Total synthesis of (−) altholactone (goniothalenol) from D-glucose

A ten steps total synthesis of (−) altholactone, enantiomer of an antitumor pyrone isolated from Goniothalamus species, is described starting from D-glucose.     

A pyrone remodeling strategy to access diverse heterocycles: application to the synthesis of fascaplysin natural products

The synthesis of diverse N-fused heterocycles, including the pyrido[1,2-a]indole scaffold, using an efficient pyrone remodeling strategy is described. The pyrido[1,2-a]indole core was demonstrated to be a versatile scaffold that can be site-selectively functionalized. The utility of this novel annulation strategy was showcased in a concise formal synthesis of three fascaplysin congeners.

The synthesis of diverse N-fused heterocycles, including the pyrido[1,2-a]indole scaffold, using an efficient pyrone remodeling strategy is described.     

Application of directed metallation in synthesis. Part 3: Studies in the synthesis of (±)-semivioxanthin and its analogues

The synthesis of several analogues of (±)-semivioxanthin including five thiophene analogues, using directed metalation are reported. The strategy consisted of the synthesis of functionalized naphthalene or benzo[b]thiophene as building blocks followed by annelation of the pyrone.     

Iodine,a protecting group for the synthesis of psoralen derivatives oxygenated in pyrone ring : Synthesis of 4,5-dimethoxypsoralen and 3,4,5-trimethoxypsoralen (halfordin)

A convenient synthesis of psoralen derivatives oxygenated in pyrone ring, viz 4,5-dimethoxy-7-oxo-7H-furo [3,2-g] [1] benzopyran (4,5-dimethoxypsoralen) and 4,5,6-trimethoxy-7-oxo-7H-furo [3,2-g] [1] benzopyran (halfordin) (3,4,5-trimethoxypsoralen) is described by blocking the 8-position with iodine, Claisen migration followed by cyclisation.     

Regioselectivity in nickel(0) catalyzed cycloadditions of carbon dioxide with diynes

The regioselectivity of Ni(0)-catalyzed cycloadditions of CO₂ (1 atm) with various asymmetrical diynes to afford pyrones was explored. The use of 1,3-bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) provided high regioselectivity when one terminal substituent on the diyne was a methyl group and the other was medium or large in size (R_L=i-Pr, t-Bu, or TMS). In contrast, the use of a relatively small N-heterocyclic carbene, 1,3-dimesitylimidazol-2-ylidene (IMes), afforded high selectivity only when R_L was large (TMS). X-ray crystal analysis of the major isomer indicated that the relatively large R_L group was in the 3-position of the pyrone.     

Total Synthesis and Stereochemical Revision of Phacelocarpus 2‐Pyrone A

The first total synthesis of phacelocarpus 2‐pyrone A is reported. The original natural compound was tentatively assigned (by NMR spectroscopy) as containing two cis‐alkenes and a trans‐vinyl ether connected to a 2‐pyrone ring motif. Our computational predictions indicated that a cis‐vinyl ether motif was equally feasible. Attempts to prepare the trans‐vinyl ether were met with no success. The all cis‐target compound was synthesised in nine steps, employing key regio‐ and stereoselective reactions including Au^I‐catalysed vinyl etherification, Wittig alkenylation and end‐game Stille macrocyclisation. Analysis of the NMR data enabled identification and confirmation of the correct structure of phacelocarpus 2‐pyrone A, containing a cis‐vinyl ether. Our studies pave the way for future development of methodologies to these structurally distinct pyrone skipped‐polyenyne natural products.     

Revisited synthesis of fused diazepines from 3-(3-aryl-3-oxopropenyl)chromen-4-ones and binucleophilic amino enones in a three-step domino process

The synthesis of tetracyclic 11,12,13,14b-tetrahydrodi-benzo[b, f]pyrido[1,2-d][1,4]diazepines was revisited via a catalyst-free three-step domino reaction involving a pyrone ring-opening/aza-Michael addition/intramolecular cyclization, the reactants having been o-arylenediamine–dimedone adducts and 3-(3-aryl-3-oxopropenyl)chromen-4-ones. The 3-positioned exocyclic α,β-enone fragment on the chromone moiety is involved in the cyclization into the final products at the last step of the process.     

