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Oxidation Trifluoride (DAST) Selectfluor™ Vol. 5 No. 1 Synthetic Methods Iodotoluene difluoride Diethylaminosulfur Oxidation trifluoride (DAST) Selectfluor™ Ishikawa’s Reagent Other Reagents for Fluorination Dess–Martin Periodinane Bis(pyridine)iodonium tetrafluoroborate Magnesium bis(monoperoxyphthalate) hexahydrate Chloramine-T Other Reagents for Oxidation Advancing Science 2 Introduction Oxidation is one of the most common transformations in all of organic chemistry. As a result, the number of oxidizing agents and reactions at the research chemist’s disposal number in the hundreds; Sigma-Aldrich is proud to offer a new series of ChemFiles—called however, few provide extended utility across a broad range of Synthetic Methods—to our Organic Chemistry and Drug Discovery substrates while remaining able to selectively oxidize one group customers. Each piece will highlight a specific motif or selected over another. Fewer yet remain effective under mild conditions. At organic transformation, and the range of products Sigma-Aldrich Sigma-Aldrich, we are committed to being your preferred supplier offers in this area. Our first installment focuses on oxidation for reagents used in oxidation—and we’d like to highlight some chemistry. In the future, we will highlight other synthetic methods new, popular, and versatile oxidation reagents in this ChemFile. such as asymmetric synthesis and reduction. We trust you will find these pieces useful, and we welcome your comments for Listed on the following pages is a selection of oxidation reagents future installments. If you cannot find a product for your specific available from Sigma-Aldrich. For a complete listing of oxidation research in organic synthesis or drug discovery, we encourage your reagents, please visit sigma-aldrich.com/oxidation. For a regular input to help us increase our product range. “Please bother us” at update on the latest products from Sigma-Aldrich, please visit [email protected] with your suggestions. sigma-aldrich.com/newprod. 8 Iodotoluene difluoride F F The importance of selectively fluorinating I C H O compounds in medicinal chemistry, biology, and O tol-IF2 5 11 C5H11 organic synthesis is well appreciated and provides a I Et3N-5HF, CH2Cl2 major impetus to the discovery of new fluorinating F agents that can operate according to an efficient, safe, and mild criteria. Elemental fluorine, and many Scheme 1 electrophilic fluorinating agents have been used CH3 Iodotoluene difluoride in synthesis; however, most of these fluorinating agents are highly aggressive, unstable, and require special equipment and care for safe handling. By contrast, iodotoluene O O O O tol-IF2 R X difluoride (tol-IF2) is easy to handle, and is less toxic than many R X CH2Cl2, 40°C F fluorinating agents. Sigma-Aldrich is pleased to introduce this new X = R', OR', NR'2 reagent for oxidation and fluorination. Scheme 2 A new methodology for the synthesis of fluorinated cyclic ethers was recently reported, which utilized tol-IF2 to achieve a fluorinative ring-expansion of four-, five-, and six-membered rings, an example 1 of which is illustrated in Scheme 1. O O S 1eq tol-IF2 S OR OR CH Cl , 0°C 2 2 F Selective monofluorination of b-ketoesters, b-ketoamides, and diketones takes place without requirement of HF-amine complexes O O under mild conditions (Scheme 2). The formation of difluoro 1) 2eq tol-IF , CH Cl , 0°C S 2 2 2 F 2 O O products were not detected in these reactions. 2) H2O 3) toluene, ∆ When one equivalent of tol-IF2 is reacted with phenylsulfanylated esters, the a-fluoro sulfide results through a Fluoro-Pummerer Scheme 3 reaction. In contrast to diethylaminosulfur trifluoride, two equivalents produces the a,a-difluoro sulfide, and three equivalents of tol-IF2 produces the a,a-difluoro sulfoxide. This behavior was exploited in the one pot synthesis of a 3-fluoro-2(5H)-furanone 3 (Scheme 3). O O S 1eq tol-IF2 S CH N CH3 N 3 X CH2Cl2, 0°C Treatment of phenylsulfanylated lactams with one equivalent n n of tol-IF2 results in the unsaturated heterocycle in moderate to X = H, PhS good yields. When two equivalents of the reagent were used, the O O lactams were fluorinated in the a - and b-positions resulting in the S 2eq tol-IF2 S CH CH3 N 3 4 N F diastereomeric difluoride (Scheme 4). X CH2Cl2, 0°C n F n X X = H, PhS 4-Iodotoluene difluoride 65,111-7 1 g 31.50 5 g 108.00 Scheme 4 TO ORDER: Contact your local Sigma-Aldrich office (see back cover), sigma-aldrich.com call 1-800-558-9160 (USA), or visit sigma-aldrich.com. US $ 3 Diethylaminosulfur trifluoride (DAST) H O H O F 1) HCl , aq F HO2C CO2H DAST F dioxane O NO NO CH2Cl2 F DAST