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In addition to hydrogen-bond-donor templates, we expanded
the template approach by reversing the “code” of the assembly
process to involve a CCF that functions as a hydrogen-bond-
acceptor [21]. In particular, co-crystallization of 2,3-bis(4-
methylenethiopyridyl)naphthalene (2,3-nap) with fumaric acid
(fum) generated 2(2,3-nap)·2(fum), with C=C bonds parallel and
separatedby3.84Å(Fig.7.2b).UVirradiationproduced
-
1,2,3,4-cyclobutanetetracarboxylicacidinupto70%yield.The
study involving the code reversal also showed how carboxylic acid
groups, which are important functional groups in organic synthetic
chemistry, could be directly “incorporated” within a cyclobutane
product.
As res, 1,8-nap, and 2,3-nap are symmetrical, we next developed
an unsymmetrical CCF as a template. From the studies of induced
π-stacking,weexpectedRebek'simide(Reb-im)tofunctionasan
unsymmetricaltemplate.Eachdiferent“hand”ofReb-imwould
assembleandinteractwithtwodiferent4-pyridylgroupsof4,4
rctt
′
-
bpe. Indeed, co-crystallization of Reb-im with 4,4
′
-bpe produced
′
2(Reb-im)·2(4,4
-bpe) (Fig. 7.2c) [22]. The molecular assembly
involvedacombinationofO-H∙∙∙NandN-H∙∙∙NforceswhereinReb-
im adopted an antiparallel orientation within the hydrogen-bonded
structure. The solid reacted to give 4,4
′
-tpcbstereospecificallyand
in quantitative yield.
7.4.1
Complex Targets
ThefactthatCCFsbasedonrescoulddirectreactivitywithinfinite
assemblies prompted us to explore the construction of a more
complextargetintheformofa[2.2]paracyclophane(Scheme7.7).The
cyclophane framework, which was studied extensively by Cram [23],
has relevance to physical organic chemistry and materials science
owing to the photo- and electrochemical properties. A retrosynthetic
analysis of the cyclophane suggested that the molecule could be
constructed using a res template from the bifunctional diene 1,4-
bis[2-(4-pyridyl)ethenyl]benzene (1,4-bpeb). Co-crystallization of a
reswith1,4-bpebwouldyieldthediscreteassembly2(res)∙2(1,4-
bpeb) with the two dienes preorganized for photoreaction. UV
irradiation would give tetrakis(4-pyridyl)-1,2,9,10-diethano[2.2]-
paracyclophane (4,4
′
-tppcp).
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