Organic Chemistry of Natural Products Table of contents –1/10– ＃2 – Terpenes (terpenoid)

Examples of terpenes

O

HO H

pinene limonene geraniol citral

O OH H H H

H HO O HO camphor menthol β-selinene cafestol

OH

retinol β-carotene

H

H H HO

cholesterol Isoprene units

Classifcation of terpenes Overview of terpene biosynthesis Table of contents –2/10– key starting materials and intermediates: O O OH O O O OH O OH H O P OH O P OH CoAS HO OH O OH OH O OH OH acetyl-CoA pyruvic acid glyceraldehyde 3-phosphate mevalonic acid deoxyxylulose 5-phosphate

O O O O O O O P O P OH O P O P OH O P O P OH OH OH OH OH OH OH isopentenyl pyrophosphate dimethylallyl pyrophosphate geranyl pyrophosphate (IPP) (DMAPP) (GPP) Table of contents –3/10– Biosynthesis of isopentenyl pyrophosphate (IPP) via the mevalonate pathway (①)

O O OH cf. × 3 CoAS HO OH acetyl-CoA mevalonic acid

O O O P O P OH coenzyme A (HSCoA) OH OH IPP Table of contents –4/10– Biosynthesis of dimethylallyl pyrophosphate (DMAPP) from IPP (②)

Biosynthesis of isopentenylThe pyrophosphate Mevalonate and Methylerythritol(IPP) via the non-mevalonate Phosphate Pathways: pathway Terpenoids and Steroids 191

nucleophilic attack of H enamine onto aldehyde H CO2H O OH OH O H3C OH OP H3C O CO2 OP OP OH O OH pyruvic acid R1 R1 OH E1 N S D-glyceraldehyde E1N S E1 1-deoxy-D-xylulose 5-P thiamine PP 3-P (TPP) see Figure 2.16 1 R2 R2 R N S TPP anion TPP/pyruvate-derived regenerated enamine 2 OH R N P reduction of aldehyde to alcohol; O fosmidomycin the aldehyde intermediate remains enzyme-bound reverse aldol aldol O OH reaction OH reaction OH OH NADPH OP OP OP ≡ OP OP E2 HO E2 O O O O OH O OH OH OH H H 1-deoxy- 2-C-methyl- D-xylulose 5-P D-erythritol 4-P

nucleophilic attack of phosphate compare formation E3 CTP hydroxyl on diphosphate; of UDPglucose, O formation of phosphoanhydride Figure 2.28 NH2 NH2 P O OH O O N O O N OH OH ATP P P N O P P N O O O O CH2 O O O CH2 OH O OH OH O OH OH OH E4 OH OH

2-phospho-4-(CDP)- HO OH 4-(CDP)-2-C-methyl- HO OH 2-C-methyl-D-erythritol D-erythritol

E5 CMP OPP O OH OPP O O isopentenyl PP P O (IPP) + P H+ H O OH OPP E7 E6 E8 OH H OH OH OPP OPP 2-C-methyl-D-erythritol- 4-hydroxy-3-methyl- 2,4-cyclophosphate but-2-enyl diphosphate OPP H H H major minor dimethylallyl PP product product (DMAPP)

E1: 1-deoxy-D-xylulose 5-phosphate synthase (DXP synthase) E5: 2-C-methyl-D-erythritol-2,4-cyclodiphosphate synthase (IspF) E2: 2-C-methyl-D-erythritol 4-phosphate synthase; E6: 4-hydroxy-3-methylbut-2-enyl diphosphate synthase (IspG) 1-deoxy-D-xylulose 5-phosphate reductoisomerase (IspC) E7: 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (IspH) E3: 4-diphosphocytidyl-2-C-methyl-D-erythritol synthase (IspD) E8: isopentenyl diphosphate isomerase (IPP isomerase) E4: 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase (IspE)

Figure 5.6 Table of contents –5/10– Oligomerization of terpene pyrophosphates (③)

Maturation of monoterpenes - E/Z isomerization, cyclization, and rearrangement (④)

E/Z isomerization (④-1)

O P P O P P geranyl pyrophosphate neryl pyrophosphate GPP NPP generation of linear monoterpenes Table of contents –6/10–

cyclization - e.g. biosynthesis of limonene, α-terpineol, and α-phellandrene (④-2)

limonene O P P

NPP OH α-terpineol

α-phellandrene Table of contents –7/10– bicyclization and rearrangement (④-3)

O P P

GPP camphene Table of contents –8/10– 212 Medicinal Natural Products: A Biosynthetic Approach. 3rd Edition Maturation of sesquiterpenes - further complicated structures

i

H guaiyl cation e.g. matricin, thapsigargin H E 10 germacryl cation ii e.g. parthenolide E

