Scholarly article on topic 'Diterpenoids of terrestrial origin'

Diterpenoids of terrestrial origin Academic research paper on "Chemical sciences"

Share paper
Academic journal
Nat. Prod. Rep.
OECD Field of science

Academic research paper on topic "Diterpenoids of terrestrial origin"

View Online / Journal Homepage

NPR Dynamic Article Links

Cite this: DOI: 10.1039/c2np20051a


Diterpenoids of terrestrial origin

James R. Hanson*

Received 24th April 2012 DOI: 10.1039/c2np20051a

Covering: 2011. Previous review: Nat. Prod. Rep., 2011, 28, 1755-1772

This review covers the isolation and chemistry of the diterpenoids from terrestrial as opposed to marine sources and includes labdanes, clerodanes, abietanes, pimaranes, kauranes, gibberellins, cembranes and their cyclization products and taxanes. There are 161 references.

1 Introduction

2 Acyclic and related diterpenoids

3 Bicyclic diterpenoids

3.1 Labdanes

3.2 Clerodanes

4 Tricyclic diterpenoids

4.1 Abietanes

4.2 Pimaranes

5 Tetracyclic diterpenoids

6 Macrocyclic diterpenoids

6.1 Cembranes, casbanes and their cyclization products

6.2 Lathyranes, jatrophanes and their cyclization products

6.3 Taxanes

7 Miscellaneous diterpenoids

8 References

1 Introduction

This report follows the pattern of its predecessor.1 It covers the isolation and chemistry of diterpenoids that have been isolated from terrestrial sources between January and December, 2011. The isolation of diterpenoids from marine sources is very thoroughly covered in the reports on marine natural products.2 A number of reviews have appeared on specialized topics including the norcembranoids from Sinularia species,3 furanocembranoids from corals4 and on the marine 2,11-cyclized cembranoids, which include the cladiellins, asbestinins and briarellins.5 The tan-shinones have also been reviewed.6 Reviews of theoretical and stereochemical studies of terpene biosynthesis contain sections on the diterpenoids.7'8 Interest in the stereochemical consequences of the various cyclases makes it increasingly important to establish the absolute stereochemistry of novel diterpenoids, particularly as both enantiomeric series are known. It is also important for evaluating the biological activity of the diterpenoids.

Department of Chemistry, University of Sussex, Brighton, BN1 9QJ, UK

2 Acyclic and related diterpenoids

An asymmetric synthesis of (+)-geranyllinaloisocyanide based on (—)-lactic acid methyl ester has led9 to the assignment of its absolute stereochemistry. The anti-malarial activity of Aphana-mixis grandifolia has been associated10 with the presence of acyclic diterpenoids, including melidianolic acid A 1.

3 Bicyclic diterpenoids

3.1 Labdanes

A review has appeared of some diterpene lactones with the labdane, halimane and clerodane skeleta.11 6a,7b-Dihydroxy-labda-8(17),12E,14-triene has been obtained from Fritillaria ebeiensis.12 Leoheteronin F, which was isolated from Leonurus heterophyllus, has been shown13 to be 9a,15-dihydroxylabd-13E-en-7-one and to possess cholinesterase inhibitory activity. 15-Hydroxylabd-8-en-7-one has been associated14 with the leishmanicidal activity of Aeonium lindleyi. The spiro-ether preleosibirone A 2 has been isolated15 as a mildly narcotic constituent of the Chinese medicinal plant, Leonurus sibiricus. Examination of the Indian plant, Rauwolfia tetraphylla which, as a member of the Apocynaceae is well-known as a source of indole alkaloids, has afforded16 a labdane butenolide 3. Further studies of the fruits of the 'chaste tree', Vitex agnus-castus, has yielded17 viteagnusin I 4. The novel labdane 5 with an unusual acetal, has been obtained18 from a Brazilian plant Acritopappus longifolius (Asteraceae). A Diels-Alder adduct labdanecaryophyllic acid 6, involving a labda-12,14-diene and a sesquiterpenoid caryophyllic acid, has been isolated19 from the bark of Calocedrus macrolepis (Cupressaceae). A number of labdanes, including a (4-3a)-methyl rearrangement product

metasequoic acid C 7, and 12a-hydroxy-8,15-isopimaradien-18-oic acid, have been found20 in Metasequoia glyptostroboides. Examination of Austroeupatorium inulifolium has afforded a series of norlabdanes exemplified by inulifolinone A 8.21

Sclareol has continued to provide a valuable starting material of known chirality for partial syntheses, which have included methyl isocopalate,22 spongidines A and D,23 and the ses-terterpenes, 16-deacetoxyscalarafuran24 and its 12-epimer.25 The anti-inflammatory and anti-tumour biological activity of andrographolide has stimulated a number of structure:activity studies in this area.26-28 The diacetate of 14-deoxy-11,12-dehy-droandrographolide has been isolated29 from Andrographis wightiana.

The potential application of forskolin derivatives in the treatment of cardiovascular disease has led to the synthesis of some triazole derivatives.30 The biological activity of bi- and tricyclic diterpenoids from Salvia species has been reviewed.31

3.2 Clerodanes

New clerodanes, including 6a,7a-dihydroxykolavenol 9 and 16,18-dihydroxykolavenic acid lactone have been isolated from Ptychopetalum olacoides,32 Phlomis bracteosa,33 and Cyphos-temma greveana.34 The Lamiaceae continue to provide a variety of clerodanes, which often have a common oxygenation pattern and differ only in the nature and position of esters. These have included the ajubractins A-D (e.g. A, 10) from Ajuga bracteosa35 the ajugaciliatins A-J from A. ciliata,36,37 15-deme-thoxyscupolin I from Lagochilus leicanthus38 and some phytotoxic clerodanes from Salvia miniata.39 The latter also included a 9:10-seco compound 11. Further investigations of the Chinese medicinal herb, Scutellaria barbata has yielded scutabata H40 and some more nicotinyl esters known as the barbatellarines41 and scutebarbatines.42 The typical clerodane side chain has been partially degraded in scutebarbatine M 12. A series of clerodanes have previously been obtained from the Mediterranean golden germander, Teucrium polium and further related examples, the teupolins VI and VII, have now been reported.43

The biological application of salvinorin A as an opioid receptor probe has stimulated further studies on the preparation of derivatives, such as Diels-Alder adducts of the furan ring.44 The synthesis of salvinorin F has been reported.45 An enantioselective synthesis of salvileucalin B has been described46 in which a key step was a copper-catalyzed arene cyclopropanation to form the unusual norcaradiene core. The full account of the total synthesis of (—)-saudin, which led to the determination of its absolute configuration, has appeared.47

The anti-inflammatory activity of the clerodane benzoyl ester 13 obtained from Dodonaea polyandra has been reported.48 Further clerodanes have been isolated from Dioscorea bulbifera49 whilst a series of cis-clerodanes, the casearupestrins, have been obtained50 from Casearia rupestris. The absolute stereochemistry of these diterpenoids needs to be established.

