Tandem cycloaddition trans

The introduction of a substituent on the C2 position changed the reaction pathway. In the case of the trans- 161b, tandem [2 + 2]-cycloaddition at the internal C=C bond and [3,3]-sigmatropic rearrangement proceeded exclusively [133, 135]. 

The Lewis acid-promoted tandem inter[4 + 2]/intra[3 + 2]-cycloaddition of the (fumaroyloxy)nitroalkene (124) with the chiral /i-silylvinyl ether (125) is the key step in the total synthesis of (+)-crotanecine (126), the necine base of a number of pyrrolizidine alkaloids (Scheme 46).237 The tandem inter[4 + 2]/intra[3 + 2]-cycload-ditions of nitroalkenes (127) with dipolarophiles attached to the /f-carbon of a vinyl ether (128) provides a method of asymmetric synthesis of highly functionalized aminocyclopentanes (129) (Scheme 47).238 trans-2-( 1 -Methyl-phenylethyl)cyclohex-anol has been developed as a new auxiliary in tandem 4 + 2/3 + 2-cycloadditions of nitroalkenes.239 The scope and limitations of the bridged mode tandem inter-[4 + 2]/intra[3 + 2]-cycloadditions involving simple penta-1,4-dienes are described in detail.240 A tandem intermolecular/intramolecular Diels-Alder cycloaddition was successfiilly used to synthesize a B/C cA-fused taxane nucleus (130) in 50% overall... 

A process that can be considered a tandem [4 + 2]/[3 + 2] sequential cycloaddition has been developed by Kozikowski et al. (Scheme 16.6) [34]. In this example the [4 + 2] cycloaddition between diene 24 and dienophile 23 forms cycloadduct 25, which is isolated as the trans isomer 26. Upon dehydration of the nitro group with an isocyanate [35], the resulting nitrile oxide participates in an intramolecular [3 + 2] cycloaddition to afford the final product in 65% yield. Kozikowski and Jung reported similar reactions that involve both [4 + 2] and [3 + 2] cycloadditions, but even more intervening steps are inserted between them [36]. 

The Fused Mode Cycloaddition The fused mode of the tandem intermolecular-[4- -2]/intramolecular-[3 + 2] cycloaddition of nitroalkenes is the most extensively studied of all the variants. Figure 16.10 correlates the configuration of the nitroso acetal product with the requirements for the configuration of the starting materials. If present, the substituents at C(4), C(5), and C(6) are established during the inter-molecular [4 + 2] cycloaddition as outlined earlier. The trans... 

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