C-X fragment

The most commonly employed strategy for a solid-phase [2 -i- 3] cycloaddition is to support the dipolarophile as a C-C or C-X fragment. The 1,3-dipole is then generated in solution and, without isolation, reacted with the aforementioned supported dipolarophile. However, there are also examples in which the 1,3-dipole, a C-C-X, or a C-X-C fragment are supported and allowed to react with the dipolarophile in solution. 

The most common C-X fragments are imines, which can be readily obtained from a polymeric support aldehyde by many high-yielding methods [281, 282, 283]. Dor-wald et al. reported Wang resin-supported 3-enaminoamides [284]. Yamazaki generated enamines from support-bound phosphonoesters [148]. Imines serve as starting materials for [2 -r 2]-cycloadditions, [2 -i- 3]-dipolar cycloadditions, and het-ero Diels-Alder reactions (HD). 

The C-C-C-X fragments have a wide applicability on solid phases. Among the most versatile building blocks are a,jl-unsaturated carbonyl compounds. These can be used as building blocks for hetero-Diels-Alder reactions vdth inverse electron demand. As in the case of the C-C-C-C fragments, a vari-... 

Another C-C-C-X fragment is shikimic acid-derived 4,5-epoxy-3-hydroxycy-clohex-l-ene-l-carboxylic acid [270]. The latter can be bound onto an amino group which is bound to TentaGel via a Gysen Linker (367) [322]. Subsequent esterification with alkyl or aryl nitrone acids according to Tamura et al. [323] was concurrent with [2 -i- 3]-cycloadditions to the allylic double bond. As a result, tricyclic lactones (370) were obtained, which could readily be modified in many ways to yield a wide variety of polyfunctional structures (Scheme 77). 

Compounds of this class were not reviewed in CHEC(1984) and CHEC-II(1996) and, accordingly, for completeness, this section includes references to older work. In general, six-membered heterocycles containing the C-B-C-X fragment (X is any heteroatom) in which the C-B bonds are simple two-center two-electron bonds are rare much more common are the polyhedral carboranes where multicenter bonding predominates. Nevertheless, several stable representatives of 1,3,5-triboracyclohexanes are known. 

Fig. 2.4. If one superposed the Xr-C-X2 and Xj-C -X fragments of both molecules, the other two substituents could not match X in place of X3 and Xj in place of X4. The two molecules represent two enantiomeric isomers. A wave function that describes one of the enantiomers does not have a definite parity and is therefore illegal .

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