Diterpenoids James R. Hanson School of Molecular Sciences University of Sussex Brighton Sussex BN I 9QJ,UK Reviewing the literature published in 1994 (Continuing the coverage of literature in Natural Product Reports 1995 Vol. 12 p. 207) 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 4.3 Vouacapanes 4.4 The Chemistry of the Tricyclic Diterpenoids 5 Te tracyclic Di terpenoids 5.1 Kauranes 5.2 Trachylobanes 5.3 Aphidicolanes 5.4 Gibberellins 6 Macrocyclic Diterpenoids and their Cyclization Products 6.1 Cembranes 6.2 Taxanes 7 Miscellaneous Diterpenoids 8 References 1 Introduction This report follows the pattern of its predecessors and covers the literature published in 1994.l The period has again been dominated by studies on the tumour-inhlbitory diterpenoid TaxoP (also known as paclitaxel) and related compounds.Although outside the scope of this review it is important to record the total synthesis of Taxo1.@2.3 An increasing number of diterpenoids are being detected possessing structures derived by rearrangement and cleavage of the normal diterpenoid carbon skeleta. This has led to the revision of some previously accepted structures. Some recent aspects of the chemistry of the diterpenoids have been re~iewed.~ 2 Acyclic and Related Diterpenoids Phytoene- 1,2-diol(l) has been obtained5 from Artemisia annua (Compositae) the source of the antimalarial sesquiterpenoid artemisinin.Some linear diterpenoids containing a furan ring e.g. 12-hydroxyambliofuran (2) have been isolated6 from a Western Australian sponge. 3 Bicyclic Diterpenoids 3.1 Labdanes Further investigations of the conifer Cryptomeria japonica (Taxodiaceae) (sugi) have yielded' a number of labdanes related to imbricatolic acid (3). ent-Sclareol 13-o-P-D-XylO- pyranoside has been obtained* from Conyza trihecatactis whilst examination of the resin 'Ladano' obtained from Cistus creticus (Cistaceae) afforded9 some labdan- 15-yl esters of malonic acid. (3) The seeds of Aframomum species (Zingiberaceae) are used as a West African food spice.Aframodial (4) (from A. danielli),l0 aulacocarpin A (5) and aulacocarpinolide (6) from A. aulac~carpos)~~ have been isolated from these plants. The synthesis of two epimeric 6,7,8-trihydroxylabdadieneshas confirmed12 the structure (7) of crotomachlin a trio1 obtained from Croton macrostachys. (4) (5) Examination of the Mexican plant Gymnosperma glutinosum (Compositae) gavel3 the ent-labdanetriol (8). Continuing investigations of Halimium viscasum (Cistaceae) have yielded 59 NATURAL PRODUCT REPORTS 1996 some nor-labdanes including (9)14 and some rearranged labdanes (10).15 The roots of the Tibetan plant Phlomis medicinalis (Labiatae) which have been used in folk medicine have afforded16 a series of labdane glycosyl esters the phlomisosides e.g.(1 1).Labdane butenolides have also been obtained1' from the rhizomes of Hedychium coronarium (Zingiberaceae) e.g. (12) whilst the amoenolides e.g. (13) were isolated 18,l9 from Amphiachyris amoena (Compositae). Further studies20 of the medicinal plant Andrographis paniculata have yielded 14-epiandrographolide isoandrographolide (1 4) and some dimeric derivatives which were potent inducers of phagocytosis. Examination of the leaves of Leonotis ocymifolia var. raineriana (Labiatae) gave2' the known labdane leonitin and a relative (15). OPoH 0 A number of labdanes have been obtained from marine organisms. Thus the furan blanesin (16) was isolatedz2 from the Mediterranean sponge Raspaciona aculeata and chloro- lissoclimide (1 7) was from the tunicate Lissoclinum voeltzkowi.The ptychantins e.g. (18) are relatives of forskolin which were isolated24 from the liverwort Ptychanthus striatus. Apiculol lp-hydroxy- 13-epimanoyl oxide was from the roots of the mangrove Rhizophora apiculata whilst some known 13-epimanoyl oxide derivatives were detected2s in a further study of' Ladano' resin (Cistus creticus). Stimulated by the biological activity of the forskolin series the microbiological hydroxylation of ribenone (19) at C-1 C-6 and C-1 1 by Curvularia lunata has been rep~rted.