José F. Quílez del Moral

Weakening C−O Bonds: Ti(III), a New Reagent for Alcohol Deoxygenation and Carbonyl Coupling Olefination

Investigations detailed herein, including density functional theory (DFT) calculations, demonstrate that the formation of either alkoxy- or hydroxy-Ti(III) complexes considerably decreases the energy of activation for C-O bond homolysis. As a consequence of this observation, we described two new synthetic applications of Nugents reagent in organic chemistry. The first of these applications is an one-step methodology for deoxygenation-reduction of alcohols, including benzylic and allylic alcohols and 1,2-dihydroxy compounds. Additionally, we have also proved that Ti(III) is capable of mediating carbonyl coupling-olefination. In this sense, and despite the fact that for over 35 years it has been widely accepted that either Ti(II) or Ti(0) was the active species in the reductive process of the McMurry reaction, the mechanistic evidence presented proves the involvement of Ti(III) pinacolates in the deoxygenation step of the herein described Nugents reagent-mediated McMurry olefination. This observation sheds some light on probably one of the mechanistically more complex transformations in organic chemistry. Finally, we have also proved that both of these processes can be performed catalytically in Cp(2)TiCl(2) by using trimethylsilyl chloride (TMSCl) as the final oxygen trap.

Enantioselective Total Synthesis of the Potent Anti-inflammatory (+)-Myrrhanol A

The first total synthesis of potent anti-inflammatory polypodanes (+)-myrrhanol A (1), (+)-myrrhanone A (2), (+)-myrrhanone B (3), and (+)-myrrhanol B (4) has been achieved. Key steps in our convergent, highly stereocontrolled route are a Ti(III)-mediated radical cyclization of a chiral monoepoxide to furnish a bicyclic synthon that combines stereospecifically with an acyclic vinyl iodide via an intermolecular B-alkyl Suzuki-Miyaura cross-coupling.

First total synthesis of (-)-achilleol B: reassignment of its relative stereochemistry.

The first total synthesis of (-)-achilleol B was achieved using a convergent approach with a longest linear sequence of 14 steps. Three key steps were employed, including an enantioselective Robinson annelation for the construction of the bicyclic moiety. The monocyclic synthon was prepared through a Ti(III)-mediated cylization of a chiral monoepoxide obtained via asymmetric dihydroxylation of geranylacetone. The asymmetric preparation of these subunits also permitted us to achieve the enantioselective synthesis of elegansidiol, achilleol A, and farnesiferol B.

Sesquiterpenes from Juniperus thurifera L. Stereochemistry in Unusual Cedrane and Duprezianane Series

The analysis of the essential oil from the wood of Juniperus thurifera has provided the isolation of seven new natural products, including the first described examples of 2-epi-cedranes and 2-epi-duprezianane. The co-occurrence in the oil of the three cedrane-type arrangements (cedrane, 1,7-diepi-cedrane and 2-epi-cedrane) has permitted, after a thorough 2D NMR analysis the unequivocal spectro- scopical assignment of each of the three cedrane skeletons, and subsequently the development of an unambiguous method for establishing their stereochemistries. A useful pattern for distinguishing between the two duprezianane stereochemistries is also given. q 2000 Elsevier Science Ltd. All rights reserved.

Total Synthesis of (+)-seco-C-Oleanane via Stepwise Controlled Radical Cascade Cyclization

An asymmetric concise total synthesis of the (+)-seco-C-oleanane 1 was accomplished. The successful route to this natural product involves as the key step a stepwise regio- and stereocontrolled catalytic radical polyene cascade cyclization from preoleanatetraene oxide (16), a process mediated by Cp(2)TiCl. The use of this single-electron-transfer complex permits mild cyclization conditions without using unnecessary prefunctionalizations and stops the process at the bicyclic level. Theoretical data revealed high activation energy for the third ring closure, which would account for the control of the cyclization. This process also led to natural (-)-achilleol B, camelliol A, and (+)-seco-β-amyrin as minor compounds.

Modified steroids: Pb(OAc)4 mediated one-pot multistage transformations of steroidal unsaturated 1,2-diols

Abstract Lead tetraacetate treatment of steroidal 1,2-unsaturated diols provided a convenient route to molecules possessing functionalities that are appropriate for the synthesis of a great variety of skeleton modified steroids, such as A-nor or A-nor-B-homosteroids, which could present interesting biological activities.

Structural diversity from the transannular cyclizations of natural germacrone and epoxy derivatives: a theoretical-experimental study.

Treatment of germacrone (1) with different electrophiles, and of its epoxy derivatives germacrone-4,5-epoxide (2), germacrone-1,10-epoxide (3) and isogermacrone-4,5-epoxide (4) with Brönsted/Lewis acids and Ti(III), gives rise to a great structural diversity. Thus, by using a maximum of two steps, the production of more than 40 compounds corresponding to 14 skeletons is described. Computational calculations rationalizing the structural divergence produced are also described. Finally, since some of the compounds generated are bioactive natural sesquiterpenes, the mechanisms of formation of these substances will provide new insights in their biosynthesis.

Protecting-Group-Free Synthesis of Chokols

As a result of a combined theoretical and experimental study, we describe a two-step protocol for the preparation of an optically pure, multifunctional, cyclopentanic core shared by a number of natural products. This process is based on a hitherto unreported Ti(III)-mediated diastereoselective cyclization in which the hydroxy-directed template effect played by the Ti(III) species was found to be crucial for the stereoselective outcome of the reaction. The viability of this concept was confirmed with the first protecting-group free synthesis of three enantiopure chokols, namely, chokols K, E, and B.

Preparation of bioactive podolactones via a new Pd-catalysed bislactonisation reaction. Synthesis of oidiolactone C

A new route towards podolactone skeleton compounds, including the recently discovered Oidiolactone C, is described, using Pd-catalysed elimination of allylic trifluoroacetates and Pd-catalysed bislactonisation as key steps. # 2000 Elsevier Science Ltd. All rights reserved.

A CONVENIENT PREPARATION OF TAXOID RIGHT-HALF BUILDING BLOCKS

Abstract Wieland–Miescher ketone derived unsaturated diols 4 reacted with Pb(OAc)4 to furnish tricyclic enolether intermediate 6 which upon ozonolysis gave access to useful synthetic intermediates such as bicyclic lactone 7, methyl furanoside 8 and triol 9 (taxoid right-half precursors), depending on the solvent used during the ozonolysis and the nature of the following synthetic operation.