title: Stereospecific Synthesis of 1,6-Dioxadecalins and 2,2'-Linked Ditetrahydrofurans by Rearrangement of Steroidal Spiroacetals.
abstract: The reduction of steroidal spiroacetal methanesulfonate derivatives, containing the 1,6-dioxaspiro[4.5]decan-10-yl ring system, with DIBALH promotes a new rearrangement to give steroidal 1,6-dioxadecalin (octahydropyrano[3,2-b]pyran) or 2,2'-linked ditetrahydrofuran (octahydro[2,2']bifuranyl) derivatives. To study the scope and selectivity of the reaction, several steroidal spiroacetals such as (23R,25R)-3beta-methoxy-5alpha-spirostan-23-yl methanesulfonate (2) and its 23S-isomer (5) and (22R,23R,25R)-3beta-acetoxy-16beta,23:23,26-diepoxycholest-5-en-22-yl methanesulfonate (15) and its 22S-isomer (19) have been synthesized. Compound 2 was rearranged with absolute regio- and stereoselectivity to give (22S,23S,25R)-3beta-methoxy-16beta,23:22,26-diepoxy-5alpha-cholestane (3) which possesses a cis-fused 1,6-dioxadecalin ring system. The reaction of compound 5 gave exclusively (22S,23R,25R)-3beta-methoxy-23,26-epoxy-5alpha-furostane (6) in which the spiroacetal was converted into a ditetrahydrofuran subunit. The two other isomeric spiroacetals 15 and 19 were also mainly transformed into (22S,23S,25R)-3beta-acetoxy-16beta,23:22,26-diepoxycholest-5-ene (16) and (22R,23R,25R)-3beta-acetoxy-23,26-epoxyfurost-5-ene (20), respectively. A mechanism is proposed to explain the regio- and sterospecificity observed in the rearrangement.
authors.affiliation: Instituto de Productos Naturales y Agrobiología del C.S.I.C., Carretera de La Esperanza 3, 38206-La Laguna, Tenerife, Spain, Instituto de Bio-Orgánica, Universidad de La Laguna, Tenerife, Spain, and LURE, Université Paris-Sud, Paris, 91405 ORSAY, Cedex, France.
authors.forename: Carmen
authors.initials: C
authors.lastname: Betancor
authors.forename: Rosa L.
authors.initials: RL
authors.lastname: Dorta
authors.forename: Raimundo
authors.initials: R
authors.lastname: Freire
authors.forename: Angeles
authors.initials: A
authors.lastname: Martín
authors.forename: Thierry
authors.initials: T
authors.lastname: Prangé
authors.forename: Ernesto
authors.initials: E
authors.lastname: Suárez