La presente se refiere a derivados de fenilalanina que son inhibidores de la enzima dipeptidilpeptidasa -IV (abreviadamente en inglés "inhibidores de DP-IV") y que son útiles en el tratamiento o prevención de enfermedades en las que esta implicada la enzima dipeptidilpeptidasa-IV, tal como la diabetes y particularmente la diabetes tipo 2. También se refiere a composiciones farmacéuticas que comprende estos compuestos y el uso de estos compuestos y composiciones en la prevención o tratamiento de dichas enfermedades en las que está implicada la enzima dipeptidilpeptidasa-IV. Reivindicación 1: Un compuesto de fórmula estructural (1) en la que X es CH2, S, CHF, o CF2; W es CH2, CHF, o CF2; R2 se selecciona del grupo constituido por; metilo, etilo, CH2-ciclopropilo, COOH, COOMe, COOEt, CONHMe, CONMe2, CONH2, CONHEt, CONMeCH2Ph, pirrolidin-1-ilcarbonilo, azetidin-1-ilcarbonilo, 2-fluoroazetidin-1-ilcarbonilo, morfolin-4-ilcarbonilo, y [(tetrazol-5-il)amino]carbonilo; y R4 se selecciona del grupo constituido por 4-fluorofenilo, 2,4-difluorofenilo, 3,4-difluorofenilo, 2-clorofenilo, 2-fluorofenilo, 3-(metolsulfonol)fenilo, 3-(etoxicarbonil)fenilo, 3-carboxifenilo, 3-(aminocarbonil)fenilo, 3-[(terc-butilamino)carbonil]fenilo, , 3-[(fenilamino)carbonil]fenilo, 3-[(tiazol-2-ilamino)carbonil]fenilo, 3-[(tetrazol-5-ilamino)carbonil]fenilo, 3-[[(trifluorometil)sulfonil]amino]fenilo, 3-(tetrazol-5-il)fenilo, 4-fluoro-3-(tetrazol-5-il)fenilo, 2-fluoro-5-(tetrazol-5-il)fenilo, 3-(5-oxo-4,5-dihidro-1,2,4-oxadiazol-3-il)fenilo, 4-fluoro-3-(5-oxo-4,5-dihidro-1,2,4-oxadiazol-3-il)fenilo, 3-(5-oxo-4,5-dihidro-1,3,4-oxadiazol-2-il)fenilo, 3-(5-oxo-4,5-dihidro-1H-1,2,4-triazol-3-il)fenilo, 3-(1,3,4-oxadiazol-2-il)fenilo, 3-(1,2,4-triazol-3-il)fenilo, 3-[5-(trifluorometil)-1,2,4-triazol-3-il]fenilo, 3-(5-etoxi-1,2,4-triazol-3-il)fenilo, piridin-3-ilo, 6-fluoro-piridin-3-ilo, 6-metoxipiridin-3-il, 6-oxo-1,6-dihidropiridin-3-ilo, 1-metil-6-oxo-1,6-dihidropiridin-3-ilo, 1-etil-6-oxo-1,6-dihidropiridin-3-ilo, 5bromo-1-metil-6-oxo-1,6-dihidropiridin-3-ilo, imidazo[1,2-a]piridin-6-ilo, [1,2,4]triazolo[4,3-a]piridin-6-ilo, 3-(trifluorometil)[1,2,4]triazolo[4,3-a]piridin-6-ilo, 3-oxo-2,3-dihidro[1,2,4]triazolo[4,3-a]piridin-6-ilo, 2-metil-3-oxo-2,3-dihidro[1,2,4]triazolo[4,3-a]piridin-6-ilo, 4-aminoquinazolin-6-ilo, 2-(acetilamino)imidazo[1,2-a]piridin-6-il, 3-aminoimidazo[1,2-a]piridin-6-ilo, 3-carboxipirazolo[1,5-a]piridin-5-ilo, 5-bromo-6-oxo-1,6-dihidropiridin-3-ilo, [1,2,4]triazolo[1,5-a]piridin-6-ilo, [1,2,4]triazolo[1,5-a]piridin-7-ilo, y pirazolo[1,5-a]pirimidin-5-il.This refers to