has proven itself to be an extremely NH 2) propene N CO2H F 2 H F F popular reagent for nucleophilic fluorination, F3C CF3 F3C CF3 oxide S due to its ease of handling and versatility. Scheme 5 N It has regularly been employed in a myriad selective fluorinations of alcohols, alkenols, carbohydrates, ketones, sulfides, epoxides, thioethers, and cyanohydrins. In addition some novel organic 5 OH F cyclizations are possible when DAST is employed as a reagent. DAST LHMDS F F Ar CF3 CH Cl , -70°C Ar CF THF, 15°C 2 2 3 Ar F Fluorodeoxygenation was achieved using DAST in a preparatively simple synthesis of 5,5-difluoropipecolic acid from glutamic acid 6 (Scheme 5). Scheme 6 1,2,2-Trifluorostyrenes can be synthesized using a sequential reaction on the parent a-(trifluoromethyl)phenylethanol with DAST, followed by dehydrohalogenation with lithium bis(trimethylsilyl)amide Br Br Br Br (LHMDS). This method achieves the trifluorostyrene without requirement of palladium coupling (Scheme 6).7 N N DAST OH F CH2Cl2, 0°C trifluoride (DAST) DAST was recently used to obtain fluorinated analogues of N N Diethylaminosulfur 3,6-dibromocarbazole piperazine derivatives of 2-propanol BocN BocN (Scheme 7). A series of these analogues are described as the Br Br first small and potent modulators of the cytochrome c release Br Br triggered by Bid-induced Bax activation in a mitochondrial assay.8 N N DAST O F CH Cl , 0°C The synthesis of a,a-difluoroamides via direct fluorination was 2 2 F recently reported using DAST as the fluorinating reagent in a one- N N BocN pot reaction (Scheme 8). Decreasing the molar ratio of DAST to BocN substrate resulted in the formation of the respective a-ketoamide.9 Scheme 7 a-Fluorosulfides and secondary alcohols were coupled by Yb(OTf)3 to generate O,S-acetals, which are key intermediates in the assembly of ciguatoxins. In their synthesis, hydrogen was directly converted to fluorine using DAST and a catalytic amount of SbCl to 3 O 10 O F F make the a-fluorosulfide (Scheme 9). DAST (1eq) DAST (2eq) NEt2 OH NEt2 CH2Cl2, rt CH2Cl2, rt O O O Diethylaminosulfur trifluoride (DAST) 23,525-3 1 g 20.30 Scheme 8 5 g 62.90 25 g 194.50 125 g 583.00 250 g 900.00 BnO F BnO Me H Me H O O PhS OBn DAST, SbCl3 PhS OBn TIPSO OBn TIPSO OBn BnO BnO OH Me H H O BnO O O OBn H H SPh O OBn BnO O Yb(OTf)3, DTBMP, MS 4Å H TIPS BnO H Scheme 9 Ready to scale up? For competitive quotes on larger quantities or custom synthesis, contact SAFC at 1-800-244-1173 (USA), or visit safcglobal.com. Order: 1.800.558.9160 Technical Service: 1.800.231.8327 4 Selectfluor™ ™ CH2Cl Selectfluor (1-chloromethyl-4-fluoro- + 1,4-diazoniabicyclo[2.2.2]octane Cl N Br Me Sn Cl + - bis(tetrafluoroborate), or F-TEDA) is a 3 N 2 BF4 N 1) n-BuLi, THF, 30min, -78°C N user-friendly, mild, air- and moisture- N 2) Me3SnCl, THF, 24h, rt F N N N stable, non-volatile reagent for H H electrophilic fluorination. Selectfluor™ is capable of introducing fluorine into organic substrates in one step, with a remarkably F NH2 broad scope of reactivity.11 Many of these reactions display excellent F Cl N SelectfluorTM N N regioselectivity. HO O N CH CN, 7h, rt 3 N N H CH3 Recently, the synthesis of a potent and noncytotoxic nucleoside HO OH inhibitor of Hepatitis C virus RNA replication was accomplished ™ ™ using Selectfluor (Scheme 10). This ribonucleoside shows a significantly improved enzymatic stability profile compared to the Scheme 10 parent 2’-C-methyladenosine.12 Allylic fluorides can be prepared via a sequential cross-metathesis/ electrophilic fluorodesilylation route (Scheme 11). This route avoids Selectfluor TM R cross-metathesis Selectfluor R the formation of by-products that result from allylic transposition, R TMS TMS CH3CN, 48h, rt which is observed when nucleophilic displacement or ring-opening F reaction with DAST is attempted.13 Selectfluor™ is also capable of effecting other transformations. One Scheme 11 recent example uses Selectfluor™ as an alternative to silver or mercury salts in the rearrangement of iodides to alcohols (Scheme 12). In the case of an iodochloride (X=Cl), substitution only occurs at the 14 iodide and not the chloride. I OH CO Bn TM N 2 Selectfluor BnO2C R = H, Me R N X = F, Cl CH3CN / H2O, 48h, rt R Selectfluor™ has even been employed as an efficient catalyst for the X X preparation of b-hydroxy thiocyanates using ammonium thiocyanate under mild conditions (Scheme 13). In all cases, the reactions Scheme 12 proceed rapidly at room temperature without the formation of the corresponding thiiranes, which are generally the exclusive products 15 when Lewis acids such as Sc(OTf)3 and InCl3 are used.
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