Studies toward the total synthesis of clavulactone

Synthetic studies directed toward a total synthesis of clavulactone are reported. In light of the analysis made in our previous work, cyclopentane 4a (a key intermediate in the present work) was synthesized through a radical-mediated ring closure of a rationally designed substrate 25. Using HWE reactions, the lower and upper side-chains of 4a were converted into an allyl chloride and an allyl cyanohydrin, respectively. Subsequent treatment of the allyl chloride/cyanohydrin in a highly diluted THF solution with sodium bis(trimethylsiliyl)amide led to intramolecular alkylation and thus completed a major endeavor in synthesizing the dolabellane framework, construction of the eleven-membered ring. SmI(2)-mediated lactonization as a model reaction for the formation of the alpha,beta-unsaturated delta-lactone segment of clavulactone is also described.     

Studies toward the total synthesis of the hirsutellones

A strategy of tandem ketene-trapping/IMDA toward the total synthesis of the hirsutellones was attempted. The AB ring moiety of the hirsutellones was constructed with the proper stereochemistry.     

Studies toward the total synthesis of nagelamide K

A stereocontrolled strategy toward the synthesis of nagelamide K has been developed. The dimeric imidazole acrylate, diimidazolidenesuccinate, was constructed as a synthetic precursor by a Ni-catalyzed coupling reaction; the microwave-promoted intramolecular aza-Michael addition afforded the imidazo[1,5-a]pyridine core structure of nagelamide K in high stereoselectivity. A detaurine-dediamino analogue of nagelamide K has been prepared.     

Studies toward the total synthesis of Wiedemannic acid

[reaction: see text] We report a novel and efficient diastereoselective synthesis of wiedemannic acid analogue 30 in 16 steps from 7 using a tandem oxy-Cope/Claisen/ene reaction as the key step. Comparison of NMR data between wiedemannic acid (1) and analogue 30 leads us to believe that the reported stereochemistry at the ring junction of 1 is incorrect.     

Studies toward the total synthesis of axinellamine and massadine

Intramolecular Diels-Alder reactions of several N-O linked 4-vinylimidazole dimers provide the expected adduct in moderate to good yield as a single, all trans stereoisomer, along with smaller amounts of the inverse electron demand adduct. Oxidative rearrangement of the cycloadducts occurs on treatment with Davis' reagent, providing a single spiro imidazolone in good yield and with excellent levels of stereocontrol albeit epimeric at the spiro center found in axinellamine and massadine.     

Studies toward the total synthesis of vinigrol. synthesis of the octalin ring

[reaction: see text] Herein, we disclose our results regarding various strategies toward the assembly of the octanyl ring of vinigrol. Attempts to generate the problematic eight-membered ring through a ring expansion or via unification of the terminal olefins using the ring-closing metathesis were not successful. The cyclooctane ring was created via a sequential hydroxy Diels-Alder/Claisen rearrangement reaction of diene 55 and N-benzylmaleimide.     

Studies toward the total synthesis of azaspiracids: synthesis of the FGHI ring domain

Synthesis of the FGHI ring domain of azaspiracids, the causative agents for a new type of shellfish poisoning, azaspiracid poisoning (AZP), has been achieved. The synthesis features dithiane anion-epoxide coupling for convergent fragment assembly.     

Studies directed toward the first total synthesis of acremodiol and acremonol

Studies directed toward the synthesis of acremodiol and acremonol resulted in the synthesis of two macrodiolides 1, 1a, and 2 besides 3. The attempted synthesis of 1 and 2 confirmed that the absolute stereochemistry defined in the earlier report is incorrect. Compound 1 was synthesized by RCM-mediated macrocyclization. Attempted synthesis of 2 failed to give good yields in the cyclization, and 1a and 2 were synthesized by the Yamaguchi macrolactonization method.     

Studies toward the total synthesis of the cytotoxic sponge alkaloid pyrinodemin A

[structure: see text]. The syntheses of the proposed structure of pyrinodemin A (1) and its cis double bond positional isomer (C15'-C16') in racemic form are described. The key reaction involved an intramolecular nitrone/double bond cycloaddition. Our results suggest that neither 1 nor its double positional isomer is the correct structure of pyrinodemin A     

Studies toward the total synthesis of popolohuanone E: enantioselective synthesis of 8-O-methylpopolohuanone E

[structure: see text]. 8-O-methylpopolohuanone E (2) was synthesized in a highly convergent manner starting from the cis-fused decalin derivative accessible from the (-)-Wieland-Miescher ketone analogue. The synthetic method features a biogenetic-type annulation of the phenolic and quinone segments to regioselectively construct the central tricyclic ring system as the key step.     

