Enantiocontrolled Construction of Tricyclic Furan Derivatives via an Asymmetric Diels−Alder Reaction

Studies toward the total syntheses of cucurbitacins B and D

Synthetic efforts toward the convergent construction of the tetracyclic triterpenoids cucurbitacins B and D are described. The results of a Diels-Alder study examining the effects of steric and electronic variations of 2-methyl-2-cyclohexenone on the endo/exo-diastereoselectivity of the reaction are presented. The diastereomer of the core of the cucurbitacins, epimeric at C8, C9, and C10, 51, was synthesized via a highly regio- and stereoselective Diels-Alder reaction of the diene 4 and the novel dienophile 50.

First Comprehensive Investigation of Suzuki Couplings of Alkenyl Nonaflates with Aryl and Alkenyl Boronic Acid Derivatives by Using Classical Conditions and Microwave Heating

Alkenyl nonaflates (nonafluorobutanesulfonates) are excellent substrates in a variety of palladium-catalysed coupling reactions. We herein demonstrate that bicyclic nonaflates generated from 8-heterobicyclo[3.2.1]octan-3-one derivatives can be coupled with aryl or alkenyl boronic acids in a very efficient manner. The resulting densely functionalised bicyclic skeletons are highly suitable for further synthetic elaboration. The thermal Suzuki couplings provided the expected products in moderate to good yields. Microwave (MW) irradiation dramatically shortened reaction times and gave superior results. Bisboronic ester 19 was also coupled with bicyclic nonaflates, for example, with 14, and double Suzuki-coupling products, such as 22, were isolated in good yields. We demonstrated the great synthetic potential of aryl-substituted 8-heterobicyclo[3.2.1]octene derivatives, such as 15, by the stereoselective conversion of this compound into highly substituted furanose 31 or substituted pyran derivative 33, which were obtained in short and efficient reaction sequences.

A facile synthesis of enantiopure tricyclic furanyl and pyranyl derivatives via tungsten-mediated cycloalkenation and Diels-Alder reaction

We report the synthesis of chiral furanyl and pyranyl dienes 1 and 2 based on cycloalkenation of chiral tungsten alkynol complexes. These two dienes bear a chiral 1,3-dioxolane group to control diastereoselective Diels-Alder reactions with electron-deficient olefins. The chiral 1,3-dioxolane substituents of the cycloadducts were degraded into hydrogen atoms to make these molecules possess common furan and pyran rings. Dienes 1 and 2 are good building blocks for enantiopure forms of tricyclic oxygen compounds.