Formal Synthesis of Natural 1,3‐Polyol/α‐Pyrone Diastereoisomers

A stereoselective formal synthesis of diastereoisomers of 1,3‐polyol/α‐pyrone antifungal natural products isolated from Ravensara anisata has been achieved involving epoxide opening and asymmetric allylation as key steps.     

Preparation of 2,3-disubstituted indoles by sequential Larock heteroannulation and silicon-based cross-coupling reactions

A simple and convergent synthesis of 2,3-disubstituted indoles has been developed using a sequential Larock indole synthesis and silicon-based, cross-coupling reaction. Substituted 2-iodoanilines reacted with an alkynyldimethylsilyl tert-butyl ether to afford indole-2-silanols under the Larock heteroannulation conditions after hydrolysis. The corresponding sodium 2-indolylsilanolate salts successfully engaged in cross-coupling with aryl bromides and chlorides to afford multi-substituted indoles. The development of an alkynyldimethylsilyl tert-butyl ether as a masked silanol equivalent enabled a smooth heteroannulation process and the identification of a suitable catalyst/ligand combination provided for a facile cross-coupling reaction.     

One-pot reaction of ortho-acylphenols and terminal alkynoates for synthesis of 2-alkyl-substituted chromanones

A facile synthesis of 2-alkyl-substituted chromanones from ortho-acylphenols and terminal alkynoates is described. The method contains two consecutive processes in one-pot reaction through a DABCO-catalyzed condensation reaction and a KOBu^t-mediated intramolecular cycloaddition to afford the desired products.     

Copper-catalyzed synthesis of 2-aminobenzimidazoles from carbonimidoyl dichlorides and amines

A new protocol for the synthesis of a variety of 2-aminobenzimidazole derivatives has been developed. O-haloaryl carbonimidoyl dichloride reacted with anilines to generate an o-haloaryl guanidine intermediate, which underwent copper catalyzed ring closure to afford 2-aminobenzimidazole derivative in moderate yields     

A convenient one-pot synthesis of 3,5-diarylisoxazoles via oxidative cyclisation using catalytic CuBr2 and oxone

A facile one-pot synthesis of 3,5-diarylisoxazoles from α,β-unsaturated ketones and hydroxylamine hydrochloride is reported. The reaction is efficiently promoted by catalytic CuBr₂ and Oxone to afford the desired products mostly in high yields and in relatively short time. The mild nature of the synthesis and short reaction time are notable advantages of the developed protocol. This protocol is effective toward various substrates having different functionalities.     

Z-Selective synthesis of o-bromoacetophenone N-tosylhydrazones and formation of 3-methylindazoles in aqueous ethanol

A practical and effective Z-selective synthesis of o-bromoacetophenone N-tosylhydrazones is developed. Subsequent cyclization of Z-tosylhydrazones to furnish 3-methylindazoles is accomplished with the aid of copper and DMEDA in aqueous ethanol. Cyclization reactions are complete at ambient temperature in 10 min to afford the desired compounds in excellent yields.     

Stereodivergent reduction of enelactams embedded in hexahydroindoles. Synthesis of trans-3-substituted-cis-3a-methyloctahydroindoles

A stereodivergent synthesis of cis- and trans-octahydroindole derivatives from the ethylene acetal of methyl 1-benzyl-3a-methyl-2,5-dioxo-2,3,3a,5,6-hexahydro-1H-3-indoleacetate is reported. Under ionic reduction conditions the enamide group was reduced to afford a trans-ring fused product, while a hydrogenation process led to the formation of a cis-ring fused lactam, which was transformed into a building block for daphniphyllum alkaloid synthesis after an epimerization at C-3.     