~' The 'Cup-plant' Silphium perfoliatum (Compositae) has been used medicinally in North America. Extraction of the leaves afforded2s chloro- silphanol (20) and silphanepoxol (21). The X-ray crystallo- graphic studies of (20) suggested a need to re-examine the structures of the carterochaetols.Some highly modified labdanes the clutyenes e.g. (22) and (23) have been obtainedz9. 30 from the medicinal plant Cluytia richardiana (Euphorbiaceae). These compounds may be visualized as 6,7 :9,1 O-biseco-6,ll 1,9-bicyclo- and 6,7-seco- 6,ll -cyclo-labdanes. Pallavicinin (24) is a 7,8-secolabdane isolated3' from the liverwort Pallavicinia subciliata. ?H 0 A number of aspects of the chemistry of the labdanes have been examined particularly in the light of the perfumery properties of diterpenoid ethers in this series. These studies include the oxidation of the side chain of sclareol to give ambraketal (25) using ruthenium tetr~xide~~ and an exam- ination of the products of the ring opening of this acetal with boron trifl~oride.~~ The synthesis of some biologically active drimanes including polygodial (26) from sclareol has been NATURAL PRODUCT REPORTS 1996J.R. HANSON reported.34 These drimanes have also been obtained from other OAc labdane~.~~ The preparation of methyl isocopalate from sclareol and of norambreinolide from abienol have been described.36. 37 Some studies on the partial synthesis of prehispanolone from hispanolone have been reported."* 3.2 Clerodanes Many clerodanes have been isolated from the Labiatae and this work has been revie~ed.~~~*~ An account of work on the furanoclerodanes from Teucrium species has been pre~ented.~~ lariaceae). The cytotoxic clerodane (36) has been isolated51 Croton sonderianus (Euphorbiaceae) is a widespread Brazilian from the stem bark of Polyalthia barnesii (Annonaceae).medicinal shrub which is used as a remedy for gastric Copaiba oil has been to contain 7a-acetoxyhardwickiic disturbances. 6a-Hydroxyannonene (27) and the corresponding acid (37). 6a,7/?-diol have been isolatedg2 from a population growing in c6" North East Brazil. Heteroscyphic acid A (28) the 6a-acetoxy derivative and the 3a,4a-epoxide have been obtainedg3 from cultured cells of the liverwort Heteroscyphus planus (Junger-manniales). Roseostachenol (29) is a neoclerodane which was isolated4* from Stachys rosea (Labiatae). Extraction of the Mexican plant Gymnosperma glutinosum (Compositae) afforded not only labdanes but also clerodanes including dihydro- tucumanoic acid (30).45 Further examination of Cistus populi- fulius (Cistaceae) has yielded46 some clerodanes functionalized on ring A including (3 1).Pulicaria salviifolia (Compositae) (fleabane) contains4' the clerodane salvicinin (32). The clero- dane (33) was among the constituents of the dried seed pods of Sindora sumatrana (Leguminosae).*8 p I' OH (29) (30) Vi' HQC OH H0& (33) Although triterpenoids of the limonin variety are common in the Meliaceae diterpenoids are rare. However the clerodane 'OAc HOzC 137) Investigations on Salvia and Teucrium (Labiatae) species have continued to yield novel clerodanes. NMR studies have been on a number of compounds derived from Mexican sources.New clerodanes include infuscatin (38) from S. inf~scata,~* salviandulin D (39) from S. lavand~loides,~~ salvimadrensin (40) from S. madren~is,~~ 3-deacetylteu-micropodine (4 1) from Teucrium polium ssp. au~asianum,~' and teutrifidin (42) from T. triJid~m.