Structure Determination of a Chloroenyne from Laurencia majuscula Using Computational Methods and Total Synthesis.

Despite numerous advances in spectroscopic methods through the latter part of the 20th century, the unequivocal structure determination of natural products can remain challenging, and inevitably, incorrect structures appear in the literature. Computational methods that allow the accurate prediction of NMR chemical shifts have emerged as a powerful addition to the toolbox of methods available for the structure determination of small organic molecules. Herein, we report the structure determination of a small, stereochemically rich natural product from Laurencia majuscula using the powerful combination of computational methods and total synthesis, along with the structure confirmation of notoryne, using the same approach. Additionally, we synthesized three further diastereomers of the L. majuscula enyne and have demonstrated that computations are able to distinguish each of the four synthetic diastereomers from the 32 possible diastereomers of the natural product. Key to the success of this work is to analyze the computational data to provide the greatest distinction between each diastereomer, by identifying chemical shifts that are most sensitive to changes in relative stereochemistry. The success of the computational methods in the structure determination of stereochemically rich, flexible organic molecules will allow all involved in structure determination to use these methods with confidence.

Asymmetric Total Syntheses and Structure Confirmation of Chlorofucins and Bromofucins.

Substrate-controlled asymmetric total syntheses and structure confirmation of (+)-(3E)- and (-)-(3Z)-chlorofucin [(E)-1 a and (Z)-1 a], and (+)-(3E)- and (-)-(3Z)-bromofucin [(E)-1 b and (Z)-1 b] were accomplished. Our syntheses feature as key steps haloetherification (either 'conventional' or 'one-pot organoselenium-mediated') of α,α'-trans-γ,δ-unsaturated oxocene alcohol 9 and our (E)- and (Z)-selective cross-metathesis (CM) protocols. More importantly, a rationale is provided for the strikingly different pathways followed by α,α'-trans-γ,δ-unsaturated oxocene alcohol 9 and its α,α'-cis isomer 9' in the presence of different electrophiles during the intramolecular electrophilic addition reactions.

Stereoselective Substrate-Controlled Asymmetric Syntheses of both 2,5-cis- and 2,5-trans-Tetrahydrofuranoid Oxylipids: Stereodivergent Intramolecular Amide Enolate Alkylation.

The concise, highly stereoselective, substrate-controlled asymmetric total syntheses of both 2,5-cis- and 2,5-trans-tetrahydrofuranoid nematocidal oxylipids from the Australian brown algae Notheia anomala have been accomplished in a stereodivergent fashion. The highly stereoselective intramolecular amide enolate alkylation strategy provides access to both stereoisomers of the 3-hydroxy-2,5-disubstituted tetrahydrofuran core of these marine natural products through chelate and nonchelate control, which is driven by the C3-hydroxy protecting group. This approach offers an optional and highly stereoelective access to any of the eight possible stereoisomers of the 2,5-disubstituted-3-oxygenated tetrahydrofuran skeleton, an important structural feature which is present in many biologically active natural products.

Substrate-Controlled Asymmetric Total Syntheses of Microcladallenes†A, B, and C Based on the Proposed Structures.

Substrate-controlled asymmetric total syntheses of (+)-microcladallenes A, B, and C have been accomplished based on the proposed structures. The syntheses of microcladallenes A and B confirmed the structures and absolute configurations of both natural products. However, the synthesis of microcladallene C, which includes seven stereogenic centers and an (R)-bromoallene in its compact C15 framework, brought the realization that its proposed structure must be revised. The introduction of C12-bromine into these natural products with retention of configuration relied on TiBr4 -mediated nucleophile-assisting leaving group brominations, the stereochemical outcome of which could be attributed, at least in part, to an oxonium or halonium ion formation-fragmentation sequence through intricate neighboring group participation. In addition, the pivotal ß-oriented vicinal cis-dichloride function in microcladallene C was elaborated through a novel tandem Cl2 -induced electrophilic cyclization/imidate chlorination process. The positive rotations of these natural products with an (R)-bromoallene constitute exceptions to Lowe's rule for reasons yet to be determined.

Asymmetric total synthesis of (+)-bermudenynol, a C15 Laurencia metabolite with a vinyl chloride containing oxocene skeleton, through intramolecular amide enolate alkylation.

A substrate-controlled asymmetric total synthesis of (+)-bermudenynol, a compact and synthetically challenging C15 Laurencia metabolite that contains several halogen atoms, is reported. The oxocene core, which contains a vinyl chloride, was constructed by an efficient and highly stereoselective intramolecular amide enolate alkylation (IAEA). This result showcases the broad utility of the IAEA methodology as a useful alternative for cases in which the ring-closing metathesis is inefficient.

Efficient synthesis of dichlorodenafil, an unapproved sildenafil analogue appearing in non-prescription supplements.

We have developed an efficient synthesis of dichlorodenafil (4), an unapproved sildenafil analogue isolated from dietary supplements. Our sequence employs POCl(3)-mediated chlorination of readily available chloroacetyl compound 7 followed by selective hydrolysis of the chloro-heterocycle function. Our synthesis confirms the structure of the illegal additive, and will provide regulatory agencies with ready access to authentic standard samples of dichlorodenafil (4) to aid in their mission to protect the public from unapproved and potentially harmful erectile dysfunction (ED) drug analogues that are added to herbal and dietary supplements without providing users with appropriate toxicological or pharmacological information.

Total Synthesis and Structure Confirmation of (-)-Asimitrin, a C37 Annonaceous Acetogenin with a Hydroxylated Adjacent Bis-Tetrahydrofuran Core.

The total synthesis and structure confirmation of the potent cytotoxic agent (-)-asimitrin (1), a C37 annonaceous acetogenin having a hydroxylated adjacent bis-tetrahydrofuran (THF) core, are described. The present synthesis features a highly stereoselective, chelate-controlled intramolecular amide enolate alkylation (IAEA) for the synthesis of key intermediate 17-hydroxy-16,17-erythro-16,19-trans-THF 6, our direct ketone synthesis/l-Selectride reduction protocol for stereoselective introduction of the C(21)-C(34) unit, Sharpless asymmetric dihydroxylation (SAD), and internal Williamson etherification for construction of the 20,23-trans-THF ring.

Stereoselective total synthesis of (3Z)- and (3E)-elatenynes.

We describe here the highly stereoselective total synthesis of the Laurencia C15 acetogenins (3Z)- and (3E)-elatenynes having a 7,12-dibromo-6,9-cis-10,13-cis adjacent bis-tetrahydrofuran (THF) core. The present synthesis features a highly stereoselective, protecting group-dependent, chelate-controlled intramolecular amide enolate alkylation (IAEA) for the synthesis of key intermediate 7-hydroxy-6,7-cis-6,9-cis-THF intermediate 10, deployment of the sequential ate complex (n-BuLi/DIBAL-H) reduction/Keck allylation/cross metathesis (CM) protocol for the stereoselective introduction of the C(10)-C(15) unit, a sequential Sharpless asymmetric dihydroxylation (SAD)/intramolecular Williamson etherification for the construction of the 10,13-cis-THF ring, and a modified Nakata chloromethanesulfonate-mediated SN2 displacement for the 7,12-dibromo functionality. Furthermore, our strategy based on chelate-controlled IAEA methodology would provide access to any member of the C15 adjacent bis-THF acetogenin class.

Concise substrate-controlled asymmetric total syntheses of dioxabicyclic marine natural products with 2,10-dioxabicyclo-[7.3.0]dodecene and 2,9-dioxabicyclo[6.3.0]undecene skeletons.

We report a completely substrate-controlled approach to the asymmetric total synthesis of representative dioxabicyclic bromoallene marine natural products with either a 2,10-dioxabicyclo[7.3.0]dodecene or 2,9-dioxabicyclo[6.3.0]undecene skeleton from commercially available glycidol as a common starting material. The former include (-)-isolaurallene (1), the enantiomeric form of natural (+)-neolaurallene (2), and (+)-itomanallene A (3c), and the latter are (+)-laurallene (4) and (+)-pannosallene (5a). In addition, our first syntheses of 3c and 5a established the structure and absolute stereochemistry of both natural products. Our general approach to establish the α,α'-relative stereochemistry of the medium-ring (oxonene or oxocene) and tetrahydrofuran, respectively, involved the judicious pairing of our protecting-group-dependent intermolecular amide enolate alkylation (either chemoselective chelation-controlled or dianion alkylation) with either our intramolecular amide enolate or nitrile anion alkylation. Remarkable selectivity was achieved through the use of the appropriate alkylation steps, and this approach offered us optional access to any of these dioxabicyclic bromoallene marine natural products. In addition, a computational analysis was performed to investigate conformational effects on the rate of oxonene formation via RCM, a key step in these approaches. The results suggested an alternative rationale for reactivity based on the avoidance of eclipsing torstional interactions in the AS2-type ring conformation.

