A ring-closing metathesis approach for the synthesis of (±)-pregabalin

Abstract  

Efficient utilization of a Mannich-type reaction and the ring-closing metathesis (RCM) approach that leads to a convenient synthesis of 3-(aminomethyl)-5-methylhexanoic acid (pregabalin) is described.     

New strategy for the synthesis of ladybird beetle azaphenalene alkaloids using a combination of allylboration and intramolecular metathesis. Total synthesis of (±)-Hippocasine and (±)-epi-Hippodamine

A new strategy for assembly a tricyclic skeleton of ladybirds azaphenalene alkaloids (coccinellides) was developed based on the combination of allylboration reaction and intramolecular metathesis. The first key step is the 1,2-organolithiation of 4-picoline with (4,4-dieth-oxybutyl)lithium with subsequent reductive allylation with triallylborane leading to trans-2-allyl-6-(4,4-diethoxybutyl)-4-methyl-1,2,3,6-tetrahydropyridine. The 4,4-diethoxybutyl substituent was further converted to 4-acetoxy-5-hexenyl in four steps, then, the product obtained was involved in the second key step, the intramolecular allylic amination upon treatment with a [Pd] or an [Ir] catalyst giving diastereomeric bicyclic terminal dienes (～1: 1), which were separated by chromatography. The stereochemistry of one of the dienes is the same as that in alkaloid Hippocasine. The third key step (the intramolecular metathesis reaction) includes the final assembly of the azaphenalene system. The tricyclic derivative obtained contains two differently substituted C=C bonds, selective hydrogenation of one of which (Pd/C) leads to (±)-Hippocasine, whereas exhaustive hydrogenation gives (±)-epi-Hippodamine.     

Total synthesis of (±)-cephalosol

A concise and efficient total synthesis of (±)-cephalosol has been completed (5 steps from known ester 5, 39% overall yield), featuring a Cu(II)-promoted haloisocoumarin formation and sequential Suzuki cross-coupling/intramolecular oxo-Michael addition.     

Total synthesis of (±)-platencin

A novel route to (±)-platencin is reported, in which the highly stereoselective alkylative quaternization of a cyclohexenone scaffold via 1,4-diastereoinduction and two radical carbon-carbon bond-forming reactions that involve titanium(III)-mediated cyclization and stannyl-radical-mediated skeletal rearrangement are utilized.     

Total synthesis of (±)-sanadaol

The first total synthesis of (±)-sanadaol, a structurally unique marine diterpene, was achieved in a fully stereocontrolled manner.     

Total synthesis of (±)-meloscine

The total synthesis of (±)-meloscine was completed in a highly stereoselective manner starting from the known 4-(2-aminophenyl)-2,3-dihydro-N-methoxycarbonylpyrrole. The crucial step in this total synthesis involves the efficient construction of the tetracyclic framework of the target natural product by the intramolecular Pauson-Khand reaction.     

Total synthesis of (±)-isophellibiline

The total synthesis of (±)-isophellibiline is described. This represents the first synthesis of a member of the nonaromatic homoerythrinan family of alkaloids. The tetracyclic ring system of the natural product was quickly assembled by a strategy that features a retrocycloaddition/cycloaddition reaction of an amidodioxin, an intramolecular Heck reaction and a 6π-electrocyclic ring closure of a dienoic acid.     

Total synthesis of (±)-paniculide-a

An efficient synthesis of the paniculide A precursor 4 has been achieved beginning with 3-methylglutaric anhydride, and utilizing as the key step an intramolecular Diels-Alder reaction of an acetylenic oxazole to give a 2-methoxyfuran. Acid hydrolysis then provided the requisite butenolide ring characteristic of the paniculides.     

Total synthesis of (±)-chloranthalactone A

A total synthesis of (±)-chloranthalactone A was completed. It features substrate-controlled epoxidation of ketone and highly diastereoselective intramolecular cyclopropanation to construct the cis, trans-3/5/6 tricyclic skeleton.     

Total synthesis of (±)-chamobtusin A

The total synthesis of chamobtusin A, the first diterpenoid alkaloid isolated from the whole Pinales, is described. Key features of the synthesis include a stereoselective intramolecular Michael addition to install a key stereocenter and an oxidative manipulation to prepare a 2H-pyrrole ring.     

Total synthesis of (±) maoecrystal V

A concise first total synthesis of (±) maoecrystal V (1) is reported. The synthesis features a Wessely oxidative dearomatization of a phenol, an intramolecular Diels-Alder reaction, and a Rh-catalyzed O-H bond insertion as key steps.     

Total synthesis of (±)-arteannuin B

The title compound has been synthesized beginning with 4-methyl-2-cyclohexen-1-one, using intramolecular hydroxyl-directed reactions to achieve regio- and stereoselectivity at important stages.     

Total synthesis of (±)-dragmacidin E

The bis indole sponge alkaloid dragmacidin E was synthesized in racemic form over 25 steps starting from 7-benzhydroxyindole. Key steps include (a) a Witkop cyclization to facilitate construction of the indole-spanning seven-membered ring and (b) a cyclodehydrative pyrazinone synthesis that unites the two indole-containing sectors.     

Total synthesis of (±)-aspidophylline A

We report the total synthesis of (±)-aspidophylline A, one of many complex furoindoline-containing alkaloids that has not been synthesized previously. Our route features a number of key transformations, including a Heck cyclization to assemble the [3.3.1]-bicyclic scaffold as well as a late-stage interrupted Fischer indolization to install the furoindoline and construct the natural product's pentacyclic framework.     

Total synthesis of (±)-sacidumlignan D

The first total synthesis of (±)-sacidumlignan D featuring a Zn-mediated Barbier reaction and reverse Wacker oxidation to form the key γ-lactone, its diastereoselective α-methylation followed by reduction cyclization, was documented.     

Total synthesis of (±)-Scrodentoid A

An efficient total synthesis of scrodentoid A was accomplished starting from a Hagemann's ester and a substituted (2-bromoethyl)benzene. Key reactions in this synthesis include C-alkylation of the Hagemann's ester, 6-endo-Trig cationic cyclization of an enone and Lewis acid promoted isomerization of a cis-fused ketone.     

Total synthesis of (±)-cylindricine C

A concise synthesis of (±)-cylindricine C and its C(13)-epimer is described. Starting from 1-octyne, cylindricine C and 13-epi-cylindricine C were prepared in 11% and 15% yields, respectively. The synthesis involves the preparation of the central tricyclic moiety via a radical α-iodoketone carboazidation/bis-reductive amination sequence. Inversion of the stereochemistry at C(13) and C(5) was efficiently achieved on late stage intermediates.