Crotonic condensation intramolecular

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). 

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 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... 

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