Crotonic condensation

More serious limitations and precautions apply to compounds in which not all three R, R, and R" groups are aromatic. Autocondensation of benzylideneacetone (111) yields an unstable chloroferrate which may be 113 or 115, according to whether a Michael addition to 112 or a crotonic condensation to 114 is first involved. Since compound 113 could readily be prepared from 2,6-dimethyl-4-phenylpyrylium and benzaldehyde, the structure of the reaction product should be easily soluble. Another equivocal product is formed from two moles of benzylideneacetone, but a definite structure (116) results from chalcone and benzylideneacetone. ... 

Catalytic activities of zeolites were also studied for the dehydration of the azeotropic mixture of water and dimethylvinylcarbinol with formation of isoprene (30), for the hydrogenation of ethylene (31), the crotonic condensation of n-butyl aldehyde (32), and many other reactions (33-41)-... 

The reaction is carried out in vapour phase (250°C) using a flow system (see methods section). This procedure turned out to be essential in order to mantain the hydrogen transfer as the main reaction pathway. A batch experiment carried out in an autoclave actually showed a wide range of condensation products besides some saturated ketone [6]. Reactions of ketones over oxide catalysts can lead to a variety of products due inter alia to aldol condensation, intramolecular dehydration and intermolecular disproportionation [16]. However, the presence of a good hydrogen donor such as a secondary alcohol and vapour phase conditions favour the transfer hydrogenation as the major reaction [16,17]. In our reaction conditions, products attributable to crotonic condensations and subsequent 1,4 Michael addition [18] were observed by g.l.c.-m.s. (Table 1). 

In the case of ketoximes and acetylene in an aqueous alkaline medium the following condensation processes leading finally to pyridines are conceivable. (i) Dimerization of ketoximes with elimination of hydroxylamine (analog of crotonic condensation) (Scheme SO), (ii) Acetaldehyde condensation with the oxime of a,/3-un saturated ketone 107. 

Figure 5 Schematic free-energy profile for two modes offormation of 94 from the reaction of 90 with 91 (a) conventional sequence of independent Michael addition-crotonic condensation via formation of covalent 92a (b) a one-pot Robinson annulation without proton quenching of carbanionic intermediate 92...

Beside the Grignard and other C-C bond-forming reactions, a number of functional group transformations may also serve as an entry into allylic systems. Some of them, namely the reduction of a, -unsaturated carbonyl compounds (products of crotonic condensation), halogenation of alkenes at the allylic position with Af-bromosuccinimide (NBS) and epoxide isomerization, are shown in Scheme 2.56. 

An equally high ring-size selectivity is observed in the cyclization of 1,4-diketones such as 282 (Scheme 2.109). Here again an intramolecular crotonic condensation may yield two products, containing either a three- or a five-membered ring. The reaction, however, invariably follows the latter pathway and serves as a reliable method to synthesize cyclopentenone derivatives. A similar reaction for 1,6-diketones, for example 283, ultimately yields acylcyclo-pentenones. The competing pathway leading to the cycloheptenone derivative is virtually blocked. 

The catalytic behaviour of the cations is affected by the zeohte matrix [6], The initial activities and the rates of deactivation of the same metal-forms of different zeohtes, including the most active Cd-forms are different. Crotonic condensation of product acetaldehyde is foUowed by formation of carbonaceous deposits resulting in catalyst deactivation [6], For some catalysts, such as Cd-LTA, Cd-X,FAU, Cd-Y,FAU fast deactivation does not permit to determine the initial activities and to characterize unambiguously the specific activity of Cd ions in different zeohte matrices. It was found [7] that at low temperatures, e.g., at about 298 K, the acetaldehyde and the secondary products formed in the reaction of zeohtic water of a... 

The methods of cyclization with the participation of nonaromatic carbonyl and dicarbonyl compounds that are widely used in total steroid synthesis are not very diverse they comprise only the intramolecular alkylation of ketones and crotonic condensation. 

Bond. This group of syntheses takes place by three main routes, the difference between which depends on the lype of reactions leading to the closure of ring C. This ring can be formed by an intramolecular crotonic condensation of diketones (Schemes 24 and 25), by the cyclization of aromatic acids, (Schemes 26-28), or, finally, by diene condensations of tricyclic ABD dienes with acyclic dienophiles (Schemes 29 and 30). 

A preliminary communication of the first synthesis by Wettstein s group (Scheme 66) was published in 1955 [131]. By this method, the atom is first introduced by etho grcarbonylation and then the C20, and C21 atoms by alkylation with methallyl iodide, and, finally, the Cg and Cxg atoms with ethoxyethnylmagnesium bromide. Ring D is formed by the intramolecular crotonic condensation of the 16,20-dicarbonyl compounds. 

In this section we must also mention the syntheses of cyclopentano-phenanthrene derivatives described in Scheme 88. These include, in the first place, attempts to apply the Robinson synthesis (Chapter II, Schemes 24 and 25) to the preparation of tetracyclic products with an angular methyl group at CiQ. By this method, for example, from the acetylmethylcyclo-hexene (63) and 1,5-decalindione (4) it was expected to obtain the diketone (65) through a sequence of Michael addition reactions and intramolecular crotonic condensation. However, it was found that the presence of the ce-methyl group in the acetylcyclene (63) suppressed the Michael reaction and... 

The introduction of ring A was effected by crotonic condensation of the aldehyde (92) with 4-hydroxycyclohexanone, which led to a mixture of equal amounts of (96) and (97), these being separated by chromatography on alumina. The epimer (97) corresponding to the natural series was converted by the Wittig reaction into the corresponding methylene derivative (98), the isomerization of which on irradiation with UV light led to vitamin D3 (99) ... 

A. Dimerization of two molecules of ketoxime with abstraction of hydroxyl-amine (analog of crotonic condensation) to afford oxime of a,p-unsaturated ketoxime (Scheme 1.152)... 

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