Efficient synthesis of novel cytotoxic cis-fused α-methylene γ-lactones from 7,14-dihydroxy-ent-kaurenes by transformation under Mitsunobu reaction conditions

Facile transformation of 7,14-dihydroxy-ent-kaurenes such as excisanin A (8), kamebanin (9), and kamebakaurin (10), which are abundant in plants of the genus Rabdosia species (Labiatae), to ent-abietanes was accomplished under the Mitsunobu reaction conditions. The δ, ε-unsaturated cis-fused α-methylene γ-lactones (17, 18, and 25) thus prepared showed a moderate cytotoxic activity on P388 murine leukemia cells.     

Studies directed towards the stereoselective total synthesis of ilexlactone via a tandem ring-closing enyne metathesis protocol

Studies directed towards the stereoselective total synthesis of ilexlactone resulted in the synthesis of bicyclic systems 1a, 1b and ent-1a through tandem ring-closing enyne metathesis using Grubbs’ catalyst. The structures of synthetic 1a, 1b and ent-1a revealed that the proposed structure for ilexlactone is incorrect.     

Asymmetric Michael addition using N-cinnamoyl- and N-crotonyl-trans-hexahydrobenzoxazolidin-2-ones

Preparation of N-cinnamoyl- and N-crotonyl-oxazolidin-2-ones 2 and 3 or ent-2 and ent-3 from (4S,5S)- and (4R,5R)-trans-hexahydrobenzoxazolidin-2-ones 1 or ent-1 are reported. Stereoselective copper promoted conjugated additions of Grignard reagents to chiral N-enoyl amides 2 and 3 or ent-2 and ent-3 in the presence of Zn(II) salts afforded the 1,4-addition products 4-11 and the corresponding enantiomers.     

An efficient approach to 2-substituted N-tosylpiperdines: asymmetric synthesis of 2-(2-hydroxy substituted)piperidine alkaloids

We have developed an efficient and a general approach to chiral 2-substituted N-tosylpiperidines starting from chiral α-substituted-N-tosylaziridines. Using this approach, we have synthesized (+)-coniine. The synthesis of chiral N-tosyl-2-piperidinylethanol 15 and ent-15, was achieved from l- and d-aspartic acids, respectively in few steps. Piperidine 15 was converted into 2-(2-hydroxysubstituted)piperidines of type 2 in optically active form. By applying this strategy, asymmetric syntheses of halosaline (R,R)-2a, (+)- and (−)-sedamine 2b, (+)- and (−)-allosedamine 2c, (+)- and (−)-sedridine 2d, (+)- and (−)-allosedridine 2e, (+)-tetraponerine T-3 3a, T-4 3c, T-7 3b, and T-8 3d have been achieved in high yields. These stereoisomers can be interconverted via Mitsunobu inversion in excellent yields.     

Asymmetric synthesis of pentenomycin I, epipentenomycin I, and their analogs

The synthetic utility of the intramolecular acylation of α-sulfinyl carbanions as an efficient and general synthetic approach for the preparation of (−)-pentenomycin I (1) and (−)-epipentenomycin I (5) and their enantiomers (ent-1 and ent-5), starting from chiral (2S,5S,6S)-ester 6 and ent-6, respectively, has been demonstrated. Easy accesses to pentenomycin analogs have also been demonstrated through the Pummerer, Suzuki-Miyaura, and Sonogashira reactions.     

Tandem radical cyclization-based strategy for the synthesis of oxa- and aza-cages: a case of fragmentation versus cyclization

A new strategy for the synthesis of oxa- and aza-cage compounds based on tandem radical cyclizations is described. The iodides 1 lead to oxa-cages 3 after two tandem radical cyclizations. The ester 10aa on reaction with n-Bu₃SnH and AIBN gives rise to the oxa-cage 12aa after two tandem 5-exo-trig cyclizations. On the other hand, reaction of the ketones 17aa and 21 under similar conditions furnished the oxa-cages 20aa and 23, respectively, via a double 5-exo-trig tandem radical cyclization followed by fragmentation.     