43 Рч

4 Tricyclic diterpenoids

4.1 Abietanes

The norditerpenoid isolophanthin C, podocarpa-8,11,13-triene-3b,13-diol, along with the corresponding abieta-8,11,13-triene-3b,15-diol (isolophanthin A) were isolated51 from Isodon lophanlhoides varn. gerardianus during an investigation into its anti-viral constituents, which showed anti-hepatitis B activity. The synthesis and anti-plasmodial activity of some 8p,13p-dihydroxypodocarpane derivatives has been reported.52

A number of phenolic and quinonoid abietanes, which have been isolated, possess cytotoxic and anti-inflammatory activity. These include ramentoxide 14 from Amentotaxus formosana,53 taxusabietane A 15 from Taxus wallichiana,54 triptobenzene Y 16 from Tripterygium wilfordii55 and the quinol 17 from Hyssopus cuspidatus.56 The latter inhibited leukotriene excretion. Examination of Isodon rubescens, which is widely used in Chinese folk medicine, has afforded57 the unusual b-lactone, rubesanolide A 18. The biologically-active quinones 19 and 20 have been isolated from Platycladus orientalis58 and Pygmacopremna herbacea,59 respectively. Further studies have been reported60-62 on the synthesis and biological activity of the tanshinones from the Chinese folk medicine, Dan-shen (Salvia miltiorrhiza). Given the well-established chemistry and availability of the resin acids, such as abietic acid and podocarpic acid, there is a good case for the partial synthesis and further structure:activity studies in this area. The synthesis of (—)-taiwaniaquinone H and (—)-taiwa-niaquinol B and of triptodiolide from abietic acid, have been reported.63'64

Further examples of abietane rearrangement products have been isolated, including calocetriol 21 from Calocedrus macro-lepis varn. formosana65 and salviskinone A 22 from Salvia prze-walskii.66 The synthesis of the seco-ring A abietane quinone, prionoid E, has been reported.67 Some 17(15/16)-abeo-abie-tanes, including 23, have been obtained from Clerodendrum kaichianum.68,69 Investigations70 into the inhibition of the oncogenic transcription factor AP-I by compounds from Podocarpus latifolius have yielded some sempervirol analogues, including cycloinumakiol 24.

4.2 Pimaranes

Isopimaric and sandaracopimaric acids have been identified71 as the constituents of the Chinese traditional medicine, Biota orientals, which act as modulators of GABAa receptor activity and decrease locomotor activity in mice. A number of novel pimaranes have been isolated from fungi, including some new virescenoside glycosides from Acremonium striatisporum72 and the 19-glucopyranoside of 3b.19-dihydroxyisopimara-7,15-diene from an endophytic Paraconiothyrium species,73 which had been isolated from a beech tree. The eutypellones (e.g. A, 25) were obtained74 from an endophytic Eutypella species whilst the smardaesidins (e.g. A, 26) were obtained75 from a Smardaea species, which was an endophyte of a 'fire moss', Ceratodon purpureus. Whether these diterpenes have a role in the endo-phyte:plant relationship, for example, as anti-bacterial agents, is not clear. A norpimarane glycoside, xylopimarane 27 with an aromatic ring B, has been isolated76 along with the pimarane,

sphaeropsidin C, from a Xylaria species. The anti-bacterial activity of the sphaeropsidins has been examined77 towards the bacterium, Xanthomonas oryzae, the causative organism of rice bacterial blight.

(Verbenaceae) has been the source of some further icetexanes, such as latifolional 34 and a dimer premnalatifolin A.86'87

Cassane diterpenoids are characteristic constituents of Eryth-rophleum and Caesalpinia species. Further examples have been found in E. fordii,78 and C. echinata79 whilst the pulcherrins D-R (e.g. D, 28) were anti-inflammatory constituents of the roots of C. pulcherrima.80 The absolute configuration of the 7,8-seco-7,8-epoxycassane 29 obtained from another member of the Legu-minosae, Acacia schaffneri, was established81 by making use of vibrational circular dichroism measurements on the corresponding lactone.

The pimarane dienone 30, isolated from Croton insularis (Euphorbiaceae) slowly decomposed to spruceanol 31 and may form a biogenetic link between the pimarane and cleistanthol series.82 A stereocontrolled synthesis has been reported83 of 3b-hydroxy-9b-pimara-7,15-diene, which is a possible biosynthetic intermediate in the formation of the momilactones possessing a syn-9:10 relationship.

Cuzcol 32 is a rosane which was obtained84 from Maytenus cuzcoina whilst the simiranes (e.g. A, 33) are erythroxylanes that were obtained85 from the bark of a Brazilian tree, Simira eli-ezeriana (Rubiaceae). The Indian plant Premna latifolia

5 Tetracyclic diterpenoids

ent-Kaurenoic acids, which are widespread amongst the Asteraceae, have been used88 to group various Leontopodium species. L. alpinum is the Alpine Edelweiss which has a long tradition of use in European folk lore. These acids have also been evaluated89 for their anti-cariogenic activity. Stevioside is now widely used as a non-nutritive sweetener, particularly for anti-diabetic drinks. Its biological activity has been reviewed90 whilst some derivatives have shown moderate anti-tubercular activity,91 Some further steviol glycosides have been isolated from Stevia rebaudiana.92'93 The bio-transformation of an iso-steviol ring D lactam has been investigated.94 The pterisolic acids A-F, which were isolated95 from the fern, Pteris semi-pinnata, include some steviol relatives (e.g. C, 35). ent-11p-Acetoxy-18-hydroxykaur-16-ene from Croton tonkinensis has been shown96 to stimulate osteoblast formation. Among other ent-kauranes that have been isolated are 15a-angeloyloxy-16,17-epoxykauran-19-oic acid from Chromoleana odorata97 wedelidin A (3a-cinnamoyloxy-17-hydroxykaur-15-ene-19-oic acid) from Wedelia trilobata98 and 19-acetoxy-3b,16a,17-trihydroxykaurane from the Chinese raspberry, Rubus corchorifolius.99

