Sterically controlled diastereoselectivity in thio-Staudinger cycloadditions of alkyl/alkenyl/aryl-substituted thioketenes

Diastereoselectivity in [2 + 2] cycloadditions of alkyl/alkenyl/arylthioketenes and imines, named as thio-Staudinger cycloaddition, was rationalized and verified.

Stereochemistry and mechanistic insights in the [2t + 2i + 2i] annulations of thioketenes and imines

The mechanism and stereochemistry of [2^t + 2ⁱ + 2ⁱ] annulations between one thioketene molecule and two imine molecules have been studied experimentally and theoretically.

Stereochemistry and Mechanistic Insight in the [2(k)+2(i)+2(i)] Annulations of Ketenes and Imines

The stereochemistry and mechanistic insight in the annulations of one ketene molecule with two imine molecules ([2(k)+2(i)+2(i)] annulation) are studied by using six-membered 3,4-dihydroisoquinoline as an imine probe. A concerted hetero-Diels-Alder cycloaddition mechanism is proposed to explain the stereochemical outcomes. In most cases, the zwitterionic 2-aza-1,3-butadiene-type intermediates, generated from ketenes and imines, undergo endo hetero-Diels-Alder cycloaddition with the second imine molecule. For ketenes with electron-donating substituents, (2,4)-cis-(4,5)-cis-[2(k)+2(i)+2(i)] annuladducts formed stereospecifically, while, for ketenes with electron-accepting substituents, (2,4)-cis-(4,5)-trans-[2(k)+2(i)+2(i)] annuladducts are generated stereospecifically. The [2(k)+2(i)+2(i)] annulations of aryloxyketenes and 3,4-dihydroisoquinoline give stereodivergent products due to the occurrence of the stepwise nucleophilic annulation. However, in the [2(k)+2(i)+2(i)] annulations of seven-membered cyclic imine dibenzo[b,f][1,4]oxazepine, the zwitterionic aza-butadiene-type intermediates exclusively undergo exo hetero-Diels-Alder cycloadditions with another molecule of imine to yield (2,4)-trans-(4,5)-trans-[2(k)+2(i)+2(i)] annuladducts stereospecifically, regardless of the ketene substituents. The mechanistic model not only discloses the nature of the [2(k)+2(i)+2(i)] annulations, but also can be used to explain and predict the stereochemistry of the [2(k)+2(i)+2(i)] annuladducts from different ketenes and imines.

Base-switched annuloselectivity in the reactions of ethyl malonyl chloride and imines

The [2 + 2] and [2 + 2 + 2] annuloselectivity in the reactions of ethyl malonyl chloride with imines was controlled by different bases, providing a simple synthesis of β-lactam-3-carboxylates and 2,3-dihydro-1,3-oxazin-4-ones.