phenylalanine derivatives that are inhibitors of the enzyme dipeptidylpeptidase-IV (abbreviated in English "DP-IV inhibitors") and that are useful in the treatment or prevention of diseases in which the enzyme dipeptidylpeptidase-IV is involved , such as diabetes and particularly type 2 diabetes. It also refers to pharmaceutical compositions comprising these compounds and the use of these compounds and compositions in the prevention or treatment of said diseases in which the enzyme dipeptidylpeptidase-IV is involved. Claim 1: A compound of structural formula (1) wherein X is CH2, S, CHF, or CF2; W is CH2, CHF, or CF2; R2 is selected from the group consisting of; methyl, ethyl, CH2-cyclopropyl, COOH, COOMe, COOEt, CONHMe, CONMe2, CONH2, CONHEt, CONMeCH2Ph, pyrrolidin-1-ylcarbonyl, azetidin-1-ylcarbonyl, 2-fluoroazetidin-1-ylcarbonyl, morpholin-4-yl-carbonyl and [(tetrazol-5-yl) amino] carbonyl; and R4 is selected from the group consisting of 4-fluorophenyl, 2,4-difluorophenyl, 3,4-difluorophenyl, 2-chlorophenyl, 2-fluorophenyl, 3- (metolsulfonol) phenyl, 3- (ethoxycarbonyl) phenyl, 3-carboxyphenyl, 3- (aminocarbonyl) phenyl, 3 - [(tert-butylamino) carbonyl] phenyl,, 3 - [(phenylamino) carbonyl] phenyl, 3 - [(thiazol-2-ylamino) carbonyl] phenyl, 3 - [(tetrazol- 5-ylamino) carbonyl] phenyl, 3 - [[(trifluoromethyl) sulfonyl] amino] phenyl, 3- (tetrazol-5-yl) phenyl, 4-fluoro-3- (tetrazol-5-yl) phenyl, 2-fluoro -5- (tetrazol-5-yl) phenyl, 3- (5-oxo-4,5-dihydro-1,2,4-oxadiazol-3-yl) phenyl, 4-fluoro-3- (5-oxo- 4,5-dihydro-1,2,4-oxadiazol-3-yl) phenyl, 3- (5-oxo-4,5-dihydro-1,3,4-oxadiazol-2-yl) phenyl, 3- ( 5-oxo-4,5-dihydro-1H-1,2,4-triazol-3-yl) phenyl, 3- (1,3,4-oxadiazol-2-yl) phenyl, 3- (1,2, 4-triazol-3-yl) phenyl, 3- [5- (trifluoromethyl) -1,2,4-triazol-3-yl] phenyl, 3- (5-ethoxy-1,2,4-triazol-3- il) phenyl, pyridin-3-yl, 6-fluoro-pyridin-3-yl, 6-methoxypyridin-3-yl, 6-oxo-1,6-dihydropyridin-3-yl, 1-methyl-6-oxo- 1,6-dihydropyridin-3-yl, 1-ethyl-6-oxo-1,6-dihydropyridin-3-yl, 5-bromo-1-methyl-6-oxo-1,6-dihydropyridin-3-yl, imidazo [1,2-a] pyridin-6- ilo, [1,2,4] triazolo [4,3-a] pyridin-6-yl, 3- (trifluoromethyl) [1,2,4] triazolo [4,3-a] pyridin-6-yl, 3 -oxo-2,3-dihydro [1,2,4] triazolo [4,3-a] pyridin-6-yl, 2-methyl-3-oxo-2,3-dihydro [1,2,4] triazolo [4,3-a] pyridin-6-yl, 4-aminoquinazolin-6-yl, 2- (acetylamino) imidazo [1,2-a] pyridin-6-yl, 3-aminoimidazo [1,2-a] pyridin-6-yl, 3-carboxypyrazolo [1,5-a] pyridin-5-yl, 5-bromo-6-oxo-1,6-dihydropyridin-3-yl, [1,2,4] triazolo [1 , 5-a] pyridin-6-yl, [1,2,4] triazolo [1,5-a] pyridin-7-yl, and pyrazolo [1,5-a] pyrimidin-5-yl.