Four-membered rings

A similar situation exists with four-membered rings—the cis coupling is larger than the trans but they are generally both smaller than those in larger rings. A good example is the amino acid in the margin, the skeleton of the penicillins. The NMR spectrum contains three 1H signals in the middle regions. There is a singlet at δH 4.15 ppm that clearly belongs to the isolated green proton and two doublets at δH 4.55 and 5.40 ppm that must belong to the black protons. The coupling constant between them is 5 Hz and they are cis-related. There are now large numbers of β-lactam antibiotics known and one family has the opposite (trans) stereochemistry around the four-membered ring. The typical member is thienamycin. We will analyse the spectrum in a moment, but fi rst look at the differences—apart from stereochemistry—between this structure and the last. The sulfur atom is now outside the five-membered ring, the acid group is on a double bond in the same ring, and the amino group has gone from the β-lactam to be replaced by a hydroxyalkyl side chain. Turning to the spectrum and the key question of stereochemistry, this is what the Merck discoverers said in their original article: ‘1H NMR spectra of thienamycin (and derivatives) ... show small vicinal coupling constants J ≤ 3 Hz for the two β-lactam hydrogens. Past experi ence with penicillins...shows the cis relationship of the β-lactam hydrogens to be always associated with the larger coupling.’ As we have just seen penicillins have J ~ 5 Hz for these hydrogens. The NMR spectrum of a thienamycin derivative with protecting groups on the amine and carboxylic acids is shown below. Try your hand at interpreting it before you read the explanation. Your aim is to find the coupling constant across the four-membered ring.

The simple answer is 2.5 Hz. The signals at 3.15 and 4.19 ppm are the protons on the β-lactam ring and the 9 Hz extra coupling is to the CH2 in the five-membered ring. If you went into this spectrum in detail you may have been worried about the 12.5 and especially the 18 Hz couplings. These are 2J (geminal) couplings and we will discuss them in the next sec tion. The full assignment is shown below.

We should emphasize that a coupling constant of 5 or 2.5 Hz in isolation would not allow us to assign stereochemistry across the four-membered ring but, when we have both, we can say with confidence that the larger coupling is between cis Hs and the smaller coupling between trans Hs.

اخر الاخبار

اخبار العتبة العباسية المقدسة

مجلة تسليم تدعو الباحثين والأكاديميين للمشاركة في أعدادها القادمة

المجمع العلمي يواصل إقامة محاضراته الأسبوعية في مقام الإمام المهدي (عجل الله فرجه)

جامعة الكفيل تطلق الدورة التدريبية الصيفية لطلبة كلية الهندسة التقنية

استعدادًا لزيارة الأربعين العتبة العباسية المقدسة تعقد الورشة التنسيقية الرابعة الخاصة بمراكز إرشاد التائهين

المزيد

الآخبار الصحية

دراسة علمية تكشف مفاجأة حول الأسباب الحقيقية للسمنة

دراسة: الأطباء يتجاهلون "سببا شائعا" لارتفاع ضغط الدم

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المزيد

الآخبار التكنلوجية

ISOFIX: معيار الأمان الذهبي لمقاعد الأطفال في السيارات

هل المحرك الأكبر دائمًا الأفضل للسيارة؟

اختبارات ناجحة لمحرك "VK-800" الروسي وتطويره لطائرة بايكال الخفيفة

رئيس إنفيديا: الذكاء الاصطناعي الصيني "محفز للتقدم العالمي"

المزيد

مواضيع ذات صلة

Diastereotopic protons in acyclic compounds

Spotting diastereotopic protons in the NMR spectrum

How to tell if protons are homotopic, enantiotopic, or diastereotopic

The size of the geminal coupling constant

Geminal (2J) coupling

Heteroatoms in rings have axial and equatorial lone pairs

Ring size and NMR

Baldwin’s rules and ring opening

Combatting ΔS‡—the Thorpe–Ingold effect

Thermodynamic control over ring size

Making heterocycles: ring-closing reactions

Related effects in other types of compounds