Heteroatom-directed Wacker oxidations. A protection-free synthesis of (-)-heliophenanthrone

The first enantioselective six-step synthesis of (-)-heliophenanthrone has been achieved without any protection-deprotection protocol at an overall yield of 28%. Key features of this synthesis comprise a heteroatom-directed Wacker oxidation of an internal cyclic olefin in addition to asymmetric Brown allylation and ring closing metathesis (RCM).     

(R)-2,3-O-Cyclohexylideneglyceraldehyde, a Versatile Intermediate for Asymmetric Synthesis of Homoallyl and Homopropargyl Alcohols in Aqueous Medium

Zn-mediated allylation and propargylation of (R)-2,3-O-cyclohexylideneglyceraldehyde (1) in aqueous medium following Luche's procedure afforded the anti homoallylic 3 and homopropargylic 8 alcohols in good yield and with high stereoselectivity. Crotylation of 1 under similar conditions afforded appreciable amounts of erythro-5 and threo-6 alcohols. In each case, the diastereo alcohols are separable by column chromatography. Compound 8 on appropriate chemical manipulation of its functionalities gave the (S)-enantiomer of (R)-13, a useful synthon of LTB(4). Compound 3 on chemical elaboration afforded a diversely functionalized triol derivative 15, a potentially useful synthon for many bioactive compounds.     

Synthesis of the anti-influenza agent (-)-oseltamivir free base and (-)-methyl 3-epi-shikimate

A new enantioselective synthesis of the anti-influenza agent (-)-oseltamivir free base (7.1% overall yield; 98% ee) and (-)-methyl 3-epi-shikimate (16% overall yield; 98% ee) has been described from readily available raw materials. Sharpless asymmetric epoxidation and diastereoselective Barbier allylation of an aldehyde are the key reactions employed in the incorporation of chirality, while the cyclohexene carboxylic ester core was constructed through a ring closing metathesis reaction.     

Asymmetric synthesis of (-)-adaline

[reaction: see text] An enantioselective total synthesis of (-)-adaline has been achieved starting from a chiral 6,6-disubstituted piperidone derivative previously prepared by diastereoselective allylation of a chiral tricyclic N-acyl-N,O-acetal. The key steps include lithium ion-activated SN2-type alkynylation of the tricyclic N,O-acetal leading to exclusive formation of the (6S)-ethynylpiperidine and ring-closing olefin metathesis of the (2R,6S)-cis-2,6-dialkenylpiperidine for constructing the bridged azabicyclononane.     

A stereoselective synthesis of (R)-5-hydroxy-3-(4-methoxyphenethyl)cyclohex-2-enone,towards total synthesis of Prelunularin

A first chiral strategy for synthesis of (R)-5-hydroxy-3-(4-methoxyphenethyl) cyclohex-2-enone reported. Key transformation includes Barbier allylation, ionic hydrogenation, Jeffery reaction, Katsuki Sharpless asymmetric epoxidation, reductive opening of epoxide and ring closing metathesis.     

Total synthesis of (+/-)-alpha-isosparteine, (+/-)-beta-isosparteine, and (+/-)-sparteine from a common tetraoxobispidine intermediate

The three title alkaloids were separately prepared in stereocontrolled fashion from a common tetraoxobispidine precursor, 3,7-diallyl-2,4,6,8-tetraoxo-3,7-diazabicyclo[3.3.1]nonane (16). Bisimide 16 was generated from malonate via acid promoted cyclization of the Knoevenagel condensation adduct 1,1,3,3-propanetetracarboxamide. (+/-)-alpha-Isosparteine (dl-2) was elaborated from 16 in 28% overall yield by a two-directional synthetic sequence composed of four reactions: double addition of allylmagnesium bromide, ring-closing olefin metathesis (RCM), hydrogenation, and borane mediated reduction. (+/-)-beta-Isosparteine (dl-3) was targeted along similar lines by a strategic reversal in allylation and reduction operations on the core synthon. Thus, 16 was advanced to dl-3 in five steps and 12% overall yield by a reaction sequence commencing with sodium borohydride mediated reduction and followed by double Sakurai-type allylation of the resulting bishemiaminal. The synthesis of dl-3 was concluded by RCM and then global reduction (H2, Pd/C; LiAlH4). The final target, (+/-)-sparteine (dl-1), was secured in six steps and 11% overall yield from 16 by monoreduction and Sakurai allylation, followed by allyl Grignard addition and then RCM and global reduction as before. Reasons for the inherent C2-type regioselectivity of net double nucleophilic additions to tetraoxobispidines are discussed and enantioselective oxazaborolidine mediated reduction of the N,N'-dibenzyl congener of 16 is reported.     