E,E-farnesyl cation 11 eudesmyl cation e.g. α-santonin

humulyl cation e.g. humulene E

caryophyllyl cation e.g. caryophyllene ≡ W–M E 1,3-hydride shift

H

nerolidyl cation bisabolyl cation amorphyl cation e.g. bisabolene, α-bisabolol e.g. artemisinic acid a 6

a E 7 a carotyl cation W–M Z b b 1,3-hydride shift 10 E,Z-farnesyl cation H b 11 cis-germacryl cation cadinyl cation e.g. α-cadinene

n n = ring size generated

cis-humulyl cation

Figure 5.31 Table of contents –9/10– Synthesis of squalene, the precursor of triterpene

O P P × 2 farnesyl pyrophosphate FPP

squalene

The Mevalonate and Methylerythritol Phosphate Pathways: Terpenoids and Steroids 235 Mechanism FPP 1′ 3′ E1 * 2′ PPO FPP 1 * electrophilic addition allylic cation giving tertiary cation

OPP H H * * loss of proton with formation of cyclopropane ring

loss of diphosphate H * OPP gives primary cation * H

* * H H 1,3-alkyl shift generates presqualene PP W–M new cyclopropane ring 1,3-alkyl and more favourable shift tertiary cation * * H * cation quenched by * H attack of hydride H (NADPH) H bond cleavage produces alkene and favourable allylic cation NADPH E1

H H * *

E1: squalene synthase squalene

Figure 5.60 squalene (Figure 5.60); in general, FPP is formed from The formation of presqualene PP in Figure 5.60, is MVA (see page 192). Squalene is a hydrocarbon origi- initiated by attack of the 2,3-double bond of FPP onto nally isolated from the liver oil of shark (Squalus sp.), the farnesyl cation, which is mechanistically equivalent to but was subsequently found in rat liver and yeast, and normal chain extension using IPP. The resultant tertiary these systems were used to study its biosynthetic role as cation is discharged by loss of a proton and formation of aprecursoroftriterpenesandsteroids.Severalseedoils acyclopropanering,givingpresqualenePP.Anexactly are now recognized as quite rich sources of squalene, e.g. analogous sequence was used for the origins of irregular Amaranthus cruentus (Amaranthaceae). During the cou- monoterpenes (see page 205). Obviously, to then form pling process, which on paper merely requires removal squalene, C-1s of the two FPP units must eventually be of the two diphosphate groups, a proton from a C-1 po- coupled, whilst presqualene PP formation has actually sition of one molecule of FPP is lost and a proton from joined C-1 of one molecule to C-2 of the other. To NADPH is inserted. Difficulties with formulating a plau- account for the subsequent change in bonding of the two sible mechanism for this unlikely reaction were resolved FPP units, a further cyclopropane cationic intermediate when presqualene diphosphate,anintermediateinthe is proposed. Loss of diphosphate from presqualene PP process, was isolated from rat liver. Its characterization would give an unfavourable primary cation, which via as a cyclopropane derivative immediately ruled out all Wagner–Meerwein rearrangement can generate a tertiary the hypotheses current at the time. carbocation and achieve the required C-1–C-1′ bond. 236 Medicinal Natural Products: A Biosynthetic Approach. 3rd Edition Table of contents –10/10– 236 Medicinal Natural Products: A Biosynthetic Approach. 3rd Edition Synthesis of steroids from squalene

squalene squalene E1 O2, FAD sequence of W–M 1,2-hydride NADPH E1 O2, FAD sequenceand 1,2-methyl of W–M shifts 1,2-hydride NADPH and 1,2-methyl shifts

H H H H cyclizations H H H H H cyclizations H ≡ 3 H H 2 ≡ 3 H HO H O 2 HO H HO H O HO (3S)-2,3-oxidosqualene protosterylH cation protosterylH cation (squalene oxide) protosteryl cation (3S)-2,3-oxidosqualene animalsprotosteryl cation (squalene oxide) plants fungi animals plants fungi loss of proton loss of proton leads lossgives of alkeneproton H lossto cyclopropane of proton leads H gives alkene H to cyclopropane H H E1: squalene epoxidase H E1: squalene epoxidase HO HO H HO H HO H lanosterol H cycloartenol lanosterol cycloartenol Figure 5.61 FigureDetailed 5.61 mechanism of the cyclization step

protonation of epoxide electrophilic addition electrophilic addition protonationallows ring of opening epoxide to electrophilicgives tertiary addition cation electrophilicgives tertiary addition cation allowstertiary ring cation opening to gives+ 6-membered tertiary cation ring gives+ 6-membered tertiary cation ring tertiary cation + 6-membered ring + 6-membered ring

H HO HO HO H O H H HO HO HO O H H H 2,3-oxidosqualene electrophilic addition H 2,3-oxidosqualene electrophilicgives tertiary addition cation gives+ 5-membered tertiary cation ring + 5-membered ring electrophilic addition W–M rearrangement; H gives tertiary cation H electrophilic addition W–Mring expansionrearrangement; at expense of H H H gives tertiary cation ringtertiary expansion → secondary at expense cation of H tertiary → secondary cation

H H H HO H HO H HO H H H H HO HO HO protosterH yl cation H H protosteryl cation Figure 5.62 SummaryFigure 5.62