A contribution to the confusion surrounding the reaction of ketenes with imines to produce .beta.-lactams. A comparison of stereoselectivity dependence on the method of ketene generation: acid chloride/triethylamine vs photolysis of chromium carbene complexes

Abstract

The stereoselectivity of the reaction of imines of benzaldehyde and cinnamaldehyde with ketenes generated by the reaction of optically active oxazolidinone acid chlorides with triethylamine and complexed ketenes generated by photolysis of optically active oxazolidine-and oxamlidinonechromium-carbene complexes in the presence and absence of added triethylamine was compared. The absolute stereochemistry was determined primarily by the structure of the chiral auxiliary. The relative (cis-trans) stereochemistry was determined primarily by the structure of the imine and the free or bound character of the ketene. Triethylamine addition to reactions of carbene complexes afforded results that closely paralleled that of acid chloride generated ketenes. A mechanistic scheme accounting for these results and the general trends observed in other ketenelimine cyclizations is provided.

Facile synthesis of novel monocyclic trans- and cis-3-oxo/thio/seleno-4-pyrazolyl-β-lactams

“…[48][49][50] In the present work, all the 2-benzyl/phenylthio/seleno ethanoic acids (Moore ketenes having S/Se-alkyl or aryl groups) on treatment with pyrazolyl substituted imines gives trans-β-lactams as suggested in the literature. [48][49][50] Whereas, the Bose-Evans ketenes (having O-alkyl or O-aryl groups) should give cis-β-lactams, which have been achieved in the synthesis of cis-3-methoxy/acetoxy-β-lactams with low yield. However, this cycloaddition did not afford the targeted products at lower temperature, therefore high temperature selectively favors the formation of predominantly trans-β-lactams.…”

“…Further, competition between the rate of isomerisation and direct ring closure, temperature and substituents plays an important role towards the stereoselectivity of these Staudinger cycloadditions which is well documented in literature. 39,[48][49][50] The crystal structure of trans-1-(4′-methoxyphenyl)-3-methoxy-4-(5′-chloro-3′-methyl-1′-phenyl-1H-pyrazol-4′-yl)azetidin-2-one 5p was established by X-ray crystallographic analysis (Figure 1). 45 It was crystallized from dichloromethane-hexane (3:1) as a colorless crystalline solid suitable for single crystal X-ray diffraction.…”

Substituent effects of rate constants for thermal [4π + 2π] cycloreversion of spiro-fused β-lactam oxadiazolines

“…4Although the result is readily accounted for in terms of [2n + 2~ pericyclic reaction is a concerted reaction that occurs through 2n] cycloaddition, the actual mechanism is still a controversial a cyclic transition state. subject (7). 3~h e facile thermal cycloreversion of A3-1 ,3,4-thiadiazolines to Details of the syntheses, including isomer ratios and spectra, N2 and thiocarbonyl ylides, for example (1).…”

“…Moreover, a phenyl substituent in the oxadiazoline ring is strongly rate enhancing. 7 The different reactivities of diastereomers (Sf and 8g; 8k and 81) are more difficult to explain. Although ylides from diastereomeric oxadiazolines must be identical in energy, the 'separation of the diastereomers of 13 was not achieved and only a crude estimate of the effect of a phenyl substituent, in the oxadiazoline ring, was possible.…”

Photoinduced Reactions of Metal Carbenes in Organic Synthesis

“…The optically active oxazolidine carbene (Table 5) gave excellent yields and high ee values when allowed to react with imidates, oxazines, thiazines, and (9) 166 L. S. Hegedus Table 4 Reaction of chromium aminocarbenes with imines Photoinduced Reactions of Metal Carbenes in Organic Synthesis 167 Table 5 Reaction of optically active aminocarbenes with imines aliphatic imines, but modest yields of mixtures of cis and trans isomers with aryl or a,b-unsaturated imines [50]. In contrast, the corresponding oxazolidinone ketene (from the acid chloride [51]) gave excellent yields and ee values with aryl and a,b-unsaturated imines but very low yields of b-lactams with other imines.…”