Synthetic applications of homoiodo allylsilane II. Total syntheses of (−)-andrographolide and (+)-rostratone

The first total synthesis of (−)-andrographolide (1), an ent-Labdane diterpenoid lactone from Asian medicinal herb Andrographis paniculata, was achieved via the biomimetic cyclization of an epoxy homoiodo allylsilane precursor 7. Asymmetric total synthesis of (+)-rostratone (25), an antipodal Labdane diterpenoid, was also accomplished via similar biomimetic cyclization of a readily accessible epoxy homoiodo allylsilane precursor 18.     

Synthesis of exo enol ether-cyclic ketal isomers of substituted furanmethanol structures related to marine furanocembranoids

Oxidations of the 2-alkenylfurans 8a and 8b, using peroxy reagents, lead to the dienedione 9 and the furan epoxide 10, respectively. Treatment of the epoxide 10 with p-TSA in MeOH produces the enol ether cyclic ketal 12, which is rapidly isomerised to the furanmethanol ether 15, isolated in 80% yield. By contrast, when the propanol-substituted furan epoxide 23 was kept in CDCl₃ containing traces of HCl for 2 h, a 3:2 mixture of Z- and E-isomers of the enol ether spiro ketals 25a and 25b was produced in >92% yield; after 24 h this mixture of isomers underwent dehydration leading to the corresponding enol ether triene 26 (70%). When a solution of the dienedione 9 in H₂O-THF containing p-TSA was stirred at 25 °C for 20 h, the tertiary alcohol 27 was produced which, after a further 20 h was converted into the furan vicinal diol 29. Likewise, when the ‘cembranoid’ dienedione 31 was treated with p-TSA-H₂O, the hydroxymethyl-substituted furanobutenolide 33 was produced in 40% yield. It is probable that the enol ether cyclic hemiketals 28 and 32/34, which are related to 12 and 25, and also to the naturally occurring cembranoids 1 and 2 found in corals, are transient intermediates in the conversions leading to 29 and 33 from 9 and 31, respectively.     

Asymmetric synthesis of both enantiomers of syn-(3-trifluoromethyl)cysteine derivatives

The use of several chiral trifluoromethylated building blocks 1a, 1b, 9a and 9b was attempted to synthesize of syn-(3-trifluoromethyl)cysteine. A novel and efficient enantioselective synthesis of both enantiomers of syn-(3-trifluoromethyl)cysteine derivatives 12a and 12b was successfully achieved.     

Concise route to defined stereoisomers of the hydroxy acid of the chondramides

The use of Kobayashi vinylogous aldol reaction in the reaction with acetaldehyde led to anti-aldol product 11. After reductive removal of the chiral auxiliary, the primary alcohol was converted to the allyliodide 14. This compound could be engaged in an Evans alkylation reaction, leading eventually to hydroxy acid 19. Inclusion of a Mitsunobu inversion reaction on the sequence starting with ent-11 led to hydroxy ester 30, featuring a 6,7-syn-configuration. These hydroxy acids should help to elucidate the correct stereostructure of the chondramide depsipeptides.     

Synthesis of (−)-lentiginosine, its 8a-epimer and dihydroxylated pyrrolizidine alkaloid from d-glucose

The d-glucose derived α,β-unsaturated ester 5 on 1,2-acetonide deprotection, oxidative diol cleavage followed by treatment with N-benzylamine in the presence of NaBH₃CN undergoes reductive amination and a concomitant intramolecular conjugate addition reaction leading to the formation of dihydroxypyrrolidine-ester 6a and monohydroxypyrrolidine-γ-lactone 6b. Intermediates 6a and 6b were efficiently converted to (−)-lentiginosine 3a, its 8a-epimer 3b, and pyrrolizidine azasugar 4 in good overall yield.     