~* The stereochemistry of 0Aoj p HO-ys" _..-0 OH (34) has now been found49 in Cipadessa fruticosa. The bH norclerodane (35) along with some other clerodanes has been ~H~OAC CH&AC obtained50 from Linaria saxatilis var. glutinosa (Scrophu- opening of the epoxide ring of some 4a,18-epoxyclerodanes from Teucrium species has been examined.59 The structure of montanin E (43) was confirmed by X-ray crystallography. Some further reactions of eriocephalin have also been investigated.60 These compounds continue to attract interest as insect anifeedants.The biological activity of a range of clerodanes from Baccharis Teucrium and Salvia species has been examined in this context.61 Scutalpin E (44) has been obtained@ from Scutellaria alpina. CH20H CH20Ac (43) (44)R = CCMe=CHMe 0 A number of cis-clerodanes have been isolated. Thus the unsaturated ketone (45) has been from Vellozia bicolor (Velloziaceae). Tinospora species (Menispermaceae) have yielded a number of these compounds. Thus menisperma- cide (46) was obtained64 from T. malabarica and tinotufolin C (47) from T. tuberculata.65 The cordifolisides e.g. (48) are glycosidic constituents of T. cordifolia.66 0 GluO'* C02Me C02Me (47) (48) The clerodane ring system undergoes various rearrangements and examples of structures arising by these reactions have been isolated from natural sources.These include teubrevin A (49) from Teucrium b~evifolium,~' peronemin A from Peronema canexens (Verbenaceae),68 salvileucantholide (50) and salvian- NATURAL PRODUCT REPORTS 1996 dulin E (51) from S. le~cantha.~'Further studies of different populations of S. rhyacophila have afforded7* the 5,6-secoclerodane 7-epirhyacophiline (52). 4 Tricyclic Diterpenoids 4.1 Abietanes The crystal structure of abietic acid has been rep~rted.~' Some dehydroabietic acid derivatives including (53) have been is~lated'~ from Nepeta teydea (Labiatae). Examination of the leaves of Juniperus chinensis (Cupressaceae) aff~rded'~ (54) whilst 3~-hydroxy-l-oxototarol (55) was obtained'* from the roots.13P-Hydroxyabiet-8( 14)-en- 19-a1 has been isolated75 from the berries of J. foetidissima. More highly oxidized abietanes including (55) were the constituents of the leaves of Cryptomeria japonica (Taxodiaceae). Examination of Lepechinia caulescens (Labiatae) which is used in Mexican folk medicine for the treatment of stomach ailments gave" the epoxy acid (56). (53) (54) C02H The catechols (57) montbretol and salvinolone have been shown to be identi~al.'~ A number of abietanes have been isolated from Salvia (Labiatae) species. Thus columbaridione (58) and 1 1,12-di-0-methylrosmanol (59) have been 0 ,' 0 '*.* H @.&$ (57) (58) OMe I NATURAL PRODUCT REPORTS 1996-5.R. HANSON obtained5" from S. columbariae whilst pomiferin F (60) and G (61) have been obtainedg1 from S. pomferu and nemorosin (62) was obtained" from S. nemorosa. Some rearrangement products include coulterone (63) from S. co~lteri,~~ and tilifolidione (64) from S.tiliaef~lia.~~ Model studies on the biogenetic relationships between these oxidized abietanes have been reported.s5 A number of these compounds form the antimicrobial constituents of the roots of the Labiatae. Thus the quinone (65) has been founds6 in the roots of Plectranthus heteroensis. Further studies on Sulvia sclureu have revealed8' the presence of the highly oxidized abietane 2,3-dehydro- salvipisone (66) whilst safficinolide (67) and sageone (68) were obtaineds* from S.oficinalis. Paramiltioic acid (69) in which ring C has been cleaved was isolatedsg from S.paramiltiorrhiza. HO> Ho..