![]() ![]() However, some instructors will specify the stereochemistry of the anomeric carbon as either the α-form or the β-form. Usually, you can put the anomeric -OH in any position you like. If you’re curious about the stability of the anomeric -OH in the “up” or a “down” position, look up “anomeric effect” to dive into the topic deeper. While, there are factors that can make one orientation more favorable than the other one, I’m not going to go over those here to keep this post as much to the point as possible. This means that it can be either facing “up” or “down” depending on how the cyclization happened. The anomeric carbon has a special place in the chemistry of carbohydrates because it doesn’t have a set stereochemistry. The Stereochemistry of the Anomeric Carbon (the α-form or the β-form)īy now I have defined every stereocenter in the molecule except for the anomeric carbon. I kept the same color scheme to make it a little easier to see what’s going on and track the groups as we go from one structure to another. The “left” groups on carbons 2, 3, and 4 in the Fischer projection go onto the “up” positions in Haworthĭ-glucopyranose, L-idopyranose, and L-glucopyranose.The “right” groups on carbons 2, 3, and 4 in the Fischer projection go onto the “bottom” positions in Haworth.If your sugar was D, then the carbon #6 is going to be looking “up” (down for the L sugar).Number your atoms 1 through 5 starting from the anomeric carbon and going clockwise.Draw a Haworth’s “stem” like in the picture above.Now, to make a cyclic hemiacetal from your linear Fischer projection, you’ll need to follow a few simple steps: So make your (and your instructor’s) life easier and show the wedges if you draw your molecule in ANY way that is not a canonical/traditional representation. I can tell you from the perspective of an instructor: every time I have to guess, I take off points in my classes. So if you have to flip it around in some sort of a way to, maybe, make a disaccharide or something like that, please, remember to show the stereochemistry! While this might be a small thing, it CAN cost you some points on the exam. Fischer Projectionsĭrawing Haworth projection in a different way might be ambiguous. Carbohydrates is a typical mid first semester biochemistry topic. You’ll also cover this topic in any introductory biochemistry class. You’ll also need to know it for the MCAT even if your organic chemistry professor doesn’t cover it in class. Finally, this is a very typical type of an exam question! So you wanna make sure you really know how to convert Fischer to Haworth and chair and back. Also, six-membered cyclic forms (pyranoses) are also prevalent in nature, so they are very important. Why focusing on the aldohexoses and pyranoses specifically? Glucose, galactose, and mannose are among the most common carbohydrates in living cells. You might wanna check it out before you continue with this post so you don’t get lost in the discussion when I use the terminology. If you’re not sure what exactly these terms mean, I go over the fundamental nomenclature of carbohydrates in this blog post over here. Pyranoses: six-membered cyclic structures that result from the hemiacetal formation between the aldehyde on the 1st carbon in the molecule and the -OH group on the 5th carbon.Examples of aldohexoses are glucose and galactose which are essential for the living organisms on Earth. Aldohexoses: carbohydrates that have an aldehyde functional group and contain six carbons.While other structures are possible (and you’re definitely going to encounter them in your course), I’m specifically going to be focusing on the: In this post I want to go over the three most typical forms of the carbohydrates: Fischer projection for the open-chain molecules, Haworth projections focusing on cyclic pyranoses, and your regular chair conformations. The Stereochemistry of the Anomeric Carbon (the α-form or the β-form).Steps for Converting Fischer to Haworth. ![]()
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