Tandem electrocyclization

Diol 4 had been prepared from diol 7 by a cascade initiated by semihydrogenation presumably intermediate diol 8 undergoes in situ Sk/6k tandem electrocyclization. We prepared diol 7 in one step as described and... 

Modifying our strategy, we used a conventional coupling/tandem electrocyclization method to obtain diol 4. Thus, we prepared ( ,Z)-iododiene 10 and ( ,Z)-boronate 11 and subjected these intermediates to Suzuki conditions to obtain bicyclooctadiene 12. Deprotection released diol 4 (Scheme 5). 

Raju, R., Piggott, A.M., Conte, M.M., and Capon, R.J. (2010b) Heronamides A-C, new polyketide macrolactams from an Australian marine-derived Streptomyces sp. A biosynthetic case for synchronized tandem electrocyclization. Org. Biomol. Chem.. 8,4682-4689. 

A pseudo-C(2) symmetric trans-diphenyl oxazoline group has been reported to act as a chiral auxiliary in the 8k, 6k tandem electrocyclization of a substituted tetraene 1-carboxylic acid in a process where both s-cis and s-trans conformations favour the transition states with the same helical twist (Scheme 29) ... 

Annulation of pyridine to indole is accomplished by a tandem aza-Wittig/electrocyclization strategy as shown in Eq. 10.68."... 

Strategies based on two consecutive specific reactions or the so-called "tandem methodologies" very useful for the synthesis of polycyclic compounds. Classical examples of such a strategy are the "Robinson annulation" which involves the "tandem Michael/aldol condensation" [32] and the "tandem cyclobutene electrocyclic opening/Diels-Alder addition" [33] so useful in the synthesis of steroids. To cite a few new methodologies developed more recently we may refer to the stereoselective "tandem Mannich/Michael reaction" for the synthesis of piperidine alkaloids [34], the "tandem cycloaddition/radical cyclisation" [35] which allows a quick assembly of a variety of ring systems in a completely intramolecular manner or the "tandem anionic cyclisation approach" of polycarbocyclic compounds [36]. 

Using the tandem aza-Wittig/bir-electrocyclization strategy, an isoquinoline synthesis starts with salicylaldehyde, which is converted with azido-... 

The tandem aza-Wittig/67r-electrocyclization principle has been used for the synthesis of 1,3,5-benzotriazepines. After pioneering work done by... 

This multicomponent reaction was used by the same group to prepare a ring-expanded analogue 114 of the marine sesquiterpenoid frondosin B through a remarkable reaction cascade [102]. An unprecedented tandem 1,7-hydrogen shift, 8 7r-electrocyclization converting 112 to 113 was proposed to explain the formation of this ring-expanded species (Scheme 8.47). 

Fig. 7.9 Electrocyclic tandem reaction as generated by RAIN. Rearrangement of a benzocyclobutane 8 into a isochromanone lTvia intermediate compounds 9 and 10.

According to RAIN, only one reaction pathway namely the electrocyclic tandem reaction8— 9—-10—-11 leads from 8 to II under the conditions given below. 

The corresponding reaction of 2-alkenyl-3-alkadienyM-nitropyrrole 1156 also leads to 3-nitroindoles 1158 (21-90%) via intermediate 1157, however, now together with 3-nitrotetrahydroindole derivatives 1159 (Scheme 225) <1998T1913>. The latter compounds are formed by a tandem 6jt-electrocyclization-intramolecular Diels-Alder reaction, and are the predominant (or only) products when nitrobenzene is replaced by triglyme (70-78% yield). 

Three enantiomerically pure starting materials ensure remote stereochemical control Tandem iminium ion formation and vinyl silane cyclisation Part IV - Tandem Pericyclic Reactions Electrocyclic Formation of a Diene for Diels-Alder Reaction Tandem Ene Reactions Tandem [3,3]-Sigmatropic Rearrangements Tandem Aza-Diels-Alder and Aza-Ene Reactions... 

The cyclobutene ring first opens in an electrocyclic reaction 152. This must be conrotatory as it is a four electron process but there is no stereochemistry at this stage. Then an intramolecular Diels-Alder cycloaddition 153 closes the new six-membered ring. This is a particularly favourable reaction as the formation of the alkene completes a benzene ring. It would not be possible to prepare such an unstable diene so a tandem process is necessary. 

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