Lewis acid and photochemically mediated cyclization of olefinic .beta.-keto esters

Abstract

Olefinic ß-keto esters 8, 17, and 20, prepared by either carbomethoxylation of the parent ketone or alkylation of the dianion of methyl acetoacetate with the allylic bromide, underwent cyclization with stannic chloride in dichloromethane to give mono-and bicyclic keto esters 10 and 18, respectively. The latter was transformed via ketone 24 to A8(u)-podocarpen-13-one ( 27) and to keto acid 29, an intermediate in the synthesis of ambreinolide. Keto ester 46, prepared by condensation of the dianion of methyl acetoacetate with ß-cyclocitral, gave 56 as the major product upon irradiation in neutral solution but, in the presence of sodium methoxide, a mixture (ca. 1:1) of 45 and 65 was produced in 66% yield. Enol derivatives 52, 53, and 54 of 46 underwent 1,5-hydrogen migration upon photolysis to give 55, 58, and 59. The dark transformation of 56 to 47 with triethylamine revealed a pathway to this pyranoid structure unrecognized in previous photochemical studies of ß-ionone, and the conversion of 47 to 45 and 65 upon irradiation in the presence of sodium methoxide suggests a mechanistic hypothesis for these light-induced reactions involving a manifold of equilibrating enolates which terminate at 45.

Synthesis and Absolute Configuration of Zonarol. A Fungitoxic Hydroquinone from the Brown Seaweed Dictyoptfris Zonarioides(1)

“…The precursor to 6 was a known keto ester ( + ) -5 , which was prepared by cyclization of an acyclic keto ester 4 ( 6 ) . The keto ester 4 was previously synthesized from geranyl bromide and methyl acetoacetate by White, & z d ( 6 ) . …”

Synthesis of Racemic β-Chamigrene, a Spiro[5.5]undecane Sesquiterpene

“…It was anticipated that the spiroketone 2 would be formed either by a direct Diels-Alder reaction on the alkene moiety of 4 or by a Claisen rearrangement of the bicyclic compound 3 , derived from the cyclohexanone 4 and isoprene in a hetero Diels-Alder reaction. Compound 4 has been prepared from the ketoester 5 by Adams et al [6] Following a literature procedure [7] 5 was prepared from the commercially available ketone 6 in 60% yield. According to Adams, both ester and keto functions were reduced to the corresponding diol and subsequently the secondary hydroxyl group selectively protected as the tosylate.…”

A biomimetic approach towards phorone sesterterpenoids

“…The synthesis of a drimane aldehyde such as 14 commenced by treatment of commercially available β-ionone with NaH in the presence of dimethylcarbonate to give the ester 15, which could be accessed in >20 g. Photochemically-mediated cyclisation of the ester 15 gave enone 16 (Scheme 3). 10 Enone 16 was then readily converted to the diol 18 in three steps by reduction with LiAlH 4 , allylic alcohol oxidation and then nucleophilic addition using an excess MeLi. 9 Diol 18 was then oxidised with m-CPBA to give the epoxide 19 as a single diastereomer.…”