The main topic of thesis is the use of organocatalysis to synthesize cyclobutanones derivatives. Cyclobutanone derivatives are useful molecular building blocks for the construction of complex molecular structures. Surprisingly, however, the use of organocatalysts to functionalize cyclobutanones is rare, especially when the substrate bears substituents. In this thesis, we present the enantioselective transformations and functionalizations of substituted cyclobutanones which employ readily-available amino acids (or derivatives) and thiourea derivatives as organocatalysts. - The first transformation involves the enantioselective aldol reaction between 2-hydroxy-cyclobutanone with a selection of aromatic aldehyde. The results show that the 2-hydroxycyclobutanone is particularly amenable to solvent-free L-threonine-catalyzed direct aldol reactions with reasonable stereocontrol. - After, we synthesized 2,3-disubstituted cyclobutanones through direct aldol reactions involving 3-substituted cyclobutanones and aryl aldehydes catalyzed by N-phenylsulfonyl (S)-proline and via asymmetric nitro-Michael reaction of 3-substituted cyclobutanones and several nitrostyrenes catalyzed by thiourea derivatives. In the first case the relative aldol products were obtained with an unprecedented control of all three contiguous stereocenters in the latter the relatives γ-nitro cyclobutanones were obtained in good yield but in modest enantioselectivity. - The last case concerns the conversion of 3-substituted cyclobutanones into 4-substituted-5-hydroxy-γ-lactam using as electrophile nitrosobenzene and L-proline as catalysts. This reaction involves a ring-expanding O-nitroso-aldol–cyclization domino sequence. The synthetic protocol provides access to the five-membered ring system in good yield, and the formation of two new stereogenic centers is achieved with complete stereochemical control. Thus, the main topic of the first chapter is the organocatalysis, applications of the most common organocatalysts are discussed. The vast majority of organocatalytic reactions use chiral amine as catalysis (asymmetric aminocatalysis). Different types of organocatalysis involve the use of Br¿nsted acids and bases, Lewis acids, hydrogen bond-mediated catalysis, phase transfer and N-heterocyclic carbene catalysis. The second chapter deals with the reactivity of cyclobutanones. High electrophilicity and ring strain make the cyclobutanone and its derivatives a good substrate for ring transformation reactions. Characteristic reactions of functionalized cyclobutanones involve the ring opening, ring contraction and ring expansion reactions. In the third chapter, the synthesis of 2,2-disubstituted cyclobutanones via direct aldol reaction of 2-hydroxycyclobutanone with several aldehydes catalyzed by primary amines is presented. The results show that the 2-hydroxycyclobutanone is particularly amenable to solvent-free L-threonine-catalyzed direct aldol reactions with reasonable stereocontrol. In the fourth chapter we describe the synthesis of 2,3-disubstituted cyclobutanones through direct aldol reactions of 3-substituted cyclobutanones and aryl aldehydes, catalyzed by N-phenylsulfonyl (S)-proline and through asymmetric nitro-Michael reaction of 3-substituted cyclobutanones and several nitrostyrenes, catalyzed by derivatives of thiourea. In this last chapter an organocatalyzed enantioselective desymmetrization reaction of 3-substituted cyclobutanones is presented using nitrosobenzene as an electrophile and proline derivatives as catalysts. This reaction give an original 5-hydroxy-γ-lactam in good yield and with the generation of two new stereogenic centers.