Isodon (Lamiaceae) species, many of which are used in Chinese traditional medicine, have continued to be a fruitful source of kauranoid diterpenes. Recent isolates include isoadenolin A 36 from I. adenolomus.100 isodohenrins A-E (e.g. A, 37) from I. henryi,101 and isorubesin A 38 from I. rubescens.102'103 The stereochemistry of the 15,16-seco-ent-kaurane rubescensin S, has been revised to 39.104 Dimeric ent-kauranoids with a bridge between C-17 and C-170 have been isolated from I. excisus105 and I. leucophyllus.106

Examination of Euphorbia fischeriana, which is used in Chinese traditional medicine, has yielded some atisenes, including alboatisin A 40.107 The inhibitory activity of some stemodin analogues and rearrangement products against lipid peroxidation, cyclo-oxygenase activity and human tumour cell proliferation, has been reported.108 Some aphidicolin analogues have been isolated from the fungus, Tolypocladium inflatum109 whilst four aphidicolin biosynthetic genes have been expressed in Aspergillus oryzae.110

6 Macrocyclic diterpenoids

6.1 Cembranes, casbanes and their cyclization products

Serratol 43, which was isolated118 from Boswellia serrata (Indian frankincense) has been shown to have anti-protozoal activity. Examination of the leaves of the South African tree, Croton gratissimus has afforded119 ten new cembranolides exemplified by the methyl ether 44. The NMR spectra of some casbanes have been assigned,120 including that of 45 obtained from Croton argyrophyllus.

The bio-transformation of the ent-trachylobane, trachinodiol by Mucor plumbeus gave not only hydroxylated products but also led to cleavage of the cyclopropane ring and the formation of ent-kaur-11-enes.111

The synthesis of probes for the photo-affinity labelling of gibberellin receptors has been described.112 An interesting example of neighbouring group participation has been observed113 in the reaction of 7-oxo-ent-kaur-16-en-18-oic acid with diacetoxyiodobenzene. This brought about ring contraction and the formation of 4-epigibberellin A12 isolated as its methyl ester. A review of glycosyl conjugates of plant hormones includes a discussion of gibberellin glycosides.114

Some highly acylated 3,4-seco-grayanotoxins have been obtained115'116 from the fruits of Pieris formosa (Ericaceae) and include pierisformotoxin G 41. Lyonin A 42 is a 9,10-seco-grayanotoxin, which was isolated117 from another member of the Ericaceae, Lyonia ovalifolia.

6.2 Lathyranes, jatrophanes and their cyclization products

Diterpenoids with the lathyrane and jatrophane skeleta together with their further cyclization products are very common as the bioactive constituents of the Euphorbiaceae. Many plants from this family are used in traditional folk medicines whilst others are noted for their toxicity. The lathyrane 46 was among those isolated121 from the Chinese plant Euphorbia micractina whilst esulatin I 47 was an anti-proliferative agent that was obtained122 from E. esula. The euphodendrophanes A-F (e.g. A, 48) are a group of polyesters, which include nicotinic acid derivatives, that were obtained123 from the Montenigran spurge, E. den-droides. Examination of an Iranian species, E. bungei gave124 the jatrophane 49.

The total synthesis of jatrophane diterpenes and their evaluation as MDR modulators has been reported.125 Jatropha curcas is another member of the Euphorbiaceae which has been grown in very arid conditions for the production of oil from its seeds. The tricyclic jatrophodione A 50 is one of a number of compounds that have been isolated126 from this plant.

The daphnane and tigliane diterpenoids are also common constituents of the Euphorbiaceae. Trigoheterin A 51 is an example which was obtained127 from the Chinese plant, Trig-onostemon heterophyllus whilst the benzoate ortho-esters trig-ohownin A 52128 and trigonosin A 53129 were among the constituents of T. howii and T. thyrsoideum, respectively. The related stelleralides A-C, which have potent anti-HIV activity, were isolated130 from Stellera chamaejasme (Thymeleaceae). These compounds have a structural similarity to prostratin, which is also under investigation for its anti-HIV activity.

The biological activity of the constituents of the Chinese drug, Euphorbia fischeriana (Lang-du) have been reviewed,131 The tigliane glycoside, fischeroside A 54 was among the glycosides that have recently been described132 from this source. Tigliane diterpenoids, such as phorbol, are also responsible for the skin irritant properties of the milky latex of plants from the spurge family and for their activity as tumour promoters. Another

phorbol relative is the polyhydroxy ester, 55, that was isolated133 from E. grandicornis.

The myrsinanes, which possess a related carbon skeleton, are also found in the Euphorbiaceae. Some premyrsinane polyesters134 and the myrsinol euphorbiaproliferins A-J (e.g. A, 56)135 have been found in E. falcata and E. prolifera, respectively.

6.3 Taxanes

There has been continued interest in the taxanes. A novel taxa-4(20),12-diene and a 2(3 / 20)-abeo-taxane 57 have been isolated136 from the needles of Taxus canadensis whilst a further taxane has been obtained137 from T. cuspidata. A retro-aldol fragmentation reaction of 13-oxotaxyunnansin A has been observed,138 which has application in the synthesis of paclitaxel analogues. Biosynthetic studies139 have suggested that both the mevalonate and non-mevalonate pathways may be operative when paclitaxel is produced by fungal endo-phytes, such as Paraconiothyrium species. Mass spectroscopic analysis of the fluctuations in the metabolites of Taxus seedlings has been used140 to derive biosynthetic sequences amongst the taxanes.

7 Miscellaneous diterpenoids

Examination of Jatropha curcas has afforded141 spirocurcasone 58 as well as the rhamnofolane, curcusone E 59. The unusual cyclobutene salvileucalin C 60 and the related diene salvileucalin D 61 have been obtained142 from the Mexican sage, Salvia leu-cantha. The stereochemistry of leubethanol 62 has been assigned.143 This serrulatone from the root bark of the Mexican medicinal plant, Leucophyllum frutescens was reported to be active against resistant strains of tuberculosis. 17-Acetoxy-13a-hydroxyazorellane has been isolated144 from Azorella mod-reporica whilst a further fusicoccane was obtained145 from the South American liverwort, Porella chilensis.

