A cunning way to get unusual regioselectivity is to make the reaction intramolecular. The synthesis from benzene of the cyclic ketone known as tetralone may look difficult as we must get an ortho relationship on the benzene ring. But if we make the final bond in the ring by a Friedel–Crafts acylation there is no problem. The alkyl group is ortho, para-directing and the acid cannot reach the para position.

Notice the use of a cyclic anhydride in the first Friedel–Crafts acylation. It doesn’t matter where the acylation occurs and the reaction stops there as the ring is deactivated by the ketone and the carboxylic acid released in the reaction is much less electrophilic than the anhydride. The ketone is then reduced to a CH₂ group by the Clemmensen method and polyphosphoric acid is used to carry out the intramolecular acylation step. A more subtle approach is to use a ‘tether’—something that holds two reagents together and is afterwards cleaved. An example is halo lactonization. The idea is simple. A halogen, say bromine, attacks an alkene and the bromonium ion intermediate is captured intramolecularly by the anion of a carboxylic acid. The reaction therefore uses bromine and NaHCO₃—a weak base, but one strong enough to deprotonate a carboxylic acid. The anion attacks the more highly substituted end of the bromonium ion, and forms a five-membered ring.

Although any halogen might be used in this reaction, iodine is the most versatile and the reaction is commonly called iodolactonization. The tether is the C–O bond of the lactone and this can be cleaved with an alkoxide.

The reaction with methoxide needs some explanation. Attack on the carbonyl group cleaves the lactone, releasing an alkoxide that cyclizes to form an epoxide. A second molecule of methoxide now attacks the epoxide, opening it from the less hindered end as we should expect in an anionic reaction.

Another example shows that reaction may occur at the other end of the iodonium ion. Attack at the tertiary carbon would be difficult sterically and, in any case, would give an unstable four-membered ring. The lactone formed has the iodine β to the carbonyl group and so eliminates easily in base (pyridine works well) by the E1cB mechanism to give the unsaturated lactone. Although the relative stereochemistry of the iodolactone is controlled by the inversion in the opening of the iodonium ion, it is irrelevant as it disappears in the elimination step.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم العلاقات العامة يختتم فعاليات محطات فتية الكفيل الثقافية في محافظة نينوى

ما الخدمات التي قدّمها موكب أشبال الطفّ لزائري عاشوراء؟

العتبة العباسية المقدسة تنظّم مسيرات عزائية لإحياء ذكرى عاشوراء في نينوى

قسم الشؤون الفكرية يحيي ذكرى استشهاد الإمام الحسين (عليه السلام) في القارة الإفريقية

المزيد

الآخبار الصحية

تناول الجبن.. مفيد للجسم أم خطر يهدد صحتك؟

طبيبة أسنان تحذر.. لا تتجاهل هذه العلامة بعد التنظيف

يجب أن تأكل المزيد من الشمام هذا الصيف.. إليك السبب ؟

تأثير صحي كبير.. كم بيضة يجب أن تتناول يوميا؟

المزيد

الآخبار التكنلوجية

أداء خلايا البيروفسكايت الشمسية تحت الماء قد يزيد كفاءة تحويل الكهرباء

مسافة الأمان: أساس القيادة الآمنة وتجنب الحوادث

هل القيادة السريعة تعني استهلاك اكثر للوقود؟

لماذا يعتبر كوكب المريخ غير صالح للحياة؟

المزيد

مواضيع ذات صلة

E-selective reduction of alkynes using sodium in liquid ammonia

Amine synthesis using functional group interconversions

Functional group interconversion

Multiple step syntheses: avoid chemoselectivity problems

Synthons are idealized reagents

The E-selective Wittig reaction

The Z-selective Wittig reaction

Stereoselectivity in the Wittig reaction depends on the ylid

The Peterson reaction is stereospecific

The one-step Julia olefination

The Julia olefi nation is stereoselective

The Julia olefination is regiospecific and connective