The search for tolerant Lewis acid catalysts. Part2: enantiopure cycloalkyldialkylsilyl triflimide catalysts

A series of 2-aryl-and arylmethyl-3-dialkylphenylsilyl cycloalkanones have been prepared and resolved. The pure enantiomers were reduced to the corresponding cycloalkane derivatives. These were used for the in situ generation of enantiopure cycloalkylsilyl triflimides by protodesilylation with bis( trifluoromethanesulfonyl) imide. The association of a bulky leaving group with a silicon atom carrying large substituents was again shown to favour the complexation of silicon with carbonyl groups: all these trialkylsilyl triflimides showed a high catalytic activity for Diels -Alder reactions. Enantiomeric excesses up to 59% were observed. This is the highest enantioselectivity ever observed for a Diels-Alder reaction catalysed by a silicon Lewis acid. (c) 2007 Elsevier Ltd. All rights reserved

Large ring 1,3-bridged 2-azetidinones: experimental and theoretical studies

The relationship between angular strain and (re)activity of bicyclic 2-azetidinones is still an open question of major concern in the field of penicillin antibiotics. Our study deals with original 13-membered-ring 1,3-bridged 2-azetidinones related to the carbapenem family, and featuring a "planar amide" instead of the "twisted amide" typical of penam derivatives. The bicycles 11 and 12 were obtained from acetoxy-azetidinone 7, via the key-intermediate 10, by using the RCM (ring closing metathesis) strategy. Theoretical predictions and experimental results of hydrolysis showed that the large bicycle 12, endowed with high conformational flexibility, is more reactive than the bicycle 11, including a C=C bond of E configuration, and the monocyclic 2-azetidinone precursor 10. The processing of 2-azetidinones 10-12 in the active site of serine enzymes has been computed by ab initio methods, considering three models. Due to geometrical parameters of the enzymic cavity (nucleophilic attack from the alpha-face), precursor 10 was predicted more active than 11 and 12 in the acylation step by Ser-OH. Indeed, bicycles 11 and 12 are modest inhibitors of PBP2a, while 10 is a good to excellent inhibitor of PBP2a and R39 bacterial enzymes. (C) 2008 Elsevier Masson SAS. All rights reserved

Kristalna struktura 14-okso-13,14-seko-5α-holest-13(18)-en-3β-il-acetata i određivanje konfiguracije njegovih acetoksi derivata

The crystal structure of 14-oxo-13,14-seco-5α-cholest-13(18)-en-3β-yl acetate (2), obtained (in addition to the (E)-Δ12-isomer 3) by oxidative fragmentation of the C(13)-C(14) bond of 14α-hydroxy-5α-cholestan-3β-yl acetate (1), was determined by X-ray analysis. In addition, the configurations of the acetoxy derivatives 4-6, formed by the thermal lead tetraacetate oxidation of 1, were deduced from the relevant 1H-NMR parameters.Kristalna struktura 14-okso-13,14-seko-5α-holest-13(18)-en-3β-il-acetata (2). koji se dobiva (pored (E)-Δ12-izomera 3) oksidativnom fragmentacijom C(13)-C(14) veze 14α-hidroksi-5α-holestan-3β-il-acetata (1) određena je analizom X-zraka. Pored toga, konfiguracije njegovih acetoksi derivata 4-6, koji se grade pri termičkoj olovo-tetraacetatnoj oksidaciji, izvedene su na osnovu odgovarajućih 1H-NMR parametara

Transannular Photocyclization of (E)-3B-Acetoxy-5,10-seco-1(10)-cholesten-5-one*

UV irradiation of the (E)-5,10-seco-steroidal ketone 1 in acetone solution results, in addition to the previously describedE/Z isomerization and intramolecular Paterno-Buchi reaction(leading to the la,5:x-oxetane 4 in about 42% yield and 1~,5~-oxetane5 in 2-3% yield), in atransannular cyclization (accompaniedby acetic acid elimination) producing the anthrasteroidal enone 3 (in about 7% yield). The structureof 3 was deduced from spectral data and confirmed by X-ray analysis