43 Рч

A number of microbial metabolites have been found with diterpenoid skeleta, including 17-hydroxycyclooctatin 63 from a Streptomyces species,146 conidiogenone H 64 from a marine strain of Penicillium chrysogenum147 and an indolic diterpene, 17-oxoeujindole 65 from Eupenicillium javanicum.148 The fungal metabolite, pleuromutilin, has continued to attract interest for the development of antibiotics for the treatment of MRSA infections. A series of rearrangements have been uncovered in the course of these studies.149 Further derivatives have also been prepared.150,151

The unusual skeleta, structural complexity and the biological activity of the diterpenoids have acted as the stimuli for a number of syntheses that have been reported during the year. These include syntheses of pseudolaric acid A,152 isofregenedadiol,153 the guanacastepenes N and O,154 scabronine G,155 the cyanthi-wigins B,F and G,156 basiliolide,157 przewalskin B,158 vigulariol,159 kempene-2,160 and rippertenol.161

8 References

1 J. R. Hanson, Nat. Prod. Rep., 2011, 28, 1755-1772.

2 J. W. Blunt, B. R. Copp, M. H. G. Munro, P. T. Northcote and M. R. Prinsep, Nat. Prod. Rep., 2011, 28, 196-268.

3 Y. Li and G. Pattenden, Nat. Prod. Rep., 2011, 28, 429-440.

4 Y. Li and G. Pattenden, Nat. Prod. Rep., 2011, 28, 1269-1310.

5 A. J. Welford and I. Collins, J. Nat. Prod., 2011, 74, 2318-2328.

6 Y. Dong, S. L. Morris-Natschke and K.-H. Lee, Nat. Prod. Rep., 2011, 28, 529-542.

7 D. J. Tantillo, Nat. Prod. Rep., 2011, 28, 1035-1053.

8 J. S. Dickschat, Nat. Prod. Rep., 2011, 28, 1917-1936.

9 Y. Ichikawa, Y. Matsuda, K. Okumura, M. Nakamura, T. Matsuda, H. Kotsuki and K. Nakano, Org. Lett., 2011, 13, 2520-2523.

10 A. Astulla, Y. Hirasawa, A. Rahman, I. Kusumawati, W. Ekasari, A. Widyawaruyanti, N. C. Zaini and H. Morita, Nat. Prod. Commun., 2011, 6, 323-326.

11 L. Silva, A. C. Gomes and J. M. L. Rodilla, Nat. Prod. Commun., 2011, 6, 497-504.

12 J. Xu, C. Liu, P. Guo, Y. Guo, D.-Q. Jin, X. Song, Z. Sun, L. Gui and Y. Ma, Fitoterapia, 2011, 82, 772-776.

13 T. M. Hung, T. C. Luan, B. T. Vinh, T. D. Cuong and B. S. Min, Phytother. Res., 2011, 25, 611-614.

14 M. L. Kennedy, F. Cortes, J. E. Pinero, S. Castanys, A. Lopez-Arencibia, F. Garnarro, I. L. Bazzocchi and I. A. Jimenez, Planta Med., 2011, 77, 77-80.

15 H. Wum, F. R. Fronczek, D. Ferreira, C. L. Burandt and J. K. Zjawiony, J. Nat. Prod., 2011, 74, 831-836.

16 G. Brahmachari, L. C. Mandal, D. Gorai, A. Mondai, S. Sarkar and S. Majhi, J. Chem. Res., 2011, 35, 678-680.

17 S. N. Chen, J. B. Friesen, D. Webster, D. Nikolic, R. B. van Breemen, Z. J. Wang, H. H. S. Fong, N. R. Farnsworth and G. F. Pauli, Fitoterapia, 2011, 82, 528-533.

18 F. P. Ferreira, E. H. Marturano, C. A. Carollo and D. C. Rodrigues de Oliveira, Tetrahedron Lett., 2011, 52, 297-299.

19 C.-H. Hsien, M.-H. Tseng, R.-N. Pan, J.-Y. Chang, C.-C. Kuo, T.-H. Lee and K.-H. Kuo, Tetrahedron Lett., 2011, 52, 515-517.

20 L.-B. Dong, J. He, Y.-Y. Wang, X.-D. Wu, X. Deng, Z.-H. Pan, G. Xu, L.-Y. Peng, Y. Zhao, Y. Li, X. Gong and Q.-S. Zhao, J. Nat. Prod., 2011, 74, 234-239.

21 Y. Saito, S. Matsuo, S. Sutoyo and M. Tori, Helv. Chim. Acta, 2011, 94, 313-326.

22 S.-K. Hua, J. Wang, X.-B. Chen, Z.-Y. Xu and B.-B. Zeng, Tetrahedron, 2011, 67, 1142-1144.

23 P. Basabe, A. Bianco, I. S. Marcos, D. Diez, O. Badero, M. Martin and J. G. Urones, Tetrahedron, 2011, 67, 3649-3658.

24 W.-Y. Fan, Z.-L. Wang, Z.-G. Zhang, H.-C. Li and W.-P. Deng, Tetrahedron, 2011, 67, 5596-5603.

25 Z.-L. Wang, Z.-G. Zhang, H.-C. Li and W.-P. Deng, Tetrahedron, 2011, 67, 6939-6943.

26 S. Panneerselvam, G. Arumugam and N. S. S. M. Karthikeyan, Nat. Prod. Res., 2011, 25, 1560-1564.

27 S.-P. Guan, L.-R. Kong, C. Cheng, J. C. W. Lim and W. S. F. Wong, J. Nat. Prod., 2011, 74, 1484-1490.

28 H.-W. Chen, A.-H. Lin, H.-C. Chu, C.-C. Li, C.-W. Tsai, C.-Y. Chao, C.-J. Wang, C.-K. Lii and K.-L. Li, J. Nat. Prod., 2011, 74, 2408-2413.

29 J. M. Sudhana, R. Munikishore, M. V. Reddy, D. Gunasekar, A. Blond and B. Bodo, Nat. Prod. Commun., 2011, 6, 1569-1572.

30 M. K. Reddy, K. S. Kumar, P. Sreenivas, G. L. D. Krupadanam and K. J. Reddy, Tetrahedron Lett., 2011, 52, 6537-6540.

31 M. C. Bonito, C. Cicala, M. C. Marcotullio, F. Maione and N. Mascolo, Nat. Prod. Commun., 2011, 6, 1205-1215.

32 W. Tang, K. Harada, M. Kubo, H. Hioki and Y. Fukuyama, Nat. Prod. Commun., 2011, 6, 327-332.

33 J. Hussain, R. Ullah, A. Khan, F. U. Khan, Z. Muhammad and M. R. Shah, Nat. Prod. Commun., 2011, 6, 171-173.

34 S. Cao, Y. Huo, P. Brodie, J. S. Miller, R. Randrianaivo, E. Rakotobe, V. E. Rasamison and D. G. I. Kingston, Chem. Biodiversity, 2011, 8, 643-650.

