تسجيل الدخول

تسجيل

Ketene [2 + 2] cycloadditions

المؤلف:  Jonathan Clayden , Nick Greeves , Stuart Warren

المصدر:  ORGANIC CHEMISTRY

الجزء والصفحة:  ص899

2025-07-23

556

Ketene [2 + 2] cycloadditions

Ketene itself is usually made by high-temperature pyrolysis of acetone but some ketenes are easily made in solution. The very acidic proton on dichloroacetyl chloride can be removed even with a tertiary amine and loss of chloride ion then gives dichloroketene in an ElcB elimination reaction. If the elimination is carried out in the presence of cyclopentadiene a very efficient regio- and stereospecific [2 + 2] cycloaddition occurs.

The most nucleophilic atom on the diene adds to the most electrophilic atom on the ketene and the cis geometry at the ring junction comes from the cis double bond of cyclopentadiene. It is impressive that even this excellent diene undergoes no Diels–Alder reaction with ketene as dienophile. The [2 + 2] cycloaddition must be much faster.

But what do we do if we want the product of a ketene [4 + 2] cycloaddition? We must use a compound that is not a ketene but that can be transformed into a ketone afterwards—a masked ketene or a ketene equivalent. The two most important types are nitroalkenes and compounds such as the ‘cyanohydrin ester’ in the second example.