Total synthesis of the tricyclic marine alkaloids (-)-lepadiformine, (+)-cylindricine c, and (-)-fasicularin via a common intermediate formed by formic acid-induced intramolecular conjugate azaspirocyclization

A very short and efficient enantioselective total synthesis of the tricyclic marine alkaloids (-)-lepadiformine (3), (+)-cylindricine C (1c), and (-)-fasicularin (4) has been developed utilizing the formyloxy 1-azaspiro[4.5]decane 5 as a common intermediate. The key strategic element for the synthesis was the formic acid-induced intramolecular conjugate azaspirocyclization, which proved to be a highly efficient and stereoselective way to rapid construction of the 1-azaspirocyclic substructure of these natural products in a single operation. Thus, the common intermediate 5, synthesized in two steps with 70% overall yield starting from the known (S)-N-Boc-2-pyrrolidinone 7 via the conjugate spirocyclization using an acyclic ketoamide 6, was utilized for the concise and stereoselective total synthesis of (-)-lepadiformine (3), which was accomplished in seven steps with 45% overall yield from 5 (31% yield from 7). The developed strategy based on the conjugate spirocyclization was also applied to the stereoselective total synthesis of (+)-cylindricine C (1c), which was achieved in 10 steps from 5 in 18% overall yield (12% yield from 7). Further application of this approach using 5 led to the synthesis of (-)-fasicularin (4), wherein an extremely efficient method for the introduction of the thiocyanato group via an aziridinium intermediate at the last step was developed. Thus, the highly efficient first enantioselective total synthesis of (-)-fasicularin was accomplished in nine steps with an overall yield of 41% from 5 (28% yield from 7).     

Expedient, stereocontrolled synthesis of (+)-compactin lactone via intramolecular Reformatsky reaction

A concise, stereodefined synthesis of compactin lactone is described. Samarium(II) iodide mediated intramolecular Reformatsky reaction is the key step in the synthesis.     

Total synthesis of (-)-20-epiuleine via stereocontrolled one-pot asymmetric azaelectrocyclization followed by novel 1,4-addition reaction

The first asymmetric total synthesis of an indole alkaloid, (-)-20-epiuleine, containing the 2,3,4-trisubstituted piperidine core, was achieved using a stereocontrolled one-pot asymmetric 6π-azaelectrocyclization followed by a stereoselective 1,4-addition reaction of the unsaturated ester with a Grignard reagent resulting from the novel neighboring participation of the hydroxyl group in cis-aminoindanol as a chiral nitrogen source.     

Total synthesis of marine sponge bis(indole) alkaloids of the topsentin class

The synthesis of four natural bis(indole) alkaloids of topsentin class 1 and 2 is described. Their bis(indole) alpha-carbonylimidazoline and subsequently bis(indole) alpha-carbonylimidazole moieties have been built via the condensation between indolic alpha-ketothioimidate salts 4 and 1-(indol-3'-yl)-1,2-diaminoethane 3. This compound results from the beta-amino indolic hydroxylamine 5 by a two-step sequence. This is the first total synthesis of compounds 1d, 2a, and 2b.     

Total synthesis of LL-Z1640-2 utilizing a late-stage intramolecular Nozaki-Hiyama-Kishi reaction

A total synthesis of LL-Z1640-2 (2), a potent and selective kinase inhibitor, has been completed. The key step of the convergent synthesis utilized a late-stage intramolecular Nozaki-Hiyama-Kishi (NHK) reaction to close the macrocycle at the C6′-C7′ bond.     

Total synthesis of (-)- and ent-(+)-vindoline and related alkaloids

A concise 11-step total synthesis of (-)- and ent-(+)-vindoline (3) is detailed based on a unique tandem intramolecular [4 + 2]/[3 + 2] cycloaddition cascade of a 1,3,4-oxadiazole inspired by the natural product structure, in which three rings and four C-C bonds are formed central to the characteristic pentacyclic ring system setting all six stereocenters and introducing essentially all the functionality found in the natural product in a single step. As key elements of the scope and stereochemical features of the reaction were defined, a series of related natural products of increasing complexity were prepared by total synthesis including both enantiomers of minovine (4), 4-desacetoxy-6,7-dihydrovindorosine (5), 4-desacetoxyvindorosine (6), and vindorosine (7) as well as N-methylaspidospermidine (11). Subsequent extensions of the approach provided both enantiomers of 6,7-dihydrovindoline (8), 4-desacetoxyvindoline (9), and 4-desacetoxy-6,7-dihydrovindoline (10).     

