Bridged bicyclic rings

There are broadly three kinds of bicyclic compounds. If we imagine adding a second five membered ring to one already there, we could do this in a bridged, fused, or spiro fashion, as you see in the margin. Bridged bicyclic compounds are just what the name implies—a bridge of atom(s) is thrown across from one side of the ring to the other. Fused bicyclic compounds have one bond common to both rings, while spiro compounds have one atom common to both rings. You will notice that these three types of bicyclic compounds with five-membered rings have different numbers of atoms added to a ‘parent’ five-membered ring. The bridged com pound has two extra atoms, the fused compound three, and the spiro compound four. These are marked in green with the original five-membered ring in red. We shall consider stereoselectivity in each of these types of bicyclic ring systems, starting with bridged structures. The bridged ring shown in the margin is known as norbornane: it’s a simple but very important skeleton on which many other structures are based, and it’s worth spending a moment learning how to draw it convincingly. The instructions in the box overleaf tell you how! Another way of looking at norbornane is as a six-membered ring held in a boat conformation by a one-carbon bridge. The bridge has to be axial at both bridgehead positions (or it wouldn’t be able to form a ring) so the cyclohexane has no choice but to be a boat.

A selection of important bridged bicyclic compounds is shown in the margin. Bridged structures (sometimes called cage structures) are generally very rigid, spending most of their time in a single, well-defined conformation, and this rigidity is reflected in the stereo chemistry of their reactions. For example, attack on norbornanone occurs predominantly from the side of the one-atom bridge (the green arrow) rather than the two-atom bridge (the orange arrow).

This selectivity is completely reversed in camphor because the one-atom bridge then carries two methyl groups. One of these must project over the line of approach of the hydride reducing agent.

The two methyl groups on the bridge of the camphor molecule are key features in stereo selective reactions—take them away and the result often changes dramatically. This bi cyclic system, with and without methyl groups, has been so widely used to establish stereochemical principles that the two faces of, say, the ketone group in camphor, or the C=C double bond in norbornene (the alkene derivative of norbornane) have been given the names endo and exo. These refer to inside (endo) and outside (exo) the boat-shaped six-membered ring shown in black. In general, reactions of norbornane-type structures occur from the less hindered exo face, but the methyl groups of camphor reverse this selectivity to favour endo attack:

In a similar style, epoxidation of the two alkenes is totally stereoselective, occurring exo in norbornene and endo when methyl groups are present on the bridge.

These stereoselectivities would be remarkable in a simple monocyclic compound, but in a rigid bridged bicyclic structure they are almost to be expected.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تحيي ذكرى شهادة الإمام السجاد (عليه السلام)

العتبة العباسية المقدسة تنشر لافتات خاصة بذكرى شهادة الإمام زين العابدين (عليه السلام)

قسم شؤون المعارف ينظّم دورة مختصة بتحقيق المخطوطات لطلبة الحوزة العلمية النجف الأشرف

قسم التطوير ينظم دورات وورشا تدريبية لطلبة أكاديمية التطوير الإداري

المزيد

الآخبار الصحية

دراسة علمية تكشف مفاجأة حول الأسباب الحقيقية للسمنة

دراسة: الأطباء يتجاهلون "سببا شائعا" لارتفاع ضغط الدم

نظامك الغذائي غني بالبروتين؟.. احذر هذه الآثار الجانبية

لا تتجاهلها.. أعراض تنذرك بأنك مهدد بالإصابة بالسكري

المزيد

الآخبار التكنلوجية

ISOFIX: معيار الأمان الذهبي لمقاعد الأطفال في السيارات

هل المحرك الأكبر دائمًا الأفضل للسيارة؟

اختبارات ناجحة لمحرك "VK-800" الروسي وتطويره لطائرة بايكال الخفيفة

رئيس إنفيديا: الذكاء الاصطناعي الصيني "محفز للتقدم العالمي"

المزيد

مواضيع ذات صلة

Reactions of cis-decalins

cis-Fused rings

Reactions that break open bridged molecules can preserve stereochemistry

Regard five-membered rings with two or more sp3 carbons as flat

Five-membered ketones are flexible

Only axial attack is possible with cyclohexenes

Axial or equatorial attack is possible on a cyclohexanone

Diastereotopic protons in acyclic compounds

Spotting diastereotopic protons in the NMR spectrum

How to tell if protons are homotopic, enantiotopic, or diastereotopic

The size of the geminal coupling constant

Geminal (2J) coupling