Total synthesis and structure confirmation of elatenyne: success of computational methods for NMR prediction with highly flexible diastereomers.

Elatenyne is a small dibrominated natural product first isolated from Laurencia elata. The structure of elatenyne was originally assigned as a pyrano[3,2-b]pyran on the basis of NMR methods. Total synthesis of the originally proposed pyrano[3,2-b]pyran structure of elatenyne led to the gross structure of the natural product being reassigned as a 2,2'-bifuranyl. The full stereostructure of this highly flexible small molecule was subsequently predicted by Boltzmann-weighted DFT calculations of (13)C NMR chemical shifts for all 32 potential diastereomers, with the predicted structure being in accord with the proposed biogenesis outlined below. Herein we report two complementary total syntheses of elatenyne, which confirm the computer-predicted stereostructure. Additionally, the total syntheses of (E)-elatenyne and a related 2,2'-bifuranyl, laurendecumenyne B, are reported. This work has not only allowed the full structure determination of all of these natural products but also provides excellent supporting evidence for their proposed biogenesis. The total synthesis of elatenyne demonstrates that DFT calculations of (13)C NMR chemical shifts coupled with biosynthetic postulates, comprise a very useful method for distinguishing among large numbers of highly flexible, closely related molecules.

Stereocontrolled total synthesis of (+)-trans-dihydronarciclasine.

A highly stereoselective and efficient total synthesis of trans-dihydronarciclasine from a readily available chiral starting material was developed. The synthesis defines two of the five stereogenic centers of the natural product by an amino acid ester-enolate Claisen rearrangement. The other three stereogenic centers are created in a highly stereocontrolled fashion via a six-ring vinylogous ester intermediate, which is generated from the γ,δ-unsaturated ester functional group of the Claisen rearrangement product in an efficient three-step sequence. This concise total synthesis exemplifies the use of a highly regioselective Friedel-Crafts-type cyclization to form the B ring via an isocyanate intermediate derived from an N-Boc group, which is superior to the conventional method using an imino triflate intermediate. This same N-Boc group is employed to give high selectivity in the Claisen rearrangement earlier in the sequence.

Highly Stereodivergent Construction of a C2-Symmetric cis,cis- and trans,trans-2,6-Dioxabicyclo[3.3.0]octane Framework by Double Intramolecular Amide Enolate Alkylation: Total Synthesis of (+)-Laurenidificin and (+)-Aplysiallene.

The highly stereoselective construction of C2-symmetric cis,cis- and trans,trans-2,6-dioxabicyclo[3.3.0]octane (fused bis-tetrahydrofuran) skeletons 4a and 4b has been accomplished via a novel stereodivergent double intramolecular amide enolate alkylation of common cyclization substrate 5 through the judicious choice of "chelate" versus crown ether-promoted "nonchelate" control. Application of this methodology has provided access to substrate-controlled concise total syntheses of (+)-laurenidificin (3) and (+)-aplysiallene (ent-2), which possess cis/cis- and trans/trans-fused bis-tetrahydrofuran cores, respectively.

Highly Stereoselective Asymmetric Total Synthesis of (-)-Jimenezin via Sequential Intramolecular Amide Enolate Alkylation.

A highly stereoselective asymmetric total synthesis of (-)-jimenezin (1), a potent anticancer acetogenin, was efficiently completed with the key feature being a sequential intramolecular amide enolate alkylation (IAEA). Our investigation to probe the origin of the complete stereoselectivity in the second IAEA step to form the conformationally flexible tetrahydrofuran with perfect stereocontrol identified the presence of the oxygen atom in the adjacent tetrahydropyran ring to be crucial.

Regioselective inversion of the hydroxyl group in D-ribo-phytosphingosine via a cyclic sulfate and bis-sulfonate intermediate.