Assessing synthetic strategies: total syntheses of (+/-)-neodolabellane-type diterpenoids

Two strategies, namely a cross-metathesis/ring-closing metathesis and Pd-catalyzed Stille allylation/Nozaki-Hiyama-Kishi coupling, are examined for the preparation of neodolabellane-type diterpenoids 1 and 2. Whereas the first approach possessed synthetic limitations, the latter was successfully employed to provide compounds 1 and 2 in 8.8% (14 steps) and 8% (15 steps) overall yields, respectively.     

Combining α-amidoalkylation reactions of N-acyliminium ions with ring-closing metathesis: access to versatile novel isoindolones spirocyclic compounds

A novel approach to diversely spirocyclic isoindoles has been developed by using N-acyliminium/ring-closing metathesis strategy. Spirocyclization precursors, diolefinic, and enyne spiro-fused-isoindole derivatives have been obtained by a regioselective reduction of the spiro-imide compounds, followed by the allylation of the N-acyliminium intermediates (generated from the acetoxylactam compounds). Ruthenium catalyzed ring-closing metathesis of the above unsaturated derivatives provided novel spiroisoindoles.     

A short enantioselective total synthesis of the phytotoxic lactone herbarumin III

The phytotoxic lactone herbarumin III has been synthesized in 11% overall yield. The approach applied uses Keck's asymmetric allylation and Sharpless epoxidation to build the key fragment. Esterification with 5-hexenoic acid and a ring closing metathesis was used to arrive at the target.     

An effective enantioselective approach to the securinega alkaloids: total synthesis of (-)-norsecurinine

[reaction: see text] A highly versatile approach to the enantioselective synthesis of securinega alkaloids is presented. Crucial steps are a palladium-catalyzed enantioselective imide alkylation, a vinylogous Mannich reaction, and a ring-closing metathesis process. Through this strategy, the synthesis of (-)-norsecurinine has been accomplished in nine steps and 11% overall yield.     

Enantiopure eta4-(1-sulfinyldiene)iron(0) tricarbonyl complexes as templates for carbocycle construction via ring-closing metathesis

[reaction: see text] Enantiomerically pure 1-(1Z,3E)-sulfinyl iron(0) dienols have been elaborated to bis-olefins capable of undergoing ring-closing metathesis chemistry. Using Grubbs' ruthenium carbene catalyst, a six-membered carbocycle (with one chiral center) and seven-, eight-, and nine-membered carbocycles (with two chiral centers) have been prepared. Novel transformations include the LiClO4-promoted allylation of an alkylidene malonate and a reductive sulfur-carbon bond cleavage to convert a vinyl sulfoxide into an alkene with SmI2.     

Total syntheses of (S)-(-)-zearalenone and lasiodiplodin reveal superior metathesis activity of ruthenium carbene complexes with imidazol-2-ylidene ligands

Total syntheses of the bioactive orsellinic acid derivatives zearalenone 3 and lasiodiplodin 1 are reported based on a ring-closing metathesis (RCM) reaction of styrene precursors as the key steps. These and closely related macrocyclizations are catalyzed with high efficiency by the "second generation" ruthenium carbene catalyst 5 bearing a N-heterocyclic carbene ligand, whereas the standard Grubbs carbene 4 fails to afford any cyclized product. Only the (E)-isomer of the macrocyclic cycloalkene is formed in all cases. The substrates for RCM can be obtained either via a Stille cross-coupling reaction of tributylvinylstannane or, even more efficiently, by Heck reactions of the aryl triflate precursors with pressurized ethene. Furthermore, the synthesis of 1 via RCM is compared with an alternative approach employing a low-valent titanium-induced McMurry coupling of dialdehyde 47 for the formation of the large ring. This direct comparison clearly ends in favor of metathesis which turned out to be superior in all preparatively relevant respects.     

