Stereochemistry of the dien 

This is slightly more complicated as the diene can be cis, cis, or cis, trans (there are two of these if the diene is unsymmetrical), or trans, trans. We shall look at each case with the same dienophile, an acetylene dicarboxylate, as there is then no stereochemistry in the triple bond! Starting with cis, cis-dienes is easy if we make the diene cyclic.

The diene has two sets of substituents—inside and outside. The inside one is the bridging CH2 group and it has to end up on one side of the molecule (above in the last diagram) while the two green hydrogens are outside and remain so. In the final diagram they are below the new six-membered ring. With a trans, trans-diene we simply exchange the two sets of substitu ents, in this example putting Ph where H was and putting H where the bridging CH2 group was. This is the reaction:

The green Ph groups end up where the hydrogens were in the fi rst example—beneath the new six-membered ring—and the hydrogens end up above. It may seem puzzling at fi rst that a trans,trans-diene gives a product with the two phenyls cis. Another way to look at these two reactions is to consider their symmetry. Both have a plane of symmetry throughout and the products must have this symmetry too because the reaction is concerted and no significant movement of substituents can occur. The orange dotted line shows the plane of symmetry, which is at right angles to the paper.

The remaining case—the cis, trans-diene—is rarer than the first two, but is met sometimes. This unsymmetrical diene means the two substituents clearly end up on opposite sides of the new six-membered ring.

The red R group may seem to get in the way of the reaction but, of course, the dienophile is not approaching in the plane of the diene but from underneath. It is difficult to find a convincing example of this stereochemistry as there are so few known, partly because of the difficulty of making E,Z dienes. One good approach uses two reactions you met in Chapter 29 for the control of double-bond geometry. The cis double bond is put in first by the addition of methanol to butadiyne and the trans double bond then comes from LiAlH4 reduction of the intermediate acetylenic alcohol.

The acetate of this alcohol is used in a Diels–Alder reaction with the interesting dienophile DEAD (diethyl azodicarboxylate—in orange). The product is formed in excellent yield and has the trans stereochemistry that was predicted. The amide nitrogen atoms are planar, so there is no question of stereochemistry there. DEAD itself can equilibrate rapidly between E and Z isomers, but the E predominates.

Now to the most interesting cases of all, when both the diene and the dienophile have stereochemistry.

اخر الاخبار

اخبار العتبة العباسية المقدسة

بيان لمكتب المرجعية العليا حول نشر صور السيد السيستاني في الأماكن العامة

متحف الكفيل: إنجاز المرحلة الرابعة من مشروع متحف العتبة العسكرية المقدسة

تضمن عرضًا مسرحيًّا.. شعبةُ الخطابة النسويّة تقيم مجلس عزاء بذكرى شهادة الإمام الحسن (عليه السّلام)

الأمين العام للعتبة العبّاسية المقدّسة يتفقد مجموعة مواكب أمّ البنين (عليها السلام) لخدمة الزائرين

المزيد

الآخبار الصحية

تسارع الشيخوخة.. متى يبدأ وأي الأعضاء أكثر تأثرا؟

9 مسببات شائعة للالتهاب المزمن قد تهدد صحتك

باحثون يطورون علاجا لهشاشة العظام.. النتائج مذهلة

تحمي من أمراض عديدة.. 6 فوائد لتناول البطاطا الحلوة

المزيد

الآخبار التكنلوجية

عودة التعاون الفضائي بين روسيا وأميركا.. وبحث ملفات "هامة"

المعلمون في خطر.. خاصية جديدة في "تشات جي بي تي" تشرح الدروس

بشرى لمرضى الشلل.. شريحة في الدماغ قد تتيح التحكم بالأدوات

مدينة غارقة.. هل يخفي قاع البحر الكاريبي سرّ حضارة مفقودة؟

المزيد

مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct