Kiliani fischer. 22.8: Lengthening the Chain 2022-10-14
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24.7: Reactions of Monosaccharides
This was commonly accomplished by the Ruff procedure. Chain Shortening and Lengthening 1. So, there's 0 new information here. To refresh your memory about the chemistry of acetals, quickly review Section 19. Sections 20 mins 0 completed Learn 9 mins 0 completed Learn 18 mins 0 completed Learn 6 mins 0 completed Learn 12 mins 0 completed Learn 19 mins 0 completed Learn Summary 13 mins 0 completed Learn 11 mins 0 completed Learn Summary 9 mins 0 completed Learn Summary 9 mins 0 completed Learn Summary 11 mins 0 completed Learn 8 mins 0 completed Learn 7 mins 0 completed Learn Summary 18 mins 0 completed Learn 12 mins 0 completed Learn 7 mins 0 completed Learn 24 mins 0 completed Learn Summary 14 mins 0 completed Learn 28 mins 0 completed Learn 10 mins 0 completed Learn 24 mins 0 completed Learn 12 mins 0 completed Learn 12 mins 0 completed Learn 30 mins 0 completed Learn 12 mins 0 completed Learn We know thataldose aldehydes are susceptible to the same nucleophilic addition reactions that we learned in carbonyl chemistry. So, why did I call this reaction the modern Kiliani-Fischer synthesis? Note that glycosides are in fact acetals, and that glycoside formation is therefore analogous to acetal formation.
Now, you don't know what this reducing agent is but I'm just letting you know that, that's going to be part of the synthesis is to find a reducing agent that only reduces it one step and not two steps, cool? This is shown in the following diagram by a red dot where the symmetry axis passes through the projection formula. For example, if successive iterations of the Kiliani—Fischer synthesis are used, the overall yield drops approximately exponentially for each additional iteration. Thus, ribose, xylose, allose and galactose yield achiral aldaric acids which are, of course, not optically active. Note: The sugar is able to reduce an oxidizing agent, and is thus called a reducing sugar. It was completely reduced, we would add a ton of H's.
Alright, guys so you have to love saccharides because saccharides have alcohol they have carbonyls there's so many different things you can do with them they're so versatile and one thing that you should always keep in mind is that the reaction that we learned about aldehydes and ketones back in your carbonyl section of organic chemistry apply, many of those reactions apply to saccharide because we have an aldehyde present, okay? The formation of esters and ethers is quite straightforward and should not require further clarification. So, if we just use, for example, h2 and palladium, let's say, if we just use that as catalytic hydrogenation, what we would expect to get is ch2, don't draw this by the way because this is the wrong answer, NH2, okay? The D- L series is available by starting the sequence with L-glyceraldehyde. A large class of hydroxylated, aromatic oxonium cations called anthocyanins provide the red, purple and blue colors of many flowers, fruits and some vegetables. So it says, the cyano group can then be reduced and hydrolyzed to form a new chain length in aldehyde, okay? Now, I have one, two, three, four, five, six, carbons and I still have an aldehyde present, what can aldehydes do when you expose them to a CN negative, they can react again and they can do another cyanohydrin, so the whole idea here is that you can repeat this cycle as many times as you want and get your carbon chain to be longer, longer and longer and you can theoretically just keep on going forever, okay? Because of the 2º hydroxyl functions that are also present in these compounds, a mild oxidizing agent such as hypobromite must be used for this conversion equation 1. The active functional groups on monosaccharides are essentially carbonyls and hydroxyls.
An interesting alternative technique, known as the Wohl degradation has also been used. If a sugar is oxidized by these reagents it is called reducing, since the oxidant Ag + or Cu +2 is reduced in the reaction, as evidenced by formation of a silver mirror or precipitation of cuprous oxide. The neat thing about this reaction is that the cyano group can then be reduced and hydrolyzed to form a new, chain-lengthened aldehyde. Remember, a Fischer projection formula may be rotated by 180º in the plane of projection without changing its configuration. As is generally true for most acetals, glycoside formation involves the loss of an equivalent of water. For example, if successive iterations of the Kiliani—Fischer synthesis are used, the overall yield drops approximately exponentially for each additional iteration. The complex changes that occur when wine is fermented and stored are in part associated with glycosides of anthocyanins.
