4468 3. Synthesis of Amino(hydroxy) Acid Derivatives 4474 3.1. β-Amino Acids 4474 3.2. R-Hydroxy-β-amino Acids 4480 3.3. R-Amino Acids 4482 4. Synthesis of β-Amino Ketone Derivatives 4484 5. Synthesis of γ-Amino Alcohol Derivatives 4485 6. Miscellaneous 4486 7. Conclusions 4489 8. Abbreviations 4489 9. Acknowledgments 4489 10. References 4489

The allene moiety represents an excellent partner for the [2+2] cycloaddition with alkenes and alkynes, affording the cyclobutane and cyclobutene skeletons in a single step. This strategy has been widely studied under thermal, photochemical and microwave induced conditions. More recently, the use of transition metal catalysis has been introduced as an alternative relying on the activation of the allenic component. On the other hand, the intramolecular version has attracted much attention as a strategy for the synthesis of polycyclic compounds in a regio-and stereoselective fashion. This critical review focuses on the most recently developed [2+2] cycloadditions on allenes along with remarkable early works accounting for the mechanism, the regio-and diastereoselectivity of the cycloadducts formed (103 references).

Metal-mediated carbonyl allylation, allenylation, and propargylation of optically pure azetidine-2,3-diones were investigated in both anhydrous and aqueous environments. Different metals promoters showed varied regioselectivities on product formation during allenylation/propargylation reactions of the keto-beta-lactams. The stereochemistry of the new C3-substituted C3-hydroxy quaternary center was controlled by placing a chiral auxiliary at C4. In this way, the coupling of azetidine-2,3-diones with a variety of propenyl-, propynyl-, and allenylmetal reagents offers a convenient asymmetric entry to potentially bioactive 3-substituted 3-hydroxy-beta-lactams.

This review presents the reactivity of a variety of substituted bis(allenes) under different reaction conditions, showing the high number of complex structures that can be obtained.