The endo and exo products are really two different diastereomers. A number of researchers attribute the prefernece to a "secondary molecular orbital interaction" between the diene and the dienophile, whereas others describe the interaction as a London dispersion interaction, in which the weak intermolecular attractions stabilise the transition state in one geometry. The reason for this difference has something to do with an interaction between the delocalised π-system of the diene and the substituent groups attached to the dienophile. Often, the margin is substantial one might see 90% endo product or greater in some cases, although the ratio is sometimes much lower. In fact, as the diagram shows, the endo product is usually the favored one. This approach leads to the curled-up endo product. In the other case, the two molecules can be directly on top of each other one molecule appears to be folded underneath the other. This approach leads to the zig-zag exo product. The two reactants can approach each other such that one appears to be trailing behind the other, and in this case they appear to be facing the same direction, as far as the orientation of the oxygen atoms goes. Here we are adding furan, the diene, to maleic anhydride, the dienophile. Let's look at these two modes of addition with real molecules. The endo product that results has a kind of C-shape. Two cyclic molecules approach each other so that there is maximum overlap between their faces. The exo product that results has a sort of Z-shape.Īn endo addition looks more like this, schematically. The two molecules approach with minimal overal between their faces they combine edge-on. These two outcomes are called "exo" and "endo" addition.Īn exo addition looks something like this, schematically. There are different ways the two original rings can combine, leading to different stereochemical outcomes. When two cyclic structures combine in a Diels Alder reaction, a third ring is formed in between the original ones. Often, there are already rings in the molecules undergoing reaction, and a new one is being added. It allows the construction of six-membered rings, which are very common in biological small molecules which are frequently synthetic targets. The Diels Alder reaction is probably the most common cycloaddition. Illustration of an axon releasing dopamine by exocytosis.\) For a detailed animation of cellular secretion, see. Newly made membrane proteins and membrane lipids are moved on top the plasma membrane by exocytosis. Exocytosis occurs when a cell produces substances for export, such as a protein, or when the cell is getting rid of a waste product or a toxin. Pinocytosis is a type of endocytosis.Įxocytosis describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell, as shown in Figure below. Transmission electron microscope image of brain tissue that shows pinocytotic vesicles. When the channel is closed, the liquid is encircled within a pinocytic vesicle. Pinocytosis, or cellular drinking, occurs when the plasma membrane folds inward to form a channel allowing dissolved substances to enter the cell, as shown in Figure below.The plasma membrane engulfs the solid material, forming a phagocytic vesicle. Phagocytosis, or cellular eating, occurs when the dissolved materials enter the cell.At this point a membrane-bound sac, or vesicle, pinches off and moves the substance into the cytosol. The membrane folds over the substance and it becomes completely enclosed by the membrane. \)Įndocytosis is the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane.
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