Controlled distribution of lipids across various cell membranes is crucial for cell homeostasis and regulation. We developed an imaging method that allows simultaneous in situ quantification of cholesterol in two leaflets of the plasma membrane (PM) using tunable orthogonal cholesterol sensors. Our imaging revealed marked transbilayer asymmetry of PM cholesterol (TAPMC) in various mammalian cells, with the concentration in the inner leaflet (IPM) being ~12-fold lower than that in the outer leaflet (OPM). The asymmetry was maintained by active transport of cholesterol from IPM to OPM and its chemical retention at OPM. Furthermore, the increase in the IPM cholesterol level was triggered in a stimulus-specific manner, allowing cholesterol to serve as a signaling lipid. We found excellent correlation between the IPM cholesterol level and cellular Wnt signaling activity, suggesting that TAPMC and stimulus-induced PM cholesterol redistribution are crucial for tight regulation of cellular processes under physiological conditions.

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Numerical calculations are carried out for natural convection induced by a temperature difference between a cold outer square enclosure and a hot inner circular cylinder. A two-dimensional solution for unsteady natural convection is obtained, using the immersed boundary method (IBM) to model an inner circular cylinder based on the finite volume method for different Rayleigh numbers varying over the range of 10 3-10 6. The study goes further to investigate the effect of the inner cylinder location on the heat transfer and fluid flow. The location of the inner circular cylinder is changed vertically along the center-line of square enclosure. The number, size and formation of the cell strongly depend on the Rayleigh number and the position of the inner circular cylinder. The changes in heat transfer quantities have also been presented.

[reaction: see text] Two pairs of complexity-generating reactions with an essential product-substrate relationship along a synthetic pathway are demonstrated. This pathway illustrates a key element in a planning algorithm for diversity-oriented synthesis. This element facilitates the efficient synthesis of structurally complex compounds, and it can be integrated with ones that provide structurally diverse compounds.

Towards overcoming the size limitations of conventional two-photon fluorescence microscopy, we introduce two-photon imaging based on microelectromechanical systems (MEMS) scanners. Single crystalline silicon scanning mirrors that are 0.75 mm x 0.75 mm in size and driven in two dimensions by microfabricated vertical comb electrostatic actuators can provide optical deflection angles through a range of approximately16 degrees . Using such scanners we demonstrated two-photon microscopy and microendoscopy with fast-axis acquisition rates up to 3.52 kHz.

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In the preceding communication, 1 we described the synthesis of a potentially general precursor (2, Scheme 1) of the highly promising chemotherapeutic agent Taxol 2 (1, Scheme 2) and its analogues. Our strategy for the elaboration of this ABbicyclic precursor into the ABC-tricyclic core of the taxanes was predicated on the view 1b that epimerization of the C7 center of Taxol 3 proceeds through the intermediacy of the AB-bicyclic enolaldehyde or its ketone isomer, leading to the intriguing possibility that the C-ring of Taxol could self-assemble under exceptionally mild conditions from a considerably less complex AB-bicyclic ketoaldehyde precursor (e.g., 9). In this communication, the viability of this aldol cyclization strategy is demonstrated in a synthesis of Taxol (1), representing the shortest sequence yet reported for the preparation of this important natural product. 4,5 The elaboration of our general taxane precursor (2, Scheme 1) into Taxol started with its homologation with Ph 3 PC(H)-OMe (91%) 6 followed by a one-step hydrolysis of the enol ether and acetonide groups (HCl, NaI) to provide aldehyde 3 (94%). 7 Selective protection of the C9 hydroxyl was then accomplished in 92% yield with TESCl and pyridine. Dess-Martin periodinane oxidation 8 of the C10 alcohol and introduction of C20 with [Me 2 NCH 2 ]I (g0.1 M) 9 and Et 3 N (excess) was conducted in one operation to produce enal 4 in 97% yield. The remaining carbons of the taxane skeleton were then introduced through the addition of 4 to a solution of allylmagnesium bromide and ZnCl 2 (89%) 10 which after BOM (benzyloxymethyl) protection (N,N-diisopropylethylamine solvent) provided the ether 5 as a single diastereomer. 10c The presence of ZnCl 2 in the former reaction completely suppressed addition of the Grignard reagent to the cyclic carbonate. Removal of the C9 silyl group (NH 4 F, MeOH) 11 provided an unstable hydroxyketone (93% over two steps) which was reacted immediately with PhLi 12 to form the C2 benzoate providing, after in situ acetylation, the acetate 6 in 79% yield. Transposition of the acetoxyketone under kinetic 5a or equilibrating conditions (Et 2 NH, KOAc, DMF) 13 resulted in limited success. However, when the guanidinium base 7 14 was employed for this transposition, the desired acetoxyketone 8 and recyclable 6 were obtained in 80% as a 4:3 equilibrium mixture. The monosubstituted alkene in 8 was then cleaved through addition of an ozone solution to form aldehyde 9 in 86% yield.The viability of the key aldol cyclization was addressed at this point. Previous studies in our laboratory 1b,15 showed that ketoaldehydes similar to 9 but incorporating a C1-C2 cyclic carbonate did not undergo aldol cyclization, preferring instead

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