The first stereocontrolled synthesis of 12-methyl-hexahydrobenzo[c]phenanthridine alkaloids

12-Methyl B/C hexahydrobenzo[c]phenanthridines have been synthesized stereoselectively starting from chiral nonracemic 2-aryl-4-pentenoic acids prepared by asymmetric allylation of (+)-(S,S)-pseudoephedrine-based arylacetamide enolates. Subsequent transformations (Friedel–Crafts acylation, stereocontrolled reductive amination, Pictet–Spengler cyclization and PPA catalyzed cationic cyclization) led to the synthesis of enantiomerically enriched hexahydrobenzo[c]phenanthridines in which the sequential formation of all the new stereogenic centres was controlled by the starting chiral acids.     

Total synthesis of the marine alkaloid (-)-lepadin B

An enantioselective total synthesis of (-)-lepadin B has been developed starting from (2S,4S)-2,4-O-benzylidene-2, 4-dihydroxybutanal. The key steps in the synthesis include the use of an aqueous intramolecular acylnitroso Diels-Alder reaction to afford the trans-1,2-oxazinolactam and Suzuki cross-coupling reaction to elaborate the (E,E)-octadienyl unit.     

Total synthesis of the marine metabolite (-)-clavosolide D

The first total synthesis of the marine natural product (-)-clavosolide D is described confirming the structure of the unsymmetrical 16-membered diolide glycosylated by permethylated d-xylose moieties. Following efficient assembly of the two tetrahydropyrans using stereoselective Prins cyclizations, the side chains were introduced via an allylation/isomerization/anti cyclopropanation sequence; the final macrolactonization step was achieved under Yamaguchi conditions.     

Total synthesis of complex cyclotryptamine alkaloids: stereocontrolled construction of quaternary carbon stereocenters

Our ability to access the more complex members of the cyclotryptamine family of alkaloids, and to exploit their disparate biological activities, is limited by the synthetic challenge posed by their oligomeric, polyindoline structures. A recurring structural theme within these molecules is the presence of multiple quaternary stereocenters in close proximity to one another. Over the last decade, we have developed a set of transformations that allow rapid access to polyindolines, a number of which exploit the ability of catalytic levels of palladium to orchestrate carbon-carbon bond formation with impressive levels of regio- and stereocontrol. This review tells the story behind the development of this toolbox of synthetic methods, and their validation through the total synthesis of a number of structurally complex cyclotryptamine alkaloids. It also highlights an aspect of asymmetric catalysis that has received little attention, the ability of catalytic asymmetric reactions to selectively elaborate complex, polyfunctional molecules.     

Total synthesis of natural (+)-sesbanimide a and (-)-sesbanimide b

The first total synthesis of natural (+)-sesbanimide A (1) and (-)-sesbanimide B (2), potent antitumor alkaloids isolated from the seeds of the leguminous plant, Sesbania drummondii, has been accomplished starting from D-(+)-xylose. This total synthesis involves efficient construction of the optically active AB-ring system from D-(+)-xylose, introduction of the C₅-unit into the AB-ring system in a form of exo-methylene-γ-lactone, and elaboration of the labile C-ring system at the last stage of the synthesis. The absolute configurations of natural 1 and 2 could be obviously established by our total synthesis.     

Total synthesis of (-)-ulapualide A, a novel tris-oxazole macrolide from marine nudibranchs, based on some biosynthesis speculation