The selective synthesis of D-xylo- and D-lyxo-phytosphingosines from commercially available D-ribo-phytosphingosine is described. Thermolysis of the N-carbonyl protected cyclic sulfate led to an inversion of configuration of the proximal hydroxyl group to give the xylo-isomer, whereas the corresponding bis-sulfonate resulted in an inversion of configuration of the distal hydroxyl group to give the lyxo-isomer. This study allowed the comparison between a cyclic sulfate and a bis-sulfonate in an intramolecular substitution reaction involving a carbonyl oxygen nucleophile.

Syntheses of sulfur and selenium analogues of pachastrissamine via double displacements of cyclic sulfate.

Bioisosteric analogues of pachastrissamine that contain sulfur and selenium atoms replacing the oxygen in the ring system, were efficiently prepared from a cyclic sulfate intermediate by sequential intermolecular and intramolecular S(N)2 displacement reactions of the dianions. The analogues exhibited cytotoxicities comparable to that of pachastrissamine.

Asymmetric Total Synthesis and Determination of the Absolute Configuration of (+)-Srilankenyne via Sequence-Sensitive Halogenations Guided by Conformational Analysis.

This first asymmetric total synthesis of (+)-srilankenyne (1), a halogenated C15 tetrahydropyran acetogenin isolated from Aplysia oculifera, features a sequence-sensitive process guided by conformational analysis to solve the challenging problem of introducing halogens. A competing semipinacol rearrangement during the installation of C(12)-bromide was suppressed by our A1,3 strain-controlled bromination protocol with support from X-ray crystallographic and computational studies. The C(10)-chloride was then placed by the Nakata chloromesylate-mediated chlorination.

Asymmetric total synthesis of trilobacin via organoselenium-mediated oxonium ion formation/SiO2-promoted fragmentation.

An asymmetric total synthesis of trilobacin (1), an annonaceous acetogenin with potent anticancer activities, was accomplished wherein the construction of its erythro-bis(2,2')-tetrahydrofuran core 2 featured a novel organoselenium-mediated oxonium ion formation/SiO(2)-promoted fragmentation of alpha,alpha'-cis-oxocene 3.

Arylpiperazine-containing pyrimidine 4-carboxamide derivatives targeting serotonin 5-HT(2A), 5-HT(2C), and the serotonin transporter as a potential antidepressant.

Pyrimidine usually has good pharmacokinetic properties as a drug substance and considerable efforts have been devoted to develop pyrimidine derivatives into drug candidates. Arylpiperazine-containing pyrimidine 4-carboxamide derivatives were synthesized and evaluated for binding to serotonin receptors and transporter. Pyrimidine derivatives showed good antidepressant activity in FST (forced swimming test) animal model and also displayed no appreciable inhibitory activity against hERG channel blocking assay. Herein SAR studies of pyrimidine derivatives targeting serotonin receptors and transporter will be disclosed.

Design and synthesis of piperidine-containing sphingoid base analogues.

We report an approach that allows the efficient synthesis of the designed sphingoid base analogues in which the conformational restriction is introduced by incorporation of a cyclic moiety between the 2-amino group and the C-4 carbon atom of the sphingoid base. Our synthesis features a tandem enyne/diene-ene metathesis reaction that provides access to the designed framework. The diene moiety of the metathesis product is stereoselectively elaborated for the synthesis of the designed analogues. The preliminary biological evaluation indicates that the designed constrained analogues are much more effective than prototype natural sphingoid bases at inhibiting cancer cell growth.

Simultaneous determination of anti-diabetes/anti-obesity drugs by LC/PDA, and targeted analysis of sibutramine analog in dietary supplements by LC/MS/MS.

The safety of dietary supplements is questionable as there have been occasional reports of products contaminated with illegal adulterants. The present study was carried out to develop trustworthy methodologies to screen for six anti-diabetic drugs (phenformin, rosiglitazone, glipizide, glimepiride, glybenclamide and gliclazide) and six anti-obesity drugs (ephedrine, fenfluramine, T3, T4, fluoxetine and sibutramine) in dietary supplements. A simultaneous determination method of the 12 drugs by liquid chromatography coupled with a photodiode array (LC/PDA) was established and was validated for linearity (r(2) > 0.99), precision (RSD <13.3%), recoveries (88.8-115.9%) and reproducibility. Sibutramine and its analogs, N-desmethylsibutramine, were subject to further investigation by LC/MS/MS because they were one of the major illegal adulterants. Our proposed method to monitor illegal drug adulterations in dietary supplements using LC/PDA is a simple and reliable, and therefore applicable to routine drug-adulteration screening.