Resin-bound vinyl ether

Isoxazoles have also been obtained through an elimination reaction (Scheme 11.40). Cycloaddition of nitrile oxide to resin-bound vinyl ether gave resin-bound 2-isoxazoUne, which aromatized to isoxazole after the elimination of the polymer support. A regioselective reaction gave isoxazoles in high purity and with 36—83% yield. Enhanced diversity was obtained with the Suzuki coupling reactions to the R group prior to the cycloadditions. 

Thepreparadonof resin-bound nitroalkenes via a micro wave-assisted Knoevenagel reacdon of resin-bound nitroacedc acid v/ith aryl and alkyl subsdnited Mdehydes is reported. The potendiil of these resin-bound nitroalkenes for apphcadon in combmatorlM chemistry is demonstratedby a Diels-Alder reacdon v/ith 2,3-dimethylbutadienefScheme 8 9) Iris Mso used for one-pot three-component tandem [4t-2 /[3t-2 reacdons v/ith ethyl vinyl ether and styrene... 

The preparation of resin-bound nitroalkenes via a microwave-assisted Knoevenagel reaction of resin-bound nitroacetic acid with aryl and alkyl substituted aldehydes is reported. The potential of these resin-bound nitroalkenes for application in combinatorial chemistry is demonstrated by a Diels-Alder reaction with 2,3-dimethylbutadiene (Scheme 8.9). It is also used for one-pot three-component tandem [4+2]/[3+2] reactions with ethyl vinyl ether and styrene 46... 

Bicyclic nitroso acetals were able to be synthesised by employing ethyl vinyl ether (dienophile), styrene (dipolarophile) and the previously discussed resin-bound ni-troalkenes in a one-pot tandem [4+2]/[3+2]. As illustrated in Scheme 7.30, several aromatic and aliphatic substituents could be introduced to the bicyclic scaffold. Reductive cleavage of the cycloadducts with lithium aluminium hydride (LLAIH4) gave rise to the 3a-methyl alcohol substituted nitroso acetals in moderate overall yields. All these examples demonstrate that resin-bound nitroalkenes can be readily synthesised by microwave synthesis and thereafter can be used as starting materials, in a variety of high pressure-promoted cycloadditions. 

A survey of the literature reports regarding the synthesis of (3-lactam using carbohydrate precursors has recently been published by Furman et al. [215]. They have shown that the carbohydrates were used either as chiral tools or chiral auxiliaries. A few solid phase approaches have been outlined in the survey using the vinyl ether bound to the polymeric support through a sulfonyl linker. Two alternative modes of attachment of the vinyl ethers to the resin have been reported. 

As shown in Table 9.3, both (hetero) aromatic and aliphatic R substituents can be introduced via the Knoevenagel condensation. In a parallel fashion, ethyl vinyl ether (10) and styrene (74) reacted with the resin-bound nitroalkenes 73a-e at a pressure of 15 kbar for 20 h (50 °C), after which the resin-bound domino adducts 75a-e were formed (Scheme 9.30). After the domino [4 + 2j/ [3 + 2] cycloadditions... 

The emfo-selective reaction of a resin-bound heterodiene with a chiral vinyl ether catalysed by Eu(fod)3 occurs with excellent facial selectivity the catalyst can be recycled <01TL8849>. 

Later, the same research group reported a similar [4h-2]/ [3+2] sequential cycloaddition of resin-bound nitroalkenes, ethyl vinyl ether, and styrene under high-pressure conditions [88]. The cycloadducts were obtained in moderate yields (29-51%) as a mixture of diastereomers. 

Lysek and coworkers [111] reported the preparation of P-lactams via a [2+21-cycloaddition reaction of CSl with resin-bounded chiral vinyl ethers 168 followed by the intramolecular alkylation of the fS-lactam nitrogen atom to give the corresponding diastereomeric clavam 169 (Scheme 3.48). 

Bisdiazaphospholanes have been linked to organic resins via the correspond-ingtetraacyl fluoride derivatives (Scheme 2.147) [69]. Compared with the relevant homogeneous catalysts, comparable regio- and enantioselectivities were observed in the AHF of styrene, vinyl acetate, 0-silyl protected allyl ethers, and dihydro-furane. Excellent recyclability with only trace levels of rhodium leaching were reported under batch or flow conditions. In contrast, the silica-bound catalyst I exhibited poorer enantioselectivities. 

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