Five-membered heterocycles act as dienes in Diels–Alder reactions

All of the reactions of pyrrole, furan, and thiophene we have discussed so far have been variations on reactions of benzene. But heterocycles also do reactions totally unlike those of benzene and we are now going to explore two of them.

The first is a reaction you will meet in detail. It is known as the Diels–Alder reaction, and although it has a number of subtleties we will not discuss here, it has a simple cyclic mechanism in which six electrons (three curly arrows) move around to form a new six membered ring. Here is an example with the Boc derivative of pyrrole. The electron-deficient Boc group makes pyrrole less nucleophilic and promotes the Diels–Alder reaction with an alkynyl sulfone. Benzene, and even many other heterocycles, will not do this sort of reaction.

The product is a useful intermediate in the synthesis of the analgesic epibatidine. Selective reduction of the non-conjugated double bond is followed by addition of a pyridine nucleophile (a lithium derivative can be prepared from a bromopyridine) to the vinyl sulfone.

Furan is particularly good at Diels–Alder reactions but it gives the thermodynamic product, the exo adduct, because with this aromatic diene the reaction is reversible.

Aromaticity prevents thiophene taking part in Diels–Alder reactions, but oxidation to the sulfone destroys the aromaticity because both lone pairs become involved in bonds to oxy gen. The sulfone is unstable and reacts with itself but will also do Diels–Alder reactions. With an alkyne, loss of SO₂ gives a substituted benzene derivative.

Similar reactions occur with α-pyrones. These are also rather unstable and barely aromatic and they react with alkynes by Diels–Alder reactions followed by reverse Diels–Alder reactions to give benzene derivatives with the loss of CO₂.

اخر الاخبار

اخبار العتبة العباسية المقدسة

أهالي كربلاء يحيون اليوم السابع لذكرى شهادة الإمام الحسين (عليه السلام)

مشروع أمير القراء الوطني يشهد مشاركة أكثر من 100 طالبٍ في مرحلته الثالثة

قسم الشؤون الفكرية يصدر دليلًا يوثّق القصص الفائزة في مسابقة فتوى الدفاع المقدسة

قسم الشؤون الفكرية يعلن عن توفير عددٍ من المكتبات الإلكترونية المتخصّصة للمؤسّسات الدينية والمكتبات العامّة

المزيد

الآخبار الصحية

نصائح عملية للحد من التعرق المفرط في الصيف

أطباء يحذرون: مكمل غذائي شائع قد يدمر الجهاز العصبي

تحذير طبي من تأثير جانبي "غير متوقع" لحقن التخسيس

حمامات الثلج قد تكون خطيرة وأحيانا مميتة.. ما السبب؟

المزيد

الآخبار التكنلوجية

أكبر جدارية شمسية في العالم تدخل موسوعة غينيس.. مشروع ضخم

تسارع دوران الأرض يثير القلق.. وتحذيرات من "كوارث محتملة" ؟

"سوذبيز" تطرح أكبر قطعة من المريخ على الأرض في مزاد علني

أوبن أيه آي تعتزم إطلاق متصفح إنترنت يعمل بالذكاء الاصطناعي

المزيد

مواضيع ذات صلة

There are thousands more heterocycles out there

Fusing rings to pyridines: quinolines and isoquinolines

Putting more nitrogen atoms in a six-membered ring

Benzo-fused heterocycles-Indoles are benzo-fused pyrroles

Five-membered rings with two or more nitrogen atoms Imidazole

Nitrogen anions can be easily made from pyrrole

Electrophilic addition may be preferred to substitution with furan

Furan and thiophene are oxygen and sulfur analogues of pyrrole

Five-membered aromatic heterocycles are good at electrophilic substitution

Pyridine as a catalyst and reagent

Pyridine N-oxides are reactive towards both electrophilic and nucleophilic substitution

Pyridones are good substrates for nucleophilic substitution