& C02H (62) 0 OH 0 (63) (64) "OH OH I The villosins A-C e.g. (70) were obtainedgo from Teucrium divaricutum. Their formation involves a rearrangement of the isopropyl side chain of the abietanes. The Chinese anticancer drug Tripterygium wilfordii (Celastraceae) has continued to be the source of abietanes including the compounds (71)g1and (72).92 The microbiological hydroxylation of abietanes such as (73) by Aspergillus fumigutus has been exploredg3 as a route to these compounds. CH20H 0& 0 (73) 4.2 Pimaranes The structure of the A9'11)-i~~pimaradiene (74)94 and 14a-hydroxysandaracopimara-7,15-dien- 19-oic acid (75)95 ( = callyphyllin from Callicarpa macrophyllag6) have been con- firmed by X-ray crystallography.The phytoalexin oryzalexin F (76) has been producedg7 in rice leaves by UV irradiation. Fermentation of an unidentified coelomycete producedgs a diol maxikdiol (77) which is a powerful agonist of the maxi-K channels. As such it could have therapeutic benefit in the treatment of diseases such as asthma. The rhizomes of Kuempferia pulchra (Zingiberaceae) have been showng9 to contain 2a-acetoxysandaracopimaradien-1a-01 (78). The rearranged isopimarane (79) has been isolatedlo0 from Satureja gilliesii (Labiatae). C02H (75) =*\ & 'I H 4.3 Vouacapanes 6a-Acetoxyvouacapane (80) has been isolatedlOl from the seeds of Dipteryx lacunifera (Leguminoseae).The oil from the fruit of the Brazilian plant Pterodon polygalaeforus is reputed to inhibit the penetration of the skin by the cercaria of Schistosoma mansoni. Examination of this oil has revealedlo2 14,15- epoxygeranylgeraniol as an active component together with 6a-7P-dihydroxyvouacapan- 17p-oic acid (8 1) and its esters. Caesaldekarin A (82) has been isolatedlo3 from the roots of Caesalpinia major (Fabaceae) which are used as an anthelmintic. The erythroxylon diterpenoid 7/3-hydroxyfagonene (83) has been isolatedlo4 from Fagonia bruguieri (Zygophyllaceae). Some dolabrane diterpenoids with a cis A/B ring fusion including NATURAL PRODUCT REPORTS 1996 5 Tet racycl ic Di terpenoids 5.1 Kauranes Euphoranginol B (88) and C (89) are two ent-kauranes that have been obtained114 from Euphorbia wangii (Euphorbiaceae).ent-Kaur- 16-ene-3/3 1SP-diol(88) was also obtained115 from the roots of Gelonium multijlorum (Euphorbiaceae). ent-Kaur- 16-en-19-oic acid has been reported116 to be a trypanocidal component of Mikania obtusa with activity against Chagas disease. The X-ray crystal structure of (16R)-ent-kaurane-2,12- dione (90) obtained from the rhizomes of Alisma orientale has been described.lI7 Methods for the separation and quanti- fication of the sweet diterpenoids stevioside and rebaudioside in plant extracts using HPLC and TLC methods have been described.11* 0 (84) have been isolatedlo5 from the wood of Endospermum diadenum (Euphorbiaceae).$pcH*oH I OH 0@...rcH20H 0 Examination of the constituents of Werneria ciliolata has \ \ OH 4.5 The Chemistry of the Tricyclic Diterpenoids The oxidative decarboxylation of dehydroabietic acid by hydrogen peroxide and a mercury(I1) salt has been described.lo6 The modification of podocarpic acid via the intermediate (85) has been reported.lo7 The copper(1) mediated oxygenation of the aromatic ring of podocarpic acid and totarol has givenlo* o-catechols. Various methods for the benzannulation and cyclopenta-annulation of podocarpic acid derivatives have been reported involving orthometallation and aryne ~hemistrylO~-~~~ and leading to compounds such as (86) and (87) reminiscent of steroids.OMe OH revealedlls the presence of some dimeric diterpenoids based on a combination of ent-manoyl oxide and kauren-17-al. Liverworts are a rich source of diterpenoids. The 6,7- secokaurene (9 1) has been obtained120 from Jungermannia exserifolia ssp. cordifolia. Some dimeric diterpenoids were also found in this species. AcO.. The genus Rabdosia (Isodon) (family Labiatae) comprises about 150 species a number of which figure in Chinese traditional medicine. Examination of these has continued to yield novel diterpenoids. 121Recent isolates include megathyrin A (92) from I. megathyrsus,122 rabdoternin D (93) from R. ternif~lia,~~~ nervosanin A (94) from I. nervo~us,~~~ Me02C NATURAL PRODUCT REPORTS 1994-J.R. HANSON longirabdolide A (95) and C (96) from R.longit~ba,~~~*~~~ and e.g. (104) which underwent further rearrangement in acid to rabdoshikoccin A (97) from R.shikoki~na.'~~ Examination of R. eriocalyx revealed the presence of monomeric compounds such as maoecrystal L (98)128 and dimeric The antibacterial activity of R.trichocarpa particularly against oral micro-organisms has been associated130 with the presence of these di terpenoids. n (97) The toxic principle of Iphiona aucheri (Asteraceae) has been to be atractyloside. 16a 17-Isopropylidene-3- oxophyllocladene has been as a constituent of the Indian medicinal plant Callicarpa macrophylla (Verbenaceae). Bengalensol (99) isolated from Coflea bengalensis has been to be a 16-epicafestol derivative.(99) Rearrangements of the tetracyclic diterpenoids have continued to attract interest. The Favorski rearrangement of the chloroenol-lactone (100) has been to give the isomer (101) of gibberellin A12. The aromatic compound (102) was formed135 in the rearrangement of methyl 9P-hydroxy-11- 0x0-ent-kauran- 19-oate. Acetolysis of ent-7a 18-diacetoxy-14P- mesyloxybeyer-l5-one (103) gave136 12( 13-14)abeo compounds afford (105). The microbiological transformation of ent- 16P 17-epoxy-7- hydroxykaurane by Gibberella fujikuroi gavel3' ent-7a,11a,16p 17-tetrahydroxykaurane rather than gibberellins. The presence of a 16,17-diol appeared to inhibit oxidation at C-19 -a key step in gibberellin biosynthesis.The biotrans- formation of some ent-beyerenones by Rhizopus nigricans and Curvularia lunata has been examined.138 5.2 Trachylobanes The trachylobane diterpenes have been reviewed.13 15-Oxotrachylobanic acid has been isolated140 from a West African population of the tree Xylopia aethiopica (Annonaceae). 5.3 Aphidicolanes Further total syntheses of the tumour inhibitory diterpenoid aphidicolin have been recorded.141* 16p 18-Dihydroxy- aphidicolan- 17-oic acid (106) has been isolated143 from the fungus Cephalosporium aphidicola. The scope of biosynthetically-directed transformations with this fungus has been examined in the context of the biotransformation of sclare01,~~~ ent- 19-hydroxykaur- 16-en- 15-0ne,l,~ some stemo- dane diterpen~ids,,~~ and some aphidicolanes with different substituents on ring A.147 5.4 Gibberellins The quantitative analysis of the gibberellin plant hormones by isotope dilution mass spectrometry has been described.148 Systems for the reverse phase HPLC of permethylated free and glycosylated gibberellins have been reported.,, Details have appeared of the identification of gibberellins A, A and A, in Phaeosphaeria sp. L487,150.151 of GA, and GA, in the growing shoots of the yam Dioscorea b~lbifera,'~~ a range of of gibberellins in Dalbergia d~lichopetala,~~~ and of GA, in clover bro~rnrape.'