35 A. Castro, J. Coll and M. Arfan, J. Nat. Prod., 2011, 74, 1036-1041.

36 P. Guo, Y. Li, J. Xu, Y. Guo, D.-Q. Jin, J. Gao, W. Hou and T. Zhang, Fitoterapia, 2011, 82, 1123-1127.

37 P. Guo, Y. Li, J. Xu, C. Liu, Y. Ma and Y. Guo, J. Nat. Prod., 2011, 74, 1575-1583.

38 M. Furukawa, H. Suzuki, M. Makino, S. Ogawa, T. lida and Y. Fujimoto, Chem. Pharm. Bull., 2011, 59, 1535-1540.

39 A. Bisio, G. Damonte, D. Fraternale, E. Giacomelli, A. Salis, G. Romussi, S. Cafaggi, D. Ricci and N. De Tommasi, Phytochemistry, 2011, 72, 265-275.

40 F. Zhu, Y.-T. Di, X.-Y. Li, L.-L. Liu, Q. Zhang, Y. Li, X.-J. Hao and H.-P. He, Planta Med., 2011, 77, 1536-1541.

41 H. Lee and S. H. Shim, Helv. Chim. Acta, 2011, 94, 643-649.

42 S.-J. Dai, W.-B. Peng, L. Shen, D.-W. Zhang and Y. Ren, Nat. Prod. Res., 2011, 25, 1019-1024.

43 A. Fiorentino, B. D'Abrosca, S. Pacifico, M. Scognamiglio,

G. D'Angelo, M. Gallicchio, A. Chambery and P. Monaco, Phytochemistry, 2011, 72, 2037-2044.

44 A. Lozama, C. W. Cunningham, M. J. Caspers, J. T. Douglas, C. M. Dersch, R. B. Rothman and T. E. Prisinzano, J. Nat. Prod., 2011, 74, 718-726.

45 A. Hagiwara, T. Nojima, Y. Suka, T. Hoshi and T. Suzuki, Nat. Prod. Commun., 2011, 6, 333-335.

46 S. Levin, R. R. Nani and S. E. Reisman, J. Am. Chem. Soc, 2011, 133, 774-776.

47 R. K. Boeckman, M. R. del R. Ferreira, L. H. Mitchell, P. Shao, M. J. Neeb and Y. Fang, Tetrahedron, 2011, 67, 9787-9808.

48 B. S. Simpson, D. J. Claudie, J. P. Gerber, S. M. Pyke, J. Wang, R. A. McKinnon and S. J. Semple, J.Nat. Prod., 2011, 74, 650-657.

49 K. Kidyu, H. Thaisuchat, P. Meepowpan, S. Sukdee, N. Nuntasaen, S. Punyanitya and W. Pompimon, Nat. Prod. Commun., 2011, 6, 1069-1072.

50 G. M. Vieira Jr, L. A. Dutra, P. M. P. Ferreira, M. O. de Moraes, L. V. C. Lotufo, G. D. Pessoa, R. B. Torres, N. Boralle, V. D. S. Bolzani and A. J. Cavalheiro, J. Nat. Prod., 2011, 74, 776-781.

51 L.-B. Yang, L. Lin, S.-X. Huang, J.-X. Pu, Y. Zhao, Y.-B. Ma, J.-J. Chen, C.-H. Leng, Z.-M. Tao and H. D. Sun, Chem. Pharm. Bull., 2011, 59, 1102-1105.

52 R. M. Young, W. L. Popplewell, M. R. Caira, R. L. van Zyl and M. T. Davies-Coleman, J. Chem. Res., 2011, 35, 18-23.

53 H.-L. Chen, K.-W. Lin, K.-H. Gan, J.-P. Wang, S.-J. Won and C.-N. Lin, Fitoterapia, 2011, 82, 219-224.

54 I. Khan, M. Nisar, M. R. Shah, H. Shah, S. N. Gilani, S. M. Abdullah, M. Ismail, N. Khan, W. A. Kaleem, M. Qayum,

H. Khan and M. Ullah, Fitoterapia, 2011, 82, 1003-1007.

55 J. Xu, J. Lu, F. Sun, H. Zhu, L. Wang, X. Zhang and Z. Ma, Phytochemistry, 2011, 72, 1482-1487.

56 M. Furukawa, M. Makino, E. Ohkoshi, T. Uchiyama and Y. Fujimoto, Phytochemistry, 2011, 72, 2244-2252.

57 J. Zou, L. Pan, Q. Li, J. Zhao, J. Pu, P. Yao, N. Gong, Y. Lu, T. P. Kondratyuk, J. M. Pezzuto, H. H. S. Fong, H. Zhang and H. D. Sun, Org. Lett., 2011, 13, 1406-1409.

58 S.-Y. Fan, Y.-H. Pei, H.-W. Zeng, S.-D. Zhang, Y.-L. Li, L. Li, J. Ye, Y.-X. Pan, H.-L. Li and W.-D. Zhang, Planta Med., 2011, 77, 1623-1630.

59 K. Satish, M. V. P. S. Vishnuvardhan, V. L. Nayak, G. Srihari, M. Subrahamanyan, T. P. Rao, S. Ramakrishna, K. Ravikumar,

B. Sridhar and M. M. Murthy, Bioorg. Med. Chem. Lett., 2011, 21, 4581-4584.

60 M. M. Cummings, R. W. Clawson, S. Sharma, R. A. Byerly, N. G. Alchmedev and B. C. B. Soderberg, Tetrahedron, 2011, 67, 4753-4757.

61 X. Wang and J. H. K. Young, Fitoterapia, 2011, 82, 539-545.

62 S. Slusarczyk, S. Zimmermann, M. Kaiser, A. Matkowski, M. Hamburger and M. Adams, Planta Med., 2011, 77, 1594-1596.

63 X. Liao, L. M. Stanley and J. F. Hartwig, J. Am. Chem. Soc., 2011, 133, 2088-2091.

64 B. Zhou, H. Tang, H. Feng and Y. Li, Tetrahedron, 2011, 67, 904909.

65 C.-L. Hsieh, M.-H. Tseng, R.-N. Pan, J.-Y. Cheng, C. C. Kuo, T.-H. Lee and Y. H. Kuo, Chem. Biodiversity, 2011, 8, 1901-1907.

