Rate Dependence on Inductive and Resonance Effects for the Organocatalyzed Enantioselective Conjugate Addition of Alkenyl and Alkynyl Boronic Acids to ß-Indolyl Enones and ß-Pyrrolyl Enones.

Two key factors bear on reaction rates for the conjugate addition of alkenyl boronic acids to heteroaryl-appended enones: the proximity of inductively electron-withdrawing heteroatoms to the site of bond formation and the resonance contribution of available heteroatom lone pairs to stabilize the developing positive charge at the enone ß-position. For the former, the closer the heteroatom is to the enone ß-carbon, the faster the reaction. For the latter, greater resonance stabilization of the benzylic cationic charge accelerates the reaction. Thus, reaction rates are increased by the closer proximity of inductive electron-withdrawing elements, but if resonance effects are involved, then increased rates are observed with electron-donating ability. Evidence for these trends in isomeric substrates is presented, and the application of these insights has allowed for reaction conditions that provide improved reactivity with previously problematic substrates.

Synthesis of 2-D-L-Tryptophan by Sequential Ir-Catalyzed Reactions.

Herein, we report a practical synthesis of 2-D-L-tryptophan via sequential Ir-catalyzed C-H borylation, and Ir-catalyzed C-2-deborylative deuteration steps. In this synthetic sequence, deprotection of the Boc and methyl ester groups proved challenging, due to replacement of deuterium with hydrogen. However, mild deprotection conditions were developed to avoid this D/H scrambling. Further, 2-D-L-Tryptophan is stable in many buffers used for biological studies.

Biomimetic synthesis and studies toward enantioselective synthesis of flindersial alkaloids.

A strategy allowing both stereocontrol and control over structural isomer formation has been defined for the antimalarial flindersial alkaloids. The recently reported flinderoles were demonstrated to be derived from the natural product borrerine. The structural isomers of flinderoles, the borreverines, were also produced in vitro along with the flinderoles through the dimerization of borrerine in acidic conditions. This result is thought to replicate the biosynthesis of these compounds. Flinderoles A, B, and C, desmethylflinderole C, isoborreverine, and dimethylisoborreverine can each be synthesized in three steps from tryptamine. Furthermore, progress toward a concise enantioselective synthesis of flinderoles A, B, and C is described. This work includes enantioselective conjugate addition to an unprotected indole-appended enone.

Enantioseparation of flinderoles and borreverines by HPLC on Chirobiotic V and V2 stationary phases and by CE using cyclodextrin selectors.

Racemic mixtures of the promising anti-malarial bisindole alkoids, flinderole A-C, desmethyl flinderole C, borreverine and isoborreverine, are baseline-separated for the first time by HPLC using vancomycin-based stationary phases and partially separated by capillary electrophoresis (CE) using cyclodextrin selectors. The HPLC results compare the performance of Chirobiotic V and V2 in the polar organic and reversed phase modes and their complementary selectivity is discussed. The performance of the cyclodextrin selectors in CE, while less effective, are discussed in terms of their selectivity in normal and reversed polarity modes.

Biomimetic synthesis of the antimalarial flindersial alkaloids.

A biomimetic strategy for the synthesis of the antimalarial flindersial alkaloids is described. Flinderoles A, B, and C, desmethylflinderole C, isoborreverine, and dimethylisoborreverine were all synthesized in three steps from tryptamine. The key step is an acid-promoted dimerization of the natural product borrerine. This approach is thought to mirror the biosynthesis of these compounds.

Mechanistic investigations of the ZnCl2-mediated tandem Mukaiyama aldol lactonization: evidence for asynchronous, concerted transition states and discovery of 2-oxopyridyl ketene acetal variants.

The ZnCl(2)-mediated tandem Mukaiyama aldol lactonization (TMAL) reaction of aldehydes and thiopyridyl ketene acetals provides a versatile, highly diastereoselective approach to trans-1,2-disubstituted ß-lactones. Mechanistic and theoretical studies described herein demonstrate that both the efficiency of this process and the high diastereoselectivity are highly dependent upon the type of ketene acetal employed but independent of ketene acetal geometry. Significantly, we propose a novel and distinct mechanistic pathway for the ZnCl(2)-mediated TMAL process versus other Mukaiyama aldol reactions based on our experimental evidence to date and further supported by calculations (B3LYP/BSI). Contrary to the commonly invoked mechanistic extremes of [2+2] cycloaddition and aldol lactonization mechanisms, investigations of the TMAL process suggest a concerted but asynchronous transition state between aldehydes and thiopyridyl ketene acetals. These calculations support a boat-like transition state that differs from commonly invoked Mukaiyama "open" or Zimmerman-Traxler "chair-like" transition-state models. Furthermore, experimental studies support the beneficial effect of pre-coordination between ZnCl(2) and thiopyridyl ketene acetals prior to aldehyde addition for optimal reaction rates. Our previously proposed, silylated ß-lactone intermediate that led to successful TMAL-based cascade sequences is also supported by the described calculations and ancillary experiments. These findings suggested that a similar TMAL process leading to ß-lactones would be possible with an oxopyridyl ketene acetal, and this was confirmed experimentally, leading to a novel TMAL process that proceeds with efficiency comparable to that of the thiopyridyl system.

Asymmetric synthesis, structure, and reactivity of unexpectedly stable spiroepoxy-beta-lactones including facile conversion to tetronic acids: application to (+)-maculalactone A.

A novel class of small spirocyclic heterocycles, spiroepoxy-beta-lactones (1,4-dioxaspiro[2.3]-hexan-5-ones), is described that exhibit a number of interesting reactivity patterns. These spiroheterocycles, including an optically active series, are readily synthesized by epoxidation of ketene dimers (4-alkylidene-2-oxetanones) available from homo- or heteroketene dimerization. An analysis of bond lengths in these systems by X-ray crystallography and comparison to data for known spirocycles and those determined computationally suggest that anomeric effects in these systems may be more pronounced due to their rigidity and may contribute to their surprising stability. The synthetic utility of spiroepoxy-beta-lactones was explored, and one facile rearrangement identified under several conditions provides a three-step route from acid chlorides to optically active tetronic acids, ubiquitous heterocycles in bioactive natural products. The addition of various nucleophiles to these spirocycles leads primarily to addition at C5 and C2. The utility of an optically active spiroepoxy-beta-lactone was demonstrated in the concise, enantioselective synthesis of the antifouling agent, (+)-maculalactone A, which proceeds in five steps from hydrocinnamoyl chloride by way of a tetronic acid intermediate.