~~ The possibility was considered that this parasite obtained its GAS from the host plant. GA, (107) and GA, (108) have detected in developing wheat grain and their partial synthesis has been rep0~ted.l~~ 16a 17-Dihydroxy- 16,17- dihydroGA,- 17-O-~-~-ghcopyranoside (109) has been isolated from rice The surprisingly high gibberellin-like biological activity of some tetracyclic diterpenoids of Elaeo-selinum species has been re~0rded.l~~ 6 Macrocyclic Diterpenoids and their Cycl iza tion Products 6.1 Cembranes Soft corals have continued to be a source of cembrane diterpenoids.Some relatives e.g. (1 lo) of sarcophytol A have been from Sarcophyton trocheliophorum (Alcyonaceae) whilst sarcotol(ll1) with a novel 13-membered ring has been isolated159 from another Sarcophyton species. The occurrence of the cembranolide (1 12) has been reportedlG0 in a Lobophyturn species.Some cembranes have been isolated161 from the resin of Western Australian plants that are adapted to desert conditions including Eremophila gilesii and E. viscida. The koumbalones A (1 13) and B are casbane diterpenes that have been found162 in Maprounea africana. oO:* OH 6.2 Taxanes There has been an enormous upsurge of work that has been reported on taxanes in the light of their anticancer activity. Reviews have appeared on the chemistry of tax01,'~~ the phytochemistry of the yew,164 and on structure-activity relationships in this series.165 The determination of the structures of taxanes by tandem mass spectrometryf66. 167 and laser desorption/ionization time of flight mass spectrometry168 has been reported. Rearranged taxanes of the 11(15-+1)- and 2(3+20)-abeo types (114) and (115) respectively have been isolated16g from Taxus baccata.AcO. Ad-*'OH 20 NATURAL PRODUCT REPORTS 1996 Desacetyltaxine A has also been isolated170 from this source whilst cell culture lines of T. baccata have yielded some new biologically-active ta~0ids.l~' The taxane (I 16)172 and the taxuspines e.g. (117) and (1 18),173 have been obtained from T. cuspidata. The revision of the structures of some taxanes from T. brevifolia has been The 11(15-tl)-abeo-taxane structure (1 19) has been e~tablished"~ for taxuchin A obtained from T. chinensis whilst the taxchinins E-K have been assigned similar structures. The rearranged taxane ( and some further taxol derivatives have been isolated from Taxus xmedia c~1tivars.l~~ The structure of wallifoliol(121) from T.wallichiana represents another variant of the underlying taxane skeleton. Yunantaxusin A (122) and some relatives have been from T. yunnanensis whilst yunnanexane has been isolatedlB0 from cell cultures of T. chinensis. AcO Y Et-y-C-Me 0 HO.. Ad OAc AcO.. AcO. PhOCO OAc NMR and molecular modelling studies of taxol analogues in aqueous and non-aqueous solution have been reported.181 A great deal of novel chemistry of taxol and its relatives has been reported in studies on structureactivity relationship^'^^-^^^ including studies on the facile hydrolysis of the C-2 benzoatels5 and the synthesis of C-2 taxol analogues.ls6 Modifications have been reported at C_4,187,188 C-9,189,190 C-10 191-194 C-14195,196 and C-19.1g7 A number of rearrangements of the taxane skeleton have been rep~rted'~~-~~~ and some photochemical NATURAL PRODUCT REPORTS 1996-5.R. HANSON reactions have been 203 The electrochemical modification of taxanes has been described.204* 205 Various improvements to the methods for introducing the side chain and modifications of its structure have been reported.20s-z12 The synthesis of photoaffinity analogues of taxol bearing photo- reactive substituents and their use in labelling tubulin has been described.213. 