66 A. Ohsaki, S. Kawamata, M. Ozawa, A. Kishida, X. Gong and

C. Kuroda, Tetrahedron Lett., 2011, 52, 1375-1377.

67 F. Dong, J. Xu, M. Zhao, H.-Y. Liu, Y. Ye and J.-S. Zhang, Helv. Chim. Acta, 2011, 94, 1326-1334.

68 M. Xu, L. Shen, K. Wang, Q. Du and N. Wang, Nat. Prod. Commun., 2011, 6, 3-5.

69 M. Xu, L. Shen, K. Wang and Q. Du, Helv. Chim. Acta, 2011, 94, 539-544.

70 K. P. Devkota, R. Ratnayake, N. H. Colburn, J. A. Wilson, C. J. Henrich, J. B. McMahon and J. A. Beutler, J. Nat. Prod., 2011, 74, 374-377.

71 J. Zaugg, S. Khom, D. Eigenmann, I. Baburin, M. Hamburger and S. Hering, J. Nat. Prod., 2011, 74, 1764-1772.

72 S. S. Afiyatullov, A. I. Kalinovsky and A. S. Antonov, Nat. Prod. Commun., 2011, 6, 1063-1068.

73 Y. Shiono, M. Kikuchi, T. Koseki, T. Murayama, E. Kwon, N. Aburai and K. I. Kimura, Phytochemistry, 2011, 72, 1400-1405.

74 M. Isaka, S. Palasarn, W. Prathumpai and P. Laksanacharoen, Chem. Pharm. Bull., 2011, 59, 1157-1159.

75 X.-N. Wang, B. P. Bashyal, E. M. K. Wijeratne, J. M. U'Ren, M. X. Liu, M. K. Gunatilaka, A. E. Arnold and A. A. L. Gunatilaka, J. Nat. Prod., 2011, 74, 2052-2061.

76 M. Isaka, A. Yangchum, P. Auncharoen, K. Srichomthong and P. Srikitikulchai, J. Nat. Prod., 2011, 74, 300-302.

77 A. Evidente, V. Venturi, M. Masi, G. Degrassi, A. Cimmino, L. Maddau and A. Andolfi, J. Nat. Prod., 2011, 74, 2520-2525.

78 D. Du, L. Fang, J. Qu, S. Yu, S. Ma, H. Lu, J. Liu, Y. Liu, J. Wang and X. Wang, Planta Med., 2011, 77, 1631-1638.

79 B. B. Cota, D. M. de Oliveira, E. P. de Siqueira, E. M. S. Fagundes, A. M. C. Pimenta, D. M. Santos, A. Rabello and C. L. Zani, Fitoterapia, 2011, 82, 969-975.

80 O. Yodsaoue, C. Karalai, C. Pongliamanont, S. Tewtrakul and S. Chantrapromma, Tetrahedron, 2011, 67, 6838-6848.

81 J. J. Manríquez-Torres, J. M. Torres-Valencia, M. A. Gómez-Hurtado, V. Motilva, S. García-Mauriño, J. Avila, E. Talero, C. M. Cerda-García-Rojas and P. Joseph-Nathan, J. Nat. Prod., 2011, 74, 1946-1951.

82 L. A. Maslovskaya, A. I. Savchenko, V. A. Gordon, P. W. Reddell, C. J. Pierce, P. G. Parsons and C. M. Williams, Org. Lett., 2011, 13, 1032-1035.

83 A. Yajima, K. Toda, Y. Okada, H. Yamane, M. Yamamoto, M. Hasegawa, R. Katsuta and T. Nukada, Tetrahedron Lett., 2011, 52, 3212-3215.

84 M. J. Nuí nñez, C. P. Reyes, I. A. Jimíenez, H. Hayashi, H. Tokuda and I. L. Bazzocchi, Phytochemistry, 2011, 72, 385-390.

85 M. F. de Araujo, I. J. C. Vieira, R. Braz-Filho and M. G. de Carvalho, Nat. Prod. Res., 2011, 25, 1713-1719.

86 G. Suresh, K. S. Babu, V. R. Subba Rao, M. S. A. Rao, L. Nayak and S. Ramakrishna, Tetrahedron Lett., 2011, 52, 1273-1276.

87 G. Suresh, K. S. Babu, V. R. Subba Rao, M. S. A. Rao, L. Nayak and S. Ramakrishna, Tetrahedron Lett., 2011, 52, 5016-5019.

88 S. Safer, S. S. Cicek, V. Pieri, S. Schwaiger, P. Schneider and V. Wissemann, Phytochemistry, 2011, 72, 1379-1381.

89 B. B. de Andrade, M. R. Moreira, S. R. Ambrosio, N. A. J. C. Furtado, W. R. Cunha, V. C. G. Heleno, A. N. Silva, M. R. Simao, E. M. P. da Rocha, C. H. G. Martins and R. C. S. Veneziani, Nat. Prod. Commun., 2011, 6, 777-780.

90 G. Brahmachari, L. C. Mandal, R. Roy, S. Mondal and A. K. Brahmachari, Arch. Pharm. (Weinheim), 2011, 344, 5-19.

91 R. R. Sharipova, I. Y. Strobykina, G. G. Mordovskoi, R. V. Chestnova, V. F. Mironov and V. E. Kataev, Chem. Nat. Compd., 2011, 46, 902-905.

92 V. S. P. Chaturvedula, J. Rhea, D. Milanowski, U. Mocek and I. Prakash, Nat. Prod. Commun., 2011, 6, 175-178.

93 V. S. P. Chaturvedula and I. Prakash, Nat. Prod. Commun., 2011, 6, 1059-1062.

94 B.-H. Chou, L.-M. Yang, S.-F. Chang, F.-L. Hsu, L.-H. Wang, W.-K. Lin, P.-C. Liu and S.-J. Lin, J. Nat. Prod., 2011, 74, 13791385.

95 F. Wang, Y.-J. Li, F. C. Ren, G.-Z. Wei and J. K. Liu, Chem. Pharm. Bull., 2011, 59, 484-487.

96 T.-T. Dao, K.-Y. Lee, H.-M. Jeong, P.-H. Nguyen, T. L. Tran, P.-T. Thuong, B.-T. Nguyen and W.-K. Oh, J. Nat. Prod., 2011, 74, 2526-2531.

