Fusing rings to pyridines: quinolines and isoquinolines

A benzene ring can be fused on to the pyridine ring in two ways, giving the important heterocycles quinoline, with the nitrogen atom next to the benzene ring, and isoquino line, with the nitrogen atom in the other possible position. Quinoline forms part of quinine and isoquinoline forms the central skeleton of the isoquinoline alkaloids. In this chapter we need not say much about quinoline because it behaves rather as you would expect—its chemistry is a mixture of that of benzene and pyridine. Electrophilic substitution favours the benzene ring and nucleophilic substitution favours the pyridine ring. So, nitration of quinoline gives two products—the 5-nitroquinolines and the 8-nitroquinolines—in about equal quantities (although you will realize that the reaction really occurs on protonated quinoline).

This is obviously rather unsatisfactory but nitration is actually one of the better behaved reactions. Chlorination gives ten products (at least!), of which no fewer than five are chlorinated quinolines of various structures. The nitration of isoquinoline is rather better behaved, giving 72% of one isomer (5-nitroisoquinoline) at 0 °C.

To get reaction on the pyridine ring, the N-oxide can be used—as with pyridine itself. A good example is acridine, with two benzene rings, which gives four nitration products, all on the benzene rings. Its N-oxide, on the other hand, gives just one product in good yield— nitration takes place at the only remaining position on the pyridine ring.

In general, these reactions are not of much use and most substituents are put into quino lines during ring synthesis from simple precursors, as we will explain in the next chapter. There are a couple of quinoline reactions that are unusual and interesting. Vigorous oxidation goes for the more electron-rich ring, the benzene ring, and destroys it leaving pyridine rings with carbonyl groups in the 2- and 3-positions.

A particularly interesting nucleophilic substitution occurs when quinoline N-oxide is treated with acylating agents in the presence of nucleophiles. These two examples show that nucleophilic substitution occurs in the 2-position and you may compare these reactions with those of pyridine N-oxide. The mechanism is similar.

In considering quinolines and indoles with their fused rings we kept the benzene and hetero cyclic rings separate. Yet there is a way in which they can be combined more intimately, and that is to have a nitrogen atom at a ring junction.

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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

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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؟

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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

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مواضيع ذات صلة

There are thousands more heterocycles out there

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

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

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