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A versatile asymmetric synthesis of bicyclic pyrazolidinones through alkaloid‐catalyzed formal [3+2]‐ and [3+2+2]‐cycloadditions of ketenes with azomethine imines is described. The methodology was found to be tolerant of ketene and a variety of monosubstituted ketenes (R=alkyl, OAc). The products were formed in good to excellent yields (71–99 % for 24 examples, 39 examples in all), with good to excellent diastereoselectivity in many cases (dr 3 : 1 to 27 : 1 for 22 examples), and with excellent enantioselectivity for most examples (≥93 %eefor 34 products). In the case of most disubstituted ketenes, the reaction proceeded through a [3+2+2]‐cycloaddition to form structurally interesting bicyclic pyrazolo‐oxadiazepinediones with moderate diastereoselectivity (dr up to 3.7 : 1) and as racemic mixtures (3 examples). The method represents the first unambiguous example of an enantioselective reaction between ketenes and a 1,3‐dipole.

Catalytic hydrogenolysis of theZ‐isomer of a series of aryl‐substituted ketene heterodimer β‐lactones facilitated access to deoxypropionate derivatives with adrranging from 54:46 to 86:14, favoring theanti‐isomer, and with excellent transfer of chirality (91 → 99 %eefor 13 examples). Although X = 4‐F was determined to provide optimal diastereoselectivity (dr86:14), a non‐linear relationship between diastereoselectivity and aryl substituentσvalues was found. For cases where apara‐ orortho‐EWG was present on the aryl ring of the ketene heterodimer, formation of significant amounts of β‐lactone (20–44 %) as by‐product was observed. The results of a number of control reactions point toanti‐β‐elimination and ananti‐selective hydrogenation of anE‐isomer olefin intermediate being key steps in the reaction mechanism. The synthetic potential of the deoxypropionate derivative products was demonstrated by oxidative conversion into a 1,5‐difunctionalized deoxypropionate motif.