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Date: 5-8-2018
1072
Date: 1-12-2019
861
Date: 22-10-2020
710
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Four-Membered Rings
Reactions
Reactions of four-membered heterocycles also show the influence of ring strain. Some examples are given in the following diagram. Acid-catalysis is a common feature of many ring-opening reactions, as shown by examples 1, 2 & 3a. In the thietane reaction (2), the sulfur undergoes electrophilic chlorination to form a chlorosulfonium intermediate followed by a ring-opening chloride ion substitution. Strong nucleophiles will also open the strained ether, as shown by reaction 3b. Cleavage reactions of β-lactones may take place either by acid-catalyzed acyl exchange, as in 4a, or by alkyl-O rupture by nucleophiles, as in 4b. Example 5 is an interesting case of intramolecular rearrangement to an ortho-ester. Finally, the β-lactam cleavage of penicillin G (reaction 6) testifies to the enhanced acylating reactivity of this fused ring system. Most amides are extremely unreactive acylation reagents, thanks to stabilization by p-π resonance. Such electron pair delocalization is diminished in the penicillins, leaving the nitrogen with a pyramidal configuration and the carbonyl function more reactive toward nucleophiles.
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إجراء أول اختبار لدواء "ثوري" يتصدى لعدة أنواع من السرطان
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دراسة تكشف "سببا غريبا" يعيق نمو الطيور
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جامعة الكفيل تناقش تحضيراتها الخاصة بالامتحانات النهائية
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المجمع العلمي يستأنف برنامج (عرش التلاوة) الوطني
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أرباح مصرف الراجحي ترتفع إلى 4.4 مليار ريال في الربع الأول
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الأمانة العامة للعتبة العبّاسية تشارك في مُلتقى أمناء العتبات المقدّسة داخل العراق
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