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Date: 26-7-2019
888
Date: 14-7-2019
1867
Date: 10-9-2020
1438
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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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اكتشاف تأثير صحي مزدوج لتلوث الهواء على البالغين في منتصف العمر
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زهور برية شائعة لتر ميم الأعصاب التالفة
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جمعيّة العميد وقسم الشؤون الفكريّة تدعوان الباحثين للمشاركة في الملتقى العلمي الوطني الأوّل
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الأمين العام المساعد لجامعة الدول العربية السابق: جناح جمعية العميد في معرض تونس ثمين بإصداراته
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المجمع العلمي يستأنف فعاليات محفل منابر النور في واسط
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برعاية العتبة العباسيّة المقدّسة فرقة العبّاس (عليه السلام) تُقيم معرضًا يوثّق انتصاراتها في قرية البشير بمحافظة كركوك
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