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The mechanism for the 2,5-diphenylpyrrolidine-catalyzed enantioselective alpha-chlorination of aldehydes with electrophilic halogenation reagents has been investigated by using experimental and computational methods. These studies have led us to propose a mechanism for the reaction that proceeds through an initial N-chlorination of the chiral catalyst-substrate complex, followed by a 1,3-sigmatropic shift of the chlorine atom to the enamine carbon atom. The suggested reaction course is different from previously proposed mechanisms for organocatalytic enamine reactions, in which the carbon-electrophile bond is formed directly. Furthermore, the rate-determining step in the overall reaction was determined and the presence of nonlinear effects was probed.

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on behalf of the ASSERT (Aortic Stentless versus Stented valve assessed by Echocardiography Randomized Trial) InvestigatorsBackground-Aortic valve replacement (AVR) is the established treatment for severe aortic stenosis. In response to the long-term results of aortic homografts, stentless porcine valves were introduced as an alternative low-resistance valve.We conducted a randomized trial comparing a stentless with a stented porcine valve in adults with severe aortic stenosis. Methods and Results-The primary outcome was change in left ventricular mass index (LVMI) measured by transthoracic echocardiography and, in a subset, by cardiovascular MR. Measurements were taken before valve replacement and at 6 and 12 months. Patients undergoing AVR with an aortic annulus Յ25 mm in diameter were randomly allocated to a stentless (nϭ93) or a stented supra-annular (nϭ97) valve. There were no significant differences in mean LVMI between the stentless versus stented groups at baseline (176Ϯ62 and 182Ϯ63 g/m 2 , respectively) or at 6 months (142Ϯ49 and 131Ϯ45 g/m 2 , respectively), although within-group changes from baseline to 6 months were highly significant. Changes in LVMI measured by cardiovascular MR (nϭ38) were consistent with the echo findings. There was a greater reduction in peak aortic velocity (PϽ0.001) and a greater increase in indexed effective orifice area (PϽ0.001) in the stentless group than in the stented group. There were no differences in clinical outcomes between the 2 valve groups. Conclusions-Despite significant differences in indexed effective orifice area and peak flow velocity in favor of the stentless valve, there were similar reductions in left ventricular mass at 6 months with both stented and stentless valves, which persisted at 12 months. (Circulation. 2005;112:2696-2702.)

A series of novel ketimines with intrinsic protecting group anchoring was synthesized and allowed to react with various silylketene acetals in the presence of 5-10 mol % of a chiral Zn(OTf)(2)-(R,R)-Ph-pybox-aqua complex. The corresponding optically active quaternary alpha-amino acid derivatives were obtained in high yields and with enantioselectivities ranging from 34 % up to 95 % ee. The catalyst was studied by (1)H NMR spectroscopy and X-ray crystallography, and a dynamic equilibrium of two species was identified in solution. These are a homo-chiral 1:2 metal-ligand complex and a 1:1 metal-ligand complex, of which the latter is expected to be the actual catalyst of the diastereo- and enantioselective reaction. A strong positive nonlinear effect was observed due to the formation of a catalytically inactive 1:2 metal-ligand hetero-chiral complex. On the basis of DFT calculations and the absolute stereochemistry of the products, simultaneous coordination of the imino electrophile and a single molecule of H(2)O to the chiral Lewis acid complex is proposed. Coordination of the imine-nitrogen atom in the axial position of an octahedral complex can account for the facial selectivity as well as the diastereoselectivity observed.

Diameters of the exposed femoral artery of anesthetized dogs were continuously measured with ultrasonic elements of lead zirconate titanate. In 13 of 16 dogs vascular diameters increased following a sudden increment in arterial blood flow induced by the injection of vasodilat i ng agents (acetylcholine, histamine) peripheral to the recording level, by opening an arteriovenous shunt, or after tetanic stimulation of die sciatic nerve. The dilatation response occurred despite slight reductions in femoral blood pressure, and was present after ganglionic blockade, blockade of alpha and beta receptors with phenoxybenzamine and propranolol, atropinization and injection of an antihistamine. The dilatation response was also observed after transsection of the femoral artery distal to the recording level and is therefore not dependent on the retrograde propagation of nervous or myogenous impulses along the vascular wall. ADDITIONAL KEY WORDSarterial diameter arterial smooth muscle arteries autorcgulation blood circulation blood pressure ultrasonic vasodilator agents vasomotor system• A conduit vessel, such as the femoral artery, varies in diameter with blood pressure (1). If this were the only mode of regulation, an increase in blood flow induced by arteriolar dilatation would result in an increased pressure drop along the artery. As a consequence of the reduction in arterial blood pressure towards the periphery, the vascular diameter of the conduit vessel would decrease. However, several previous investigators (2-4), using plethysmographic methods, have found that increased blood flow was associated with an increase in the cross section of the femoral artery. Similar dilatation responses were obtained whether femoral blood flow was raised by stimulation of the sciatic nerve, injection of acetylcholine or injection of other