So, really the steps that you should already know from what we've already learned in organic chemistry are this step and this step, both of these steps come from carbonyl chemistry, the only set that should still be a big question mark for you, is this step, and that's fine because I'm going to explain that step more in a little bit okay, cool? The Tollens' test is commonly used to detect aldehyde functions; and because of the facile interconversion of ketoses and aldoses under the basic conditions of this test, ketoses such as fructose also react and are classified as reducing sugars. So, Pd over BaSo4, this combination you're just going to memorize it probably put it on a flash card, this is what we call a poisoned reducing agent and why, what poison means guys, if you ever hear the term poisoned, it means weaker, okay? Before starting with ribose, which is a pentose, right? The D-glyceraldehyde 1 leads to the D-erythrose 2a and D-threose 2b. The following equation illustrates the application of this procedure to the aldopentose, arabinose. In addition, the process requires having a supply of the previous sugar in the series, which may itself require substantial synthetic work if it is not readily available. So guys, it turns out that we were able to design a better reducing agent that would work on the CN and turn it into an amine, okay? While it does provide access to every possible stereoisomer of any desired aldose, the process is limited in by its low yield and use of toxic reagents. So, ideally we could turn a pentose into a hexose by just adding more and more cyanohydrins, okay? Two examples of naturally occurring glycosides and one example of an amino derivative are displayed above.
From the structures in the previous diagram, we see that pyranose rings prefer chair conformations in which the largest number of substituents are equatorial. This reaction is illustrated for glucose and methanol in the diagram below. An alternative chain shortening procedure known as the Wohl degradation is essentially the reverse of the Kiliani-Fischer synthesis. Now, it turns out that we don't want this because we only want to reduce it to a double bond not to a single bond because we want to keep a carbonyl, okay? So, guys it turns out that we have learned in the past how to reduce nitriles or cyano groups, okay? The process only provides direct access to aldoses, whereas some sugars of interest may instead be ketoses. What we learned is that if you react in aldehyde or a ketone with HCN what's going to end up happening is that you get a CN negative that comes in and attacks and kicks electrons up to the O, right? So, let's go ahead and figure out how to do that, okay? The remaining epimer is therefore mannose. In naming of glycosides, the "ose" suffix of the sugar name is replaced by "oside", and the alcohol group name is placed first. Let's now learn a reaction that involves the formation of a cyanohydrin.
Study Notes While several reactions are covered in this section, keep in mind that you have encountered them in previous sections. In practice, the Kiliani—Fischer synthesis is usually used for production of sugars that are difficult or impossible to obtain from natural sources. So, guys, what you need to know is that the modern Kiliani-Fischer uses this palladium and Barium sulfate and you should probably know each intermediate step, how you're reducing to an amine and then how an amine is being hydrolyzed in an aqueous solution back to the aldehyde, cool? Fischer looked for and discovered a second aldohexose that represented the end group exchange for the epimer lacking the latent C2 symmetry A. So, then it will look like this it's going to go back to being an aldehyde and as we know this h is still there. Glycoside Formation Acetal derivatives formed when a monosaccharide reacts with an alcohol in the presence of an acid catalyst are called glycosides. So, what are these newer reagents.
The upper equation shows the general form of the osazone reaction, which effects an alpha-carbon oxidation with formation of a bis-phenylhydrazone, known as an osazone. And then obviously this H is still present. Those lead to the D-ribose 3a and D-arabinose 3b , and D-xylose 3c and D-lyxose 3d , respectively. Now, specifically, when we expose an aldehyde to HCN hydrogen cyanide, aldoses can reversibly transform into cyanohydrin, okay? While it does provide access to every possible stereoisomer of any desired aldose, the process is limited in by its low yield and use of toxic reagents. Thus, allitol and galactitol from reduction of allose and galactose are achiral, and altrose and talose are reduced to the same chiral alditol.
