The requirements of an enzyme analogue are discussed in terms of (i) binding, (ii) chirality, and (iii) functionality. The ability of 18-crown-6 derivatives to complex with primary alkylammonium cations indicates the potential of the 18-crown-6 constitution to provide the binding requirement of an enzyme analogue. Attention is drawn to the fact that carbohydrates provide not only functionality but also a relatively inexpensive source of chiral bismethylenedioxy units for incorporation into the 1 8-crown-6 framework. The optically pure configurationally chiral cryptands2 ) , and DD-(33) have been synthesised from L-tartaric acid and D-mannitol. The 18-crown-6 locks excluding the tetraol L L -( ~ 6) and the octaol ~D -( 3 1 ) , have been shown by l H n.m.r. spectroscopy to form complexes in CD,C12 with primary alkylammonium salts. The stability constants for complexes formed in CDCI, solution between (i) t-butylammonium thiocyanate and (ii) benzylammonium thiocyanate and some of these 18-crown-6 locks have been determined by an l H n.m.r. spectroscopic method and compared with the complexing ability of 18-crown-6 (34). l H and 13C N.m.r. spectroscopy has been used to demonstrate that the locks ~~-( 1 8 ) and ~~-( 2 8 ) exhibit enantiomeric differentiation in complexation equilibria towards (RS)-a-phenylethylammonium hexafluorophosphate [(RS)-( 8),HPFB]. The tetraol L L -( ~ 6) and the octaol ~~-( 3 1 ) have been shown by l H n.m.r. spectroscopy to form complexes in CD,OD and D20 with primary alkylammonium salts. The tetra-U-isopropylidene derivative ~~-( 2 8 ) forms strong complexes in methanolic solution with alkali metal cations.
Sheffield S3 7HFRuncorn, Cheshire WA7 4QEHISTORY records that serendipity played a crucial role pound exhibits intriguing properties. Perhaps most in the discovery by Pedersen significant is the fact that it forms stable complexes, ethers. The accidental synthesis of dibenzo-18-crown-6 both in the crystalline state and in solution,lP2p4 with of the so-called crown (l) led lS2 to the appreciation that this ' Om-3
Optically pure configurationally chiral 18-to the development of so-called host-guest chemistry2 by crown-6 and 9-crown-3 cryptands have been synthesised Cram et aL3 Carbohydrates and their derivatives are not from L-tartaric acid and D-mannitol and some of the 18-only rich in substituted bismethylenedioxy units for crown-6 derivatives have been shown to form cryptates incorporation into the 18-crown-6 constitution but they also with metal and primary alkylammonium cations.provide a relatively inexpensive source of chirality for synthesising hosts with potential for exhibiting chiral recognition towards enantiomeric guests. This prospect has now been realised by utilising L-tartaric acid and D-THE ability1 of macrocyclic polyethers of the 18-crown-6 type t o complex with primary alkylammonium salts has led
enedi-D-mannitol [ D D -( ~) ] exhibit enantiomeric diff erentiation in complexation equilibria towards ( &) -(BS)-aphenylethylammonium hexafluorophosphate.RECENTLY the first steps have been taken by chemistslJ towards synthesizing organic catalysts which will exhibit the characteristics of regioselectivity and stereoselectivity in catalysing chemical reactions that are normally associated with enzyme-catalysed transformations found in Nature.
The availability3 of optically pure configurationally chiral cryptands ~~-( 1 ) -( 3 )and 0~-( 4)-( 6) which are derived from relatively inexpensive sources of chirality and which complex primary alkylammonium cations has prompted us to examine their chiral recognition properties towards (+)-(I?)-, (-)-(S)-, and ( f )-(RS)-a-phenylethylamine Cryptands (1)-(6) all have D, symmetry and conse-
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