield and eco-friendly advantages. 

CIC Vanillin, the main component in vanilla flavour is the basic key ingredient for the creamy, sweet character. All other volatile flavouring compounds have been identified only in small traces. Among them 2-methoxy phenol and 2-methoxy-4-vinyl phenol are responsible for the phenolic, smoky odour. 4-Methoxy benzalde-hyde, 3,4-methylene-dioxy-benzaldehyde, methyl benzoate and methyl ciimamate impart the warm, powdery, aromatic floral character. Vitispirane adds a fruity, floral topnote. Natural vanilla extract blends very well with other flavourings and it has been modified in different directions ethyl vanillin is used to increase the sweet, creamy vanillin aspect. Tonka beans and coumarin add a full, dried hay, slightly caramel-like custard aspect, supported by the butter notes of diacetyl and 4-hydroxy-decanolide. 

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