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Chemical Reviews Volume 84, Number 5 October 1984 Three-Carbon Homologating Agents JOHN C. STOWELL aaperhrsnf of chemlsbj'. ulhraslly 01 Flsw (Maam, Lake Fmt, New mns,LOUL&M 70148 Rscslvad NoVembsr 7, 1983 (Uevk4 MamcrW Receivad Juw, 27. 19W contents I. Introduction 409 11. Nucleophilic Reagents 410 A. Reagents Providing Functknal Chains wlth 410 Saturated a-. 8-. and y-Carbons 1. Ethers and Alcohols 410 2. Amines 41 1 3. Bromldes 41 1 4. Propionaldehyde Homoenolate 413 Equivalents 5. Propionate Homenolate Equivalents 419 E. Reagents Rovidlng the Terminal Acetylene 421 Function C. Reagents Rovldlng a.@-Unsatuated 422 Functionality 1. Allylic Alcohols and Halides 422 2. a,@-UnsaturatedAldehydes 422 3. a,@-UnsaturatedCarboxylic Aclds. 424 John C. Stowel was bom in Passaic. NJ. in 1938. and received Esters, and Lactones hls B.S. degee h chemlsby from Rutgers UnivmW in 1960. Hls D. Reagents Providing a.@-Acetylenk 425 W.D. wee in Qganlc chemistry was granted by Massachusetts Functionality Insmule of Tedrmqly h 1964 where he waked WMDr. Frm 1. a.@-Acatylenk Alcohols 425 D. Greene on freeradical termination reactions and prepared the first diaziridinone. He pined the Central Research division of 3M 2. a,@-AcetylenicAldehydes 426 Company in St. Paul. MN. for 4 years. In 1969 he became a 3. a,@-Acetylenic Carboxylic Aclds and 426 postdoctoral fellow at The Ohio State University with Dr. Leo A. Esters Paquette. He prepared and sWIW various (CH),, and benzo (CH),, E. Reagents providing B.yUnsahmted 426 cwnpounds including Umrmal and sihw-catalyred rearrangements. Functionality In 1970 he jokd the faculty of me University of New Ohms. thm known as Lcuiskna State University in New Orleans. His research 1. @.y-Unsaturated Alcohds 426 mere included threecarbon homologating agents. three- and 2. @,y-Unsaturated Halidas 427 four-membered ring heterocycles, formimidoyi halides. sterically 3. @,y-Unsaturated Aldehydes 427 hindered amines and nitroso compounds, and heterogeneous 4. @,y-Unsaturated Acids and DerivaWes 428 concurrent strong acid and base catalysis. 111. Electrophlllc Reagents 429 A. Chlor- and Bromopropylatbn 429 subject is not indexed as such, therefore a thorough B. Hydroxypropylatlon 429 search is not posaible and I expect there are other useful C. &Electrophilic Equivalents of 430 reagents unintentionally omitted here. Nevertheless Ropionaidehyde this review will make the surprisingly large number of D. @-Electrophilic Equivalents of Rophk 430 alternative reagents more readily accessible to the Acid, Propionic Ester. or Propionitrile synthetic organic chemist. The reagents selected here carry only three carbons I. Introduction but there are many other closely related reagents car- rying methyl groups or other structures that operate hy There are over 100 reagents useful for the extension the same principles found here. Thus the reader may of organic molecules hy three carbons with some func- use this review in planning incorporation of branched tionality at the new terminus. The literature on this extensions. 0009-2665/84/0784-0409$06.50/0 0 1984 American Chemical Societv 410 Chemical Reviews, 1984, Vol. 84, NO. 5 Stowell The subject consists of two fundamental classes, the propanol was converted to the Grignard reagent in THF nucleophilic and the electrophilic reagents. Within while cooling to 30-40 "C. This was added to cyclo- these two categories the organization is based on the dodecanone and cleaved with hydrogen to give 3-(1- degree of unsaturation in the chain and the level of hydroxycyclododecyl)-2-methylpropanolin 65 % yield.6 oxidation in the functional group. One should therefore The phenylthio group, in contrast to the oxygen keep in mind that a reagent in any part of the review ethers, does not undergo a 1,3-elimination. Thus, 3- might be useful for a certain synthesis if it is followed bromopropyl phenyl sullide and magnesium in THF or with an oxidation or reduction reaction. ether gives the Grignard reagent which could be acy- lated with a mixed anhydride at low temperature or I I. Nucleophlllc Reagents added to an aldehyde or ketone to give alcohols. The phenylthio group may then be removed by a-halogen- The choice of functionality that may be placed on the ation and hydrolysis with CuO and CuClz to give a third carbon of a nucleophilic reagent is limited owing y-keto or y-hydroxy aldeh~de.~-~ to the facile intramolecular attack leading to cyclo- The analogous lithium reagents containing benzyl or propanes. Groups that would ordinarily be stable to- phenylthio ether groups can be prepared using lithium ward intermolecular attack by Grignard or lithium dispersion or lithium naphthalene.'O 3-Chloropropyl reagents may not withstand the intramolecular attack phenyl sulfide gives [3-(thiophenoxy)propyl]lithium, under similar conditions. For example, simple alkyl and 3-(benzyloxy)propyl phenyl sulfide gives [3-(ben- ethers are stable while phenyl ethers are not. However, zyloxy)propyl]lithium (eq 1). many useful reagents have been devised where the electrophilic character of the @-functionalityis reduced or where the nucleophilic site is rendered less potent by delocalization either allylically in the chain or out- PhCH20MLi PhCH20 ward to other temporary substituents. In other cases 0 the three carbons are delivered with a double or triple bond in the chain that geometrically prevents three- 70% membered ring formation. These may be reduced to Acetal protection of an alcohol function may be re- the saturated chain at a later stage. moved easily with mild acid. Eatonl1J2used ethyl vinyl ether to protect 3-bromo- and 3-chloropropanol. Both A. Reagents Providing Functional Chains with of these halo acetals react rapidly with lithium metal Saturated cy-, @-, and ?-Carbons in ether, starting at room temperature and then cooling to between -15 and -5 "C for most of the reaction. The 1. Ethers and Alcohols organolithium reagent is stable at -30 OC for months In 1904 Hamonetl showed that a Grignard reagent but at room temperature it slowly decomposes to cy- could be prepared from 3-iodopropyl n-pentyl ether and clopropane and other products. The lithium reagent that it would couple with bromomethyl n-pentyl ether adds readily to ketones,11-14 and the acetal may be hy- to give a diether. 1,6Dimethoxybutane could be pre- drolyzed rapidly at room temperature with dilute pared similarly. Yields and other details were not given. aqueous ethanolic hydrochloric acid to afford 1,Cdiols Where a hydroxyl group is the desired functionality, in high yield (eq 2). This hydrolysis is mild enough to various protecting groups have been used during the nucleophilic step. Some ethers were suitable while Li - others were not. 3-Bromopropyl phenyl ether as well sthbr. -10 *C as the iodo compound gave prompt 1,3-elimination to afford cyclopropane plus the phenoxide salt when treated with magnesium in ethe~~8A Grignard reagent could be prepared from the methyl ether, however, and it was added to a nitrile to afford a keto ether in low 96% yield. In this case the ether was cleaved with hot con- leave the tertiary alcohol function undisturbed. This centrated hydrobromic acid at a later stage and ulti- reagent also reacts with y-lactones to give spiroketals mately used to prepare a pyrrolidine ring. Even the (eq 3).15 alkyl ethers show instability at higher temperatures. For example, the Grignard reagent from 3-bromopropyl ethyl ether gives cyclopropane when heated to about 75 "C in ben~ene.~ A tertiary ether is more readily removed. Thus tert-amyl 3-bromopropyl ether gave the Grignard reagent in ether in about 80% yield. This could be carbonated to give 4-(tert-amyloxy)butyric acid (53% 63% yield) or added to benzaldehyde, acetone, or cyclo- The corresponding Grignard reagent would not form hexanone in 90% yield^.^ Heating the hydroxy ether in ether'l but it could be prepared from magnesium products with p-toluenesulfonic acid cleaves the ether turnings in THF at 0-10 "C in about 65% yield.16 but also causes cyclization to the corresponding tetra- Catalytic or larger amounts of copper salts were used hydrofurans in 70-90% yields. with the lithium or magnesium reagent to give coupling A benzyl ether can be removed by mild hydrogenol- and Michael reactions. For instance cuprous iodide and ysis but again the Grignard reagent shows some insta- the lithium reagent gave conjugate addition to cyclo- bility. The benzyl ether of 3-bromo-2-methyl-l- pentenone (50%). It is not effective with highly @- Three-Carbon Homologating Agents Chemical Reviews, 1984, Vol. 84, No. 5 411 substituted substrates but a mixed magnesium cuprate A remarkably simple alternative is now available with will serve as shown in eq 4.17 The mixed cuprate from the discovery that the Grignard reagent can be prepared directly from the chloroalkoxide. Methyl or isopropyl 0 magnesium chloride gives the alkoxide from 3-chloro- 1-propanol at -20 "C in THF. This salt is soluble enough to give 0.3 N solutions at room temperature. It will react with magnesium in refluxing THF with the aid of a small amount of 1,Zdibromoethane to give the Grignard reagent 8). reagent gives all the usual I (4) (eq This Ob reactions" including addition to ketones, coupling with allylic halides,25copper-catalyzed conjugate addition26 and alkylation in 75-90% yields. 60% (1) CH3MpCI copper tert-butylacetylide was used in a conjugate ad- CIMOH Mg9 THF - CIMgMOMgCI (8) dition to an a,@-unsaturated lactone (56%).18 The a 5 - 90% polymeric complex prepared from lithium reagent and 2. Amines an equivalent amount
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