Polar spacer synthesis

Figure 32 Enzymatic hydrolysis of a polar spacer in solid phase synthesis.

The requirements for solid-phase synthesis are diverse. The support must be insoluble, in the form of beads of sufficient size to allow quick removal of solvent by filtration, and stable to agitation and inert to all the chemistry and solvents employed. For continuous-flow systems, the beads also must be noncompressible. Reactions with functional groups on beads imply reaction on the inside of the beads as well as on the surface. Thus, it is imperative that there be easy diffusion of reagents inside the swollen beads and that the reaction sites be accessible. Accessibility is facilitated by a polymer matrix that is not dense and not highly functionalized. A matrix of defined constitution allows for better control of the chemistry. Easier reaction is favored by a spacer that separates the matrix from the reaction sites. Coupling requires an environment of intermediate polarity such as that provided by dichloromethane or dimethylformamide benzene is unsuitable as solvent. 

The key-step of the synthesis of glycolipids, and more generally of amphiphilic carbohydrates, is the covalent coupling of a hydrophilic carbohydrate with a lipophilic compound. A hydrophilic or hydrophobic spacer may be inserted between them in order to control the hydrophilic-lipophilic balance (HLB). This modulation allows to obtain variously organized systems with the same polar head and apolar tail. 

Guanine derivatives connected to pyrene with methylene spacers exhibited exciplex emission in highly polar solvents. This fact opens up a novel approach to the synthesis of fluorescent nucleic bases <04CC824>. 

The advantages of the bipolar structure of the bolas, amphiphiles with two polar heads connected by one or two long hydrophobic spacers, and the difficulty in isolating them from natural membranes, have pushed researchers toward chemical synthesis, which can mimic namral... 

The concept of surfactant self-assembly is revisited in the organic laboratory. A manuscript detailing this experiment has been submitted elsewhere (55), and is briefly sununarized here. In a single laboratory session, students synthesize and investigate the colloidal properties of a gemini surfactant - an amphiphiUc molecule with two non-polar tails and two polar heads eoimected by a spacer (see Figure 2a) (34, 55). The synthesis (a double Menshutkin reaction) and purification are straightforward and reproducible by oi nie lab students. 

The DMAP analogues bound to cross-linked PS are active in non-polar solvents for esterification of sensitive tertiary alcohols as in equations (21) and (22), dimerization of phenyl isocyanate as in equation (23), nucleophilic acyl rearrangements as in equation (24), and synthesis of dipalmitoylphosphatidylcholine from palmitic anhydride as in equation (25). The polymeric catalysts are slightly less active than DMAP, but they have been recovered and recycled three times with no loss of activity. " The spacer chain catalyst (58 n = 3, DF = 0.16-0.48, 2% DVB) was more active than catalyst (58 n= ) for acetylation of 1-methylcyclohexanol. Spacer chains (n = 4,7) and DF 0.15-0.20 gave highest activity for acyl rearrangements. A mixture of catalyst (58) and cross-linked poly(V,V-diethylaminomethylstyrene), as a proton acceptor, was more active for acetylation of linalool (equation 21), than catalyst (58) alone. 

---