Synthesis changing functional group

In the divergent method (Fig. 4), the molecule is assembled from the core to the periphery, while in the convergent method, the molecule is synthesized from the periphery to the core. In either method, the synthesis requires a stepwise process, attaching each layer to the previous one, purifying and then changing functional groups for the next stage of reaction. 

In this case, the identity and the location of the functional group have changed. During our synthesis, the functional group (Br) must be relocated, AND it must be converted into a triple bond. Moving the functional group can be achieved via elimination (to give the Zaitsev product) followed by addition (in an anti-Markovnikov fashion). 

Substitution of a dipeptide unit by a cychc dipeptide derivative within a peptide chain can induce certain conformational restraints that may alter the biological response via changing receptor selectivity. A facile procedure for synthesis of pyrazinone ring-containing opioid mimetics [21] has been elaborated, based on the cycHzation of readily available dipep-tidyl chloromethyl ketones [22] (Scheme 6). This method affords 2(IH)-pyrazinone derivatives containing substituents with desired functional groups at positions 3 and 6 in high yield. 

In some cases, substrate racemization can be achieved by an appropriate change of a functional group. A good example of this strategy is the synthesis of acid 10, which is the key intermediate for roxifiban - a potent selective antagonist of the platelet plycoprotein Ilb/IIa receptor (Scheme 5.8) [27, 28]. 

There are no reliable data that indicate whether this conclusion is valid for all types of catalysts or is instead specific for the catalysts studied by Fierro et al. [91], because -SO3H functional groups also show other functions to which the effect on selectivity could be related. Fierro et al. [3], in their review, demonstrated that the high selectivity for H202 can be achieved in the presence of cationic Pd species, but such proof has not been reported in literature, because the reaction mechanism is complex and changes in the surface state of Pd as the H202 synthesis reaction occurs. Therefore, there are still no unequivocal conclusions on the real nature of the active and selective sites in direct H202 synthesis. 

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