you will recall that conjugate addition to enones generates first an enolate, which is usually protonated in the work-up. But, again, more fruitful things can be done with the enolate under the right conditions.

The simplest products are formed when Nu=H, but this poses a problem of regioselectivity in the nucleophilic attack step: a nucleophilic hydride equivalent that selectively undergoes conjugate addition to the enone is required. This is usually achieved with extremely bulky hydride reagents such as lithium or potassium tri(sec-butyl)borohydride (often known by the trade names of L- or K-Selectride, respectively). In this example, K-Selectride reduces the enone to an enolate that is alkylated by methyl iodide to give a single regioisomer.

With organocopper reagents, conjugate addition introduces a new alkyl group and, if the resulting enolates are themselves alkylated, two new C–C bonds can be formed in a single step (a tandem reaction: one C–C bond formation rides behind another).  we explained that the best organocuprate additions are those carried out in the presence of Me₃SiCl: the product of these reactions is a silyl enol ether, formed regioselectively (the ‘enol’ double bond is always on the side where the enone used to be).

The silyl enol ethers are too unreactive for direct alkylation by an alkyl halide, but by con verting them to lithium enolates all the usual alkylation chemistry becomes possible. This type of reaction forms the key step in a synthesis of the natural product α-chamigrene. Conjugate addition of Me2CuLi gives an enolate that is trapped with trimethylsilyl chloride. Methyllithium converts the resulting silyl enol ether into a lithium enolate by substitution at Si. The natural product has a spiro six-membered ring attached at the site of the enolate, and this is made by alkylating with a dibromide. The first substitu tion is at the more reactive allylic bromide. A second enolization is needed to make the ring, but this can be done under equilibrating conditions because the required six-membered ring forms much faster than the unwanted eight-membered ring that would arise by attack on the other side of the ketone.

Among the most important of these tandem conjugate addition–alkylation reactions are those of cyclopentenones. With cyclopentenone itself, the trans diastereoisomer usually results because the alkylating agent approaches from the less hindered face of the enolate.

This is the sort of selectivity evident in the next example, which looks more complicated but is really just addition of an arylcopper reagent followed by alkylation (trans to the bulky Ar group) with an iodoester.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تفتتح تكية طرف باب السلالمة بعد إعادة بنائها

شعبة مدارس الكفيل الدينية النسوية تعقد جلسة تثقيفية ضمن فعاليات مخيم بنات العقيدة

مواكب العزاء الحسينية تواصل برنامجها العزائي لشهر المحرم الحرام 1447هـ

شعبة الخطابة النسوية تقيم مجلس عزاء بحلول شهر المحرم الحرام

المزيد

الآخبار الصحية

دراسة تكشف علاقة خطيرة بين عمر الأم وصحة مولودها

دراسة تحذر.. اضطراب نوم بسيط قد يسبب الموت المبكر ؟

الاستحمام بالماء الساخن.. راحة مؤقتة وأضرار دائمة للبشرة

ماذا يحدث للجسم عند تخطي وجبة الإفطار؟

المزيد

الآخبار التكنلوجية

كيف تساعد الصيانة الوقائية في الحفاظ على كفاءة التكييف في سيارتك ؟

في ظاهرة غريبة حيرت العلماء.. كوكبنا قد يسجل أقصر يوم في التاريخ هذا الصيف!

السد الصيني العملاق يعيق دوران الأرض ويغيّر شكلها ؟

السعودية تطلق 10 تجارب علمية إلى محطة الفضاء الدولية

المزيد

مواضيع ذات صلة

The Claisen ester condensation and other self-condensations

Problems with acylation at carbon

Cross-condensations

The aldol reaction

Nitroalkanes are superb nucleophiles for conjugate addition

A variety of electrophilic alkenes will accept enol(ate) nucleophiles

Conjugate addition of silyl enol ethers leads to the silyl enol ether of the product

Enamines are convenient stable enol equivalents for conjugate addition

Alkali metal (especially Na, K) enolates can undergo conjugate addition

1,3-Dicarbonyl compounds undergo conjugate addition

Using Michael acceptors as electrophiles

Enones provide a solution to regioselectivity problems