Diketones double ring closure

Potassium cyanide Lactamols from diketones Stereospecific double ring closure... 

A simple preparation of indoles from anilines is based on a specific o-alkylation of ar. amines with sulfides in the presence of an active chlorine source such as tert-hwtyl hypochlorite . A lactamol has been obtained by double ring closure of a diketone in one operation combining 4 simple steps, all of which occur stereospecifically in high yield in the same medium . Another multiple reaction is the stereospecific rearrangement of an epoxytetrahydrofuranolactone in the course of synthetic studies on tetrodotoxin. A new riboflavin synthesis via its 5-oxide has been reported . 

Aldol reactions are often used to close five- and six-membered rings. Because of the favorable entropy (p. 211), such ring closures generally take place with ease, even where a ketone condenses with a ketone. An important example is the Robinson annulation reaction which has often been used in the synthesis of steroids and terpenes. In this reaction a cyclic ketone is converted to another cyclic ketone, with one additional six-membered ring containing a double bond. The substrate is treated with methyl vinyl ketone (or a simple derivative of methyl vinyl ketone) and a base.551 The enolate ion of the substrate adds to the methyl vinyl ketone in a Michael reaction (5-17) to give a diketone that undergoes or... 

The Robinson annulation has three distinct steps the Michael addition of the enol or enolate across the double bond of the a,(3-unsaturated ketone to produce a 1,5-diketone (Michael adduct), followed by an intramolecular aldol reaction, which affords a cyclic (3-hydroxy ketone (keto alcohol), and finally a base-catalyzed dehydration which gives rise to the substituted cyclohexenone. An alternative mechanism via disrotatory electrocyclic ring closure is possible. ... 

N-Heterocyclic jS-enamino esters which are unsubstituted in the 1-position are well suited for a double condensation reaction when treated with 1,3-dicarbonyl compounds to give annelation of a [1,2-a] pyrimidine ring. Thus, the condensation of 2-aminopyrroline affords pyrrolo[l,2-a]-pyrimidines.82,114 Shvedov et al. treated 2-amino-3-pyrrolecarbonitrile 187 with / -diketones or ethyl acetoacetate74115 to give 188. No ring closure... 

Martel and co-workers 52), by a series of condensation reactions, converted XCIX to C. Pormylation of the double bond, condensation of the resulting unsaturated aldehyde with methyl cyanoacetate, followed by catalytic hydrogenation, saponification, decarboxylation, and reesterification provided Cl. Desacetamidocolchicine (LXII), identical with an authentic specimen, was produced from Cl by ring closure to CII, followed by benzoyl peroxide treatment to give CIII, alkaline hydrolysis to the enolic a-diketone, and subsequent iV-bromosuccinimide treatment. 

---