Synthesis of 5-endo-, 5-exo-, 6-endo- and 6-exo-hydroxylated analogues of epibatidine

A convenient, high-yield synthesis of N-Boc-7-azabicyclo[2.2.1]hept-5-en-2-one (7) was developed by SmI₂-mediated desulfonylation of 6. Thus, 5-endo-, 5-exo-, 6-endo-, and 6-exo-hydroxylated epibatidine analogues 2a,b and 3a,b were synthesized from 7 by using a Pd(PPh₃)₄-catalyzed reductive Heck coupling reaction and SmI₂-mediated reduction of the carbonyl group as the key steps. Other reaction conditions for the reductive Heck procedure and the reduction step were also investigated.     

A glycal approach towards an efficient and stereodivergent synthesis of polyhydroxypyrrolidines

A stereo-defined synthesis of two diastereomers of polyhydroxypyrrolidines from 3,4,6-tri-O-benzyl-d-glucal 4 involving a cleavage-recyclization strategy is reported. Hemiacetal 7 obtained from glucal 4, upon reduction with LiAlH₄ afforded diol 8. Selective acetylation of 8 to 11, followed by Mitsunobu cyclization yielded the diversely protected polyhydroxypyrrolidine 12. Oxidation of 11 and subsequent stereoselective reduction led to 20, the C-5 epimer of 11, which upon Mitsunobu cyclization gave polyhydroxypyrrolidine 21. Selective deprotection of the acetyl groups of 12 and 21 were carried out using Na₂CO₃ in MeOH. Polyhydroxypyrrolidines 12 and 21 upon heating with an excess of Mg in MeOH underwent simultaneous N-detosylation and deacetylation to afford amino alcohols 15 and 24, respectively, in quantitative yield. Catalytic hydrogenation of 15 and 24 provided quantitatively the polyhydroxypyrrolidines 2 and 3, respectively.     

3-Amino- and 3-acylamido-2-phosphonopyridines: synthesis by Pd-catalyzed P-C coupling, structure and conversion to pyrido[b]-anellated PC-N heterocycles

Palladium catalyzed cross-coupling of 3-amino- and 3-acylamido-2-bromopyridines 1a-f with triethyl phosphite allowed the synthesis of 3-amino- and 3-acylamido pyridine-2-phosphonic acid diethyl esters 2a-f, whereas nickel catalysts, although providing access to related anilido-2-phosphonates, proved inactive. Reduction of the aminophosphonate 2a with LiAlH₄ afforded 3-amino-2-phosphinopyridine (3a), which was cyclocondensed with dimethylformamide dimethyl acetal (DMFA) via phosphaalkene intermediates 4a to the novel pyrido[b]-anellated 1,3-azaphosphole 5a. Reaction of amidophosphonates 2b-f with LiAlH₄ did not result in the expected reductive cyclization, as shown by closely related anilido-2-phosphonates, but led to product mixtures containing N-secondary 3-amino-2-phosphinopyridines 3b-f as the main or major component. The conversion of 3b,d,e with DMFA to 5b,d,e provides first examples of N-substituted pyrido[b]-anellated azaphospholes. Structures were confirmed by multinuclear NMR and X-ray crystallography (for 2c, 3b).     

Synthesis of the alkaloid tyroscherin by an aldol/Curtius strategy

The alkaloid tyroscherin (2), which contains a vicinal anti-amino alcohol subunit was prepared from 4-hydroxyphenylpropionic acid (5) and meso-diol 9. After desymmetrization of diol 9 and suitable protecting group manipulations, one terminus was extended via a Claisen rearrangement giving rise to enoate ent-15. The missing carbon on the other end could be incorporated using MeMgCl/CuBr·SMe₂ leading eventually to aldehyde ent-22. The acylated oxazolidinone 32 derived from acid 5 and aldehyde ent-22 were combined in an aldol reaction. A subsequent Curtius rearrangement on the carboxylic group furnished the amino function of tyroscherin (2). In a proof of concept study the same strategy was used to prepare tyroscherin analog 28.     

