Deep-Dive Guides · Terpenoids & Steroids

Terpenoids & Steroids — the complete deep dive

One organising idea unlocks this entire topic: terpenoids are oligomers of a five-carbon isoprene building block, and steroids are simply the cyclised, degraded end of the triterpenoid line.

Free extract — the opening section of the guide, in full.

0. How to use this guide — the isoprene lens

Fix the master idea before any single molecule. A terpenoid (terpene = the hydrocarbon; terpenoid = the broader oxygenated/functionalised family, though the terms are used loosely) is built formally from isoprene units, C₅H₈ (2-methylbuta-1,3-diene). The count of C5 units names the class, and the way the units are joined plus the cyclisations of a carbocation generate the skeleton.

The three questions to ask of any terpenoid: (1) How many carbons? Divide by 5 to get the number of isoprene units and hence the class (C10 = mono, C15 = sesqui, C20 = di, C30 = tri, C40 = tetra). (2) Where are the isoprene units, and are they head-to-tail or tail-to-tail? Mark the branch methyls — the "head" is the branched (isopropyl) end, the "tail" is the unbranched end. (3) What cation cyclised, and how? Almost every ring in a terpenoid traces to an electrophilic (carbocation or protonated-epoxide) cyclisation of an acyclic prenyl chain.

Class	Carbons	Isoprene units	Precursor	Type examples
Hemiterpene	C5	1	IPP/DMAPP itself	isoprene, prenol, isovaleric acid
Monoterpene	C10	2	GPP (geranyl-PP)	geraniol, limonene, α-pinene, menthol, camphor
Sesquiterpene	C15	3	FPP (farnesyl-PP)	farnesol, zingiberene, artemisinin
Diterpene	C20	4	GGPP	phytol, abietic acid, taxol core, retinol
Sesterterpene	C25	5	GFPP	ophiobolins (rare)
Triterpene	C30	6	squalene (2×FPP)	squalene, lanosterol, β-amyrin
Steroid	C27–C18 (degraded C30)	(from triterpene)	lanosterol → cholesterol	cholesterol, bile acids, hormones
Tetraterpene	C40	8	phytoene (2×GGPP)	β-carotene, lycopene, xanthophylls
Polyterpene	(C5)_n	many	—	natural rubber (cis-polyisoprene), gutta-percha (trans)

Mnemonic for the C-count ladder: 5-10-15-20-25-30-40 → Hemi, Mono, Sesqui, Di, Sester, Tri, Tetra. Note the jump from C30 (tri) straight to C40 (tetra): there is no C35 line in bulk because tri- and tetra-terpenes are made by tail-to-tail dimerisation (C15+C15 = squalene; C20+C20 = phytoene), doubling rather than adding one unit.

Read the rest of this guide

The remaining 10 sections — The isoprene rule & the C5 building blocks, Classification of terpenoids, Key terpenoid structures, Biosynthesis overview — MVA & MEP pathways and more — are part of ChemVidya Full Access, along with the other guides, the books, the study notes and 6,000+ practice questions with worked solutions.

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