Semiempirical calculations, chirality

Jimenez and co-workers conducted extensive semiempirical calculations (PM3) in order to rationalize the stereochemical outcome of some hetero-Diels-Alder reactions leading to the preparation of chiral tetrahydrotetrazine derivatives 16 <1999JOC6297>. The process was found to occur with high facial stereoselectivity when the starting... 

The 2 + 2-cycloaddition reaction of a-alkoxyketene-derived imines yields /l-lactams with quaternary stereogenic centres at C(4).33 The 2 + 2-cycloaddition of chiral aminoketenes with chiral imines yields cis-fi-lactams with the absolute stereochemistry of file C(3) and C(4) positions being controlled by the ketene partner only.34 The 2 + 2-cycloaddition of ketenes with (W)-2-/-butyldihydrooxazole (19) yields predominately the regioisomer (20) from steric control rather than the expected electronic control (Scheme 7).35 The double 2 + 2-cycloaddition reaction between ketenylidenetriphe-nylphosphorane (21) and carbon suboxide (22) produces the bis(ylidic) spirocyclobut-anedione (23) (Scheme 8).36 Semiempirical and ab initio calculations have been used to investigate the Lewis acid-promoted 2 + 2-cycloaddition leading to the formation of jS-lactones.37... 

For reactions that allow the use of g—n approximation, semiempirical methods of calculating potential energy surfaces were elaborated by Basilevsky 54 55> and by Salem 56,57>. An interesting feature of the last method, which stresses the importance of interactions between the top occupied orbital of one molecule and the lowest unoccupied orbital of the other, is that, while there is no need to calculate the whole potential surface, it is possible to derive a theoretical pathway for each reaction. Salem has recently put forward a theory of asymmetric induction, enabling the difference in energy between diastereoisomeric transition states and the diastereoisomer ratio to be calculated for an achiral reagent and a model chiral substrate 58>. 

In 2(X)0, the same group evaluated the catalytic activity of similar chiral aluminum complexes 38 in enantioselective hetero-Diels-Alder reaction of benzaldehyde and Danishefsky s diene 39 (Scheme 31) [54]. The mechanism for this hetero-Diels-Alder reaction has been investigated from a theoretical point of view using semiempirical and ab initio calculations [55]. 

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