Biomimetic synthesis of (-)-longithorone A

An enantioseletive, biomimetic synthesis of (-)-longithorone A has been achieved using an intermolecular/transannular Diels-Alder sequence which provides some support for the proposed biosynthesis. The cycloaddition precursors were two [12]-paracyclophanes that were constructed with atropisomer control during ene-yne metathesis macrocyclizations.     

Total synthesis of (−)-diospongin A and (+)-cryptofolione via asymmetric aldol reaction

Stereoselective synthesis of two distinctive pyranone skeletons diospongin A and cryptofolione has been described based on an asymmetric aldol reaction starting from Chan’s diene. The synthetic strategy involves the enantioselective Mukaiyama aldol, diastereoselective reduction of δ-hydroxy-β-keto ester, a tandem sequence of deprotection, and intramolecular oxa-Michael reaction to obtain diospongin A and an asymmetric allylation and lactone formation using ring-closing metathesis reaction to obtain cryptofolione.     

Asymmetric synthesis of (-)-dihydroxanthatin by the stereoselective Oshima-Utimoto reaction

[reaction: see text]. The catalytic stereoselective Oshima-Utimoto reaction is useful for the construction of five-membered oxacycles from simple starting materials and was employed for the preparation of the lactone group in the asymmetric synthesis of (-)-11alpha,13-dihydroxanthatin. Completion of the synthesis is facilitated by ring-closing enyne metathesis and alkene cross metathesis reactions.     

A modular approach to aryl-C-ribonucleosides via the allylic substitution and ring-closing metathesis sequence. a stereocontrolled synthesis of all four α-/β- and D-/L-C-nucleoside stereoisomers

Iridium(I)-catalyzed allylation of the enantiopure monoprotected copper(I) alkoxide, generated from (S)-5a, with the enantiopure allylic carbonates (R)-9a,b has been developed as the key step in a new approach to C-nucleoside analogues. The anomeric center was thus constructed via a stereocontrolled formation of the C-O rather than C-C bond with retention of configuration. The resulting bisallyl ethers 15a,b (≥90% de and >99% ee) were converted into C-ribosides 29a,b via the Ru-catalyzed ring-closing metathesis, followed by a diastereoselective dihydroxylation catalyzed by OsO(4) or RuO(4) and deprotection. Variation of the absolute configuration of the starting segments 5a and 9a,b allowed a stereocontrolled synthesis of all four α/β-D/L-combinations.     

On the structure of passifloricin A: asymmetric synthesis of the delta-lactones of (2Z,5S,7R,9S,11S)- and (2Z,5R,7R,9S,11S)tetrahydroxyhexacos-2-enoic acid

Stereoselective syntheses of the delta-lactone of (2Z,5S,7R,9S,11S)-tetrahydroxyhexacos-2-enoic acid, the structure reported for passifloricin A, and of its (5R)-epimer are described. The creation of all stereogenic centers relied upon Brown's asymmetric allylation methodology. The lactone ring was created via ring-closing metathesis. The NMR data of both synthetic products, however, were different from those of the natural product. The published structure of passifloricin A is thus erroneous and will require further synthetic work to be unambiguously assigned. [structure: see text]     

Total synthesis of antillatoxin

The total synthesis of natural (4R,5R)-antillatoxin and its analog (4S,5S)-antillatoxin has been achieved; the optically pure key intermediates were prepared from indium mediated allylation of either primary or secondary allylic bromide with aldehyde in aquoues media, followed by highly selective Luche's reduction and chiral resolution.     

A general enantioselective route to the chamigrene natural product family

Described in this report is an enantioselective route toward the chamigrene natural product family. The key disconnections in our synthetic approach include sequential enantioselective decarboxylative allylation and ring-closing olefin metathesis to form the all-carbon quaternary stereocenter and spirocyclic core present in all members of this class of compounds. The generality of this strategy is demonstrated by the first total syntheses of elatol and the proposed structure of laurencenone B, as well as the first enantioselective total syntheses of laurencenone C and α-chamigrene. A brief exploration of the substrate scope of the enantioselective decarboxylative allylation/ring-closing metathesis sequence with fully substituted vinyl chlorides is also presented.     

Stereoselective total synthesis of (+)-β-conhydrine from d-mannitol

An asymmetric synthesis of the piperidine alkaloid (+)-β-conhydrine has been developed starting from (R)-protected glyceraldehyde as the chiral precursor using Barbier allylation, azide nucleophilic substitution, and ring-closing metathesis as the key steps.