A simple and efficient enantioselective route to 2,6-disubstituted piperidines: synthesis of (2R,6S)-isosolenopsin A and (2S,6R)-isosolenopsin

The enantioselective synthesis of 2,6-cis-disubstituted piperidine alkaloids, (2R,6S)-isosolenopsin A 2 and (2S,6R)-isosolenopsin 5 from fire ant venom is described. Starting from the dodecanal and decanal, the synthesis presents two key steps. The first step involves Keck allylation to afford the chiral homoallylalcohol with the required stereochemistry and the second key step consists of Grubbs olefin cross metathesis. The synthesis was achieved in five steps with 44% overall yield.     

Dipyrone approach toward the synthesis of the cytotoxic natural product auripyrone A

An approach to the synthesis of the cytotoxic natural product auripyrone A 1 via the cyclization of an alcohol onto a γ-pyrone in 3 is described. The bis(pyrone) alcohol 3 was prepared efficiently from the advanced aldolate 4 via silyl ether cleavage, oxidation, pyrone formation, and PMB ether removal. Instead of providing auripyrone A 1, the attempted cyclization of 3 gave the product of 1,5-acyl migration 8. Model studies show this to be a general process; therefore, cyclization of an alcohol on such a hindered γ-pyrone under normal conditions is very difficult.     

A diastereoselective synthesis 1-trimethylsilyl-(E)-1,3-alkenynes and a simple synthesis of alkyl trimethylsilylethynyl ketones via organoboranes

A convenient, novel diastereoselective synthesis of 1-trimethylsilyl-(E)-1,3-alkenynes and a convenient synthesis of alkyl trimethylsilylethynyl ketones based on Z-1-bromo-1-alkenylboronate esters are developed. α-Bromo-(Z)-1-alkenylboronate esters readily available using literature procedures smoothly undergo a reaction with trimethylsilylethynyllithium (derived from the deprotonation of trimethylsilylethyne with n-butyllithium) in tetrahydrofuran to provide the corresponding ‘ate’ complexes. These ‘ate’ complexes undergo intramolecular nucleophilic substitution reactions to afford the corresponding (E)-1-alkenylboronate esters containing trimethylsilylethynyl moiety which upon protonolysis with acetic acid provide the corresponding 1-trimethylsilyl-(E)-1,3-alkenynes in good yields (70-82%) and in high stereochemical purities (>98%). These intermediates upon oxidation with hydrogen peroxide and sodium acetate afford the corresponding alkyl trimethylsilylethynyl ketones in good yields (66-78%).     

An efficient synthesis of fully substituted pyrazolo[3,4-b]pyridin-5-amines from α-azidochalcones

A facile, mild and efficient procedure for the synthesis of fully substituted pyrazolo[3,4-b]pyridin-5-amines is reported. A mixture of an α-azidochalcone, a 5-aminopyrazole and t-BuOK was stirred in DMF at ambient temperature for 5?min to afford the corresponding pyrazolo[3,4-b]pyridin-5-amines in good to excellent yields.     

Investigation of the regioselectivity of the Hurd-Mori reaction for the formation of bicyclic 1,2,3-thiadiazoles

A series of new pyrrolo[d][1,2,3]thiadiazole carboxylates and 5,6-dihydro-4H-cyclopenta[d][1,2,3]thiadiazoles has been synthesized via the Hurd-Mori reaction. The regioselectivity of the cyclization has been studied and trends were established to predict the cyclization direction to afford bicyclic 1,2,3-thiadiazoles. Effects promoting and disfavoring the reaction have also been investigated to guide the synthesis of scaffolds of this type.     

Rearrangement of oxazolidinethiones to thiazolidinediones or thiazinanediones and their application for the synthesis of chiral allylic ureas and α-methyl-β-amino acids

A novel rearrangement has been found between oxazolidinethiones and acyl halides under N-acylation reaction conditions to afford N-substituted 2,4-thiazolidinediones and N-substituted 1,3-thiazinane-2,4-diones. These heterocycles were used for the synthesis of chiral allylic ureas and α-methyl-β-amino acids.