A new, second generation, total synthesis of ulapualide A (1), whose stereochemistry was recently determined from X-ray analysis of its complex with the protein actin, is described. The synthesis is designed and based on some speculation of the biosynthetic origin of the contiguous tris-oxazole unit in ulapualide A, alongside that of the related co-metabolites that contain only two oxazole rings, e.g. 6 and 7. The mono-oxazole carboxylic acid 67b and the mono-oxazole secondary 55b alcohol which, together, contain all of the 10 asymmetric centres in the natural metabolite, were first elaborated using a combination of contemporary asymmetric synthesis protocols. Esterification of 67b with 55b under Yamaguchi conditions gave the ester 77 which was then converted into the omega-amino acid 18a following simultaneous deprotection of the t-butyl ester and the N-Boc protecting groups. Macrolactamisation of 18a, using HATU, now gave the key intermediate macrolactam 17, containing two of the three oxazole rings in ulapualide A (1). A number of procedures were used to introduce the third oxazole ring in ulapualide A from 17, including: a) cyclodehydration to the oxazoline 78a followed by oxidation using nickel peroxide leading to 76; b) dehydration to the enamide 79, followed by conversion into the methoxyoxazoline 78b, via 80, and elimination of methanol from 78b using camphorsulfonic acid. The tris-oxazole macrolide 76 was next converted into the aldehyde 82b in four straightforward steps, which was then reacted with N-methylformamide, leading to the E-alkenylformamide 83. Removal of the TBDPS protection at C3 in 83 finally gave (-)-ulapualide A, whose 1H and 13C NMR spectroscopic data were indistinguishable from those obtained for naturally derived material. It is likely that the tris-oxazole unit in ulapualide A (1) is derived in nature from a cascade of cyclodehydrations from an acylated tris-serine precursor, e.g.9, followed by oxidation of the resulting tris-oxazoline intermediate, i.e.10. It is also plausible to speculate that the biosynthesis of metabolites related to ulapualide A, e.g. the bis-oxazole 6 and the imide 7, involve cyclisations of just two of the serine units in 9. These speculations were given some credence by carrying out pertinent interconversions involving the bis-oxazole amide 24, the enamide 25, the imide 26, the oxazoline 27 and the tris-oxazole 30 as model compounds. An alternative strategy to the tris-oxazole macrolide intermediate 76 was also examined, involving preliminary synthesis of the aldehyde 73, containing a shortened (C25-C34) side chain from 67b and 47b. A Wadsworth-Emmons olefination reaction between 73 and the phosphonate ester 74 led smoothly to the E-alkene 75, but we were not able to reduce selectively the conjugated enone group in 75 to 76 without simultaneous reduction of the oxazole alkene bond, using a variety of reagents and reaction conditions.     

New approach to indole alkaloids based on the intramolecular Pauson-Khand reaction

The synthesis of indoles bearing alkenyl and alkynyl moieties in different positions of the nucleus is described. These compounds are used as substrates for the intermolecular Pauson-Khand reaction leading to tetracyclic cyclopentenones with formation of additional five- to seven-membered rings. Products are related to alkaloids such as mitosenes, clausines, ergotamines, or apogeissochizines.     

Total synthesis of the marine natural product (-)-clavosolide A

[Structure: see text] The total synthesis of the marine metabolite clavosolide A is reported which confirms the structure and absolute configuration of the natural product as the symmetrical diolide glycosylated by permethylated D-xylose moieties, 2.     

Total synthesis of (+/-)-taiwaniaquinol B via a domino intramolecular friedel-crafts acylation/carbonyl alpha-tert-alkylation reaction

The first synthesis of taiwaniaquinol B, a 6-nor-5(6-->7)abeoabietane-type diterpenoid exhibiting the uncommon fused 6-5-6 tricyclic carbon skeleton, was accomplished in 15 steps. A Lewis acid-promoted tandem intramolecular Friedel-Crafts/carbonyl alpha-tert-alkylation reaction was exploited as the core strategy for the synthesis of the sterically congested 1-indanone-containing tricyclic structure. This multiple carbon-carbon bond forming reaction exploits the unique reactivity of Meldrum's acid. The facile precursor synthesis makes this a useful methodology for the expedient modification and assembly of sterically congested 1-indanone-containing ring systems.     

The stereocontrolled total synthesis of (-)-O-methylpallidinine

[reaction: see text] The stereocontrolled total synthesis of (-)-O-methylpallidinine, a naturally occurring Morphinan alkaloid with a B/C-trans-hydrophenanthrene framework, has been achieved starting from the chiral bicyclo[3.2.1]octenone building block by employing a single-step dihydrophenanthrene formation reaction as the key step.     

Application of the intramolecular magnesium-ene reaction to the stereocontrolled total syntheses of (±)-12-acetoxysinularene and (±)-5-epi-12-acetoxysinularene

The sesquiterpene (±)-12-acetoxysinularene (1) and its C(5)-epimer (13) were each synthesized stereoselectively from the norbornyl-iodoacetal 7 in 5% and 7% overall yields. The critical step (10) → (11) involves the regio- and stereoselective “magnesium-ene” reaction (6) → (5).     

Total synthesis of (-)-brevisin: a concise synthesis of a new marine polycyclic ether

The first and highly efficient total synthesis of (-)-brevisin has been achieved. The title compound was synthesized in only 29 steps (longest linear sequence) from commercially available starting materials. The synthesis provided over 70 mg of a marine polycyclic ether compound.     

Total synthesis of Akuammiline alkaloid (-)-vincorine via intramolecular oxidative coupling

An asymmetric total synthesis of the Akuammiline alkaloid (-)-vincorine (18 steps from 5-methoxytryptamine, 5% overall yield) is described. The key steps include Pd-catalyzed direct C-H functionalization of indole derivatives, organocatalyzed asymmetric Michael addition of aldehydes to alkylidene malonates, and intramolecular oxidative coupling between indole and malonate moieties.