97 P. Wato, R. S. T. Kamdem, Z. Ali, S. Ahjum, A. Begum, O. O. Oluyemisi, S. N. Khan, B. T. Ngadjui, X. F. Etoa and M. I. Choudhary, Fitoterapia, 2011, 82, 642-646.

98 Y. Qiang, D.-L. Du, Y.-J. Chen and K. Gao, Helv. Chim. Acta, 2011, 94, 817-823.

99 M. Zhang, Y.-W. Ou, X.-X. Chen, Y. Cao, Y. Kuang, Z.-Q. Gong, S. Peng and Y.-J. Chen, Helv. Chim. Acta, 2011, 94, 1820-1824.

100 W. Zhao, J.-X. Pu, X. Du, J. Su, X.-N. Li, J.-H. Yang, Y.-B. Xue, Y. Li, W. L. Xiao andH. D. Sun, J. Nat. Prod., 2011,74, 1213-1220.

101 Z. Hu, R. Zhan, X. Du, J. Su, X.-N. Li, J.-H. Yang, H.-B. Zhang, Y. Li, H. D. Sun, G. P. Li and J.-X. Pu, Chem. Pharm. Bull., 2011, 59, 1562-1566.

102 X.-M. Gao, X. Luo, J.-X. Pu, Y.-L. Wu, Y. Zhao, L. B. Yang, F. He, X. N. Li, W.-L. Xiao, G.-Q. Chen and H. D. Sun, Planta Med. ,2011, 77, 169-174.

103 R. J. Xie, F.-L. Yan, G.-F. Hai, R.-J. Hou, M.-M. Ding and Y.-X. Bai, Fitoterapia, 2011, 82, 726-730.

104 M. Zhang, Y. Zhang, W. Lu and F.-J. Nan, Org. Biomol. Chem., 2011, 9, 4436-4439.

105 S. S. Hong, S. A. Lee, C. Lee, X. H. Han, S. Choe, N. Kim, D. Lee, C.-K. Lee, Y. Kim, J. T. Hong, M. K. Lee and B. Y. Hwang, J. Nat. Prod., 2011, 74, 2382-2387.

106 H.-B. Zhang, J.-X. Pu, Y. Zhao, F. He, X.-N. Li, X. Luo, L.-G. Luo and H. D. Sun, Tetrahedron Lett., 2011, 52, 6061-6066.

107 L.Yang, S. Liu, B. Zhang and Y. Suo, J. Chem. Res., 2011, 35, 692693.

108 F. A. Russell, V. Mulabagal, D. R. Thompson, M. A. Singh-Wilmot, W. F. Reynolds, M. G. Nair, V. Langer and P. B. Reese, Phytochemistry, 2011, 72, 2361-2368.

109 J. Lin, X. Chen, X. Cai, X. Yu, X. Liu, Y. Cao and Y. Che, J. Nat. Prod., 2011, 74, 1798-1804.

110 R. Fujii, A. Minami, T. Tsukagoshi, N. Sato, T. Sahara, S. Ohgiya, K. Gomi and H. Oikawa, Biosci., Biotechnol., Biochem., 2011, 75, 1813-1817.

111 B. M. Fraga, V. Gonzalez-Vallejo and R. Guillermo, J. Nat. Prod., 2011, 74, 1985-1989.

112 J. M. Crow, P. M. Chandler and L. N. Mander, Aust. J. Chem. ,2011, 64, 471-488.

113 B. M. Fraga, C. Bressa, V. Gonzalez-Vallejo, S. Suarez and R. Guillermo, Tetrahedron Lett., 2011, 52, 7138-7140.

114 A. Piotrowska and A. Bajguz, Phytochemistry, 2011, 72, 2097-2112.

115 Z.-Y. Wu, Y. D. Li, G. S. Wu, H. R. Luo, H.-M. Li and R.-T. Li, Chem. Pharm. Bull., 2011, 59, 492-495.

116 Z.-Y. Wu, H.-M. Li, Y.-D. Li and R.-T. Li, Helv. Chim. Acta, 2011, 94, 1283-1289.

117 Z.-Y. Wu, H.-Z. Li, W.-G. Wang, H.-M. Li, R. Chen, R.-T. Li and

H.-R. Luo, Chem. Biodiversity, 2011, 8, 1182-1187.

118 T. J. Schmidt, M. Kaiser and R. Brun, Planta Med., 2011, 77, 849850.

119 M. K. Langat, N. R. Crouch, P. J. Smith and D. A. Mulholland, J. Nat. Prod., 2011, 74, 2349-2355.

120 F. A. Silva-Filho, R. Braz-Filho, E. R. Silveira and M. A. S. Lima, Magn. Reson. Chem., 2011, 49, 370-373.

121 Y. Tian, W. Xu, C. Zhu, S. Lin, Y. Li, L. Xiong, S. Wang, L. Wang, Y. Yang, Y. Guo, H. Sun, X. Wang and J. Shi, J. Nat. Prod., 2011, 74, 1221-1229.

122 A. Vasas, E. Sulyok, D. Redei, P. Forgo, P. Szabo, I. Zupko, A. Berenyi, J. Molnar and J. Hohmann, J. Nat. Prod., 2011, 74, 1453-1461.

123 I. S. AljanCiC, M. PesiC, S. M. MilosavlejeviC, N. M. TodoroviC, M. Jardanin, G. MilosavlejeviC, D. PovrenoviC, J. BankoviC, N. TaniC, I. D. MarkoviC, S. RuzdijiC, V. E. Vajs and V. V. TeseviC, J. Nat. Prod., 2011, 74, 1613-1620.

124 Y. ShokoohiniC, G. Chianese, B. Zolfaghari, S. E. Sajjadi, G. Appendino and O. Taglialatela-SCafati, Fitoterapia, 2011, 82, 317-322.

125 C. SChnabal, K. Sterz, H. Muller, J. Rehbein, M. Weise and M. Hiersemann, J. Org. Chem., 2011, 76, 512-522.

126 J. J. Xu, J.-T. Fan, G.-Z. Zeng and N.-H. Tan, Helv. Chim. Acta, 2011, 94, 842-846.

127 S.-H. Dong, H.-B. Liu, C.-H. Xu, J. Ding and J.-M. Yue, J. Nat. Prod., 2011, 74, 2576-2581.

128 S.-H. Dong, C. R. Zhang, C.-H. Xu, J. Ding and J.-M. Yue, J. Nat. Prod., 2011, 74, 1255-1261.

129 S.-F. Li, T.-T. Di, S.-L. Li, Y. Zhang, F.-M. Yang, G.-Y. Sun, J. M. Simo, H.-P. He and X.-J. Hao, J. Nat. Prod., 2011, 74, 464469.

