Relay cascade

Grigg, R. Sridharan, V. (1999) Palladium Catalyzed Cascade Cyclisation - anion Capture, Relay Switches and Molecular Queues. Journal of Organometallic Chemistry, 576, 65-87. 

The alkyne relay, in particular, has frequently been used for cascade carbopaliadations with ring formation, because a carbon-carbon triple bond is more reactive toward carbopalladation than a carbon-carbon double bond. Thus, methyl -iodocinnamate 70 reacts with diphenylacetylene 71 to yield the methylenediphenylindene derivative 72 (Scheme 22). ... 

Scheme 22 Alkyne relays in inter-intramolecular carbopalladation cascades leading to ring-annelated alkylidenecyclopenta-dienes and cyclopentenes. ...

Scheme 23 Highly convergent assembly of calcitriol 83 employing an alkyne-relayed inter-intramolecular carbopalladation cascade.

Tricyclic skeletons such as 85, 87, 89 with a central benzene ring are formed in the fully intramolecular Pd-catalyzed cascade cyclization of 2-bromo-l-ene-//,w-diynes 84, 86, 88 and analogs (Scheme 24). This process involves two alkyne relays in a row and a final 67r-electrocyclization or 6-endo-trig carbopalladation with ensuing / -dehydropalladation. 

A single alkyne relay is involved in the cascade oligocyclizations of 2-bromoalka-l,w-diene-//-ynes such as 90, 92, 94 and others. Three new rings are formed in these sequences of two intramolecular Heck-type reactions and a 67r-electrocyclization leading to various oligocyclic systems such as 91, 93, 95 in a rather elegant way (Scheme 25). - ... 

Scheme 24 Two alkyne relays in sequence in intra-intramolecular carbopalladation cascades. ...

For such cascades with intramolecular carbopalladations, smaller ring sizes (5 and 6) are preferred due to the entropic term thus, 1,1-disubstituted alkenes suitably located in the substrate preferably serve as the relay without formation of the larger ring size resulting from insertion into the terminal alkene moiety. 

V. Saraf, F. Zhao, B.W. Bequette, Relay autotuning of cascaded-controlled open-loop unstable reactors, Ind. Eng. Chem. Res. 42 (2003) 4488 1494. 

Multiple cyclo olefinic relays are also suitable substrates to promote cascades of RCM-ROM-RCM reactions [9] (Scheme 16). Thus, W-protected polycyclic amines have been synthesized in moderate to good yields, but an increasing number of cyclopentene relays gave rise to less efficient cascade transformations. This ring rearrangement metathesis (RCM-ROM-RCM) has been extensively reported by Blechert and co-workers for its application in the synthesis of natural products [30-34]. 

Photogenerated cofactors can be employed to drive biocatalytic enzyme cascades including the photosynthetic carbon dioxide fixation process [184] (Figure 35). Photogenerated NADPH provides a two-electron relay for the insertion of CO2 into a-ketoglutaric acid (23) and pyruvic acid (21) in the presence of isocitrate dehydrogenase (IcitDH) and malic enzyme (MalE), respectively. In these photosystems, Ru(bpy)3 " acts as a photosensitizer, as a primary electron acceptor... 

Purely intramolecular cascade reactions using triple bonds as a kind of relay station are well known (Schemes 3-10, 3-16, 3-20, 3-21, 3-22, and 3-24). An inter-intramolecular variant was developed as an elegant access route to calcitriol (60) (Scheme 3-18) [170]. 

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