An entry to 1,6-dioxaspiro[4.6]undecanes and 1,7-dioxaspiro[5.6]dodecanes

Ketones 11a-c obtained by iterative alkylation of acetone N,N-dimethylhydrazone with iodides 6 and 8a,b or epoxide 9 followed by SiO₂/H₂O-induced cleavage of the hydrazone were quantitatively transformed into 1,6-dioxaspiro[4.6]undecanes 12a,c and 1,7-dioxaspiro[5.6]dodecanes 12b using Yb(OTf)₃ in CH₃CN.     

Methylene-2-ethynylcyclopropanes: synthesis and biological activity of (Z)- and (E)-9-{[2-ethynyl-2-(hydroxymethyl)cyclopropylidene]methyl}adenine and -guanine

Synthesis of methylene-2-ethynylcyclopropane analogues of nucleosides 12a, 12b, 13a, and 13b is described. Ethyl methylenecyclopropane carboxylate 14 was hydroxymethylated to give alcohol 15, which was reduced to diol 16. Selective protection with tert-butyldimethylsilyl group gave derivative 17, which was oxidized to aldehyde 18. Wittig reaction with CBr₄ gave dibromoalkene 19. Elimination of both bromine atoms afforded methylene-2-ethynylcyclopropane 20. Bromoselenenylation using N-bromosuccinimide and diphenyldiselenide gave intermediate 21. Alkylation of adenine and 2-amino-6-chloropurine with 21 provided the Z,E-isomeric mixtures 22a and 22c. Oxidation afforded selenoxides 23a and 23c. Mild thermolysis furnished methylenecyclopropanes Z-24a, E-24a, and 24c. Deprotection and separation of Z,E-isomers gave adenine analogues 12a and 13a, and 2-amino-6-chloropurine intermediates 12c and 13c. Hydrolytic dechlorination of 12c and 13c afforded guanine analogues 12b and 13b. Adenine Z-isomer 12a inhibits replication of Epstein-Barr virus through its cytotoxicity. The E-isomer 13a is a substrate for adenosine deaminase.     

Intramolecular Heck reaction strategy for the synthesis of functionalised tetrahydroanthracenes: a facile formal total synthesis of the linear abietane diterpene, umbrosone

Rapid annulation employing an intramolecular Heck reaction yielded the functionalised 1,1,10-trimethyl-6-methoxy-1,2,3,4-tetrahydroanthracene 4a, a key intermediate for the linear diterpenoid quinone umbrosone (1), and the related compounds 4b-d. A similar strategy was also successfully adopted for the synthesis of the 9-methyl tetrahydroanthracene ester 5 and the tetrahydrodibenzo[a,d]cycloheptene ester 6.     

Total synthesis of eurypamides, marine cyclic-isodityrosines from the Palauan sponge Microciona eurypa

Total synthesis of eurypamides A, B, and D, 1, 2, and 4, has been successfully accomplished. The Tl(NO₃)₃ (TTN) oxidation of the halogenated bisphenols, 14a, 14b, 24, and 43, effected regio-controlled cyclization to provide the corresponding diaryl ethers, 15a, 15b, 25, and 46. This investigation revealed a structural revision of eurypamide A as to possess (2″S,3″R,4″S)-configuration (47), along with the spectral data of pure 2 and 4, which were previously characterized in a mixture.     

Alkaloids from alkaloids: total synthesis of (±)-7a-epi-hyacinthacine A1 from Z-protected tropenone via Baeyer-Villiger oxidation

Baeyer-Villiger oxidations of several tropane derivatives have been investigated. Whereas tropenones 15a-c underwent exclusive epoxidation to 21a-c, the corresponding 6-oxotropane derivative 28 yielded the desired lactone 29. Baeyer-Villiger oxidation was also possible for the O-isopropylidene-protected diols 32a,b. The resulting lactones 33a,b were employed in the total synthesis of (±)-7a-epi-hyacinthacine A₁ (7a-epi-7) via an intramolecular nucleophilic alkyllithium addition to a carbamate as the key lactamization step. The target compound was prepared from tropenone 15b in 10 steps and 14% overall yield. Enzymatic resolution of pyrrolidine (±)-36 provided a formal total synthesis to both enantiomers of 7.