214 The partial synthesis of the major human metabolites of docetaxel has been 7 Miscellaneous Diterpenoids A wide diversity of diterpenoid structures continue to be isolated from marine organisms.Several spongiane e.g. (123) and isocopalane e.g. (124) diterpenoids have been obtained216 from the Mediterranean sponge Spongia zimocca. A number of dolabellane diterpenoids have been isolated217 from the brown alga Dilophus mediterraneus whilst the cyclocembrane variant (125) has been obtained21s from the coral Sarcophyton trocheliphorum. The isolation of the glucoside (126)219 from Chrozophora obliqua (Euphorbiaceae) appears to be the first report of a dolabellane diterpenoid from a higher plant. Other dolabellanes and dolestanes have been isolated220 221 from marine alga such as Dictyota pardalis. " "O OH P A number of eunicellane (cladiellane) diterpenoids have been described including palmonine F (127) from the gorgonian Eunicella verrucosa,222 the compound (128) from the coral Cladiella australi~,~~~ and litophynol A (129) from a Litophyton species.224 Gorgonian octocorals of the genus Briareum have afforded various briarane and asbestinane diterpenoids in- cluding (1 30) (1 3 1) and (1 32) from B.asbestin~m.~~~, 226 Novel ..OH #OH * OAc (127) R = Me OAc (128) R=CH2 HO'. xenicanes include helioxenicin A (133) from the blue coral and Heliopora ~oerulea~~~acalycigorgin A (1 34) from an Acalycigorgia species.228 Ad@ 0 ACO H (134) Amongst the prenylated sesquiterpenes that have been isolated are lemnabourside (1 35) from the coral Lemnalia bournei (Alcyona~eae),~~~ and the kalihinol isonitriles (1 36) and (1 37),230 231 from the sponge Acanthella cavernosa which was collected from the Seychelles.Palmatol (138) is a prenyl-bicyclogemacrane which was isolated232 from the Mediterranean octocoral Alcyonium palmatum. $ CN (1 37) The unusual structure (139) has been assigned to floridicin which was from the coral Xeniaflorida. It represents an oxidative cyclization product of xeniafaraunol A (140) from Xenia faraunensis. Examination of the soft coral Sinularia dissecta gave235 the unusual mandapamate OHC (141) the structure of which may arise through the internal Diels-Alder cyclization of a furanocembranoid compound of the pukalide series. Fungi have yielded some unusual diterpenoid structures including the 2,3-~econeodolastane trichoaurantin (142) from Tricholoma auranti~m,~~~ and the phomactins e.g.(143) which OH 0 OH (142) are platelet activating factors from a marine Phoma species.237 Some further indole diterpenoids e.g. (144) have been reported23s from Emericella purpurea. Pl Liverworts have proved to be a source of fusicoccane diterpenoids. Recent isolates include fusicogigantepoxide (145) from BryopteriaJili~ina,~~~ fusicorrugatol(l46) from Plagiochila corr~gata,~~~ fusicoauritone from Anastrophyflum and a~ritum.~~~ The unusual structure (147) has been established242 for jamesoniellide C which was isolated from Jamesoniella autumnalis (Hepaticeae). NATURAL PRODUCT REPORTS 1996 A further phorbol derivative has been from Euphorbia laterifolia whilst picrodendrin U (148) is a picro- toxane lactone which was isolated244 from Picrodendron baccatum (Euphorbiaceae) a plant which is used in folk medicine as an insecticide..o HO 8 References 1 J. R. Hanson Nat. Prod. Rep. 1995 12 207. 2 R. A. Holton C. Somoza H. B. Kim F. Liang R. J. Biediger P. Boatman S. Shido C. S. Smith S. Kim H. Nadizadeh Y. Suzuki C. Tao P. Vu S. Tang P. Zhang K. K. Murthi L. N. Gentil and J. H. Liu J. Am. Chem. SOC. 1994 116 1597 et seq. 3 K. C. Nicolaou Z. Yang J. J. Liu H. Ueno P. G. Nantermet R. K. Guy C. F. Claiborne J. Renaud E. A. Couladoures K. 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