[Show abstract][Hide abstract] ABSTRACT: The biological activity of bicyclic beta-lactam antibiotics depends strongly on the absolute configuration of the bridgehead carbon atom. Frelek and co-workers proposed an empirical helicity rule relating the configuration of the bridgehead carbon atom to the sign of the 220 nm band in the electronic circular dichroism (CD) spectrum of beta-lactams. Here we use synthetic organic chemistry, CD spectroscopy, and time-dependent density functional theory (TDDFT) to investigate the validity of this structure-property relationship for eight model compounds. For conformationally flexible beta-lactams, substantial thermal effects are found which must be included in calculations. To this end, we combine TDDFT calculations of CD with full quantum-mechanical Born-Oppenheimer molecular dynamics (MD) simulations for the first time. The CD spectra are sampled with ground-state density functional trajectories of up to 60 ps. The MD simulations show a surprisingly high sensitivity of the CD to the molecular conformation. On the other hand, the relation between CD and thermally averaged structural parameters is much less complex. While the helicity rule does not seem to hold for individual conformers, it is confirmed by the calculations for seven out of eight systems studied if thermally averaged CD spectra and structures are considered. Since thermal effects on CD can be larger than typical inherent inaccuracies of TDDFT, our results emphasize the need for a systematic treatment of conformational dynamics in CD calculations even for moderately flexible systems. Temperature-dependent CD measurements are very useful for this purpose. Our results also suggest that CD spectroscopy may be used as a sensitive probe of conformational dynamics if combined with electronic structure calculations.
[Show abstract][Hide abstract] ABSTRACT: The synthesis of oxacephalotin and oxacephamandol, which are more active than natural, sulfur-containing congeners, and the isolation of clavulanic acid, a potent inhibitor of beta-lactamase enzymes, directed attention of many academic and industrial laboratories the synthesis of oxygen analogues of penicillins and cephalosporins. The present review focuses attention on the problem of stereocontrol in the formation of a desired configuration of the bridgehead carbon atom in the title compounds. Five feasible synthetic methods leading to the basic skeletons of clavams and 5-oxacephams are discussed. Three of them involve the nucleophilic substitution at C-4 of the azetidin- 2-ones performed as inter- or intramolecular process and the remaining two involve cycloaddition reactions between ketenes and iminoethers, or between vinyl ethers and isocyanates. Owing to the general application, stereospecificity and high asymmetric induction, the last method seems to be most advantageous. The weak point of the nucleophilic substitution methodology is that a nucleophile approaches the 3-substituted azetidin-2-one ring preferentially anti to the existing substituent and in the case where there is no substituent at C-3, that the stereoselectivity of formation of the new chirality center at C-4 is low. All discussed methods are illustrated by the examples taken from the literature.
No preview · Article · Aug 2004 · Current Medicinal Chemistry
[Show abstract][Hide abstract] ABSTRACT: Due to their multifunctional character, the ethoxycarbonylation of 3-keto-5-oxa-cephams (10, 27 and 28) using ethyl cyanoformate in THF/HMPA solution led to expected products 14, 27 and 29 in a low yield only. Apart from the desired compounds, the O-acylation, C-alkylation at C-7, and the addition of CN¯ anion to the keto group were also observed.
[Show abstract][Hide abstract] ABSTRACT: Starting from (l)-ethyl lactate and 4-vinyloxy-azetidin-2-one the diastereomeric (4S,6R)- and (4S,6S)-4-methyl-5-oxa-3-methylene and 3-oxo-cephams were obtained. The formation of the cepham skeleton proceeds with a diastereomeric excess up to 80%, depending on catalyst and reaction conditions. For comparison, the corresponding racemic cephams lacking a methyl at C-4 or with a gem-dimethyl group at C-4 were synthesized.
[Show abstract][Hide abstract] ABSTRACT: The [2 + 2] cycloaddition of chlorosulfonyl isocyanate to (Z)-4-O-propenyl ethers 16, 17, 29 and 30 proceeds with an excellent stereoselectivity in the case of ether 16 and with moderate stereoselectivity in remaining cases. Adducts were transformed into corresponding oxacephams: 37 in the first case, a mixture of 37/40 in the second and third case, and a mixture of 50/51 in the last case. In all instances addition to the si-re side of the olefin dominates. Oxacephams 41 and 52 with opposite R-configuration at the bridgehead carbon C-5a can be obtained by the alternate methodology based on the alkylation of nitrogen in 4-vinyloxyazetidin-2-one by protected 6-O-triflate 24 or 25, followed by cyclization via intramolecular displacement of the vinyloxy group. Compounds 37, 40, 41, 50, 51 and 52 constitute a convenient entry leading to polyfunctionalized oxacephams.
No preview · Article · Dec 2002 · Carbohydrate Research