Terpenes are volatile constituents of plants

Terpenes were originally named after turpentine, the volatile oil from pine trees used in oil painting, whose major constituent is α-pinene. The term was rather vaguely used for all the volatile oily compounds, insoluble in water and usually with resinous smells from plants. Oils distilled from plants, which often contain perfumery or flavouring materials, are called essential oils and these too contain terpenes. Examples include camphor from the camphor tree, which is used to preserve clothes from moths, and humulene from hops, which helps to give beer its flavour.

You will notice that they are all aliphatic compounds with a scattering of double bonds and rings, few functional groups, and an abundance of methyl groups. A better defi nition (that is, a biosynthetically based defi nition) arose when it was noticed that all these compounds have 5n carbon atoms. Pinene and camphor are C10 compounds while humulene is C15. It seemed obvious that terpenes were made from a C5 precursor and the favourite candidate was isoprene (2-methylbuta-1,3-diene) as all these structures can be drawn by joining together 2-, 3-, or 4-isoprene skeletons end to end.

In fact, this is not correct. Isoprene is not an intermediate, and the discovery of the true pathway started when acetate was, rather surprisingly, found to be the original precursor for all terpenes. The key intermediate is mevalonic acid, formed from three acetate units and usually isolated as its lactone.

The first step is the Claisen ester condensation of two molecules of acetyl CoA, one act ing as an enol and the other as an electrophilic acylating agent to give acetoacetyl CoA.

We have drawn the product with stereochemistry even though it is not chiral. This is because one of the two enantiotopic thiol esters is hydrolysed while this intermediate is still bound to the enzyme, so a single enantiomer of the half-acid/half-thiol ester results.

The remaining thiol ester is more electrophilic than the acid and can be reduced by the nucleophilic hydride from NADPH. Just as in LiBH₄ reductions of esters, the reaction does not stop at the aldehyde level, and two molecules of NADPH are used to make the alcohol. This is mevalonic acid.

Mevalonic acid is indeed the true precursor of the terpenes but it is a C₆ compound and so it must lose a carbon atom to give the C₅ precursor. The spare carbon atom becomes CO₂ by an elimination reaction. First, the primary alcohol is pyrophosphorylated with ATP; then the CO₂H group and the tertiary alcohol are lost in a concerted elimination.

So is isopentenyl pyrophosphate the C₅ intermediate at last? Well, yes and no. There are actually two closely related C₅ intermediates, each of which has a specifi c and appropriate role in terpene biosynthesis. Isopentenyl pyrophosphate is in equilibrium with dimethylallyl pyrophosphate by a simple allylic proton transfer.

The two C₅ intermediates now react with each other. The dimethylallyl pyrophosphate is the better electrophile because it is allylic, and allylic compounds are good at both SN1 and SN2 reactions. Isopentenyl pyrophosphate is the better nucleophile because it can react through an unhindered primary carbon atom to produce a tertiary cation—we can draw the reaction like this:

Although this idea reveals the thinking behind the reaction, in fact it does not go quite like this. The product is one particular positional and geometrical isomer of an alkene and the cation is not an intermediate. Indeed, the reaction is also stereospecific (discovered again by proton labelling, but we will not give the rather complex details) and this too suggests a con certed process.

As soon as we start to make typical cyclic monoterpenes from geranyl pyrophosphate we run into a snag. We cannot cyclize geranyl pyrophosphate because it has a trans double bond! We could cyclize the cis compound (neryl pyrophosphate), and it used to be thought that this was formed from the trans compound as an intermediate.

It is now known that nature gets round this problem without making neryl pyrophosphate. An allylic rearrangement occurs to move the pyrophosphate group to the tertiary centre. This is an unfavourable rearrangement thermodynamically and probably occurs via the allyl cat ion and is catalysed by Mg(II). There is no longer any geometry about the alkene. The molecule can now rotate freely about a single bond and cyclization can occur. Even if only a small amount of the rearranged allylic pyrophosphate is present, that can rearrange and more can isomerize.

More interesting compounds come from the cyclization of the fi rst formed cation. The remaining alkene can attack the cation to form what looks at first to be a very unstable com pound but which is actually a tertiary carbocation with the pinene skeleton. There are many thousands of terpenes with multiple C5 units all made from mevalonic acid.

The steroids are another group of compounds derived from mevalonic acid. They include sex hormones such as testosterone and progesterone, and the cholesterol needed to build cell membranes but also implicated in the damage to arteries caused by atherosclerosis.

The elucidation of the ways in which organic chemistry underpins life, along with the use of organic chemistry to construct in the laboratory the molecules used by nature, has been one of the greatest scientific success stories of recent decades. In this chapter we have revealed but a glimpse of the immense complexity of the world of biological organic chemistry; you will find an extended version of this discussion in the three chapters on the web, and a book on biochemistry will fill in more detail. The beautiful molecular structures of nature and the reactions used to make them have provided an example for organic chemists to follow—sometimes at a distance, but always in hot pursuit. The next and final chapter of this book tells a few stories of how such scientific inspiration is the key to the future of chemistry, not only for its own sake, but also for the sake of the millions of people whose lives have been improved or even saved by the ingenuity of chemists.

اخر الاخبار

اخبار العتبة العباسية المقدسة

المجمع العلمي: نحو 300 ألف مستفيد من مشروع القراءة الصحيحة في كربلاء وباقي المحافظات

صحفيون: استحداث المركز الإعلامي لزيارة الأربعين أسهم في تسهيل عمل الإعلاميين وتوفير بيئة مهنية لإنجاز مهماتهم

موكب عميد بني هاشم (عليه السلام) يقدم 4500 وجبة طعام يوميًّا لزائري الأربعين

بمشاركة زائري الأربعين.. إقامة صلاة الجماعة عند مرقد أبي الفضل العباس (عليه السلام)

المزيد

الآخبار الصحية

حجم جزء محدد من الجسم يعكس الصحة العامة ومخاطر الأمراض

تفشي عدوى بكتيرية بفرنسا يحتمل ارتباطها بنوع من الجبن الطري

دراسة حديثة تكشف ما تفعله الشاشات بـ"قلوب الأطفال" !

متلازمة المبنى المريض.. هكذا يتحول تكييف الهواء إلى خطر صحي ؟؟

المزيد

الآخبار التكنلوجية

كيف تتعامل مع حرارة الطقس أثناء القيادة؟

لماذا يجب عليك إيقاف تكييف السيارة قبل إطفاء المحرك؟

دراسة تكشف سببا محتملا وراء زيادة خطر إصابة الأطفال بالتوحد

تشيلي.. العثور على متحجرة حيوان ثديي صغير من زمن الديناصورات

المزيد

مواضيع ذات صلة

The future of organic chemistry

The synthesis of oseltamivir

The synthesis of indinavir

Chemistry vs viruses

Aromatic polyketides

Fatty acids and other polyketides are made from acetyl CoA

Benzyl isoquinoline alkaloids are made from tyrosine

Pyrrolidine alkaloids are made from the amino acid ornithine

Alkaloids are made by amino acid metabolism

The shikimic acid pathway

Phosphoenolpyruvate

Nature’s enolate equivalents: lysine enamines and coenzyme A