title: New flexible synthesis of pyrazoles with different, functionalized substituents at C3 and C5.
abstract: Syntheses of pyrazoles featuring a functionalized side chain attached to carbon 3 and varying alkyl and aryl substituents attached to carbon 5 are presented. Installation of R = methyl, isopropyl, tert-butyl, adamantyl, or phenyl groups at C5 is reported here, starting by coupling protected alkynols with acid chlorides RCOCl, forming alkynyl ketones, which are reacted with hydrazine to form the pyrazole nucleus. Alcohol deprotection and conversion to a chloride gave 5-substituted 3-(chloromethyl)- or 3-(2-chloroethyl)pyrazoles. This sequence can be done within 2 d on a 30 g scale in excellent overall yield. Through nucleophilic substitution reactions, the chlorides are useful precursors to other polyfunctional pyrazoles. In the work here, derivatives with side chains LCH(2)- and LCH(2)CH(2)- at C3 (L = thioether or phosphine) were made as ligands. The significance of the ligands made here is that by placing a ligating side chain on a ring carbon (C3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton will be available for hydrogen bonding, depending on the steric environment created by R at C5.
authors.affiliation: Department of Chemistry, San Diego State University, 5500 Campanile Drive, San Diego, California 92182-1030, USA. grotjahn@chemistry.sdsu.edu
authors.forename: Douglas B
authors.initials: DB
authors.lastname: Grotjahn
authors.forename: Sang
authors.initials: S
authors.lastname: Van
authors.forename: David
authors.initials: D
authors.lastname: Combs
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Lev
authors.forename: Christian
authors.initials: C
authors.lastname: Schneider
authors.forename: Marc
authors.initials: M
authors.lastname: Rideout
authors.forename: Christoph
authors.initials: C
authors.lastname: Meyer
authors.forename: Genaro
authors.initials: G
authors.lastname: Hernandez
authors.forename: Lupe
authors.initials: L
authors.lastname: Mejorado