Dedicated to the memory of Klaus Hafner
Abstract
For a long time, allenes—and cumulenic systems in general—played a relatively minor role in Diels–Alder cycloadditions. This situation has changed, since allenes are more readily available and as their unique stereochemical features in [4+2]cycloadditions are more widely recognized. This review presents a comprehensive overview of allenes in Diels–Alder processes using selected examples. Allenes in dienes, dienophiles and cycloadducts are covered, inter- and intramolecular Diels–Alder cycloadditions are discussed, and stereochemical features of the addition process are described. Areas of emerging importance are also covered, including allenic components in dehydro-Diels–Alder processes, and dendralenic allenes in Diels–Alder sequences for the rapid generation of target-relevant molecular complexity. Preparatively useful methods for allenic precursor synthesis are also discussed.
1 Introduction
2 Allenic Dienes
2.1 Vinylallenes
2.2 Bisallenes
2.3 Cross-conjugated Allenes
3 Allenic Dienophiles
4 Intramolecular Diels–Alder Cycloadditions
5 Allenic Cycloadducts
6 Conclusions and Outlook
Key words
Diels–Alder addition - allenes as dienes and dienophiles - intermolecular cycloaddition - intramolecular cycloaddition - stereoselective synthesis - dehydro-Diels–Alder reactions - rapid generation of molecular complexity