Ring alkene mimics

The Z-alkene isostere mimics the cis-amide bond conformation. Its molecular volume and log p values are almost identical to those of the E-isostere (Table 1). Its use in bioactive peptides is very limited, probably because of the synthetic challenge involved. One study reports the migration of the double bond to the a,(3-position in an enkephalin analogue. 4 The Z-alkene in the Alat t[Z, CH=CH]Pro dipeptide isostere, however, was reported to be stable towards isomerization.1 1X1 orf/to-Substituted aromatic or tetrazole rings have been used more frequently as ds-amide bond mimics. 

Complexes X, XI and XlVa mimic the steps of the ring-opening polymerization of five-membered cyclic alkenes and account for the reversibility of the polymerization of cyclopentene. In the structure of compound XlVb the double bond shifts so that, once more a five-membered ring is formed. 

Bonding of the 3jt p(P)-2jt p(C) type is now generally believed to be present in the trivalent phosphorus compounds indicated in Table 6.13. Such compounds have proved invaluable in the synthesis of organophosphorus ring and cage derivatives. It is believed that the chemical behaviour of P=C bonds mimics that of alkenes very closely [15]. 

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