Effect of 9,10-cyclic acetal stereochemistry on feasible operation of the alpha-ketol rearrangement in highly functionalized paclitaxel (Taxol) precursors

Total Synthesis of (-)-Mitrephorone A Enabled by Stereoselective Nitrile Oxide Cycloaddition and Tetrasubstituted Olefin Synthesis

A highly enantioselective and diastereoselective total synthesis of the diterpenoid (−)-mitrephorone A is presented. Key to the synthesis are stereocontrolled 1,4-semihydrogenation of a 1,3-diene to a tetrasubstituted double bond, enzyme-catalyzed malonate desymmetrization, and highly diastereoselective nitrile oxide cycloaddition. The streamlined strategy is a considerable improvement to those reported earlier in terms of diastereo- and enantioselectivity. For the first time, the combination of modern Pd-cross-coupling with Cr-catalyzed reduction allows for rapid access to tetrasubstituted olefins with full stereocontrol.

Diastereoselective and regioselective synthesis of adenosine thionucleoside analogues using an acyclic approach

An acyclic approach to synthesize thiofuranoside N-glycosides bearing an adenine nucleobase is presented herein. This approach provides a significant improvement in terms of regio- and diastereoselectivity compared with the current paradigms used for their formation. Activation of acyclic dithioacetal substrates bearing either a C2′-alkoxy or fluoro group and coupling with purine nucleobases selectively generates 1′,2′-syn thioaminals. The regiochemistry of the nucleobase coupling (N7 or N9) can also be controlled by using either silylated or unsilylated purines. A subsequent SN2-like cyclization of these 1′,2′-syn acyclic thioaminals results in the desired 1′,2′-cis thionucleoside analogues.

Asymmetric total syntheses of naturally occurring α,β-enone-containing RALs, L-783290 and L-783277 through intramolecular base-mediated macrolactonization reaction

Asymmetric total synthesis of two naturally occurring α,β-enone containing RALs, L-783290 and L-783277 is described in this article. An E-selective Horner-Wadsworth-Emmons (HWE) olefination was used as a key reaction to construct the C7'-C8' olefinic unsaturation in L-783290. An enantiopure alkyne addition to the aldehyde followed by Z-selective partial reduction was employed to construct the C7'-C8' olefinic unsaturation in L-783277. Biomimetic lactonization reaction was used to construct the macrolactone core in both the target molecules.

Catalytic asymmetric total synthesis of (+)-artalbic acid

The first total synthesis of (+)-artalbic acid has been accomplished using asymmetric allylation of an acetoacetate derivative with a phase-transfer catalyst. This synthetic work was completed in 12 steps from isopropyl acetoacetate with high stereocontrol. In addition, the absolute configuration of naturally occurring artalbic acid was determined to be 7S, 9S, and 10S.

Selective oxygenation of alkynes: a direct approach to diketones and vinyl acetate

Convenient and expedient methods for the synthesis of α-diketones and β-haloenol acetates from arylalkynes using PhI(OAc)₂ as an oxidant are developed at room temperature.

A stereoselective approach to β-L-arabino nucleoside analogues: synthesis and cyclization of acyclic 1',2'-syn N,O-acetals

Reported herein is a novel and versatile strategy for the stereoselective synthesis of unnatural β-L-arabinofuranosyl nucleoside analogues from acyclic N,OTMS-acetals bearing pyrimidine and purine bases. These unusual acetals undergo a C1' to C4' cyclization where the OTMS of the acetal serves as the nucleophile to generate 2'-oxynucleosides with complete retention of configuration at the C1' acetal center. N,OTMS-acetals are obtained diastereoselectively from additions of silylated nucleobases onto acyclic polyalkoxyaldehydes in the presence of MgBr(2)·OEt(2). The strategy reported is addressing important synthetic challenges by providing stereoselective access to unnatural L-nucleosides starting from easily accessible pools of D-sugars and, as importantly, by allowing the formation of the sterically challenging 1',2'-cis nucleosides. A wide variety of nucleoside analogues were synthesized in 7-8 steps from easily accessible D-xylose.

Electron ionization mass spectral studies of bridgehead-substituted norbornan-2-ones: camphor derivatives

The electron ionization (EI) mass spectra of a series of bridgehead-substituted 3,3-dimethylnorbornan-2-ones, derived from natural (1R)-(+)-camphor, have been studied and their cleavage mechanisms rationalized on the basis of the substituent shifts as well as on the identification of relevant peaks through accurate mass measurements and collision-induced dissociation (CID) tandem mass spectrometric experiments. The fragmentation patterns are very dependent on both the structural nature and the electronic properties of the bridgehead substituent. The driving force for the main fragmentation pathways are competitive cleavages of the C(1)-C(2) and C(2)-C(3) bonds directed by the bridgehead substituent and either the gem-dimethyl or carbonyl groups. These cleavages lead to distonic ions in which the charge is preferentially located either at the C(1), C(2) or C(3) positions depending on the electronic character and structural nature of the bridgehead substituent. This charge distribution determines the subsequent rearrangements and fragmentations.

