Oxazolidine is a five-membered

heterocycle A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic organic chemistry is the branch of organic chemistry dealing with the synthesis, proper ...

ringwith the formula .The O atom and NH groups are not mutually bonded, in contrast to

isoxazolidine Isoxazolidine is the organic compound with the formula (CH2)3(NH)O. It is the parent of a family of compounds called Isoxazolidines, which are saturated C3NO heterocyclic rings where the nitrogen and oxygen occupy adjacent positions (1 and 2). T ...

. Oxazolidines (emphasis on plural) are derivatives of the parent oxazolidine owing to the presence of substituents on carbon and/or nitrogen.

Oxazoline Oxazoline is a five-membered heterocyclic organic compound with the formula . It is the parent of a family of compounds called oxazolines (emphasis on plural), which contain non-hydrogenic substituents on carbon and/or nitrogen. Oxazolines are the ...

s are unsaturated analogues of oxazolidines.

Synthesis and reactions

First synthesized in the 1800s, oxazolidines are traditionally prepared by condensation of 2-

aminoalcohol In organic chemistry, alkanolamines (amino alcohols) are organic compounds that contain both hydroxyl () and amino (, , and ) functional groups on an alkane backbone. Alkanolamine's bifunctionality and physicochemical characteristics lead to its u ...

s with aldehydes and ketones. The ready availability of chiral amino alcohols by reduction of

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s enables the synthesis of chiral oxazolidines. Oxazolidines are prone to hydrolysis, the reverse of their syntheses. Perhaps for this reason, their basicity is rarely discussed.

Uses and occurrence

Several oxazolidine derivatives occur naturally, Some occur as

post translational modification In molecular biology, post-translational modification (PTM) is the covalent process of changing proteins following protein biosynthesis. PTMs may involve enzymes or occur spontaneously. Proteins are created by ribosomes, which translate mRNA in ...

s of proteins. Others are components of

alkaloid Alkaloids are a broad class of natural product, naturally occurring organic compounds that contain at least one nitrogen atom. Some synthetic compounds of similar structure may also be termed alkaloids. Alkaloids are produced by a large varie ...

s, a few of which are highly active against some tumors. Examples include terazomine, quinocarcin, and

tetrahydroisoquinoline Tetrahydroisoquinoline (TIQ or THIQ), also known as AMPH-CR, is an organic compound with the chemical formula C9H11N. Classified as a secondary amine, it is derived from isoquinoline by hydrogenation. It is a colorless viscous liquid that is mis ...

. Oxazolidines are used as moisture scavengers in polyurethane and other systems. Oxazolidines have even been researched and used as fuel additives.

Bisoxazolidines

Bisoxazolidines contain two oxazolidine rings. They are the saturated analogues of bisoxazolines. They are used as performance modifiers in

polyurethane Polyurethane (; often abbreviated PUR and PU) is a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane term ...

coatings and paints. The rings

hydrolyze Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

in the presence of moisture to give

amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

and

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

groups, which can then react with

diisocyanate In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...

s, polyisocyanates and polyurethane prepolymers to form a coating. The amine groups will form urea linkages and the hydroxyl group will form urethane links. The use of a bisoxazolidine in a polyurethane system can prevent the unwanted reaction between isocyanate and moisture resulting in coating defects, as a result of carbon dioxide release. This moisture-triggered curing route is preferential to moisture cure. As the ring opening reaction is

catalyzed Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

by acids, usually

organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are re ...

s or anhydrides of carboxylic acids are added in a small amount. The choice of linker between the two oxazolidine rings has a large impact on the performance when used to cure isocyanates. A rigid linker group increases a polyurethanes toughness. A flexible linker group imparts flexibility and increases elongation of a coating. These differences are the reason why bisoxazolidines are used to enhance the performance of polyurethane systems. Usually the rings are linked by esters, urethanes, carbonate or have the two rings fused together. A key intermediate in manufacturing bisoxazolidines is 2- -(propan-2-yl)-1,3-oxazolidin-3-ylthanol. The hydroxy group on the molecule allows for further reaction with

hexamethylene diisocyanate Hexamethylene diisocyanate (HDI) is the organic compound with the formula (CH2)6(NCO)2. It is classified as an diisocyanate. It is a colorless liquid. It has sometimes been called HMDI but this not usually done to avoid confusion with Hydrogenate ...

for example. Depending on the linker, bisoxazolines can function as chelating ligands.

Synthesis and reactions

Uses and occurrence

Bisoxazolidines

See also

References

Further reading