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Date: 16-5-2017
1553
Date: 11-9-2020
765
Date: 22-8-2019
706
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Preparation
Several methods of preparing four-membered heterocyclic compounds are shown in the following diagram. The simple procedure of treating a 3-halo alcohol, thiol or amine with base is generally effective, but the yields are often mediocre. Dimerization and elimination are common side reactions, and other functions may compete in the reaction. In the case of example 1, cyclization to an oxirane competes with thietane formation, but the greater nucleophilicity of sulfur dominates, especially if a weak base is used. In example 2 both aziridine and azetidine formation are possible, but only the former is observed. This is a good example of the kinetic advantage of three-membered ring formation. Example 4 demonstrates that this approach to azetidine formation works well in the absence of competition. Indeed, the exceptional yield of this product is attributed to the gem-dimethyl substitution, the Thorpe-Ingold effect, which is believed to favor coiled chain conformations. The relatively rigid configuration of the substrate in example 3, favors oxetane formation and prevents an oxirane cyclization from occurring. Finally, the Paterno-Buchi photocyclizations in examples 5 and 6 are particularly suited to oxetane formation.
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حمية العقل.. نظام صحي لإطالة شباب دماغك
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إيرباص تكشف عن نموذج تجريبي من نصف طائرة ونصف هليكوبتر
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موسوعة فتوى الدفاع تزيّن جناح العتبة العباسية بمعرض طهران الدولي للكتاب
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قسم شؤون المعارف: نعمل على إحياء التراث الكربلائي
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اللجنة التحضيرية لأسبوع الإمامة تعلن عن الموقف نصف الشهري لأبحاث مؤتمر الإمام الحسين (عليه السلام)
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أهم ما تضمن البرنامج الأوَّل لمخيّم بنات العقيدة التاسع عشر
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