Total synthesis of TAK-kinase inhibitor LL-Z1640-2 via consecutive macrocyclization and transannular aromatization

The biomimetic total synthesis of LL-Z1640-2 (3) is reported without the use of phenol protection. The aromatic unit was constructed via the transannular aromatization of macrocyclic triketo-ester 2, which in turn was synthesized by macrolactonization using an intramolecular trapping of a triketo-ketene derived from dioxinone 1.

Diosphenol-Based Approach to the A-Ring Functionalization of Advanced Taxol Precursors

Several different approaches to the A-ring functionalization of an advanced, highly functionalized diosphenol precursor to Taxol are described. The first phase of the undertaking consists of an assessment of those reagents conducive to reaction at the enolic oxygen (silylation, methylation, allylation, and acylation). Transformations involving an alternative attack at the enol carbon center (bromination, selenation) have also been defined. Sodium borohydride reduction operates from the beta-face of C-14 as long as the C-1 hydroxyl is not protected so as to offer steric exclusion. Complications associated with various aspects of these methodological undertakings are addressed. The most advanced oxygenation achievements were realized by way of a noteworthy sequence involving epoxidation of the O-methyl ether, methanolysis under mildly acidic conditions, and regioselective oxidation of diol 38 to give 39.

Oxidative cleavage of p-methoxybenzyl ethers with methyl(trifluoromethyl)dioxirane

[reaction: see text] The ability of methyl(trifluoromethyl)dioxirane to cleave p-methoxylbenzyl ethers oxidatively in the presence of various additional functional groups has been investigated. These reactions, performed in aqueous acetonitrile, transform a reasonably robust aryl substituent into a dienyl aldehydo ester. The originally generated E,Z-isomer undergoes slow conversion to the more stable E,E-form at 20 degrees C.

Concise means for accessing an advanced precursor to 1-deoxypaclitaxel

A program directed toward a total synthesis of 1-deoxypaclitaxel is described. The most direct route consists of six steps from the previously described diketone 12 and proceeds in 18% overall yield. The transformations involved in reaching the target molecule 22 consist of stereoselective alpha-ketol generation, an EtAlCl(2)-catalyzed transannular hydride shift, regioselective monomesylation, and a Wagner-Meerwein 1,2-shift. The central issue of this synthesis is the sequential deployment of these highly controlled steps along the perimeter and across the interior gap of a nine-membered ring.

Second-Generation, Highly Abbreviated Route for Elaboration of the Oxetane D-Ring in a Fully Functionalized Taxane

A six-step conversion of oxirane 3 to oxetane 9 is reported. The synthetic route takes particular advantage of the acid-catalyzed ring opening of 3 to allyl alcohol 4 in a polar reaction medium and of the heightened capability of the OsO4.TMEDA complex to effect the efficient stereocontrolled dihydroxylation of this intermediate. The overall yield of the new sequence is 33%.

A delicate balance of energetics. Subtleties associated with alpha-ketol-based bridge migration to afford 9-keto-10beta-p-methoxybenzyloxytaxanes

The feasibility of the title reaction has been pursued for the purpose of advancing a concise total synthesis of Taxol. Of the two closely related series examined, the first featured an exo-methylene group at C4. The second consisted of an alpha-epoxide at that site. Strikingly, the olefinic construct proved inert to attempted alpha-ketol rearrangement. In contrast, the oxiranyl derivative isomerized smoothly. The reaction sequence associated with arrival at taxane 18 is short (15 steps from a D-camphor derivative) and notably efficient. The thermodynamic issues that are raised by this investigation have been clarified by an assessment of molecular mechanics-derived (MM3) steric energy calculations.

Chemical modification of a highly functionalized taxane. The consequences of an absent bridgehead double bond on oxetane D-ring construction

An oxetane D-ring has been fused to the framework of the highly functionalized taxane 2. The synthetic route is based on a trimethylsilyl triflate-promoted epoxide-opening step, followed by stereocontrolled, regioreversed oxirane formation and reductive transposition of this intermediate with bis(cyclopentadienyl)titanium(III) chloride. This last key step provides for the convenient implementation of additional hydroxyl groups ultimately conducive to intramolecular S(N)2 reaction. Tangential features of the route outlined herein include specific rearrangement reactions and a retro-aldol cleavage of ring A.