Studies toward the total synthesis of (+)-gelsemine and synthesis of spirocyclopentaneoxindole through intramolecular Michael cyclization

During the approach to the total synthesis of (+)-gelsemine, two novel spirocyclopentaneoxindole compounds 2 and 3 were obtained. Starting from compound 9, spirocyclopentaneoxindole 3 was efficiently and unexpectedly obtained through a Boc migration-intramolecular Michael cyclization cascade procedure. This intramolecular Michael addition strategy allows conveniently access to complex spirooxindole and spirocyclopentaneoxindole derivatives.     

Studies toward the total synthesis of garsubellin A: a concise synthesis of the 18-epi-tricyclic core

[reaction: see text] During studies directed toward the total synthesis of garsubellin A, a concise stereocontrolled synthesis of the 18-epi-tricyclic compound 3 was achieved. Key steps were a one-pot stereoselective construction of the bicyclic lactone 23 followed by a formal migration to the bicyclo[3.3.1]nonane-1,3,5-trione and an intramolecular Wacker-type tetrahydrofuran ring formation.     

Studies toward the total synthesis of phalarine: a survey of some biomimetic possibilities

Biomimetically inspired oxidative coupling strategies toward the total synthesis of phalarine (1) are described.     

Studies directed toward the total synthesis of discodermolide: asymmetric synthesis of the C1-C14 fragment

[structure: see text] A convergent and stereoselective assembly of the C1-C14 subunit of marine natural product (+)-discodermolide has been completed. The approach employs chiral allylsilane bond construction methodology to establish four of the eight stereogenic centers. Key fragment coupling is achieved via an efficient stereoselective acetate aldol reaction between C1-C6 and C7-C14 subunits.     

Progress toward the total synthesis of kalihinane diterpenoids

[see structure]. Studies toward the total synthesis of marine diterpenoids isolated from Acanthella sp., e.g., kalihinol A, are described. Efficient construction of the functionalized trans-decalin core (11) is achieved through intramolecular Diels-Alder cyclization followed by diastereoselective epoxidation and aziridination.     

Recent progress toward the total synthesis of humulanolides

The humulanolides are a series of sesquiterpene lactones, most of which have unique and challenging structure. The humulanolides have exhibited anticancer activity. The combinations of fascinating structural motifs and promising pharmacological properties have prompted significant interest in the synthetic community. In this review, we provide a summary of recent progress regarding the total synthesis of humulanolides.     

Progress toward the total synthesis of frondosin C

A straightforward approach toward the total synthesis of frondosin C is described. This strategy involves a key one-pot, microwave-assisted 5-exo cyclization-Claisen rearrangement sequence that was used for the expedient assembly of the frondosic C scaffold. Subsequent manipulation of the tetracyclic core allowed the synthesis of an advanced intermediate bearing the characteristic diene moiety in the B ring. [reaction: see text]     

Studies toward the synthesis of maoecrystal V

An approach toward the synthesis of maoecrystal V is described. The synthetic strategy for this approach was designed to address unique challenges posed by the strained tetrahydrofuran ring at the center of the target structure.     

Studies toward the total synthesis of gambieric acids: convergent synthesis of the GHIJ-ring fragment having a side chain

A stereocontrolled synthesis of the GHIJ-ring fragment having a side chain of gambieric acids, which are potent antifungal polycyclic ether natural products, has been achieved. The synthesis features convergent assembly of the tetracyclic polyether skeleton by using aldol coupling and stereoselective construction of the J-ring side chain by a cerium chloride-promoted Julia-Kocienski reaction.     

Studies toward the total synthesis of diterpene antibiotic guanacastepene A: construction of the hydroazulenic core

[reaction: see text] As a part of studies aimed toward the total synthesis of biologically important natural product guanacastepene A of contemporary interest, a new and concise route to a fully functionally endowed hydroazulenic core is delineated. The strategy involves the building of the requisite stereochemical features on a endo-tricyclo[5.2.1.0(2,6)]decane matrix and excision of the five-membered ring through a retro-Diels-Alder reaction. Generation of the seven-membered ring to access the hydroazulenic framework was achieved employing ring closure metathesis (RCM) reaction as the key step.