130 Y. Asada, A. Sukemori, T. Watanabe, K. J. Malla, T. Yoshikawa, W. Li, K. Koike, C.-H. Chen, T. Akiyama, K. Qian,

K. Nakagawa-Goto, S. L. Morris-NatsChke and K. H. Lee, Org. Lett., 2011, 13, 2904-2907.

131 Y. X. Sun and J. C. Liu, Chem. Biodiversity, 2011, 8, 1205-1214.

132 L.-L. Pan, P.-L. Fang, X.-J. Zhang, W. Ni, L. Li, L.-M. Yang, C.-X. Chen, Y.-T. Zheng, C.-T. Li, X.-J. Hao and H.-Y. Liu, J. Nat. Prod., 2011, 74, 1508-1512.

133 P. Forgo, D. Redei, Z. Hajdu, P. Szabo, L. Szabo and J. Hohmann, J. Nat. Prod., 2011, 74, 639-643.

134 E. Sulyok, A. Vasas, D. Redei, P. Forgo, Z. Kele, G. Pinke and J. Hohmann, Tetrahedron, 2011, 67, 7289-7293.

135 J. Xu, Y. Guo, C. Xia, Y. Li, J. Gao, T. Zhang, W. Hou, L. Fang and L. Gui, J. Nat. Prod., 2011, 74, 2224-2230.

136 Z.-Y. Ni, Y.-B. Wu, M.-L. Zhang, Y.-F. Wang, M. Dong, F. Sauriol, C.-H. Huo, Q. W. Shi, Y.-C. Gu, H. Kiyota and

B. Cong, Biosci., Biotechnol., Biochem., 2011, 75, 1698-1701.

137 Z.-Y. Ni, M. Dong, B. Cong, F. Sauriol, M. L. Zhang, Y.-F. Wang,

C. H. Huo, Q. W. Shi and H. Kiyota, Planta Med., 2011, 77, 281283.

138 Y. Zhao, H.-B. Zhang, J.-K. Liu, J. Su, Y. Li, Z.-J. Yao and

Q. S. Zhao, Tetrahedron Lett., 2011, 52, 139-142.

139 S. S. M. Soliman, R. Tsao and M. N. Raizadi, J. Nat. Prod., 2011, 74, 2497-2504.

140 K. Tanaka, F. Li, K. Morikawa, T. Nobukawa and S. Kadota, Phytochemistry, 2011, 72, 1760-1766.

141 G. Chianese, E. Fattorusso, O. O. Aiyelaagbe, P. LuCiano, H. C. SChröder, W. E. G. Muller and O. Taglialatela-SCafati, Org. Lett., 2011, 13, 316-319.

142 Y. Aoyagi, A. Yamazaki, R. Kato, F. Tobe, H. Fukaya, T. Nishikawa, A. Nakahashi, N. Miura, K. Monde and K. Takeya, Tetrahedron Lett., 2011, 52, 1851-1853.

143 G. M. M. Salinas, V. M. R. Galindo, S. S. Fernandez, D. C. Lankin, M. A. Munoz, P. J. Nathan, G. F. Pauli and N. Waksman, J. Nat. Prod., 2011, 74, 1842-1850.

144 J. Borqueza, G. M. M. Salinas, L. A. Loyola, A. S. Martyn, L. M. P. Rodriguez and S. S. Fernandez, Nat. Prod. Res., 2011, 25, 653-657.

145 M. Gilabert, A. N. Ramos, M. M. SChiavone, M. E. Arena and A. Bardon, J. Nat. Prod., 2011, 74, 574-579.

146 A. Kawamura, M. IaCovidou, E. Hirokawa, C. E. Soll and M. Trujillo, J. Nat. Prod., 2011, 74, 492-495.

147 S.-S. Guo, X.-M. Li, Y. Zhang, C.-S. Li and B.-G. Wang, Chem. Biodiversity, 2011, 8, 1748-1753.

148 S. Nakadate, K. Nozawa, H. Horie, Y. Fujji and T. YaguChi, Heterocycles, 2011, 83, 351-356.

149 D. P. UCCello, S. M. Miller, N. A. DieteriCh, A. F. Stepan, S. W. Chung, K. A. Farley, B. Samas, J. Chen and J. I. Montgomery, Tetrahedron Lett., 2011, 52, 4247-4251.

150 Y.-P. Yao, F.-Y. Dai, K.-K. Dong, Q. Mao, Y.-L. Wang and T. Chen, J. Chem. Res., 2011, 35, 4-7.

151 L. Wang, F.-Y. Dai, J. Zhu, K.-K. Dong, Y.-L. Wang and T. Chen, J. Chem. Res., 2011, 35, 313-316.

152 T. Xu, C.-C. Li and Z. Yong, Org. Lett., 2011, 13, 2630-2633.

153 S. E. Kurhade, A. I. SanChawala, V. Ravikumar, D. Bhuniya and

D. S. Reddy, Org. Lett., 2011, 13, 3690-3693.

154 C. M. GampeandE. M. Carreira, Angew. Chem., Int. Ed., 2011, 50, 2962-2965.

155 N. Kanoh, K. Sakanishi, E. Iimori, K. Nishimura and Y. IwabuChi, Org. Lett., 2011, 13, 2864-2867.

156 J. A. Enquist Jr, S. C. Virgil and B. M. Stoltz, Chem.-Eur. J., 2011, 17, 9957-9969.

157 H. M. Nelson, K. Murakami, S. C. Virgil and B. M. Stoltz, Angew. Chem., Int. Ed., 2011, 50, 3688-3691.

158 J. Zheng, X. Xie, C. Zhao, Y. He, H. Zheng, Z. Yang and X. She, Org. Lett., 2011, 13, 173-175.

159 M. T. Crimmins, C. S. StaufferandM. C. Mans, Org. Lett., 2011,13, 4890-4893.

160 M. SChubert and P. Metz, Angew. Chem., Int. Ed., 2011, 50, 29542956.

161 S. A. Snyder, D. A. Wespe and J. M. von Hof, J. Am. Chem. Soc., 2011, 133, 8850-8853.